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CASE FILES: BIOCHEMISTRY
Intestinal
Mucosal
Cell
Lymph
Chylomicron
Heparan sulfate
Lipoprotein
lipase
capillary
' wall
FFA-albumin
+ Glycerol
Adipose and Muscle
Cell
chylomicron
remnant
receptors
Remnant
Apoproteins (Apo)
Cholesterol (c)
Phospholipid (PL)
Liver Cell
Cholesterol Ester (CE)
Triacylglycerol (TG)
Free fatty acids (FFA)
Figure 35-1. Formation and metabolism of chylomicrons. (Abbreviations used
are defined in the enclosed box.)
mediated by apoE. The remnants are taken into the hepatocytes by endocyto-
sis and degraded in the lysosome to fatty acids, amino acids, cholesterol, glyc-
erol, and phosphate.
Chylomicrons appear in the blood stream shortly after consumption of a
meal containing fat. However, the clearance rate for chylomicrons is fast and
blood is usually free of chylomicrons following an overnight fast.
VLDLs are lipoproteins synthesized in the liver from endogenous TG, cho-
lesterol, and phospholipids, along with several apolipoproteins including, apoB-
100, apoE, and apoC-II. Following synthesis, the VLDL is secreted into the
bloodstream, where it obtains more apoE and apoC-II from HDL (Figure 35-2).
In a process similar to that of chylomicron degradation, the VLDL apoC-II
binds to and activates LPL in the capillary beds of adipose, muscle and mam-
mary tissue. LPL degrades the VLDL TG core, releasing free fatty acids and
glycerol. About half of the resulting VLDL remnants bind to receptors in liver
cells that recognized apoE and are taken up by endocytosis. The remaining
VLDL receptors are further degraded to intermediate-density lipoproteins
(IDL), which have their residual TG removed by the action of hepatic lipase to
yield low-density lipoproteins (LDL). Hepatic lipase (HL) is synthesized and
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