3 Biochemical Insights

It is appropriate to reemphasize that the famous German biochemist Otto Warburg,

who received a Nobel Prize in 1931 for later work, found that cancer cell metabolism,
by and large, is anaerobic — that is, does not require oxygen — whereas normal
cell metabolism is aerobic. Thus, cancer cell metabolism involves primarily the
conversion of blood sugar or glucose (or its polymer, glycogen) ultimately to lactic
acid or lactate by an enzyme-catalyzed anaerobic fermentation rather than the oxi-
dation of glucose ultimately to carbon dioxide and water as occurs with normal cells.
If a way can be found to nullify the specific enzyme catalysts required, then con-
ceivably this could lead to a cure for cancer. (The pioneering work of Warburg is
reviewed in a book by Hans Krebs titled Otto Warburg: Cell Physiologist, Biochemist
and Eccentric. The original papers of Warburg and coworkers are produced in The
Metabolism of Tumors: Investigations from the Kaiser Wilhelm Institute for Biology,
Berlin-Dablem, edited by Otto Warburg. Warburg’s results on tumor cell glycolysis
and respiration were first published in 1923.) However, cancer cell metabolism is
complex, and is partly aerobic, with many reaction intermediates, and has led to
much controversy, as pointed out in Alan C. Aisenberg’s The Glycolysis and Respi-
ration of Tumors, published back in 1961. A later development is that glutaminolysis,
the conversion of the nonessential amino acid glutamine mainly to lactic acid or
lactate, also occurs in tumor cells (Eigenbrodt et al., 1985, II, p. 141ff; Hoffman,
1999, pp. 23, 24). Moreover, other amino acids can be so converted, by the processes
of aminolysis. The metabolism of cancer cells is beginning to appear more and more
complicated.
In this regard, the production of lactic acid from sugars by lactic-acid-forming
bacteria is an industrial process of long standing. If the enzyme or enzymes produced
by the bacteria were to be confronted and extrapolated to the human condition, and
inhibitors in turn identified, perhaps this would lead to something important like a
cure for cancer. This notation also applies to the production of lactic acid from
another compound called glutamine. And it may provide a rationale for the action
of anticancer agents, in that they may serve to destroy or inhibit the enzyme-catalyzed
metabolism of cancer cells.
Speaking further of enzymes, in brief mention there are enzymatic treatments
that have been used against cancer. These include such enzymes as trypsin, chymo-
trypsin, ficin, papain, bromelin, fungal proteases, deoxyribonuclease, lipase, and calf
thymus extracts. The subject is reviewed in a chapter of biochemist Francis X.
Hasselberger’s Uses of Enzymes and Immobilized Enzymes, in which mention is also
made of the Wobe-Mugos enzyme mixture, as used in Germany and Europe (Has-
selberger, 1978, p. 145ff). The mixture can be given orally, rectally, or injected into
the abdominal muscles, but is most effective when injected directly into the tumor,
where it literally dissolves the tumorous tissue. The animal and plant enzymes used

83

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84 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

are obtained from beef pancreas, calf thymus gland, and the plants Pisum sativum
(a variety of pea), Lens esculenta (a variety of lentil), and the papaya tree (Hassel-
berger, 1978, p. 142).
Here, however, the emphasis will be on enzyme inhibitors per se, as they pertain
to cancer cell metabolism — that is, inhibitors or blockers for certain critical
biochemical reactions occurring in the conversion of glucose to pyruvic acid or
pyruvate, and then to lactic acid or lactate.

THE WARBURG CANCER THEORY

The theory advanced by Nobel laureate Otto Warburg, outlined earlier, is that normal
cells undergo the aerobic oxidation of glucose, whereas cancer cells undergo an
anaerobic fermentation to lactic acid or lactate (the neutralized form), followed by
the conversion of the lactic acid or lactate back to glucose. Warburg’s results were
first published in 1923, and have since produced a great deal of controversy, as the
situation is much more complicated than appears at first glance, and is becoming
more so. Some of the work along the way has been described in Alan C. Aisenberg’s
The Glycolysis and Respiration of Tumors, published in 1961. For one thing, both
anaerobic and aerobic glycolysis occurs in cancers or tumors, although the first route
is predominant. Later investigations indicate that the amino acid glutamine also plays
a role.
Concomitantly, in the liver and muscle tissue the glucose is converted to and
exists in a form called glycogen, by a process called glycogenesis. Glycogen is a
high-molecular-weight branched-chain polymer or polysaccharide composed chiefly
of glucose units or monomers. The conversions are catalyzed by various enzymes
that are highly specific to each particular conversion. In the terminology used, the
conversion of glycogen to lactic acid or lactate is called glycogenolysis, and the
reconversion of lactic acid or lactate back to glycogen may also be called glycogen-
esis, confusingly, as is the conversion of glucose to glycogen. In general, we may
speak of glycolysis, the enzymatic conversion or breakdown of sugars and other
carbohydrates.
(Glucose, also called dextrose or corn sugar, may exist in several forms. It is
what is called optically active, with the usual form exhibiting the dextrorotatory
property, and is known as dextroglucose, written D-glucose or d-glucose. In turn
there are the α and β phases, depending on the particular range of conditions. For
the purposes here, however, the word “glucose” suffices, with the chemical formula
HCO(CHOH)4CH2OH or the simple stoichiometric formula C6H12O6 — although a
more complicated ring structure is involved.)
Fermentation, in its restricted sense, is the anaerobic enzymatic conversion of
carbohydrates to other useful products. The most well-known example is no doubt
the production of ethyl alcohol or ethanol from the sugar resulting from the conver-
sion of malted grains. Although yeast is listed as the agent causing or catalyzing the
fermentation, the actual agent is an enzyme or enzymes produced by the yeast, which
may be either unknown or unspecified. Another example occurs in putrefaction, for
example, the anaerobic digestion of organic wastes, producing chiefly methane and
carbon dioxide, or so-called sewer gas.

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Biochemical Insights 85

Enzymes are proteinaceous structures at the molecular level that specifically

catalyze certain biochemical reactions, or that as coenzymes may take part in the
reaction. Some vitamins, for instance, are related to coenzymes. The subject of
enzymes or enzymology is an expanding field, with a corresponding proliferation
of books and articles.
The anaerobic conversion of glucose (or molasses) to lactic acid is an industrial
process of long standing, for which the microorganism used is named Lactobacillus
delbruckii. This bacterium is the source of the enzyme, which is not routinely
specified. (The enzyme is one form or another of lactate dehydrogenase, as will be
further detailed in the sections titled “Glycolysis,” and “Production of Lactic Acid
or Lactate” below.) The conversion is called homolactic fermentation as compared
to heterolactic fermentation, because the product is almost entirely lactic acid. The
general subject area is that of bacterial metabolism (e.g., in the McGraw-Hill Ency-
clopedia of Science and Technology).
Other microorganisms that produce homolactic conversions are notably of the
genus Streptococcus. This is most interesting in that Streptococcus is such a common
source of infections. Thus it can be projected that possibly here is a mostly unsus-
pected cause of cancer, although maybe indirectly — or at least the genus could be
supportive of cancer growth.
It may be added, moreover, as has been indicated, that a species of Streptococcus
was identified as the elusive Progenitor cryptocides, the pleomorphic microbe found
in the research by Dr. Virginia Livingston to be a source of cancer. Conversely, a
species of Streptococcus is used in what are called Coley’s toxins as an immuno-
logical treatment for cancer. This seems to be a common enough paradox, that cause
and cure are somehow entwined. That is, the causes of cancer may at the same time
cause the body to attempt to fight off the cancer. Or as the saying goes, for every
action there is a reaction.
There is also an enigma buried here. Should or should not a cancer patient take
antibiotics if an infection is indicated? Is the infectious microbe in fact causing
cancer or reacting against cancer? Or doing more of the one than the other? It is a
research area that requires a sensitive test to determine whether the cancer is on the
upswing or on the downswing (the subject of monitors).
Although various microorganisms or bacteria are listed as the agents for fer-
mentation, as noted earlier the actual agents are enzymes produced by the micro-
organisms. Thus it may be said that the sources for enzymes in general are bacteria,
fungi, and yeasts. Whether the particular enzyme involved in the body reactions is
the same enzyme involved in say, the industrial formation of lactic acid remains
conjectural at this point, for it could as well, be another. (The conversion of sugars
to lactic acid remains a long-standing industrial process as per H. Benninga’s A
History of Lactic Acid Making: A Chapter in the History of Biotechnology.) Suffice
to say that biotechnology is an expanding field, and involves among other things
the production of penicillin and the other antibiotics by fermentation processes.
There is of course an equal or surpassing interest about enzymology in medicine
proper.
Central to applying Warburg’s theory is the suppression of cancer by the use of
inhibitors that would break or inhibit the glucose–lactic acid cycle. The existing

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86 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

cancer cells would atrophy and not be replaced. Presumably the hydrazine sulfate
therapy developed by a certain Joseph Gold (of the Syracuse Cancer Research
Institute) would act in this manner. Moreover, and importantly, a believable theory,
explanation, or mechanism is provided. That the medical establishment tends to
concur can be illustrated by the fact that the hydrazine therapy was removed from
the ACS unproven methods blacklist in 1982 (Walters, Options, 1993, p. 49). How-
ever, as Michael Lerner finds in Choices in Healing, after the positive reports the
negative reports started to filter in.
(As a reminder although we may be talking of anticancer agents, in the strictest
sense we are not necessarily talking of cures. In fact, the FDA does not permit claims
to be made for cancer cures, although it is permissible to speak of cancer therapies
or cancer treatments.)
The anticancer role of nitrogen-bearing compounds may be that of substances
that inhibit or break, in one way or another, the enzyme-catalyzed reactions in the
glucose–lactic acid cycle. The topic of enzyme inhibitors is in itself an expanding
subspecialty within the field. Enzymatic reactions and enzyme-breaking actions are
involved, for instance, in the efficacy of antibiotics and in other unexpected areas.
These enzyme-breaking or enzyme-inhibiting substances may be predominantly
nitrogen-bearing compounds, as represented for instance by the poisonous alkaloids
and cyanogens of plant origin. Or they may be still other kinds of substances, for
example, the compound named nordihydroguaiaretic acid, or NDGA, the main active
ingredient of chaparral or creosote bush (Larrea tridentata), which is commonly
found in the American Southwest. For instance, in work by Dean Burk of the NCI
it was shown that NDGA inhibits both aerobic and anaerobic glycolysis, particularly
the latter (Walters, Options, 1993, p. 135). Antioxidants may also play this role. It
could even explain the beneficial action of chemotherapy drugs in small amounts.
And may take in the subject of homeopathy, not to mention enzyme therapy. In fact,
this enzyme-breaking or enzyme-inhibiting property may account for the wide vari-
ety of anticancer substances found in nature, as compiled in Hartwell’s Plants Used
Against Cancer, and those cited in other listings of herbal remedies or treatments.
If the foregoing speculations are correct, at least in part, then there is the problem
of making the enzyme-breaking or enzyme-inhibiting actions more effective. The
goal should be not merely enzyme inhibiting, but enzyme breaking, and maybe
enzyme destroying. In other words, the object is to find a selective “poison” for the
enzyme catalysts involved. That is, a poison that will act only on the particular
enzyme or enzymatic reaction, and not on other enzymes, and not on the rest of the
body.
Furthermore, in order to nullify the enzyme-catalyzed conversion or conversions,
it may be required that the enzyme-breaking substance or poison be “activated,” or
that various poison enhancers be added. The latter may be the role of creatine, for
instance, in enhancing the effectiveness of urea as an anticancer agent. In one way
or another, this may also be the role of electromagnetic radiation as an anticancer
agent.
Although microorganisms per se are easily poisoned by metallic substances,
whether this can be extrapolated to enzymes is moot — though it provides a point
of departure. And it is worth noting that such metallic substances as germanium and

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Biochemical Insights 87

selenium compounds are observed to be anticancer agents. Arsenic, for instance, is

listed in J. Leyden Webb’s Enzyme and Metabolic Inhibitors. In a general way, we
may also be speaking of so-called heavy metals and their compounds, which in
higher concentrations are toxic, but potentially may act as anticancer agents in very
low or trace concentrations (homeopathy again!) As for some well-known industrial
catalyst poisons, nickel compounds foul up the anaerobic digestion of organic wastes,
and vanadium compounds adversely affect petroleum-refining catalysts. Sulfur com-
pounds are notorious as metallic or inorganic catalyst poisons, although sulfided
metals sometimes serve as catalysts for certain kinds of reactions, such as hydroge-
nation. Even electromagnetic fields or radiation may have a catalytic or anticatalytic
effect. The subject of catalysis, in fact, often seems more of an art than a science, but
presumably involves compositional heterogeneity and electrochemical phenomena.
(It is interesting to note that sulfur-containing vegetables such as garlic and
onions, as well as the cruciferous members of the mustard family, such as cabbages
and broccoli, are considered anticancer agents in their dietary effects. These effects
conceivably may involve poisoning the enzyme or enzymes favoring the glucose–lac-
tic acid cycle, the feature of anaerobic cancer cell metabolism.)
A further development (related to Warburg’s work) concerns what is called
apoptosis, whereby billions of body cells perish each day to be replaced by new
cells. There is an enzyme called apopain that determines whether the cells live or
die. This enzymatic effect on apoptosis may possibly be inhibited or favored by an
additive, which potentially may be of interest in the treatment of cancer. This again
brings up the point that some of the myriad biologically active plant substances may
act in such a way or other ways.
As a concluding statement, the fact that a treatment works for some, at least
some of the time, is an indication that it should be perfected rather than rejected.
Nor is it always necessary to understand how a treatment works, for what we are
most interested in are results. For example, we may never know exactly how anti-
biotics work, other than that they are enzyme inhibitors. This, in spite of all the
biochemical formulas and equations, and the lengthy, technical, and encyclopedic
treatises on the subject. For every answered question always leads to further ques-
tions. Ditto for the immunizations against various viral diseases, such as polio and
smallpox — though we seem almost at a point that for some diseases the only new
cases seem to be from the vaccination itself. Nevertheless, we are most grateful that
antibiotics and immunizations most often do work, and we’ll accept the same for
any treatment of cancer.
It is commonly said that everyone has cancer cells, but they are kept in check
presumably by the action of the body’s immune system. In this respect, cancer is
like other viral diseases, such as polio, which most times may be thrown off with
the symptoms of a little fever and a runny nose, as with a common cold. The
exceptions are, of course, where the trouble starts.
The fact that cancer gains a foothold may thus be a sign that the immune system
did not work satisfactorily or was overwhelmed. Therefore, an argument can be
advanced against compromising or depleting the immune system with, say, chemo-
therapy; the body needs all the help it can get. Rather, it is the signal to go on with
something else, say, an anticancer agent that will negate further cancer growth by

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88 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

inhibiting or destroying the enzyme-catalyzed anaerobic metabolism of the cancer

cells. If this thesis is correct, then the search is for the best anticancer agents to do
this, and the correct dosage and frequency, which may have to be tailor-made for
the individual patient.
Whether this regimen may be described as therapeutic, or after the fact, there
is also an interest in prophylactic regimens before the fact, as in the development
of inoculations against cancer, similar to smallpox or other serious diseases. At the
moment, however, therapeutic regimens remain at the forefront of interest. Never-
theless, whereas Virginia Livingston’s or similar cancer vaccination theories and
practices were once ignored or blacklisted, now there is a revival of interest in this
same kind of therapy. In particular, a vaccine therapy was studied for metastatic
malignant melanoma at the John Wayne Cancer Institute of Saint John’s Hospital
and Heath Center at Santa Monica, California, as was described by Donald L. Morton
and Andreas Barth in the July/August, 1996, issue of CA — A Cancer Journal for
Clinicians, a review publication of the American Cancer Society.
The use of chemotherapy, on the other hand, notably of the drug dicarbazine,
has been judged unsatisfactory for advanced or metastasized melanoma. Accord-
ingly, cancer vaccines have been tried with a degree of success. These vaccines are
made from attenuated whole cells, cell walls, specific antigens, or nonpathogenic
strains of living organisms. (The last-mentioned source underscores the sometimes-
observed correlation between resistance to bacterial infections and to cancer.) There
are, in fact, a large number of known serum melanoma-associated antigens (MAAs)
that induce an immune response. The initial response, however, is much slower than
with cytotoxic chemotherapy agents, taking 4 to 6 weeks to manifest.
That cancer vaccines may conceivably work is not surprising, given the previ-
ously mentioned work of Dr. Virginia Livingston and others. In fact, there is a vaccine
for feline leukemia. Some of the latest findings are reviewed in Exploring the
Biochemical Revolution in a section on Arousing the Immune System Through
Vaccines, and although aimed more at viruses per se, there may be a connection to
cancer formation, since viruses are recognized as a primary cause of cancer. One
version would involve injecting the DNA from a particular microbe, which would
cause an immune response by the host. Another mode would be to inject a vaccine
into an edible fruit or vegetable. Whether the extrapolation can be made to cancer
remains to be seen.

CELL METABOLISM
As has been indicated, the biochemistry of cancer and its treatment may be funda-
mentally expressed in terms of the differences in metabolism between cancerous
cells and normal cells. By metabolism is meant the biochemical processes by which
blood sugar or glucose (or its polymer, glycogen) is converted to energy and end
products. The essential differences date back to the investigations of German bio-
chemist Otto Warburg, first published in the 1920s, as previously noted. Thus,
Warburg observed that cancer cells undergo anaerobic behavior, which does not
require oxygen, whereas normal cells undergo aerobic behavior, which requires
oxygen. The fuel or energy source is principally glucose or its equivalent, although

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Biochemical Insights 89

such amino acids as glutamine may be involved to a limited degree. It may be added
that such anaerobic processes are sometime referred to as fermentation (although
there are aerobic as well as anaerobic “fermentations”).
We may further distinguish cells as prokaryotic and eukaryotic. Prokaryotes are
bacterial cells (and blue-green algae cells), whereas eukaryotes comprise all other
cells. The eukaryotes have a membrane-enclosed nucleus or organelle, which con-
tains their DNA, which encodes the genetic information. The remainder of the cell
is, in fact, made up of membrane-enclosed organelles having different functions,
bound together as the plasma, in turn enclosed by an outer membrane. Prokaryotes
do not have a nucleus, having only a simple unicellular structure, though colonies
of independent cells may exist. On the other hand, eukaryotes may be both unicellular
and multicellular. Viruses, as distinguished from bacteria, are considered nonliving
entities because they cannot reproduce outside the host cell. For the present purposes
here, only eukaryotes or eukaryotic cells are of interest.
The foregoing brings up the matter of “genes,” which in an abstract sense are
hereditary units of action or function, about which there is apparently not a complete
consensus. In any event, each such genetic unit can evidently be shown to occupy
a specific locus or segment in a length of DNA, and in a chromosome, a gene may
change into different forms called alleles, the fundamental basis of mutations.
The partial or overall cellular conversion process by which glucose is converted
or utilized may be called glycolysis, whereas the utilization of some other reactant
or nutrient, such as the nonessential amino acid called glutamine, is called glutami-
nolysis, and so forth. For cancerous cells, the main and final end product from
glycolysis is lactic acid, also called lactate. For normal cells, the main and final end
products are carbon dioxide and water, the same as for simple combustion. There
are, of course, intermediate products, principally a compound called pyruvic acid,
or called pyruvate in its neutralized form. Both anaerobic and aerobic glycolysis
yield pyruvic acid or pyruvate as an intermediate. Its further conversion marks the
overall difference between anaerobic and aerobic glycolysis, the former yielding
lactic acid or lactate, the latter carbon dioxide and water.
More generally, the term metabolism can refer to any biochemical reaction or
reaction sequence; the involved substances are known as metabolites, and may be
reactants and reaction products (or intermediates). In its more usual usage, however,
metabolism, or primary metabolism, pertains to biochemical reaction sequences or
conversions that are the primary source of energy. These life-sustaining biochemical
reactions are designated exothermic, meaning that heat energy is given off. The
quantity of heat so furnished (or evolved) can be designated the heat of reaction.
This energy may be converted to, or make an appearance as, still other forms of
energy — kinetic energy, or energy of motion, being one form. What is called
chemical or biochemical energy is stored as biomass, namely, as carbohydrates, fats,
and/or proteins, for subsequent utilization. Carbohydrates and fats in foodstuffs are
the main energy sources, being ultimately converted to glucose by digestive pro-
cesses. Proteins or amino acids are also convertible for energy purposes.
In more restricted usage, the term metabolite is reserved only for reaction
products, with the primary metabolites being those produced from energy-giving

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90 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

biochemical reactions; whereas the products from all other kinds of reactions can
be called secondary metabolites.
Furthermore, there may be great differences in the quantity of energy furnished
by different reactions. In the case of anaerobic cancer cell metabolism, the overall
heat of reaction is minimal. In the aerobic metabolism of normal cells, the heat of
reaction is much larger — equivalent to the heat of combustion. Glutaminolysis, by
comparison, does not give off much energy.
The cancer problem and solution can be viewed as that of inhibiting, blocking,
poisoning, controlling, regulating, or modulating cancer cell metabolism without
adversely affecting normal cell metabolism. The focus thus shifts to enzymes and
enzyme inhibitors, whereby cancer cells can be selectively “starved.”
Each and every biochemical reaction in the body is catalyzed or favored by a
unique proteinaceous substance called an enzyme, and a sequence of biochemical
reactions will require a separate and distinct enzyme for each individual reaction or
reaction step. (Moreover, there may be a supporting reaction or reactions involved
as well.) Such reaction sequences are commonly called “pathways.” Some of the
reaction steps and corresponding enzymes may be more significant than others, in
that they may control the overall conversion rate. In other words, a slower reaction
or reactions in the sequence tends to control the overall conversion rate.
This subject is biochemical or organic catalysis, and for the technically inclined
there is a fairly recent four-volume elaboration of various reaction mechanisms,
titled Comprehensive Biological Catalysis: A Mechanistic Reference, edited by
Michael Sinnott and published in 1998.
As indicated, any or all of these enzymes or enzyme catalysts may be modulated,
regulated, poisoned, blocked, or inhibited by another substance or substances,
organic or inorganic, and proteinaceous or otherwise. These latter substances may
be said to deactivate the active catalytic sites, or receptor sites, as they may also be
called — a terminology adopted from the explanations of inorganic catalysis. Modern
drug therapy is based on enzyme inhibitors, antibiotics being the outstanding exam-
ple, with the antibiotics blocking vital enzyme-catalyzed pathways in bacterial
metabolism.
What are commonly regarded as poisons serve to act as enzyme inhibitors,
severely interfering with a life-sustaining process such as respiration or the func-
tioning of the heart. The drugs of conventional cytotoxic chemotherapy also act as
enzyme inhibitors, interfering, for instance, with the DNA/RNA/protein synthesis
pathway — processes that will be further described subsequently. All cells are
affected to a degree in using chemotherapy, in particular the faster-growing cells
such as in hair filaments, and notably those that comprise the immune system and
gastrointestinal tract, with anemia a possible side effect.
Thus, it is of primary importance that enzyme inhibitors be selective toward a
certain enzyme, and not adversely affect other enzymes; that is, it should not produce
pronounced side effects or adverse effects. Fortuitously, this is mostly the case for
the common antibiotics used, in the dosages prescribed, and which therefore in the
main attack bacterial rather than human cells. (Some lesser-known antibiotics are
toxic even to humans, that is, are toxic to normal human cells.) As previously

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Biochemical Insights 91

indicated, conventional cytotoxic chemotherapy drugs unfortunately attack normal

cells as well as cancer cells, and all too often more so.
In essence, therefore, the objective is to determine what substances could inter-
fere with cancer cell processes, but not normal cell processes, the criterion being
interference with cancer cell metabolism as distinguished from normal cell metab-
olism. A statement made by Donald Voet and Judith G. Voet in their treatise on
biochemistry is still relevant (Voet and Voet, 1995, p. 595): “Attempts to understand
the metabolic differences between cancer cells and normal cells may one day provide
a clue to the treatment of certain forms of this deadly disease.”
This difference is underlined by the observation of biochemist Robert A. Harris
that glucose consumption under anaerobic conditions may be 20 times greater than
under aerobic conditions (Harris, in Devlin, 1986, p. 353). That is, a much higher
utilization of glucose is required to meet the energy demands of anaerobic glycolysis,
as occurs for cancer cells — in that the glucose ends up as lactic acid or lactate
instead of carbon dioxide and water, the normal products of aerobic glycolysis. This
is reflected in the fact that anaerobic conversion has a much lower overall heat of
reaction than normal aerobic conversion.
The foregoing considerations do not touch on the causes of cancer in the first
place. The overall conventional thinking is that cancers are of genetic origin, whether
inherited or induced, their basis ultimately residing in our chromosomes and genes.
Speaking of induced cancers, attributed to external causes or sources, a general
consensus is that cancers are viral-related, radiation-related, and chemical-related.
This was reflected in the landmark three-volume treatise Origins of Human Cancer,
published back in 1977, and edited by H. H. Hiatt, J. D. Watson, and J. A. Winsten,
with the individual chapters by authorities in their respective fields.
(In Book B of the aforementioned Origins of Human Cancer, published in 1977,
there is a chapter by Wattenburg et al. on the study of inhibitors for chemical
carcinogenesis as caused by the application of such carcinogens as BP or
benzo(a)pyrene to rats and mice. The antioxidants BHA (butylated hydroxyanisole)
and BPT (butylated hydroxytoluene) acted as suppressants or anticancer agents, but,
of course, not as cures. These findings were significant as these compounds are
commercial food additives. Other compounds tested included disulfiram (Antabuse,
tetraethylthiuram disulfide), benzyl isothiocyanate, and selenium compounds, as well
as various compounds used as chemopreventives. The final verdict is still out.)
Viruses, once considered perhaps the foremost cause of cancer, were subse-
quently discounted, only to be later reconsidered. For a mechanism or theory has
been provided by which a virus invades the chromosome to produce cancer-forming
genes, or oncogenes. At the same time, radiation and chemicals certainly cause
chromosomal/gene damage, presumably creating oncogenes. As for viral causes, it
can be added that a virus itself is only a segment of protein of uncertain makeup
and characteristics, and there are even subviruses called prions. We have to accept
that these are virus-related phenomena that may never be fully understood.
Expanded, more technical versions of the foregoing subject areas were included
in Part 1 of this author’s Cancer and the Search for Selective Biochemical Inhibitors,
published in 1999. A synopsis of Part 1 comprises the remainder of this chapter.
The paradigm used is cell metabolism, which provides a systematic way to analyze

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92 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

cancer and its suppression in terms of enzymes and enzyme inhibitors. Although
matters can be taken to increasingly micro levels — say to DNA and its encoded
genetic information — enzymes and their inhibition relate more directly to the macro
world about us, and are emphasized.
The remaining contents of this chapter serve to support subsequent findings
about cancer, and illustrate the overall complexity of the subject. The challenge is
to manage and apply this information toward immolating cancer cells. There is
always the dilemma of overstatement vs. understatement. But when medical folklore
opines that a (inexpensive) herbal or other substance acts against (or even cures)
cancer, the skeptic wants to know if this could indeed be true. Understanding the
reasons for possible anticancer action requires us to explore some underlying bio-
chemistry.

ENZYME-CATALYZED METABOLIC PATHWAYS

Metabolism — that is, primary metabolism — signifies the ways by which energy
is supplied to cells; it is the means by which life and growth are sustained. Funda-
mentally, it is related to the exothermic aerobic processes of normal cell respiration,
which involve the biochemical conversion of glucose or its equivalent sugars, or
blood sugars, a process ultimately involving oxygen, and known as glycolysis, or
aerobic glycolysis. These are the enzyme-catalyzed steps that ultimately yield carbon
dioxide and water, and at the same time produce energy, signified by the term
exothermic. The energy produced may be broadly designated as thermal energy, or
heat; other manifestations also occur, particularly cellular growth or proliferation.
Cellular growth is manifested in the classes of substances known mainly as
proteins, fats, and carbohydrates. Additionally, there is the metabolic support of the
myriad body functions, of maintaining body temperature, and of permitting body
motion. In other words, the subject is that of chemical or biochemical energy stored
as biomass or body mass, of thermal energy manifested as body temperature, and
of kinetic (and potential) energy changes manifested as body motion (in an external
gravitational field).
Aerobic glycolysis first involves a ten-step conversion of glucose to pyruvic acid
or pyruvate, called the Embden–Meyerhoff–Parnas pathway, followed by its further
conversion to carbon dioxide and water via what is variously called the tricarboxylic
acid cycle, or citric acid cycle, or Krebs cycle after its discoverer. The net products
discharged from the cycle are carbon dioxide and water, with recycle of a further
product called oxaloacetic acid or oxaloacetate. Successive organic acids that contain
three carboxyl groups (-COOH), are initially involved in the cycle starting with citric
acid or a neutral salt of citric acid (citrate). Hence the designator tricarboxylic.
What we call anaerobic glycolysis also involves the same ten-step conversion
of glucose to pyruvic acid or pyruvate. However, the pyruvic acid is instead further
converted to lactic acid or lactate in a single step catalyzed by an enzyme called
lactate dehydrogenase (and sometimes called lactic acid dehydrogenase).
Although the carbohydrate glucose or its polymer glycogen is regarded as the
fundamental fuel or nutrient, other carbohydrates may be involved, even nitrogen-
containing amines, or amino acids. Thus, for example, there is the role of glutamine,

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Biochemical Insights 93

a nonessential amino acid (that is, glutamine can be produced internally by body
processes), in cancer cell metabolism. Referred to as glutaminolysis, the utilization
of glutamine may be incorporated into the metabolic loop. It is illustrative of how
both normal and cancerous cells may utilize still other fuel sources such as proteins,
which are made up of chains of amino acids. Thus, other amines or amino acids
also metabolize, though glutamine is the predominant amino acid.
Illustrative diagrams showing the principal biochemical reactions are portrayed
in Figure 3.1 through Figure 3.4. To summarize, these biochemical reactions are
divided into the following categories:

1. Glycolysis, the ten-step sequence producing pyruvic acid or pyruvate,

which occurs in all cells, normal or cancerous.
2. Anaerobic conversion of pyruvate to lactate, as occurs in cancer cells.
3. Glutaminolysis, the conversion of glutamine, which may occur simulta-
neously.
4. Aerobic tricarboxylic acid cycle, which oxidizes pyruvate to carbon diox-
ide and water as end products and liberates energy, as occurs in normal
cells.

The convention is to denote each forward reaction step by an arrow, for conve-
nience represented downward (↓). For reversible reactions, in both directions, a
double arrow is used (↑↓). The particular enzyme catalysts are specified for each
step, along with whatever supportive reactions occur, with both reactants and prod-
ucts shown. Further, a positive or negatively charged atomic or molecular entity is
called an ion (or radical), and may be designated a positive ion by a (+) superscript,
and a negative ion by a (–) superscript. An increase of positive charge (or lesser
negative charge) is termed oxidation, and a decrease in positive charge (or greater
negative charge) is termed reduction. Oxidation can also correspond to the addition
of oxygen, which takes up an electron but in the process itself becomes “reduced.”
Reduction can correspond to the addition of hydrogen, which loses an electron but
in the process itself becomes “oxidized.” In other words, both oxidation and reduc-
tion occur simultaneously, the terminology depending on which way the electrons
move. (Such oxidation-reduction reactions are sometimes abbreviated as redox reac-
tions.)
It may be further observed that the appropriate reactants, intermediates, and
products may be represented either in the acid form or the “-ate” form, the latter
designating the negative ionic form (or anion), which exists in combination with a
positively charged ion (cation), for instance, the hydrogen ion, denoted as H+. (That
is to say, in the conventions used for an electric potential difference between elec-
trodes, an anion, which is negatively charged, will travel to the positively charged
anode, and the cation, which is positively charged, will travel to the negatively
charged cathode.)
Consider for example the pyruvic acid/pyruvate product of glycolysis, which
may be written equivalently as

CH3COCOOH ⇔ CH3COCOO- + H+

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94 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

where the first term on the right is the pyruvate form. For the further production of
lactic acid/lactate, say, the equivalence is

CH3CH(OH)COOH ⇔ CH3CH(OH)COO- + H +

where the first term on the right is the lactate form. The hydrogen ion may be
replaced by some other positively charged ion, such as the sodium ion, Na+ or
potassium ion, K+. We will not be further concerned about this notation, but only
mention it here for completeness.
Plants also undergo the same metabolic respiration processes, whereby the plant
energy sources such as sugars undergo respiration to yield carbon dioxide and water.
This is accompanied by transpiration, the elimination of water vapor to the atmo-
sphere via the leaves, as derived from soil water and its nutrients, and also from
respiration. (And which influences climate, notably via the tropical rain forests.)
More details about plant biochemistry are furnished in the standard references and
textbooks, e.g., in Plant Physiology by Frank B. Salisbury and Cleon W. Ross (1985).
In plant photosynthesis the opposite of respiration occurs, whereby carbon
dioxide and water are converted to organic material, say, by what is called the Calvin
cycle, yielding first either a three-carbon compound or else a four-carbon compound,
and, in turn, the myriad other plant-generated compounds (Salisbury and Ross, 1985,
p. 195ff). The initial three-carbon compound, called 3-PGA for 3-phosphoglyceric
acid, results in what are called the C-3 plant species. By far the most numerous, the
C-3 plant species include all gymnosperms, pteridophytes, bryophytes, and algae,
as well as most trees and shrubs. Most C-4 plant species are monocotyfledons,
especially grasses and sedges. (Monocotyledon seedlings having one emerging leaf,
whereas dicotyledons have two emerging leaflets.) The C-4 plant species include
the important agricultural crops sugarcane, maize (corn), and sorghum, as well as
numerous range grasses.
Sunlight is the photochemical energy source for the photosynthetic conversion,
which can be termed endothermic, indicating that energy has to be added. Photo-
synthetic processes are also enzyme-catalyzed, the green substance, the magnesium-
containing chlorophyll, serving that purpose. Interestingly, the C-4 species are the
fastest growing and most efficient, yielding the most biomass per unit of photosyn-
thetic energy expended. Photosynthetic conversion rates range from 0.6 to 2.4 micro-
moles of CO2 converted per second per square meter of leaf surface, for agave, to
20 to 40 for corn or maize (Salisbury and Ross, 1985, p. 218).
Ideally, the C-4 plant species would represent the best prospects for controlled
or greenhouse photosynthesis from added carbon dioxide sources. Unfortunately,
there is an upper limit to the carbon dioxide concentrations that can be utilized in
greenhouse gases. Whereas normal atmospheric air may contain about 340 μl of
carbon dioxide per liter (340 parts per million, or ppm), or 0.000340 mole fraction,
or say 0.034 volume percent, the upper limit is around 1000 μl of carbon dioxide
per liter (or 1000 parts per million, or ppm), or 0.001 mole fraction, or 0.1 volume
percent, whereby toxicity occurs (Salisbury and Ross, 1985, p. 224).
Interestingly, algae can stand up to 50,000 ppm of carbon dioxide. Thus, it is a
prime candidate for the regeneration of carbon dioxide via controlled photosynthesis

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Biochemical Insights 95

in glass-tubed solar farms, to yield biomass (and water). The carbon dioxide source
would be from the steam gasification of coal or other fossil fuels to yield hydrogen
(H2) and carbon dioxide (CO2):

C + H2O(g) → H2 + CO2

where the separated hydrogen serves as a clean-burning fuel for power generation,
or even for automobiles provided the hydrogen storage problem can be successfully
resolved.
Of course, with a carbon dioxide buildup in the atmosphere from fossil fuel
combustion (causing global warming), normal atmospheric concentrations continue
to be exceeded. Thus, algae may provide a resolution.

GLYCOLYSIS
The ten-step glycolysis pathway producing pyruvic acid or pyruvate is diagrammed
in Figure 3.1. Also known as the Embden–Meyerhoff–Parnas pathway, after its
discoverers, this sequence occurs in both aerobic and anaerobic glycolysis. The main
biochemical reactions are catalyzed by enzymes, and at the same time require
supportive or complementary reactions. The faster reactions approach a condition
of chemical equilibrium. The slower reactions are controlling, and may be influenced
by inhibitors or promoters. Other chemical compounds or substances entering into
glycolysis via the supporting reactions are, for the record, designated as follows:

ATP — adenosine triphosphate

ADP — adenosine diphosphate
Pi —orthophosphate ion, in any ionization state
NAD+ — nicotinamide adenine dinucleotide, oxidized form
NADH — nicotinamide adenine dinucleotide, reduced form
H+ — hydrogen, oxidized form, or hydrogen ion

Still other substances and minutiae may enter, but are not shown. Generally
speaking, the various enzymes encountered are named according to their functions,
using the suffix “-ase,” and these organic catalysts may be regarded as proteinaceous.
The names of the enzymes involved in the ten successive glycolysis reaction
steps, as per Figure 3.1, are: (1) hexokinase, (2) hexose phosphateisomerase, (3)
phosphofructokinase, (4) aldolase, (5) triosphosphate isomerase, (6) glyceraldehyde-
3-phosphate dehydrogenase, (7) phosphoglycerate kinase, (8) phosphoglyceromu-
tase, (9) enolase, and (10) pyruvate kinase.
(Note: There is a systematic nomenclature for classifying the enzymes by func-
tion, as follows: oxidoreductases involve oxidation-reduction reactions; transferases
involve the transfer of functional groups; hydrolases involve hydrolysis reactions
with water; lyases involve the elimination of a group to form double bonds;
isomerases involve isomerization to a different structure but with the same chemical
composition; ligases involve the formation of a chemical bond simultaneously with
ATP hydrolysis. There are in turn subclasses and sub-subclasses, and a subclass that

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96 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

Glucose
(1) hexokinase ATP ADP
Glucose-6-phosphate
(2) hexose phosphofructokinase

Fructose-6-phosphate
(3) phosphofructokinase ATP ADP
Fructose-1, 6-biphosphate
(4) aldolase

Glyceraldehyde-3-phosphate
(5) triosphosphate isomerase
Dihydroxyacetone phosphate

Glyceraldehyde-3-phosphate
(6) glyceraldehyde-3-phosphate dehydrogenase Pi + NAD+ NAD+H+

1, 3-Bisphosphoglycerate
(7) phosphoglycerate kinase ADP ATP
3-Phosphoglycerate
(8) phosphoglyceromutase

2-Phosphoglycerate
(9) enolase

Phosphoenolpyruvate, or PEP (plus H2O)

(10) pyruvate kinase ADP ATP
Pyruvate

FIGURE 3.1 Glycolysis conversion sequence yielding pyruvic acid or pyruvate, with
enzymes and supportive reactions shown. (Based on information in Voet and Voet, Biochem-
istry, 1995, p. 446; Harris, in Textbook of Biochemistry, 1986, p. 334.)

will be of special interest is the kinases — as per tyrosine kinase in Chapter 10 —

and which are phosphoryl-transfer enzymes involving ATP. In the glycolysis pathway
steps 1, 3, 7, and 10 variously involve kinases. For the record, the phosphoryl group
involved in the transfers may be written symbolically as -(PO3)2-, where the super-
script signifies two negative charges.)
As has been previously indicated, one or another (or several, or even all) of the
particular enzymes may each be inhibited, blocked, poisoned, controlled, regulated,
or modulated by other substances. Thus, the glycolytic pathway is affected by such
inhibitors or poisons as 2-deoxyglucose, sulfhydryl reagents, and fluoride (Harris,
in Devlin, 1986, p. 346). Fluoride is a potent inhibitor of enolase, the enzyme for
Reaction 9 in glycolysis, as shown in Figure 3.1 (which can give pause for the
common use of fluorides for dental hygiene purposes.)
Furthermore, the activity of phosphofructokinase (PFK), the enzyme that con-
trols glycolysis by regulating Reaction 3, for instance, has been found to decrease
sharply when switching from anaerobic to aerobic metabolism. This is said to

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Biochemical Insights 97

Pyruvate

lactate dehydrogenase NADH + H + NAD+ + 2[H]

Lactate

FIGURE 3.2 Anaerobic conversion of pyruvic acid or pyruvate to lactic acid or lactate with
enzyme and supportive reaction shown. (Based on information in Voet and Voet, Biochemistry,
p. 464; Harris, in Textbook of Biochemistry, p. 334.)

account for a pronounced drop in the overall glycolysis rate for aerobic conditions
as compared to anaerobic conditions. In other words, cancer cells, which undergo
anaerobic glycolysis, will have a much greater glycolysis rate than normal cells.
Still other substances may accelerate or promote the glycolytic reactions, some
of which may be related to genetic causes. For instance, there may be a genetic
predisposition to promote the activity of the enzyme or enzymes involved in cancer
cell metabolism.

PRODUCTION OF LACTIC ACID OR LACTATE

The production of, say, lactic acid from pyruvic acid is sometimes called homolactic
fermentation. The enzyme involved in the one-step conversion is lactate dehydro-
genase in one form or another. A schematic diagram for the conversion is shown in
Figure 3.2. We omit further details about the conversion.
Another bioconversion that can occur will instead yield an alcohol. Thus, with
different enzymes (from yeast cultures), aldehyde can be produced from pyruvic
acid or pyruvate (the enzyme is pyruvate decarboxylase), then ethyl alcohol or
ethanol (the enzyme is alcohol dehydrogenase), the overall process being called
anaerobic alcoholic fermentation (Voet and Voet, 1990, p. 464). This is the classic
means of making alcoholic beverages from sugars (starches are first converted to
sugars, e.g., in what is called mash, via the enzyme amylase, as produced from
young barley sprouts by the process of malting).
In the formation of lactic acid or lactate, there may be a buildup of lactic acid
in body tissues, an occurrence called lactic acidosis (Harris, in Devlin, 1986, pp.
357, 358). Normally, the lactic acid will be oxidized to the end products carbon
dioxide (or CO2) and water (or H2O), or else will converted back to glucose in the
liver, by gluconeogenesis. (The entire process, glycolysis to lactic acid or lactate
followed by gluconeogenesis back to glycogen or glucose, is called the Cori cycle.)
Decreased oxygen availability favors an increase in lactate production and a decrease
in lactate utilization. Intense muscular exertion will also favor lactic acid or lactate
in accumulations well beyond those used in the tricarboxylic cycle. Bicarbonate can
be administered as a counter, however. Whether these phenomena could be related
to cancer growth or remission has presumably not been investigated.
Glucose may also exist in the form of its polymer, glycogen. Regarding glycogen
and its synthesis and degradation, the regulatory enzymes involved are variously
activated by a chemical compound called adenosine monophosphate, or AMP (Har-
ris, in Devlin, 1986, p. 391ff.). These enzymes are in turn further regulated by a
substance with the acronym cAMP, or cyclic AMP. It is reported, furthermore, that

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98 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

in acting as a hormone regulator, cAMP also serves as a cancer inhibitor, and is

assisted by the amino acid arginine (Boik, 1996, p. 48). These are some of the
indirect means by which cancer can potentially be controlled.
Thus, natural agents said to raise cAMP levels to inhibit cancer include the
species Andrographis paniculata (of the plant family Acanthaceae), Polyporus
umbellatus (of the fungal family Polyporaceae), Salvia multiorrhiza (of the plant
family Labiatae), Ziziphus jujuba (of the plant family Rhamnaceae), Cnidium mon-
nieri (of the plant family Umbelliferae), Actinidia chinensis (of the plant family
Actinidiaceae), Aconitum carmichaeli (of the plant family Ranunculaceae), Cinna-
momum cassia (of the plant family Lauracae), and the alkaloid caffeine. All families,
genera, or species are variously listed in Hartwell’s Plants Used Against Cancer
except for Andrographis and Actinidia.
The situation is complex, but illustrates the fact that there may be many as yet
unsuspected anticancer agents, and which act by unsuspected means.

GLUTAMINOLYSIS
For documentation purposes, a much-abbreviated reaction scenario for glutaminol-
ysis is shown in Figure 3.3. Glutamine is an amino acid, and glutaminolysis is only
part of the more general topic of the metabolism of amino acids, which is covered
in the standard texts and references on biochemistry.
That cancer cell metabolism may alternately be supported by glutaminolysis or
aminolysis is an additional complication in attempting to suppress cancer growth.

Glutamine (← H2O added)

glutaminase ↓
Glutamate (→ ΝΗ3 released)

glutamate dehydrogenase ↓
Oxoglutarate or α-Ketoglutarate
2-oxoglutarate or α-ketoglutarate dehydrogenase ↓
Succinate
succinate dehydrogenase ↓
Malate
malate dehydrogenase ↓
Pyruvate (or) Oxaloacetate

↓ ↓
Lactate Aspartate

FIGURE 3.3 Glutaminolysis as it may interface with the tricarboxylic acid cycle, with
enzymes shown. (Based on information in Eigenbrodt et al., 1985, pp. 145, 153; Voet and
Voet, Biochemistry, 1995, p. 741; Diamondstone, in Devlin, 1986, p. 583). Under carbohydrate
or glucose restrictions, all lactate will be produced from glutamine rather than from glucose
via glycolysis.

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Biochemical Insights 99

TRICARBOXYLIC ACID CYCLE

The tricarboxylic acid cycle can be thought as completing aerobic glycolysis to yield
carbon dioxide and water, plus energy, by oxidizing the pyruvic acid or pyruvate
obtained at the end of glycolysis proper. The organic acids involved in the cycle
have three carboxylic groups per molecule, and hence the name. The carboxylic
group can be denoted as -(C=O)-OH, or simply as -COOH, of which citric acid is
an example, with its formula representable as HOC(CH2COOH)2COOH.
As documented in Figure 3.4, a substance called coenzyme A or CoA enters the
tricarboxylic acid cycle. It is vital to the initiation of the cycle, and occurs in other
body processes. Its complicated chemical structure is diagrammed in most biochem-
istry textbooks (e.g., by Olson, in Devlin, 1986, p. 270; and by Voet and Voet, 1995,
p. 826). Conceptually, it can be viewed as a consortium of chemical compounds

Pyruvate (plus CoASH)

pyruvate dehydrogenase (PDH) multienzyme complex ↓ NAD+ → NADH + H+
Acetyl-CoA (with CO2 released)
Acetyl-CoA + recycled Oxaloacetate (plus H2O less CoASH)

(1) citrate synthetase ↓

Citrate (with H2O released)
(2) aconitase ↓ (with H2O released)

cis-Aconitrate
(2) aconitase ↓ (with H2O added)

Isocitrate
(3) isocitrate dehydrogenase ↓ NAD+ → NADH + H+
Oxalosuccinate
(3) isocitrate dehydrogenase ↓ (with CO2 released)
α-Ketoglutarate
(4) α-ketoglutarate dehydrogenase ↓ NAD+ → NADH + H+ (↓ CoASH; CO2 ↑)
Succinyl-CoA (← CoASH added)
(5) succinyl CoA-synthetase ↓ GDP + Pi → GTP (with CoASH released)
Succinate
(6) succinate dehydrogenase ↓ FAD → FADH2
Fumarate
(7) fumarase ↓ (with H2O added)
Malate
(8) malate dehydrogenase ↓ NAD+ → NADH + H+
Oxaloacetate (to be recycled)

FIGURE 3.4 Tricarboxylic acid cycle with enzymes and supportive reactions shown. (Based
on information in Voet and Voet, 1995, Biochemistry, p. 539; Olson, in Devlin, 1986, p. 280.)

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100 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

variously called β-mercaptoethylamine, pantothenic acid, adenine, and d-ribose.

Biosynthesis occurs in the body, starting with pantothenic acid, and is completed in
a succession of reactions, each catalyzed by a particular enzyme (Diamondstone, in
Devlin, 1986, p. 671; Voet and Voet, 1995, p. 826). Alternately, it may be considered
as existing also in what is called the thio (-SH) form and written as CoASH. Another
form is obtained by replacement of the (-H) above with an acetyl group (CH3CO-)
to yield what is referred to as acetyl-CoA, which could as well be written as acetyl-
CoAS or acetyl-SCoA. In other words, the sulfur content is always present.
(The foregoing illustrates the intrinsic and vital role of the B vitamin called
pantothenic acid in normal aerobic metabolism. Furthermore, by favoring aerobic
glycolysis, it may act as an anticancer agent by countering the anaerobic metabolism
associated with cancer. Conversely, a deficiency could discourage aerobic glycolysis,
thereby contributing to anaerobic glycolysis, that is, cancer cell metabolism. Thus,
nutritional requirements can be related to cancer formation or its inhibition.)
Some other substances entering the tricarboxylic acid cycle are the following:

NAD+ — nicotinamide adenine dinucleotide, oxidized form

NADH — nicotinamide adenine dinucleotide, reduced form
H+ — hydrogen, oxidized form, or hydrogen ion

GTP — guanosine triphosphate

GDP — guanosine diphosphate
Pi — orthophosphate ion

FAD — flavin adenine dinucleotide

FADH2 — flavin adenine dinucleotide, hydrogen, reduced form

where nicotinamide, guanosine, and flavin are variously nitrogenous compounds,

with flavin signifying the B vitamin called riboflavin, or B2. Again, we see that B
vitamins are essential to life processes.
A distinguishing feature of the substances listed previously is the presence or
absence of a phosphate group or groups, again underscoring the prominent role of
phosphates in life processes. As a point of departure, it may be noted that what are
called nucleotides form the monomeric units of the polymers that comprise nucleic
acids, and are thereby connected to the composition of DNA and RNA (Voet and
Voet, 1995, pp. 795, 796). The loss of a phosphate group or groups yields what is
called a nucleoside.

OVERALL GLYCOLYTIC CONVERSIONS

In anaerobic glycolysis, by definition, no oxygen is present and the tricarboxylic
acid cycle cannot proceed, and only lactic acid or lactate is the end product. In
aerobic glycolysis, both the tricarboxylic acid cycle and lactic acid or lactate for-
mation can occur. In the preferred limit, only the tricarboxylic acid cycle is assumed
to occur.

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Biochemical Insights 101

Here, the two routes can be compared. In the one, in the limit, lactic acid or
lactate formation can be assumed not to proceed during aerobic glycolysis, with
carbon dioxide and H2O as the final products. In the other, only lactic acid or lactate
will be the final product during anaerobic glycolysis.

AEROBIC GLYCOLYSIS
This is the normal body process whereby glucose or blood sugar is converted
ultimately to carbon dioxide and water. Following the ten-step conversion of glucose
to pyruvic acid, the pyruvic acid is then converted to the end products of carbon
dioxide and water via what is variously called the tricarboxylic acid, or citric acid,
or Krebs cycle. The conversion is strongly exothermic, fueling the body’s internal
processes and its external activities.
Consider the phenomenon referred to as spontaneous human combustion (SHC),
as publicized from time to time (e.g., as was reported in Arthur C. Clarke’s Myste-
rious Universe, shown on the Discovery Channel, for instance, on October 22, 1996,
and was mentioned in Charles Dickens’ Bleak House). If this weird phenomenon
does indeed occur, it could instead be referred to as spontaneous ignition, followed
by combustion. And if it is at least conceivable for aberrations to occur among the
enzyme-catalyzed reactions involved in the metabolism of glucose or other carbo-
hydrates to yield CO2 and H2O, then conceivably there may be a case. Ordinarily,
body metabolism reaction rates are miniscule as compared to the direct combustion
or combustive oxidation of conventional fuels. If enzyme promoters exist, however,
there is the possibility that runaway metabolic processes occur, similar to those in
the ignition and further combustion of carbonaceous materials. If so, ample air or
oxygen supply would also be required for this extremely unlikely scenario.
In further comment about spontaneous combustion, there is the example that
rags soaked in linseed oil or hydrocarbon solvents will ignite if the heat of oxidation
(or heat of combustion) is not dissipated or controlled. The practice is to store these
materials in air-tight containers or else spread them out for heat dissipation. The
storage of low-rank coals, particularly, has this problem, and coal storage piles
require monitoring. Some dried low-rank coals are even pyrophoric, bursting into
flame on exposure to air. Brazil nuts from the Brazil-nut tree, species Bertholletica
excelsa of the family Lecythidaceae, must be raked and turned when stored to avoid
spontaneous ignition and combustion. It may be said, therefore, that spontaneous
ignition and combustion is a common enough phenomenon.
As for the human body system, a fever can be induced by reducing the heat
normally dissipated to the surroundings. The phenomenon can be viewed in terms
of the caloric intake, without heat dissipation. If this heat is not dissipated, the body
temperature would continue to rise inordinately.

ANAEROBIC GLYCOLYSIS
Adding all the chemical equations involved, the overall net conversion of glucose
to lactic acid is slightly exothermic, which indicates that not much metabolic energy
results. In consequence, the anaerobic glycolysis rate must increase manyfold to

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102 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

support the equivalent bodily energy requirements. Thus, in partial confirmation,

anaerobic glycolysis rates have been observed that are 20 times aerobic glycolysis
rates (Harris, in Devlin, 1986, p. 353). A noticeable consequence is that the body
loses weight so as to meet its energy requirements.

CANCER CELL METABOLISM VS. NORMAL CELL

METABOLISM
The initial part of the glycolysis sequence or pathway producing pyruvic acid or
pyruvate is the same for both anaerobic and aerobic glycolysis. It would seem that
this part should not be inhibited or otherwise interfered with, and only the anaerobic
fermentation to yield lactic acid or lactate should be blocked, this being the main
distinguishing feature of cancer cell metabolism.
There are other factors to consider, however, such as cancer cells having an
abnormally high glycolysis rate, causing a buildup of lactic acid product. This will
in turn reduce the activity of PFK (phosphofructokinase enzyme), which falls off
under acidic conditions. Thus Reaction 3 of Figure 3.1 should expectedly be inhib-
ited, thereby reducing the glycolysis rate. In spite of this, however, the glycolytic
enzymes are still in such high concentrations that glycolysis remains at high levels
(Voet and Voet, 1995, p. 595). Even under aerobic conditions, cancer cells produce
much more lactic acid than expected, and in fact the glycolytic pathway will form
pyruvic acid or pyruvate much faster than can be utilized by the tricarboxylic cycle
of normal cells (Voet and Voet, 1995, p. 595). The conversion to lactic acid or lactate
is apparently favored whatever the conditions, indicating that cancer cell metabolism
is sustained no matter what.
The inference from the foregoing, therefore, is that the enzyme-catalyzed con-
version of pyruvic acid or pyruvate to lactic acid or lactate should somehow be
blocked.
There is also another possibility. Looking at the other reactions, that are involved
in glycolysis, consider the slower or controlling reactions, which are Reactions (1),
(3), and (10) of Figure 3.1, which are catalyzed, respectively, by hexokinase, phos-
phofructokinase (PFK), and pyruvate kinase. Their inhibition in part or in totality
could conceivably slow down the formation of pyruvic acid or pyruvate, and in turn
slow down the production of lactic acid or lactate, thereby acting against cancer cell
metabolism. This could also act against the tricarboxylic acid cycle, however; that
is, against normal cell respiration or metabolism.
A few inhibitors of these several enzymes are provided in Voet and Voet (1990).
These include glucose-6-phosphate as an inhibitor for hexokinase, as it is a reaction
product. Also, ATP and citrate are listed as inhibitors of PFK, and ATP is an inhibitor
for pyruvate kinase, as it is a conversion product. Other inhibitors are tabulated in
Appendix A of Hoffman’s Cancer and the Search for Selective Biochemical Inhib-
itors (1999). On the other hand, Voet and Voet (1990) list a few activators or
promoters for PFK, including ADP, fructose 6-phosphate (which is a reactant),
fructose 1,6-bisphosphate (which is a product), the ammonium ion, and the ortho-
phosphate ion Pi. The situation can get complicated.

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Biochemical Insights 103

An inhibitor for lactate dehydrogenase would block the formation of lactic acid
or lactate, the main distinguishing feature of cancer cell metabolism. This would
alternately favor the normal oxidation of pyruvic acid or pyruvate via the tricarbox-
ylic acid cycle. As a qualification, cancer cells also apparently undergo a degree of
oxidative metabolism, though the conversion to lactate or lactic acid occurs to a
much greater extent.
There are some other reactions that occur, additionally or alternatively, indicating
that cancer cells as well as normal cells have some escape routes for survival. These
other effects have been analyzed as follows in terms of high and low glucose
concentrations, and glutaminolysis.
A comparison of the glycolysis route for cancer cells at high and low glucose
concentrations vs. that of normal cells has been provided by Eigenbrodt et al. (1985,
p. 144). The analysis is involved, to say the very least, and here only the rudiments
will be presented as based on a previous synopsis (Hoffman, 1999, p. 21ff). The
ultimate objective would be to determine if the enzyme inhibitors for the particular
controlling enzyme reactions can be linked to anticancer agents, known or unknown.
For the record, the features are as follows, as per the effect of glucose concentrations:

(1) Regulation of glycolysis by oxygen in normal cells: Reaction 3 of Figure

3.1 is controlling under normal aerobic conditions, being slower than the
other reactions. This is partially offset by a high capacity to convert the
product of Reaction 3 on to pyruvate.
(2) Regulation of glycolysis by oxygen in tumor cells at high glucose con-
centrations: In tumor cells, there is a heightened activity of the enzyme
hexokinase in Reaction 1, of the enzyme phosphofructokinase in Reaction
3, and of the enzyme pyruvate kinase in Reaction 10, all of which produce
a high glycolytic capacity. The formation of pyruvate by Reaction 10 will
be decreased, however, by the presence of alanine, phenylalanine, and
ATP, which inhibit the enzyme pyruvate kinase. This will be eventually
offset by the buildup of reaction intermediates. Tumor cells, as compared
to normal cells, use almost the total glycolytic capacity, independent of
how much oxygen is present. That is, in tumor cells, it apparently makes
no difference whether an aerobic or anaerobic condition exists for con-
version to lactic acid or lactate to occur.
(3) Regulation of glycolysis by oxygen in tumor cells at low glucose concen-
trations or with alternative reactant energy sources other than glucose
(e.g., glutamine): At low glucose concentrations, the levels of fructose-
1,6-bisphosphate produced by Reaction 3 are very much lower than at
high glucose conditions (for instance, there is a much lower concentration
of glucose initially). The result is a deactivation of the enzyme pyruvate
kinase for Reaction 10. No conversion to pyruvate occurs, and no ATP is
synthesized. (Thus, little or no pyruvate is available for oxidation via the
tricarboxylic acid cycle, or for fermentation to lactic acid.) On the other
hand, conversion of the ATP yields the orthophosphate ion Pi, which
in turn activates glutaminase for the conversion of glutamine. Thus

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104 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

glutaminolysis proceeds as per Figure 3.3. The end result is a proliferation

of the tumor cells.

A conclusion reached by Eigenbrodt et al. (1985) is that the metabolic behavior

of tumor (or cancer) cells differs from normal cells in that the former show increased
anaerobic glycolysis and glucose uptake, enhanced glutaminolysis; enhanced nucleic
acid (DNA) synthesis capacity, and enhanced lipid synthesis (p. 143).
(The molecules called lipids include the fatty acids, which are relatively small
molecules compared to the macromolecules that are proteins, nucleic acids, and
polysaccharides (Voet and Voet, 1995, p. 16). Proteins, of first importance, are
polymers of amino acids; nucleic acids are polymers of what are called nucleotides;
and polysaccharides are polymers of sugars.)
Tumor cells, however, also show a reduced pyruvate and acetyl-CoA oxidation
rate as involved in the tricarboxylic cycle. There is a lower sensitivity to oxygen,
and a lower growth hormone requirement. Thus, as a potential anticancer measure,
there is the objective also of enhancing respiration or oxygen uptake, thereby favor-
ing the tricarboxylic acid cycle over lactic acid or lactate formation.
Thus, it may be concluded that perhaps the simplest way to inhibit cancer cell
metabolism is merely to block the enzyme lactate dehydrogenase for the conversion
of pyruvic acid or pyruvate to lactic acid or lactate.

SELECTIVE BIOCHEMICAL INHIBITORS

The cancer problem can be viewed as a search for ways to suppress cancer cell
proliferation by means of the foregoing routes (Figures 2.2 and 2.3), which pertain
to tumor cells. The agents of interest are enzyme inhibitors, which have been
explained as acting in several different ways, for example, by chemically reacting
with the enzyme, and by adsorption on the enzyme surface, either at so-called active
sites, or at least adjacent to these active sites.
Inhibitors determined for the controlling enzymes involved in cell chemistry
have been listed elsewhere (Hoffman, 1999, Table A-1 through Table A-3 of Appen-
dix A), as obtained from Jain’s Handbook of Enzyme Inhibitors (1982) and Zollner’s
Handbook of Enzyme Inhibitors (1993). The breakdown is for glycolysis, lactate
formation, and glutaminolysis. Some of the more common and simpler chemicals
or compounds serving as enzyme inhibitors for one or another of the various reaction
steps are as follows, as derived from the Jain and Zollner references. Many more,
natural or synthetic, no doubt remain to be discovered, as both the Jain and Zollner
references are dated. The sequences presented here parallel those presented by
Hoffman (1999).

GLYCOLYSIS INHIBITORS
Inhibitors may serve any of the several (ten) sequential steps involved in the glyc-
olysis pathway. A partial listing includes certain metallic or mineral substances,
notably calcium and magnesium (as the ions Ca++ and Mg++). These are among the
essential minerals found in the diet. Lithium is another inhibitor, which is notably

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Biochemical Insights 105

used in treating depression. The phosphate ion is reported to be an inhibitor, as is

the sulfate ion. Chromium acts as an inhibitor in the form of an ATP complex, where
adenosine is the principle component of ATP (and ADP).
It may be added that sulfides or disulfides, which are found to be glycolysis
inhibitors, also act as poisons or inhibitors for such inorganic catalysts as nickel or
nickel oxide catalysts.
Various sugars make the listings, denoted by the suffix “-ose.” Various amino
acids also make the listings, denoted by the suffix “-ine,” notable examples being
L-alanine and L-phenylalanine, commonly associated with nutritional supplements.
Arginine, an essential amino acid (not manufactured in the body), is said to be an
inhibitor. The suffix “pyridoxal” is listed as an inhibitor, whereby it may be noted
that pyridoxine (or pyridoxin) is called vitamin B6.
Estrogen is represented by the synthetic hormone diethylstilbestrol or DES.
Citric acid, found in citrus fruits, is listed as an inhibitor, and also occurs as a
reaction intermediate in the metabolic tricarboxylic acid or citric acid cycle. Ethanol
or ethyl alcohol makes an unexpected appearance as an inhibitor, as does glycerol.
The ubiquitous alkaloid ingredient of coffee, better known as caffeine, is listed as
an inhibitor (in fact, coffee enemas are sometimes used in folkloric cancer treat-
ments). Creatine, a nitrogenous compound found naturally in the body, is an inhibitor,
and is a known anticancer agent, for example, as used with urea in a mixture called
Carbatine.
Fatty acids make the list, especially lauric acid and unsaturated oleic acid.
Flavianic acid is a precursor in the preparation of the essential amino acid arginine
and the nonessential amino acid tyrosine. Moss (1992) has a chapter about using
arginine in the treatment of cancer. Tyrosine interestingly enters the picture as a
component of the enzyme tyrosine kinase, the latter in the role of an inhibitor for
cancerous stem cells, to be described in Chapter 10.
The chemical compound quercetin, listed in the compilation of inhibitors, is
considered an anticancer agent, at least in a folkloric sense. It occurs naturally in
chaparral or the creosote bush (Larrea tridentata); in fact, chaparral itself is con-
sidered an anticancer agent.
(We remind the reader that the term “anticancer agent” is to be distinguished
from “cure.”)
Not resolved, however, is whether the inhibition of glycolysis per se can be an
effective inhibitor of cancer cell metabolism, inasmuch as glycolysis is also involved
in normal cell metabolism.

INHIBITORS OF LACTIC ACID OR LACTATE FORMATION FROM PYRUVIC ACID

OR PYRUVATE

By virtue of what is called the law of mass action, the buildup of product itself
works against the conversion reaction. Thus, the very buildup of lactic acid or lactate
product serves as an inhibitor. (Albeit pyruvic acid is also listed as an inhibitor in
the compilations, but which is a reactant for the conversion to lactic acid.) Another
inhibitor listed is oxalic acid, a naturally occurring component of such vegetables
as spinach and rhubarb, and which becomes toxic in large amounts. The common

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106 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

chemicals salicylic acid (aspirin) and urea are both listed as inhibitors, and are both
known as anticancer agents. Serotonin, the brain neurohormone or neurotransmitter,
makes the listing. It is derived from the essential amino acid tryptophan, and is more
often known as a vital mind relaxant.

AN UPDATE ON INHIBITORS FOR CANCER CELL METABOLISM: GLUCOSE TO

LACTIC ACID
Inasmuch as anaerobic glycolysis to produce lactic acid or lactate occurs in cancerous
cells, a partial update on inhibitors is presented in Table 3.1, notably as presented
in the second edition of Zollner’s compendium (1993). Of special interest is allicin
as an inhibitor for lactate dehydrogenase. Itself a sulfur compound, it is formed from
a precursor sulfur compound called alliin by the action of the enzyme alliinase. Both
alliin and alliinase occur naturally in garlic, and the conversion to allicin normally
occurs with mastication of the garlic. Allicin is a transient sulfur-bearing compound,
decomposing on standing, and must be utilized fresh or near fresh. Garlic contains
still other organic sulfur compounds that are potential enzyme inhibitors.

GLUTAMINOLYSIS INHIBITORS
According to the previously cited reference (Hoffman, (1991), Appendix A), some
dyelike materials show up as inhibitors of glutaminolysis. Thus, the agents listed as
bromcresol (or bromocresol) green and bromcresol purple are dyes derived from
sulfonephthalein, starting with the compound called meta-cresol. Cresols in turn are
related to phenol or carbolic acid, and to the coal–tar mixture called creosote.
Phenolic-type compounds are also known to occur in chaparral or creosote bush
(genus Larrea), hence its alternate common name.
(Phenolphthalein, another organic dye (red), is used as an acid-base titration
indicator, and also in a laxative [e.g., in Ex-Lax®]. It is in itself sometimes considered
to be an anticancer agent. Having the same chemical formula but different structures,
the three cresol isomers — ortho-cresol, meta-cresol, and para-cresol — serve
variously as the starting materials for such industrial chemicals as the herbicides
DNOC and MCPA, are also used in plastics or resins, and can be converted even to
the common food antioxidant known as BHT.)
Albizzin, considered an anticancer agent, comes from a tree of the genus Albi-
zzia, family Leguminosae. It is found in India and Malaysia, and is listed in
Hartwell’s compendium Plants Used Against Cancer (1982b).
The metallic ions of copper (Cu++), lead (Pb++), and mercury (Hg++) are listed
variously as enzyme inhibitors for glutaminolysis. However, not only can they be
poisonous to humans, but they can also be poisons or inhibitors for inorganic
catalysts.
Ammonia (NH3), listed as an enzyme inhibitor, has a kinship with urea and such
other ammoniacal inorganic compounds as hydrazine sulfate. Not only may they be
potential enzyme inhibitors for glutaminolysis, but also for other metabolic reactions.
Both urea and hydrazine sulfate are known to have been tried against cancer,
apparently with mixed results.

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Biochemical Insights 107

TABLE 3.1
Update on Enzymes and Inhibitors Involved in Anaerobic Glycolysis
Hexokinase or Hexosekinase
*n-Acetylglucosamine; Allicin, *Allos 6-phosphate; *Chromioum-ATP complex; *Copper (Cu++);
*L-cystamine; Dihydrogiesenin; Disulfiram; *Disulfides; Gafrinin; Geigerinin; *Glucose 6-
phosphate; *Glutathione oxidized; Griesenin; 4′,5′,7-Hydroxy-3,6-methoxyflavonone; 4-
Hydroxypentenal, 2-(p-Hydroxyphenyl)-2-phenylpropane; Ivalin; *Glycerol; *Lauric acid; *Lyxose;
*Magnesium (Mg++); *Mannoheptulose; *Mannose; *Nucleotides; o-Phthalaldehyde; Quercetin;
Vanadate oligomer; Vermeerin; *Xylose, D-,

*6-Phosphofructokinase or Phosphofructokinase
Argeninephosphate; Caffeine; Calcium (Ca++); Citric Acid; Creatine phosphate; Ethanol, Fructose
diphosphate; Glucose 6-phosphate; Glycerol; Lauric acid; NADH; Oleic acid; Phosphoenolpyruvate
(PEP); Pyridoxal-5-phosphate; Phyrophosphate ion; Pyruvic Acid.

*Pyruvate Kinase
Alanine; Amino acids; AMP, Anions; Butyric acid; Calcium (Ca++); Creatine phosphate;
Diethylstilbestrol (DE); Fatty acids; Lithium (Li+); Phenylalanine; Phosphate; Pyhridoxal-5-
phosphate; Pyrophosphate; Pyruvic Acid, Quercetin; Sulphate; Tris.

*Lactate Dehydrogenase
Allicin (in Zollner, 1989); Arsenite ion; ATP; Butyric acid, 2-3-epoxy; 3GA-destran; Estradiol, 17-
B; Fatty acids; Glycerate; Butynoic acids; Lactic acid; L-maleic acid; Mandelic acid;
Mononucleotides; NAD; NADH.
* Appears in Appendix A of Hoffman (1999).

Source: From Zollner, H., Handbook of Enzyme Inhibitors, in two volumes, VCH, Weinheim, FRG,
1993; (*) from Zollner, H., Handbook of Enzyme Inhibitors, VCH, Weinheim, FRG, 1989; Jain, M.K.,
Handbook of Enzyme Inhibitors, Wiley, New York, 1982.

The unusual material labeled Tris has the lengthy chemical name tris(2,3-dibro-
mopropanol)phosphate, where “tris” stands for a certain amine group. It was once
used as a fire retardant for clothing or nightwear, and although an enzyme inhibitor,
it is also suspected of being carcinogenic. (This is not an unknown contradiction,
for whether a substance is anticancer or procancer may depend on the dosage as
well as other factors.)
The listings of this chapter can be viewed as incomplete, here and elsewhere,
given the many naturally occurring substances from the plant world and mineral
world, yet to be tested (not to mention synthetic compounds). The bioactive alkaloids,
for example, are prime possibilities.

THE TRICARBOXYLIC CYCLE AND RESPIRATION

Glycolysis and the tricarboxylic cycle are fundamental to respiration, that is, the
uptake and utilization of oxygen from the air. If respiration is shut down, suffocation
results. It is the way in which many poisons act, for instance, the deadly South

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108 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

American poisonous mixture called curare. Curare is derived from certain members
of the genus Strychnos, and contains a number of toxic alkaloids (strychnine being
a familiar name for such an alkaloid). Thus, enzymes in glycolysis and the tricar-
boxylic cycle proper are of fundamental concern, for their blockage or inhibition
could prove fatal. Inhibitors for enzymes involved in the carboxylic acid cycle are
listed in Appendix B of Hoffman (1999).
Arsenic poisoning requires a special mention. The enzymes affected are notably
pyruvate dehydrogenase and α-ketoglutarate dehydrogenase, as well as other
enzymes (Voet and Voet, 1995, pp. 547–548; Harris, in Devlin, 1986, pp. 348, 349).
The result, unfortunately, can be overinhibition, shutting down respiration. Chronic
arsenic poisoning occurs with smaller, cumulative dosages. Microamounts possibly
may act to stimulate the immune system, a thesis of homeopathy, for example.
(Interestingly, small amounts of arsenic have been used to put back the spring
in the step, and the shine on the coat, of older and decrepit horses, as Ben Green
writes in Horse Tradin’. Neither Jain (1982) nor Zollner (1993) include arsenic as
an enzyme inhibitor per se, but list enzymes inhibited by arsenate and arsenite ions,
that is, by compounds of arsenic.)
Both enzymes mentioned previously pertain to the tricarboxylic acid cycle
proper, and are apparently vital to sustaining respiration. Accordingly, inhibitors can
lead to undesirable and sometimes life-threatening consequences. Enzyme inhibitors
for pyruvate dehydrogenase are more commonly known, those for α-ketoglutarate
dehydrogenase less so. In the Jain reference respiration inhibitors are treated as a
special category.
Among the other notations is that acetaldehyde is to be avoided, it being a
respiration inhibitor. It is related to ethyl alcohol or ethanol and also to acetic acid,
but not necessarily to citric acid as involved in the carboxylic acid or citric acid
cycle. Alkaloids are expectedly respiration inhibitors, and anesthetics can have
respiration inhibition as a side effect. Aromatic acids such as phenol are bad news,
as are arsenate, cyanide, isothiocyanate, and thiocyanate. The heavy metals cadmium,
cobalt, copper, ruthenium, vanadate, and zinc are regarded as health risks, if not for
respiration, for other reasons.
(The cyanide and cyanate ions inhibit many enzymes, as cataloged in both Jain’s
and Zollner’s handbooks, which have extensive listings for enzymes that are inhibited
or poisoned. The copper ion in the cupric form Cu++ has many entries. Frontiersman
Jesse Chisholm, namesake for the famous Chisholm Trail, died after eating bear
grease stored in a copper container.)
The inclusion of fatty acids is a surprise, but maybe not for that called “guaiaretic
acid, nordihydro,” better known as NDGA or nordihydroguaiaretic acid. It is an
ingredient in chaparral or creosote bush, and was formerly used commercially as an
oxidation inhibitor in various applications (note that oxidation inhibitors are viewed
as beneficial for a number of purposes, including cancer suppression).
The hormone progesterone shows up as an inhibitor. Interestingly, so does
sucrose, or common table sugar.
The alkaloids papaverine and theophylline are listed as inhibitors, but not strych-
nine. However, many or most alkaloids should be suspect, depending on the dosage
level.

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Biochemical Insights 109

VITAMINS AND HORMONES AS INHIBITORS OR

PROMOTERS
If enzymes do not act or only act slowly, they can be stimulated or promoted by
small ionic or molecular entities called cofactors (Voet and Voet, 1995, pp. 337,
338). Examples of cofactors include metallic ions such as the zinc ion Zn++, and
also organic molecules called coenzymes, such as NAD+ (nicotinamide adenine
dinucleotide, in the oxidized form).
If the necessary cofactors cannot be synthesized within the body, they must be
ingested via the diet. The terminology then becomes that of vitamins, which can be
regarded as precursors of coenzymes. Nutritional deficiencies in fact served as the
impetus for discovering many vitamins, i.e., coenzymes. For instance, a component
of NAD+ is better known as nicotinamide (or niacinamide), and its carboxylic acid
analog is called nicotinic acid (or niacin). The human vitamin deficiency known as
pellagra yields to supplements of niacin or niacinamide.
The vitamins that are coenzyme precursors are all water soluble, whereas the
fat-soluble vitamins such as vitamin A and vitamin D are not components of coen-
zymes, though vital to the diet (and, strictly speaking, vitamin D can be classified
as a hormone rather than a vitamin).
Vitamins are not synthesized in the body, at least not in the amounts needed.
On the other hand, hormones are produced within the body, to be secreted by specific
glands. For example, vitamin D in the form called cholecalciferol (D3) is technically
a hormone, being synthesized in the skin from the ultraviolet irradiation of 7-
dehydrocholesterol, a metabolite or metabolic product of cholesterol. On the other
hand, an almost identical form called ergocalciferol (D2) is synthetically prepared
by the irradiation of ergesterol from yeast, and is the type used in nutritional
supplements and fortified foods. It is therefore a vitamin (and large doses are
considered toxic).
(Interestingly, vitamin C, or ascorbic acid, is a vitamin in human nutrition, since
a necessary enzyme is missing to convert what is called gulonolactone to yield
ascorbic acid (Ungar, in Devlin, 1986, p. 719), whereas in animals such as the rat
or dog, this enzyme is naturally present, so that ascorbic acid could then be called
a hormone.)
As a matter of record, Table 3.2 lists vitamins, chemical names, and stoichio-
metric chemical formulas. In many or most cases, however, the vitamin structure is
too complicated for any kind of simplified representation.
Vitamins that may act as enzyme inhibitors are listed in Table 3.3, as found in
Jain (1982) and Zollner (1993).
The effects of vitamins and various foodstuffs on cancer continue to be a subject
of interest and controversy, as are of course the effects of using other plant and
herbal substances. With regard to vitamins, Frank L. Meyskens authored the Mod-
ulation and Mediation of Cancer by Vitamins, published in 1983. This was followed
by another volume titled Vitamins and Cancer, edited by Meyskens and Prasad, and
published in 1986. A further update is furnished in Vitamins and Minerals in the
Prevention and Treatment of Cancer, edited by Maryce M. Jacobs and published in
1991.

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110 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

TABLE 3.2
Vitamins
Vitamin A Retinol C19H24-CH2OH
Vitamin B1 Thiamine hydrochloride; C12H18Cl2N4OS
thiamin chloride
Vitamin B2 Riboflavin; lactoflavin; C17H22N4O6
vitamin G
Vitamin B6 Pyridoxin C8H11NO3
Vitamin B12 Cobalamine; cyanocobalamin C63H90N14O14PCo
Folic acid Pteroylglutamic acid (PGA); C14H11N6O2-(C5H7NO3)n-OH
folacin; vitamin Bc; vitamin M where n = 1–7;
C19H19N7O6 for n = 1
Niacin Nicotinic acid; (C5H4N)COOH
3-pyridinecarboxylic acid
Pantothenic acid HOCH2C(CH3)2CH(OH)CO-
NHCH2CH2COOH
or N(α,γ-dihydroxy-β,β- C9H17NO5
dimethylbutyryl)β-alanine
Vitamin C Ascorbic acid; CO (COH)3CHOHCH2OH
antiscorbutin C6H8O6
Vitamin D Calciferol C28H44O
Vitamin E α-Tocopherol; C14H17O2 (C5H10)3 H
5,7,8-trimethyltocol
Vitamin K Phthiocol; 1,4-naphthoquinone, C11H8O3
2-hydroxy-3-methyl-

Source: Adapted from Table 1.1 in Hoffman, Cancer and the Search for Selective Biochemical
Inhibitors, CRC Press, Boca Raton, FL, 1999.)

For a general look at the subject, there is the Handbook of Vitamins, second
edition, edited by Lawrence J. Machlin and published in 1991. Another is by Sheldon
Saul Hendler and a board of medical advisors, most or all M.D.s, titled The Doctors’
Vitamin and Mineral Encyclopedia, published in 1990. This reference also covers
herbs, amino acids, and other substances, and has numerous citations in the index
about cancer, but no definite, sure-fire remedies.
Further information about herbal substances will be presented in subsequent
chapters and sections, including particulars about garlic and its compounds.
The subject of hormones, along with neurotransmission, has been included under
the category of biochemical communications in the Voet and Voet reference (Voet
and Voet, 1995, p. 1261ff). These chemical messengers called hormones serve to
communicate intercellular signals, as do nerve-transmitted electrochemical signals
in higher animals. Hormones may be divided into polypeptides and amino acid
derivatives, and the reference provides a tabulation (Voet and Voet, 1995, p. 1263).
In a manner of speaking, therefore, and based on their composition, hormones may
be regarded as proteins.

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Biochemical Insights 111

TABLE 3.3
Vitamins and Hormones as Enzyme Inhibitors
Vitamins

Vitamin A Retinol Retinol inhibits β-glucuronidase C19H24-

Retinoic acid inhibits estrogen CH2OH
sulfotransferase, glutamate
dehyrogenase, metaplasia; shows
antitumor activity. (The suffix “-
metaplasia” means development.)
β-Retinoic acid inhibits hyperplasia.
13-cis-Retinoic acid inhibits
carcinogenesis.
Vitamin A topical application increased
incidence of rous sarcomas in chickens
Vitamin A acid (retinoic acid) inhibits
alcohol dehydrogenase.
Vitamin B1 Thiamine hydrochloride; Thiamine derivatives inhibit glucose C12H18Cl2
thiamin chloride synthesis, transketolase (yeast), N4OS
phosphodiesterase (snake venom),
thiamine triphosphatase, thymidylate
kinase.
Thiamine antagonists inhibit
acetylcholinesterase.
Vitamin B2 Riboflavin; lactoflavin; Inhibits daminoacid oxidase, FAD C17H22N4
vitamin G pyrophosphylase, galactonolactone O6
dehydrogenase, glutamate racemase,
riboflavin synthetase.
*Inhibits cytochrome-B5 reductase.
Vitamin B6 Pyridoxin Inhibits alanine racemase, malate C8H11NO3
dehydrogenase, pyridoxamine
pyruvate transami.
Vitamin B6 antagonists inhibit
adenocarcinoma growth.
*Inhibits alanine racemase,
pyridoxamine-pyruvate
aminotransferase.
Vitamin B12 Cobalamine; Cobalamin analogs inhibit ethanolamine C63H90N14
cyanocobalamin deaminase. (Ethanolamine is an O14PCo
industrial solvent that selectively
absorbs the acid gases CO2 and H2S.)
Cobalamin derivatives inhibit
ribonucleotide reductase.
Hydroxy-cobalamin inhibits diol
dehydratase.

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112 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

TABLE 3.3 (CONTINUED)

Vitamins and Hormones as Enzyme Inhibitors
Vitamins

Folic acid Pteroylglutamic acid Inhibits thymidylate synthetase. C14H11N6

(PGA); folacin; vitamin Pteroylglutamate derivative inhibits O 2-
Bc; vitamin M dihydrofolate reductase. (C5H7N
*Both pteroyl-α-glutamic acid and O3)n-OH
pteroyl-γ-glutamic acid inhibit 5- where n
methyltetrahydropteroyltriglutamate- = 1–7:
homocysteine methyltransferase. C19H19N7
O6
for n = 1
Niacin Nicotinic acid; Inhibits catecholase, D-aminoacid (C5H4N)C
3-pyridinecarboxylic acid oxidase, fatty acid synthesis, lipolysis,OOH
nicotinamide deaminase, NMN
aminhydrolase, phenol oxidase,
tributyrinase.
Nicotinic acid derivatives inhibit
accumulation of nicotinic acid,
lipolysis.
Nicotinamide inhibits ADPR
polymerase, cytochrome P-450
reductase, diphtheria toxin, IMP
dehydrogenase, NAD glycohydrolase,
NAD nucleosidase, NADase,
nucleoside pyrophosphatase, mixed
function oxidation, CAMP
phosphodiesterase, poly ADPR
synthesis, prostaglandin A1
metabolism, T-RNA methylase,
xanthine oxidase, 6-phosphogluconate
dehydrogenase.
*Nicotinamide inhibits NAD ADP-
ribosyltransferase,
NAD(P)nucleosidase, unspecific mono-
oxygenase.
Pantothenic acid HOCH2C(CH3)2CH(OH)C C9H17NO5
O-NHCH2CH2COOH
or N(α,γ-dihydroxy-β,β-
dimethylbutyryl)β-
alanine

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Biochemical Insights 113

TABLE 3.3 (CONTINUED)

Vitamins and Hormones as Enzyme Inhibitors
Vitamins

Vitamin C Ascorbic acid; Inhibits adenylate cyclase, Na,K-ATPase, CO

antiscorbutin catalase, catechol o-methyltransferase, (COH)3
ferredoxin-NADP reductase, glucose-6- CHOHC
P dehydrogenase, lipase, fatty acid H2OH
oxygenase, peroxidase, cAMP C6H8O6
phosphodiesterase, tyrosinase, urea
levels.
Ascorbic acid derivatives inhibit
ascorbate-2-sulfate sulfohydro,
dehydro-ascorbic acid.
L-ascorbic acid inhibits β-
acetylhexosaminidase.
*Ascorbate inhibits o-aminophenol
oxidase, catalase, β-glucuronidase, GTP
cyclohydrolase I,
hydroxymethylglutaryl-CoA reductase,
lactoylglutathione lyase.
Vitamin D Calciferol C28H44O
Vitamin D2 inhibits
ATPase
Vitamin E α-Tocopherol; Inhibits arachidonate peroxidation, C14H17O2
5,7,8-trimethyltocol Na,K-ATPase, glutamate (C5H10)3
dehydrogenase, fatty acid oxygenase. H
Tocopherol analogs inhibit
phosphodiesterase.
*Inhibits lipoxygenase.
Vitamin K Phthiocol 1,4-Naphthoquinone, 2-hydroxy-3- C11H8O3
methyl-vitamin K1 inhibits
incorporation of glucosamine.
2-chloro-vitamin K1 inhibits
prothrombin levels.
Vitamin K3 inhibits aniline hydroxylase.

Hormones Origins

Polypeptides
Corticotropin- Hypothalamus Corticotropin analogs inhibit adenylate
releasing factor cyclase.
(CRF)
Gonadotropin- Hypothalamus Human chorionic gonadotropin inhibits
releasing factor release of A-amylase.
(GnRF)

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114 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

TABLE 3.3 (CONTINUED)

Vitamins and Hormones as Enzyme Inhibitors
Hormones Origins

Thyrotropin- Hypothalamus Inhibits growth hormone biosynthesis.

releasing factor
(TRF)
Growth hormone- Hypothalamus Growth hormone inhibits glucose
releasing factor consumption.
(GRF) Growth hormone derivative inhibits
pyruvate dehydrogenase.
Somatostatin Hypothalamus Inhibits accumulation of cAMP, cAMP
levels, parathyroid hormone action,
release of CCK, release of growth
hormone, release of insulin.
Adrenocorticotropic Adenohypophysis Inhibits DNA synthesis in adrenal tumor
hormone (ACTH) cells; inhibits replication in
adrenocortical cells.
ACTH analogs inhibit adenylate cyclase,
fatty acid synthesis.
ACTH derivatives inhibit lipolytic action,
cAMP synthesis, corticosterone
synthesis, and ACTH activity.
Follicle-stimulating Adenohypophysis
hormone (FSH)
Luteinizing hormone Adenohypophysis Leuteinizing hormone inhibits
(LH) cholesterol synthesis.
Luteinizing hormone inhibits sterol
synthesis.
Chorionic Placenta
gonadotropin (CG)
Thyrotropin (TSH) Adenohypophysis Inhibits interferon action.
Somatotropin (see Adenohypophysis
growth hormone)
Met-enkephalin Adenohypophysis
Leu-enkephalin Adenohypophysis Enkephalin inhibits neuronal firing.
J-Endorphin Adenohypophysis Inhibits acetylcholine turnover.
Has opiatelike activity in mice.
Endorphin inhibits cAMP formation.
Vasopressin Neurohypophysis Forms conductance channels across
planar by: layer.
Inhibits carbon dioxide synthesis.
Oxytocin Neurohypophysis Oxytocin analog antagonizes oxytocin
action on uterus and mammary gland.
Oxytocin analogs inhibit binding to
oxytocin recepter, uterus contraction.
Oxytocin derivatives inhibit binding of
oxytocin, oxytocin effects.

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Biochemical Insights 115

TABLE 3.3 (CONTINUED)

Vitamins and Hormones as Enzyme Inhibitors
Hormones Origins

Glucagon Pancreas Inhibits contraction of dog papillary

muscle, fatty acid synthesis, glycogen
synthesis, vasoconstriction in dog
artery.
Insulin Pancreas Inhibits lipolysis, adenylate cyclase,
binding of NSILA, cAMP levels,
cathepsin D, incorporation of
thymidine, lipase, PEP carboxykinase
synthesis, phosphorylase, protein
kinase, protein synthesis.
Gastrin Stomach
Secretin Intestine
Cholecystokinin Intestine
(CCK)
Gastric inhibitory Intestine Gastric secretion inhibitor blocks
peptide (GIP) secretion of acid.
Parathyroid hormone Parathyroid Inhibits ATP–Pi exchange, glycogen
synthesis, respiration.
Analogs inhibit bovine enzyme;
adenylate cyclase.
Calcitonin Thyroid Lowers calcium, glucose, phosphate, and
potassium levels.
Somatomedins Liver Inhibits binding to insulin receptor,
NSILA binding.
Steroids
Glucocorticoids Adrenal cortex Glucocorticoid receptor inhibitor inhibits
binding to DNA.
Mineralocorticoids Adrenal cortex Corticosteroids inhibit binding of
calcium, collagen synthesis,
prostaglandin synthesis, release of
prostaglandins.
Estrogens Gonads and adrenal cortex Estradiol, stilbesterol, and
methyltestosteron, inhibit bile acid
metabolism.
Estrogens inhibit binding of estradiol,
cortisone reduction, glucose-6-P
dehydrogenase, steroid D4-5B-
reductase, steroid NAG transferase.
Androgens Gonads and adrenal cortex Derivatives bind to specific proteins. The
resulting complex migrates into the
prostate cell nuclei, where they appear
to regulate gene transcription.
Derivatives inhibit aromatase in the
human placenta.

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116 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

TABLE 3.3 (CONTINUED)

Vitamins and Hormones as Enzyme Inhibitors
Hormones Origins

Progestins or Ovaries and placenta Inhibits aldehyde dehydrogenase,

progesterones amylase, induction of collagenase
(which is modulated by cAMP), DNA
repair and replication, DNA synthesis.
*Inhibits aldehyde oxidase, cholesterol
acyltransferase, retinol fatty-
acyltransferase.
Vitamin D or Diet and sun Vitamin D2 inhibits ATPase.
calciferol
Amino Acid
Derivatives
Epinephrine Adrenal medulla Inhibits adenylate cyclase, CA-ATPase,
uterus contraction, drug metabolism,
lipogenesis, PE N-methyl transferase,
phosphofructokinase, release of β-
glucuronidase, release of tyrosine A-KG
transaminase.
Norepinephrine Inhibits binding of penoxybenzamine in
aorta, permeability of water,
pigmentation, serotonin levels,
tryptophan levels, tryptophan 2,3-
dioxygenase, tyrosine hydroxylase,
tyrosine transaminase.
Triiodothyronine Thyroid Inhibits protein synthesis, secretion of
(T3) prolactin.
Thyroxine (T4) Thyroid Inhibits alcohol dehydrogenase,
glutamate dehydrogenase, glutamic
dehydrogenase, lipid peroxidation,
malate dehydrogenase, oxidative
phosphorylation, thyroid transaminase.
L-thyroxine inhibits nicotinamide
deaminase, cAMP phophodiesterase,
triglyceride levels.

Note: Zollner (1993) in the main does not include hormones as enzyme inhibitors. Items that may be of
particular interest are in boldface.
* The tabulation is adapted from Appendix Z in Hoffman, Cancer and the Search for Selective Biochemical
Inhibitors, CRC Press, Boca Raton, FL, 1999.

Source: From M.K. Jain, Handbook of Enzyme Inhibitors, 1965-1977, Wiley, New York, 1982; (*) from
H. Zollner, Handbook of Enzyme Inhibitors, VCH, Weinheim, FRG, 1989, 1993. The list of vitamins is
according to Table 1.1 of Hoffman (1999). The list of hormones is from Voet and Voet, Biochemistry,
New York, 1995, p. 1263.

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Biochemical Insights 117

Hormones as well as vitamins also serve as enzyme inhibitors, as also presented

in Table 3.3, which lists the enzymes inhibited corresponding to the tabulation
provided in the Voet and Voet reference (Voet and Voet, 1995, p. 1263).
Hormones are classified by the distance within which they act. Autocrine hor-
mones act on the same cell that releases them. An example is interleukin-2. Paracrine
hormones act on cells close to the cell that releases them. Examples are prostaglan-
dins and polypeptide growth hormones. Endocrines act on cells at a distance. Exam-
ples are the endocrine hormones insulin and epinephrine, which are released into
the bloodstream by endocrine glands. (The term endocrine signifies internal secre-
tions directly into the bloodstream, whereas the term exocrine signifies secretions
through a duct.) A table of various endocrine hormones is provided in the cited
reference. Most hormones consist variously of polypeptides, amino acid derivatives,
or steroids. Some examples of sources and functions are as follows.
Of note is the pancreas, which serves as an exocrine gland for producing various
digestive enzymes. It also secretes insulin and glucagon, which regulate blood
glucose levels, and somatostatin, which regulates the insulin and glucagon secretions.
The gastrointestinal hormones are secreted into the bloodstream by cells lining
the gastrointestinal tract, and are no doubt affected by the ravages of chemotherapy,
which in particular attacks the cells of the gastrointestinal tract.
Thyroid hormones regulate metabolism.
Parathyroid hormone along with vitamin D and calcitonin (a polypeptide hor-
mone) regulate calcium metabolism.
The adrenal glands, which are divided into the medulla (core) and the cortex
(outer layer), furnish catecholamines and steroids, respectively. The catecholamines
are hormonally active, and consist of norepinephrine and epinephrine. Steroids
variously affect carbohydrate, protein, and lipid metabolism, regulate the salt/water
losses of the kidneys, and affect sexual development and function. Androgens and
estrogens fall in the last-mentioned category. Another adrenal hormone of note is
ACTH, or adrenocorticotropic hormone. Cortisol is an adrenal product, which can
be converted to cortisone.
The hypothalamus and the pituitary gland act together to control much of the
endocrine system. Included is regulation of the growth hormone GH, also called
somatotropin.
No effects pertaining to cancer were noted in the foregoing Voet and Voet
reference. Two chapters on the biochemistry of hormones are provided by Frank
Ungar in Textbook of Biochemistry (Devlin, 1986). Again, no direct connection with
cancer was mentioned. There are, however, some interesting inferences about thyroid
hormone functions dealing with oxygen consumption and thermogenesis (Devlin,
1986, pp. 753, 754). Increased oxygen consumption and heat production go together,
and correlate with increased thyroid activity.
In further comment about increased thyroid activity, there is the contention that
higher body temperatures (hyperthermia) act against cancer. If so, this suggests
increased capacities for the tricarboxylic acid cycle, manifested as an increased
aerobic glycolysis rate. Noting that cancer cells in the main undergo anaerobic
glycolysis, its inhibition should favor a significant increase in aerobic glycolysis.
This in turn could presumably could raise body temperatures and act against cancer.

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118 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

The effect corresponds to the immune system raising body temperatures to induce
a fever and fight off an infection — in this case, cancer cells.
Observing that iodine is associated with increased thyroid activity, the foregoing
reference states that obesity implies what is called “reduced ATPase enzyme activity.”
(ATPases are proteins that transport ATP, or adenosine triphosphate, across cellular
membranes.) This may connect with obesity, and by a stretch of the imagination,
with cancer.

CHEMOTHERAPY DRUGS AS ENZYME INHIBITORS

Conventional chemotherapy drugs act as inhibitors for enzymes, notably for critical
enzymes that pertain to DNA/RNA/protein synthesis. The idea is that by blocking
one or another or all of these enzymes, cancer cell growth and proliferation will be
stopped. Unfortunately, these drugs are nonselective, acting against normal cells as
well, and are therefore categorized as cell toxic, or cytotoxic.
A basic consideration will involve the subject of what are called nucleotides,
which are the fundamental chemical structures contributing to cellular functions.
Divided into the two classes named purines and pyrimidines, they are precursors to
both DNA and RNA. The purine and pyrimidine nucleotides are formed de novo
(anew) in the cell from amino acids, ribose, formate, and carbon dioxide (e.g., Cory,
in Devlin, 1986).
(Deficiencies in the synthesis of these nucleotides result in such diseases as gout,
the Lesch–Nyhan syndrome, orotic aciduria, and immunodeficiency diseases, which
may include cancer.)
Thus, a large number of substances have been tested as inhibitors for the steps
in purine and pyrimidine metabolism (Cory, in Devlin, 1986, p. 674ff). Included are
both synthetic compounds and compounds isolated from plants, bacteria, fungi, etc.
The substances may be further classed as glutamine antagonists, antifolates, and
antimetabolites. Whereas the first-mentioned substances are toxic in the extreme,
the latter two classifications have been of main concern although they are toxic
enough in their own right.
Enzyme inhibitors used as chemotherapy drugs take advantage of the fact that
enzymes and enzyme inhibitors are sometimes chemically or structurally similar
(Voet and Voet, 1995, p. 355). A notable example is the chemo drug methotrexate
(or amethopterin), which is similar to the chemical compound called hydrofolate.
In consequence, methotrexate binds to an enzyme called dihydrofolate reductase.
This in turn blocks the enzyme from converting dihydrofolate to tetrahydrofolate,
the latter being essential in the biosynthesis of the DNA precursor thymidylic acid.
This action is nonselective, however, affecting both normal and cancerous cells,
although there is the hope or supposition that cancer cells may be more affected.
The subject of enzyme inhibitors and their action is complex, but a few guidelines
exist (York, in Devlin, 1986, p. 165ff). Thus, inhibitors can be classed as competitive,
noncompetitive, and uncompetitive — even a buildup of reaction product can inhibit
enzyme activity — and still other enzymes can inhibit (or promote) activity.
Competitive enzyme inhibitors are generally similar to the enzyme itself, binding
to the active enzyme sites to block activity, whereas a noncompetitive inhibitor binds

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Biochemical Insights 119

at different sites. An irreversible inhibitor chemically reacts with the enzyme, as

distinguished from the more loose association denoted as bonding.
The foregoing gets around to the purposes and action of drugs or medicines.
Generally speaking, this comes under the catchall heading of chemotherapy. The
modern concept of a drug is that of inhibiting a specific enzyme in a specific
metabolic pathway. The applications include antiviral and antibacterial action, and
also antitumor action. The foremost problem is that of side effects or adverse effects,
for inhibitors will usually affect more than one enzyme, and thus may affect other
biochemical processes in different degrees. The attempt therefore is to limit toxic
side reactions, for some degree of toxicity is apparently unavoidable. A known
exception is cell-wall biosynthesis in bacteria, which is blocked by the fungal-derived
antibiotics, which are mostly nontoxic to humans in the dosages used.
In different words, it has been said that “there are no critical metabolic pathways
that are unique to tumors, viruses, or bacteria” (York, in Devlin, 1986, p. 167). The
host cell is affected by the drug as well, but the anticipation is that the disease-
causing organism will be more quickly affected, for example, bacteria, in the case
of using antibiotics..
With respect to the previous statement that “there are no critical metabolic
pathways unique to tumors,” an exception can be made in the case of glycolysis,
for glycolysis rates are many times greater for cancer cells than for normal cells.
Not only this, but cancer cells are predisposed to produce lactic acid or lactate rather
than undergo normal metabolism. Therefore, there is the incentive for not only
slowing down glycolysis, but especially for blocking lactate or lactic acid formation
and glutaminolysis. This approach has been spelled out previously, naming the
controlling enzymes involved, and listing some known inhibitors for those enzymes.
Inhibition of the particular enzymes involved may in fact be the role played by many
unorthodox anticancer agents, both organic or herbal, and inorganic.
Modern chemotherapy can be viewed as beginning with the sulfur-containing
sulfa drugs (York, in Devlin, 1986, p. 167). The general formula for these chemical
compounds is R-SO2-NHR’, where R and R’ represent arbitrary molecular groups.
The simplest and most well known is sulfanilamide, which inhibits the action of p-
aminobenzoic acid (PABA), this being required for bacterial growth. That is, bacteria
must synthesize folic acid from p-aminobenzoic acid, but as sulfanilimide is struc-
turally similar to PABA, the necessary enzyme dihydropterate synthetase will sub-
stitute sulfanilamide for PABA. The bacterium is therefore deprived of folic acid or
folate, which prevents it from growing or dividing. Fortuitously, humans obtain folic
acid or folate from external sources, and the sulfanilamide is not otherwise harmful
at the dosage levels used.
What are called antifolate drugs pertain in general to blocking the biosynthesis
of purines and pyrimidines, the heterocyclic bases used in the further synthesis of
DNA and RNA, where folic acid is required as a coenzyme (or vitamin) for the
enzyme dihydrofolate reductase. The previously mentioned compound called metho-
trexate or amethopterin (4-amino-N10-methyl folic acid), being a structural analog
of folate or folic acid, locks up the enzyme dihydrofolate reductase, which in turn
blocks the synthesis of a thymidine nucleotide necessary for cell division.

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120 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

In this way, methotrexate has been successfully used in the treatment of child-
hood leukemia, as cell division depends on thymidine in addition to other nucle-
otides. Unfortunately, rapidly dividing cells are especially sensitive, as in bone
marrow. Not to mention that prolonged usage will cause the tumor cells to produce
ever-larger amounts of the reductase enzyme, thereby becoming resistant to the drug.
What are in general called antimetabolites show a similarity with a reactant; that
is, they are analogous with, or are analogs to, a reactant. An example is the chemo-
therapy drug 5-fluorouracil (5-FU or 5Ura), which is an analog of the fundamental
building block thymine, whereby a methyl group is replaced by fluorine. Thymine,
along with adenine, guanine, or cytosine, is one of the four basic building blocks in
the nucleotides that form the DNA molecule and its encoded genetic information.
As such, 5-FU acts as an irreversible inhibitor for the action of the enzyme thymidy-
late synthase, also called thymidylate synthetase, which is necessary in DNA syn-
thesis (Voet and Voet, 1995, pp. 812–816; Cory, in Devlin, 1986, p. 677).
The drug 6-mercaptopurine is another example. It is an analog of a compound
involved with adenine and guanine, two of the basic four constituents of nucleotides
and, in turn, of DNA synthesis. This drug therefore acts as an antimetabolite,
competing in most reactions involving adenine and guanine and their derivatives.
By acting against all cells, normal or otherwise, it also acts against cancerous cells.
The pertinent descriptor is cytotoxic, or cell toxic, meaning these drugs act
inclusively against both normal and cancerous cells. In other words, these drugs are
poisons. Further specific information about the array of drugs that have been used
is furnished by Ralph Moss in Questioning Chemotherapy (1995). The degree of
success is included, which in the main is depressing, the few positive exceptions
being for blood-related cancers such as leukemia.
The connection has been made with nucleotides and their synthesis into DNA
and RNA.
“Since nucleotides are obligatory for DNA and RNA synthesis in dividing cells,
the metabolic pathways involving the synthesis of nucleotides have been the sites
at which many antitumor agents have been directed” (Cory, in Devlin, 1986, p. 628).
Some inhibitors have already been mentioned, with others in the offing. These
inhibiting substances can be said to be not commonly known outside the specialized
chemical world, with the exception of folic acid.
Apart from chemically synthesized bioactive substances and naturally occurring
bioactive substances, the alternative is the production of bioactive substances from
natural sources, and leads in turn to the much-studied and infamous bacterium E.
coli, short for Escherichia coli.
Studied over the years from both a biochemical and a genetic standpoint, the
prokaryote E. coli is found in the colon of higher mammals, and members of the
genus Escherichia are collectively known as coliform bacteria (Voet and Voet, 1995,
pp. 4, 5). Its DNA encodes some 3000 proteins, with a resulting cellular totality of
perhaps 6000 different kinds of molecules comprising proteins, nucleic acids,
polysaccharides, lipids, and other varieties. Glucose is predominantly metabolized,
and lactose less so (Glassman, in Devlin, 1986, p. 952). If no glucose is available,
lactose suffices, with the necessary enzyme (β-galactosidase) increasing a thousand-
fold.

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Biochemical Insights 121

(Still other bacteria or prokaryotes may require only simple compounds for their
nutritional or metabolic requirements, e.g., those commonly designated by the chem-
ical formulas H2O, CO2, NH3, and H2S [Voet and Voet, 1995, pp. 4, 5]. Others may
even utilize the ferrous ion Fe2+, and for some, light energy becomes the energy
source, via the processes of photosynthesis, in which H2O and CO2 are converted
to organic matter. The latter bacteria are known as cyanobacteria (“cyano,” inferring
the presence of the -CN, or cyanide, group), described as the green, chlorophyll-
containing organisms found in or around water, formerly called blue-green algae.
Species of cyanobacteria growing on the roots of legumes convert atmospheric
nitrogen into organic nitrogen compounds. Another, less-known form of photosyn-
thesis utilizes such sulfur compounds as hydrogen sulfide, or H2S, and thiosulfates,
even hydrogen or organic compounds.)
In gene cloning, the DNA of the desired gene is introduced into the E. coli cells
(Glassman, in Devlin, 1986, pp. 986, 995). Called genetic engineering, the objectives
are to produce useful proteins such as insulin, blood-clotting factors, growth factors,
hormones, interferon, etc., and to use cloned normal human genes to replace the
defective genes causing diabetes, sickle cell anemia, and hemophilia.
As for cancer treatment, the end objective is to replace the DNA or RNA of
cancer cells with DNA or RNA from normal cells. But, of course, how this can be
achieved and how effective it will be are unanswered questions.

MORE ON FOLIC ACID

Folic acid, or folate, is frequently mentioned as a treatment against cancer. There
are many natural folate-containing foods, including leafy green vegetables and fruits
such as oranges (Moss, 1992, pp. 42–44). John Heinerman also gives special mention
to folic acid in his book The Treatment of Cancer with Herbs.
In combination with vitamin B12, its more usual function is as a preventive and
treatment for certain kinds of anemia, such as, pernicious anemia. More recently, it
has been prominently cited for the prevention of birth defects. Its role as an enzyme
inhibitor is much less publicized.
A couple of relevant books are Folate in Health and Disease, edited by Lynn
B. Bailey (1994), and Apricots and Oncogenes: On Vegetables and Cancer Preven-
tion by Eileen Jennings (1993). The latter emphasizes folic acid and beta-carotene,
and it is mentioned that folic acid deficiency may be a major cause of colon cancer
and breast cancer. It is relevant to mention here that carrots, a source of beta-carotene,
are a folkloric anticancer agent, and also contain traces of bioactive alkaloids, for
example, daucine, putrescine, and pyrrolidine, as well as other bioactive compounds.
(Folate is commonly used as a carrier for the chemotherapy drug 5-FU, although
the folate has anticancer action in itself.)
In a study by Kearney et al. (1995) of the Department of Nutrition, Harvard
School of Public Health, low folic acid levels were found to correlate with an increase
in cancerous intestinal polyps, as well as with cancer in general.
Folic acid may occur in several forms, the most common being polyglutamarate
derivatives that ultimately get converted in the intestines into tetrahydrofolate by the

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122 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

action of the enzyme dihydrofolate reductase (Chaney, in Devlin, 1986, p. 1219ff.).

Note, for instance, the chemical connection with the use of 5-FU.
Folate deficiency is said to inhibit DNA synthesis, a consequence of which is
that the maturation of red blood cells is slowed, causing anemia. As for folate
deficiency during pregnancy, this can be the side effect of drugs that interfere with
folate metabolism.
In a reiteration of the action of chemotherapy agents, in describing the formation
of tetrahydrofolate from folic acid, antifolates have been defined as compounds that
inhibit this conversion by blocking dihydrofolate reductase, which in turn blocks
the formation of thymidylate (e.g., Cory, in Devlin, 1986, p. 674ff). At the same
time, antifolates also block the formation of thymidylate by acting as an inhibitor
for thymidylate synthase, a fact that has been stressed, besides inhibiting or blocking
dihydrofolate reductase. The action is twofold. Folic acid is listed as an inhibitor
for both of these enzymes. Furthermore, the chemotherapy drug methotrexate, or
MTX, is an antifolate that is very similar chemically to folic acid.
On the other hand, as previously indicated, the action of 5-fluorouracil (5-FU,
or FUra) becomes that of an antimetabolite. Further described as a pyrimidine analog
with no biological action in itself, it is converted in the body to other cytotoxic (cell-
toxic) agents that act as the antimetabolites; that is, act against cellular metabolism.
The use of 5-FU, or FUra, in cancer therapy has therefore been noted to have
some adverse consequences or side effects (Cory, in Devlin, 1986, p. 677): “ … the
incorporation of 5-FUra into RNA has serious effects on normal RNA metabolism
and is a factor in the cytotoxicity of this agent.” That is, the RNA of normal cells,
as well as that of tumor cells, is compromised. (Significantly, the reference further
states that the nucleosides thymidine and uridine may act as antidotes, rescuing the
FUra-treated cells from the action of the FUra.)
Also mentioned is that a high concentration of the compound hydroxyurea
inhibits DNA synthesis, but with very little effect on RNA (Cory, in Devlin, 1986,
p. 678). The inhibiting action is against the enzyme ribonucleotide reductase,
blocking the formation of DNA. The similarity with urea as an anticancer agent may
not be coincidental.
All things considered, then, folic acid (or one of its derivatives) should be further
studied as an anticancer agent, especially as it has been listed as an inhibitor for the
same enzymes inhibited by the usual cytotoxic chemo agents. It is a substance natural
to the body, and not some mysterious chemical or questionable plant or animal
extract. Effective dosage levels and the best method of administration would have
to be determined and the side effects established. If most doctors are reluctant to
give even B12 or B-complex shots, megadoses of folic acid may not be on the cards.
As with other cancer treatment alternatives, the patient may have to look at avenues
other than medical orthodoxy.
(Speaking of side effects, folic acid and its analogs and derivatives inhibit other
enzymes. These other enzymes are listed in Jain’s handbook (1982), and the listing
includes thymidylate synthetase or synthase, as well as dihydrofolate reductase, the
enzymes blocked by antifolates, as previously indicated.)
The foregoing speculations seem worthy of independent clinical trials, the mea-
sure for credibility, for folic acid and some of its derivatives may be key to the

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Biochemical Insights 123

regulation and control of cancer cell growth and proliferation, although there may
not be much money in folic acid for Big Pharma.
A search of Medline will yield a few items of information about the use of folic
acid and its hazards, especially in supplemental megadoses. Thus, there have been
clinical trials conducted at the University of Arizona as part of a Southwest Oncology
Group Intergroup study (Childers et al., 1995). The comparison involved a predis-
position to cervical cancer, but found no significant difference between those tested
and the controls. Oral dosage levels of 5 mg (5000 mcg) per day were used, whereas
a vitamin supplement will have a nominal 400 mcg. (As a further reference dosage,
up to 1 g per day has been reported.) A qualifier is, of course, that the tests did not
pertain to actual, developed cervical cancers, and that the control group may have
had sufficient folic acid in their diet. Also, should the folic acid have been taken
intravenously?
Another article brings up toxic side effects that may occur with folic acid and
its analogs or derivatives, and in the context of thymidylate synthase antagonists or
inhibitors (Jackman and Judson, 1994). In it, adverse effects against the kidneys
were noted.
Another article, by S.R. Snodgrass, appeared in the Spring 1992 issue of Molec-
ular Neurobiology, which deals with vitamin neurotoxicity. Tests on animals showed
that high dosages of folate directly into the brain or cerebrospinal fluid produced
seizures and excitation. However, folate toxicity in humans is rare, though fatal
reactions to intravenous injections of the B vitamin thiamine have occurred. How-
ever, the conclusion was that megadose vitamin therapy is more hazardous to periph-
eral organs than to the nervous system. Moreover, a high vitamin intake could itself
produce diseaselike symptoms, and vitamin administration directly into the brain
should be avoided. The issues are clouded by biochemical individuality: different
patients will react differently.
As for the chemoprevention program sponsored by the National Cancer Institute,
it seems odd that investigations using these kinds of substances against cancer had
to wait until the 1990s to commence, for it was known by about 1980 at the latest
— and very possibly much sooner — that folic acid and some of its derivatives were
inhibitors or blockers for the enzymes thymidylate synthase and dihydrofolate reduc-
tase. (The year 1982, for instance, was the publication date of Textbook of Biochem-
istry, edited by Thomas M. Devlin, and also of M.K. Jain’s Handbook of Enzyme
Inhibitors.) Comprehensive clinical tests on folic acid, particularly, as an anticancer
agent should no doubt have been conducted years ago, based on the biochemical
knowledge then already available, not only on folic acid, but on many of the myriad
other possibilities mentioned in Jonathan L. Hartwell’s Plants Used Against Cancer
(1982).

ENZYME TRANSFORMATIONS
An additional problem with the proliferation of cancerous cells is that there may
not only be quantitative changes in the levels of the enzymes responsible, but
qualitative changes also (Cory, in Devlin, 1986, p. 665). Thus, the chemical structure
of the enzymes may change, with the enzymes transforming into the corresponding

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124 Cancer and the Search for Selective Biochemical Inhibitors, Second Edition

isoenzymes or isozymes, as they are then called. Several kinds of biochemical

changes have been observed, categorized as follows: (1) transformation linked, (2)
progression linked, and (3) coincidental alterations. The first-mentioned enzyme
alterations are independent of tumor growth rate, the second relate to growth rate,
and the third are not malignancy connected.
For examples of the first category, the enzymes conveniently named PRPP
amidotransferase, UDP kinase, and uridine kinase may increase in all tumors, inde-
pendently of tumor growth rate. On the other hand, the levels of thymidylate syn-
thase, ribonucleide reductase, and IMP dehydrogenase also increase, but increase
with the tumor growth rate. (PRPP stands for 5-phosphoribosyl 1-pyrophosphate;
UDP, uridine 5′-diphosphate; and IMP, inosine 5′-monophosphate.) All of the afore-
mentioned enzymes are involved in the synthesis and replication of DNA, and
whereas some enzymes produce fast growth rates, say, in normal tissue, the rates
are different for tumor tissue. What could prove significant would be for certain
critical enzymes to act to decrease the growth rates in tumors.
Inhibitors for the foregoing enzymes have been furnished elsewhere (in Appen-
dix D of Hoffman, 1999), with the exception of the enzyme UDP kinase. Interest-
ingly, the substance known as nicotinamide, a vitamin, shows up as an inhibitor for
IMP dehydrogenase.

© 2007 by Taylor & Francis Group, LLC