HDL: High-density lipoprotein; synthesized in the liver, the HDL serves as
a source of apolipoproteins for other lipoproteins, as the site of action for
the conversion of cholesterol to cholesterol ester in the plasma by the
enzyme lecithin-cholesterol acyltransferase (LCAT), and delivers cho-
lesterol esters derived from peripheral membranes to the liver. It is com-
monly called the “good cholesterol.”
LDL: Low-density lipoprotein; a product of the degradation of very-low-
density lipoproteins (VLDLs) by the action of lipoprotein lipase. LDLs
are taken up by a receptor-mediated endocytosis by both peripheral tis-
sues and the liver. It is commonly called the “bad cholesterol.”
Lipoprotein: A macromolecular particle composed of varying quantities of
protein, triacylglycerol, phospholipids, cholesterol, and cholesterol esters.
The lipoprotein structure has a phospholipid and free cholesterol skin sur-
rounding a core composed of triacylglycerol and cholesterol esters, with
the proteins imbedded on the surface. They serve as carriers of lipid in
the circulation.
VLDL: Very-low-density lipoprotein; synthesized by the liver to transport
triacylglycerol from the liver to peripheral tissues.
Although cholesterol can be synthesized in almost all cells, the liver, intes-
tine, and the steroidogenic tissues such as the adrenal glands and repro-
ductive tissues are the primary sites. In the liver, cholesterol is synthesized
in the endoplasmic reticulum (ER) along with phospholipids, triacylglyc-
erides, and apoproteins. Prelipoprotein particles are assembled in the ER and
then transferred to the Golgi. Further processing and addition of cholesterol
esters occur in the Golgi, culminating in the formation of secretory granules
containing lipoprotein particles. These vesicles then fuse with the plasma
membrane and export cholesterol from the cell via exocytosis in the form of
VLDLs (see below), which then enter the circulation. As the lipoprotein com-
plexes are transported through the bloodstream they are converted from VLDL
to IDL to LDL by the removal of triacylglycerides by lipoprotein lipase, which
is located on the surface of capillary epithelial cells.
Cholesterol and triacylglycerides (TAGs) are transported as complexes in
the form of lipoprotein particles. These lipoprotein particles contain a core of
TAGs and cholesteryl esters surrounded by a monolayer of phospholipids,
cholesterol, and specific proteins called apoproteins. The apoproteins, spe-
cific to each type of lipoprotein, enable the hydrophobic lipids to be trans-
ported in the aqueous environment of the bloodstream. They also contain
signals that target the lipoprotein particles to the cells or activate enzymes. The
lipoprotein particles vary in density depending on the lipid/protein ratio and
are named based on these densities. The higher the lipid/protein ratio, the
lower the density of the particle.
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