Food Lipids: Chemistry, Nutrition, and Biotechnology

equirement for DHA for neural function and they rely on its production from n-3
precursors by elongation/desaturation cycles or through ingestion of the intact acid
[106]. Although the exact role DHA plays in animal physiology is not understood,
the great care with which the fatty acid is preserved in certain tissues implies that it
may be an essential component of certain cells. Brain and retinal tissues are particularly
enriched in DHA.
5. Other n-3 Fatty Acids
The family of n-3 fatty acids is derived from �-linolenic acid but can be modified
by chain elongation, desaturation, � oxidation, etc. Naturally occurring but rare n-3
fatty acids include 16:3, 16:4, 18:4, 18:5, 20:2, 20:3, 20:4, 21:5, 22:3, 24:3, 24:4,
24:5, 24:6, 26:5, 26:6, 28:7, and 30:5 [104].
E. Unusual and Non-Methylene-Interrupted Fatty Acids
The vast majority of PUFAs contain multiple double bonds in a 1,4-pentadiene structure
in which a single methylene carbon is positioned between the two double bonds.
More double bonds are added as a direct result of the positional selectivity of the
subsequent desaturase enzymes. Hence, from fatty acids containing two double bonds
(linoleic acid 9,12 18:2) to those containing six double bonds (DHA; 4,7,10,
13,16,19), all exhibit methylene interruption over the entire length of the molecule.
However, there are certain naturally occurring fatty acids in which single double
bonds exist at a distance. These are the non-methylene-interrupted fatty acids (NMI-
FAs). Various NMIFAs have been described, including allenic, conjugated, allylic,
enoic, acetylenic, cyclic, branched, hydroxylated, iso and anteiso fatty acids [104].
The most common NMIFAs are fatty acids in which one of the double bonds is
ostensibly missing from the middle of a double-bond system. Conifers were shown
to contain up to 20% by weight in their seeds of the NMIFA 5,11,14-eicosatrienoic
acid and 5,11,14,17-eicosatetraenoic acid [107]. In most reported studies, the basis
of the synthesis of these fatty acids is through an active elongase enzyme that elongates
18- to 20-carbon fatty acids, bypassing the �6 desaturase. Indeed, it has been
argued that the methylene-interrupted structure is not due to explicit enzyme specificity
but rather the predisposition of substrate fatty acids [108]. Desaturation at the
�5 position of the elongated fatty acid produces a 20-carbon fatty acid without the
�8 double bond. This aggressive elongation has been argued to be the basis for the
occurrence of small quantities of 5,11,14-eicosatrienoic acid in animal tissues. While
most unusual fatty acids are not readily esterified into membrane phospholipids, for
certain structures this is not true. 5,11,14-Eicosatrienoic acid, which is the structural
analog to arachidonic acid with the absence of the �8 double bond, is esterified into
several membranes, and even shows preference for incorporation into specific phospholipids
including phosphatidylinositol [107,109]. Because the absence of the �8
double bond makes it impossible to synthesize prostaglandins or leukotrienes from
NMIFA, replacement of arachidonic acid with NMIFA in phospholipids would be
predicted to have substantial effects on eicosanoid cellular signaling proportional to
the extent of displacement of arachidonic acid. This was shown to be true for isolated
tissues [107]. Furthermore, in an animal model of genetic autoimmunity, animals fed
diets containing these fatty acids exhibited a net reduced severity of disease consistent
with a selective decrease in eicosanoid signaling [110]. Other examples of un-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.