Food Lipids: Chemistry, Nutrition, and Biotechnology

(HDLs). Each class has its own characteristic lipid and apolipoprotein composition,
size, and density, whereas each apolipoprotein has its own specific metabolic functions
(Tables 2 and 3).
IV. METABOLISM OF DIETARY FATTY ACIDS AND LIPOPROTEINS
A. Exogenous Pathway
The metabolism of dietary fatty acids, dietary cholesterol, and lipoproteins is depicted
in Figure 1. In the duodenum, dietary triacylglycerols are dissolved with the help of
bile salts, as well as small quantities of fatty acids and monoglycerides. The enzyme
pancreatic lipase then hydrolyzes the dietary triacylglycerols into mono- and diglycerides,
free fatty acids, and glycerol. The so-formed emulsion of lipids, which also
contain the dietary cholesterol, passes the mucous membrane of the intestinal cells.
Within the cell, further hydrolysis of lipids takes place and new triacylglycerols are
formed by reesterification of the free fatty acids with glycerol. The newly synthesized
triacylglycerols and cholesterylesters, derived from the dietary cholesterol, are incorporated
into chylomicrons, which enter the lymph and subsequently the blood
circulation in the subclavian vein. In the blood, the triacylglycerols from the chylomicron
core are hydrolyzed by lipoprotein lipase (LPL), an enzyme adhered to the
endothelial cells of the blood vessels and activated by apolipoprotein C-II. The free
fatty acids pass the endothelial cells and enter adipocytes or muscle cells. In these
cells, the fatty acids are respectively stored as triacylglycerols or oxidized. The core
of an emptied chylomicron mainly consists of cholesterylesters and is called a chylomicron
remnant. These remnant particles are removed from the circulation by the
hepatic remnant receptor, which has a high affinity for apolipoprotein E (apoE) from
the chylomicron surface.
Table 2 Some Physical Characteristics and Mean Composition of Lipoprotein Fractions
from Normotriglyceridemic Subjects
Chylomicrons VLDL IDL LDL HDL
Density (g/mL)
Lower limit — 0.96 1.006 1.029 1.063
Upper limit 0.96 1.006 1.019 1.063 1.21
Size (nm) 75–1200 30–80 25–35 19–25 5–12
% of total lipoprotein mass
Core components
Triacylglycerols 87 52 29 6 6
Cholesterylester 3 9 28 40 21
Surface components
Phospholipids 6 23 22 22 24
Free cholesterol 2 6 7 9 3
Apolipoprotein 2 10 14 23 46
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.