Lipoprotein system: Chylomicrons,
HDL, LDL, IDL and VLDL and their
role in distribution of lipids
Prof Dr. Naseem Aslam Channa
LIPOPROTEINS
Lipids are insoluble in water and must be transported in association
with proteins (lipoproteins) in the circulation.
Large quantities of fatty acids from meals must be transported as
triglycerides to avoid toxicity.
These lipoproteins play a key role in the absorption and transport of
dietary lipids by the small intestine, in the transport of lipids from
the liver to peripheral tissues, and the transport of lipids from
peripheral tissues to the liver and intestine (reverse cholesterol
transport).
LIPOPROTEINS
Lipoproteins are complex particles that have a central hydrophobic
core of non-polar lipids, primarily cholesterol esters and
triglycerides.
This hydrophobic core is surrounded by a hydrophilic membrane
consisting of phospholipids, free cholesterol, and apolipoproteins
(Figure 1).
Plasma lipoproteins are divided into seven classes based on size,
lipid composition, and apolipoproteins (Figure 2).
APOLIPOPROTEINS
LIPOPROTEINS
Chylomicrons
These are large triglyceride rich particles made by the intestine,
which are involved in the transport of dietary triglycerides and
cholesterol to peripheral tissues and liver.
These particles contain apolipoproteins A-I, A-II, A-IV, A-V, B-
48, C-II, C-III, and E.
Apo B-48 is the core structural protein and each chylomicron
particle contains one Apo B-48 molecule.
The size of chylomicrons varies depending on the amount of fat
ingested.
Chylomicron
A high fat meal leads to the
formation of large chylomicron
particles due to the increased
amount of triglyceride being
transported whereas in the fasting
state the chylomicron particles are
small carrying decreased
quantities of triglyceride.
The quantity of cholesterol
carried by chylomicrons also can
vary depending upon dietary
intake.
Chylomicron Remnants
The removal of triglyceride from chylomicrons by lipoprotein
lipase in peripheral tissues results in smaller particles called
chylomicron remnants.
Compared to chylomicrons these particles are enriched in
cholesterol
and are
pro-atherogenic.
Very Low-Density Lipoproteins (VLDL)
These particles are produced by the liver and are triglyceride rich.
They contain apolipoprotein B-100, C-I, C-II, C-III, and E.
Apo B-100 is the core structural protein and each VLDL particle
contains one Apo B-100 molecule.
Similar to chylomicrons the size of the VLDL particles can vary
depending on the quantity of triglyceride carried in the particle.
When triglyceride production in the liver is increased, the secreted
VLDL particles are large. However, VLDL particles are smaller
than chylomicrons.
VLDL
Intermediate-Density Lipoproteins (IDL;
VLDL Remnants)
The removal of triglycerides from VLDL by muscle and adipose
tissue results in the formation of IDL particles which are enriched
in cholesterol.
These particles contain
apolipoprotein B-100
and E.
These IDL particles are
pro-atherogenic.
Low-Density Lipoproteins (LDL)
These particles are derived from VLDL and IDL particles and they
are even further enriched in cholesterol.
LDL carries the majority of the cholesterol that is in the circulation.
The predominant apolipoprotein is B-100 and each LDL particle
contains one Apo B-100 molecule.
LDL consists of a spectrum of particles varying in size and density.
An abundance of small dense LDL particles is seen in association
with hypertriglyceridemia, low HDL levels, obesity, type 2
diabetes (i.e. patients with the metabolic syndrome) and infectious
and inflammatory states.
LDL
LDL
These small dense LDL particles are considered to be more pro-
atherogenic than large LDL particles for a number of reasons.
Small dense LDL particles have a decreased affinity for the LDL
receptor resulting in a prolonged retention time in the circulation.
Additionally, they more easily enter the arterial wall and bind more
avidly to intra-arterial proteoglycans, which traps them in the
arterial wall.
Finally, small dense LDL particles are more susceptible to
oxidation, which could result in an enhanced uptake by
macrophages.
High-Density Lipoproteins (HDL)
These particles play an important role in reverse cholesterol
transport from peripheral tissues to the liver, which is one potential
mechanism by which HDL may be anti-atherogenic.
In addition, HDL particles have anti-oxidant, anti-inflammatory,
anti-thrombotic, and anti-apoptotic properties, which may also
contribute to their ability to inhibit atherosclerosis.
HDL particles are enriched in cholesterol and phospholipids.
Apolipoproteins A-I, A-II, A-IV, C-I, C-II, C-III, and E are
associated with these particles. Apo A-I is the core structural
protein and each HDL particle may contain multiple Apo A-I
HDL
APOLIPOPROTEINS
Apolipoproteins have four major functions including
1) serving a structural role,
2) acting as ligands for lipoprotein receptors,
3) guiding the formation of lipoproteins, and
4) serving as activators or inhibitors of enzymes involved in
the metabolism of lipoproteins
Apolipoproteins thus play a crucial role in lipoprotein metabolism.
ROLE OF LIPOPROTEINS IN
DISTRIBUTION OF LIPIDS
Lipoproteins are a key part of the body's lipid distribution system,
transporting lipids between organs and tissues, and playing a role in
the absorption of dietary lipids:
Absorption: Lipoproteins help the small intestine absorb dietary
lipids.
Transport: Lipoproteins transport lipids from the liver to
peripheral tissues, and from peripheral tissues back to the liver and
intestine.
Structure: Lipoproteins are also a structural component of
nervous tissue, helping to carry electrical impulses.
LIPOPROTEINS IN LIPID DISTRIBUTION
Lipoproteins are complexes of proteins and lipids, synthesized in
the liver, intestines, and kidneys.
The proteins in the lipoprotein's external shell, called
apolipoproteins, are secreted into the body's extracellular water by
the liver and small intestine cells.
Some lipoproteins can be harmful to the heart, while others can be
protective. For example, high levels of lipoprotein (a) can increase
the risk of heart attacks, atherosclerosis, and strokes.
A lipid blood test can measure lipoprotein levels and help with
diagnosis, screening, and monitoring.
THE END
