From Wikipedia,
the free encyclopedia.
Apolipoproteins are a
class of
apoproteins, which are
proteins that depend on the
presence of other small molecules,
or
cofactors, to function. Thus,
apolipoproteins are the protein
constituents of
lipoproteins, which also
consist of
phospholipids,
triacylglycerols,
cholesterol, and cholesterol
esters. There are five major
types of apolipoproteins: A, B, C,
D, and E. Apo E is perhaps the
most well studied.
The Apo E protein is 299 amino
acids long, and a core
apoprotein of the
chylomicron, which transports
lipoproteins, fat-soluble
vitamins, and
cholesterol into the
lymph system and then into the
blood. Apo E is synthesized
principally in the liver, but has
also been found in other tissues
such as the brain, kidneys, and
spleen. Apo E was initially
recognized for its importance in
lipoprotein metabolism and
cardiovascular disease. More
recently, it has been studied for
its role in several biological
processes not directly related to
lipoprotein transport, including
Alzheimer's disease,
immunoregulation, and cognition.
The apo E gene, which encodes
the Apo E protein, is located on
chromosome 19, and consists of
four
exons and three
introns totaling 3597 base
pairs. The gene is
polymorphic, with three major
alleles, apo E-3, apo E-2, and apo
E-4, which translate into three
isoforms of the protein: E3
(normal), and E2 and E4
(dysfunctional). These isoforms
differ from each other only by
single amino acid substitutions at
positions 112 and 158, but have
profound physiological
consequences. Apo E2 is associated
with the
genetic disorder
type III hyperlipoproteinemia
and with both increased and
decreased risk for
atherosclerosis. Apo E4 has
been implicated in
atherosclerosis and
Alzheimer's disease, impaired
cognitive function, and
reduced
neurite outgrowth.
Apolipoprotein synthesis is
affected by the addition of
Nicotinic Acid (a form of the
vitamine
niacin). It can be prescribed
in large doses (2-6g/day) to lower
total triglycerides, LDL and total
Cholesterol while increasing HDL
levels. This is not to be confused
with
nicotinamide (the form of
niacin in supplements).
