Food Lipids: Chemistry, Nutrition, and Biotechnology

synthesize EPA from linolenic acid except in tissues of the central nervous system.
In contrast, fish are metabolically capable of these steps (they convert the 18:3 in
plankton to 20:5), so they are a rich source. Another fatty acid concentrated in fish
oil is docosahexaenoic acid (DHA, 22:6, �3), which can potently inhibit cyclooxygenase.
Thus, fish oil feeding has become an important paradigm in the study of
the effects of dietary fat on eicosanoid metabolism and immune responses. It should
be remembered that fish oils contain highly variable amounts of �3 fatty acids, and
many fish oils are high in saturated fatty acids [1].
Interest in eicosanoids, dietary fat, and the immune system is high because
eicosanoid metabolism probably is one of the major mechanisms by which dietary
fat modulates a variety of immune responses, although undoubtedly other mechanisms
exist. Both the concentration of precursor fatty acid in the phospholipids of
cell membranes and their rate of metabolism are controlling factors in the amount
of eicosanoids released by cells. Competition between �3 and �6 fatty acids for
lipoxygenases and cyclooxygenases results in production of eicosanoids of different
types and in varying amounts. In the prostaglandin family there are two major series,
designated E and F. The E series has a ketone and hydroxyl group added at C-9 and
C-11, while the F series has hydroxyl groups at both positions, eliminating the double
bonds from these positions (Table 1). In addition to the letter designation, there are
subseries indicated by numbers. Those prostaglandins derived from 20:3 are designated
E 1 or F 1�; those from 20:4 are E 2 or F 2�; and those derived from 20:5 are E 3
or F 3�. Therefore, the subscripts 1, 2, and 3 refer to the number of double bonds
remaining in a prostaglandin molecule.
The most common method for quantitating eicosanoids is through the radioimmunoassay
(RIA). The RIA depends on an antibody that recognizes these molecules
and does not cross-react with related compounds. Problems associated with the
measurement of eicosanoids are the short half-life in tissues and body fluids as well
as the cross-reactivity seen between some prostaglandins. For example, few methods
can distinguish between prostaglandins E 1 and E 2, so they are often simply expressed
as PGE. The short half-life of these compounds necessitates rapid isolation and
chilling of samples, addition of a metabolic inhibitor, or all three. There is little
doubt that many of the studies reporting levels of eicosanoids did not use conditions
rigorous enough to ensure the accuracy of the analytical data. Moreover, since in-
Table 1 Relationship of Precursor Fatty Acids to Eicosanoid Products a
Fatty acid
Dihomo-�-linolenic acid, all-cis-
8,11,14 C20:3 (�6)
Arachidonic acid b , all-cis-
5,8,11,14 C20:4 (�6)
Eicosapentaenoic aicd, all-cis-
5,8,11,14,17 C20:5 (�3)
a Subscripts indicate the number of double bonds.
b Can also be converted to prostacyclin (PGI2).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
Prostaglandins
E series F series Thromboxanes Leukotrienes
E 1 F 1� A 1 A 3 C 3 D 3
E 2 F 2� A 2 A 4 B 4 C 4 D 4 E 4
E 3 F 3� A 3 A 5 B 5 C 5