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Encyclopedia of Physical Science and Technology EN008C-380 June 29, 2001 16:42

Lipoprotein/Cholesterol Metabolism 647

TABLE II Chemical Composition of Plasma They are the package of dietary fat. Thus, few
Lipoproteins a chylomicrons are made in fasted people.
Class Chol PL Protein TG CE They are usually >80% triglyceride.
They are secreted into the lymphatics, rather than the

Chylomicrons 2 7 2 86 3 bloodstream.
VLDL 7 18 8 55 12 They are acted upon by lipoprotein lipase on the
IDL 9 19 19 23 29 surface of adipose and muscle capillaries.
LDL 8 22 22 6 42 After depletion of their triglyceride via the lipoprotein
HDL2 5 33 40 5 17
lipase reaction, the resulting remnants are cleared from
HDL3 4 35 55 3 13
the circulation.
a Values represent percent of dry mass. Chol, choles- Chylomicrons are not converted into LDL.
terol; PL, phospholipids; TG, triglycerides; CE, choles-
terol esters. Lipoprotein lipase catalyzes the complete hydrolysis of
triglycerides to free fatty acid and glycerol. This reaction
transferred from the chylomicron particle to these tissues? occurs in the bloodstream, while the chylomicron particle
As was the case with the transfer of lipids from the intesti- is in proximity to the endothelial surface of the capillary
nal lumen to the intestinal epithelial cells, this transfer wall. Cholesterol ester is not a substrate for lipoprotein li-
begins with a lipolytic reaction to convert an oily lipid, pase. Therefore, with the selective loss of triglycerides, the
triglyceride, into an amphipathic lipid, free fatty acid. In particle becomes more enriched in cholesterol ester. The
this case, the reaction is catalyzed by lipoprotein lipase,an free fatty acids are then taken up and oxidized in muscle or
enzyme that resides at the luminal surface of the capillary re-esterified and stored as triglyceride droplets in adipose
endothelium in adipose tissue and muscle. tissue. The abundance of the adipose form of lipoprotein
lipase is increased by insulin, thus promoting uptake of
free fatty acids and storage as cytoplasmic triglyceride
Main Features of Chylomicron Metabolism
droplets (see Fig. 4).
Chylomicrons are exclusively synthesized in the A chylomicron that has been depleted of its triglyc-

intestine. eride core is termed a chylomicron remnant. Chylomicron

FIGURE 4 Chylomicron pathway. The intestine secretes chylomicron particles into the lymphatics. They gain entrance
into the general circulation through the thoracic duct. Lipoprotein lipase, on the luminal surface of adipose and
muscle capillary endothelial cells, hydrolyzes the triglyceride core to free fatty acids and glycerol. The free fatty acids
are re-esterified and stored as triglycerides in adipose tissue or undergo β-oxidation in muscle. The lipid-depleted
chylomicrons, chylomicron remnants, are cleared by the liver through a pathway that depends on apolipoprotein-E as
a ligand for cellular receptors.