lipoproteins with the LDL receptor and the chylomicron remnant (apoE)
Lipoprotein lipase (LPL): An enzyme bound to the surface of the capil-
lary endothelium by heparan sulfate proteoglycans. LPL catalyzes the
hydrolysis of triglyceride in chylomicrons and VLDL to free fatty acids
and glycerol.
Hepatic lipase (HL): An enzyme bound to the surface of sinusoidal
endothelial cells of liver. It catalyzes the hydrolysis of mono-, di-, and
triglycerides as well as the phospholipids phosphatidyl choline and
phosphatidyl ethanolamine.
Triglycerides (triacylglycerols, TG) are safely transported in the bloodstream
packaged into lipoproteins called chylomicrons or very-low-density lipopro-
teins (VLDLs). Chylomicrons are formed in the epithelial cells of the intestine
and are responsible for the transport of dietary lipids. VLDL is synthesized in
the liver and transport endogenously synthesized lipids from the liver to
peripheral tissues. Both of these lipoproteins are composed of a core com-
posed primarily of TG enveloped by a monolayer composed of phospholipids,
free cholesterol, and apolipoproteins.
Dietary triglycerides are hydrolyzed by pancreatic lipase in the lumen of
the small intestine. Colipase, a protein secreted along with pancreatic lipase,
binds to the TG and the pancreatic lipase and improves the hydrolytic process.
TG is broken down to free fatty acids and 2-monoacylglycerols, which form
micelles along with bile salts and other lipid soluble compounds such as cho-
lesterol and fat-soluble vitamins. The free fatty acids and monoglycerides are
absorbed by the microvilli of the intestinal epithelial cells. In the epithelial
cell, the fatty acids and monoglycerides are reformed into triglycerides, which
are packaged with phospholipids, cholesterol, and apolipoprotein B-48 into
The newly synthesized chylomicrons are secreted into the lymph and enter
the bloodstream via the thoracic duct. In the bloodstream, the chylomicron
particles obtain proteins from high-density lipoproteins (HDL), including
apoC-II and apoE, which are important for the function of the chylomicron
(Figure 35-1).
In the capillary beds in adipose tissue, muscle tissue (especially cardiac
muscle) and in lactating mammary glands, apoC-II binds to and activates
lipoprotein lipase (LPL), which is bound to the endothelial surface of the cap-
illaries by heparan sulfate. LPL hydrolyzes the TG in the core of the chylomi-
cron to free fatty acids and glycerol. The fatty acids are taken up by the adipose
or muscle cells; glycerol is recycled back to the liver. In muscle, the fatty acids
are oxidized to produce ATP and in adipose they are reformed into TG for stor-
age. The TG-depleted chylomicron remnant remains in the blood stream until
it binds to the chylomicron remnant receptor located on hepatocytes, a process
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