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

646 Lipoprotein/Cholesterol Metabolism

FIGURE 2 Lipids of plasma lipoproteins. Virtually all of the triglyceride and cholesterol ester of a lipoprotein is in the
interior. Some unesterified cholesterol can also exist in the interior. However, all of the phospholipid is at the surface
of the particle and surrounds the hydrophobic core.

III. CHYLOMICRON METABOLISM terol ester-rich chylomicron particles. Triglycerides and
cholesterol esters are then packaged into the core of chy-
Since the intestine is primarily an absorptive organ, it must lomicrons, which are secreted into the lymphatics. By se-
have the means of exporting newly absorbed lipids. The creting chylomicrons into the lymphatics, they gain en-
enterocyte re-esterifies fatty acids and monoglycerides to trance into the general circulation via the thoracic duct.
form triglycerides and phospholipids. Absorbed choles- This guarantees that extrahepatic tissues, principally adi-
terol is esterified to form cholesterol esters. Under ordi- pose tissue and muscle, are the first to be exposed to the
nary circumstances, the vast majority of the core lipids newly secreted chylomicrons—if chylomicrons were se-
in the chylomicron are triglycerides; however, after a creted directly into the bloodstream, they would first be
cholesterol-rich meal, the intestine manufactures choles- delivered to the liver via the portal vein.
Chylomicrons are very large (up to 1 µm in diameter).
Thus a plasma sample containing chylomicrons is milky
in appearance. A sample of fasting plasma is typically
clear in appearance, even if there is an elevation in LDL
particles—LDL particles are not large enough to scatter
light.
In terms of net transport, the bulk of lipid flux is triglyc-
eride → adipose tissue and muscle. How is the triglyceride

TABLE I Buoyant Density and Size of Plasma
Lipoproteins
Density Diameter
Class (g/ml) (nm)
Chylomicrons 0.93 75–1200
VLDL 0.93–1.006 30–80
FIGURE 3 The domain structure of a plasma lipoprotein. The IDL 1.006–1.019 25–35
nonpolar lipids triglyceride and cholesterol ester are surrounded LDL 1.019–1.063 18–25
by the amphipathic lipids phospholipid and cholesterol. The lat- HDL2 1.063–1.125 9–12
ter are stabilized by apolipoproteins. These proteins have amphi-
HDL3 1.125–1.210 5–9
pathic α-helix and amphipathic β-sheet secondary structures.