Food Lipids: Chemistry, Nutrition, and Biotechnology

saturation of EFA occurs (primarily in the liver). The elongation and desaturation of
18:2�6 is illustrated in Figure 3. The most common of the �6 fatty acids in our
diets is 18:2�6. Often considered the parent of the �6 family, 18:2�6 is first desaturated
to 18:3�6. The rate of this first desaturation is thought to be limiting in
premature infants, in the elderly, and under certain disease states. Thus, a great deal
of interest has been placed in the few oils that contain 18:3�6, �-linolenic acid
(GLA). Relatively rich sources of GLA include black currant, evening primrose, and
borage oils. GLA is elongated to 20:3�6, dihomo-�-linolenic acid (DHGLA).
DHGLA is the precursor molecule to the 1-series prostaglandins. DHGLA is further
desaturated to 20:4�6, precursor to the 2-series prostaglandins. Further elongation
and desaturation to 22:4�6 and 22:5�6 can occur, although the exact function of
these fatty acids remains obscure.
Figure 4 illustrates analogous elongation and desaturation of 18:3�3. The elongation
of 20:5�3 to 22:5�3 was thought for many years to be via �4 desaturase.
The inexplicable difficulty in identifying and isolating the putative �4 desaturase led
to the conclusion that it did not exist, and the pathway from 20:5�3 to 22:6�3 was
elucidated as a double elongation, desaturation, and �-oxidation.
One of the main functions of the EFAs is their conversion to metabolically
active prostaglandins and leukotrienes [14,15]. Examples of some of the possible
conversions from 20:4�6 are shown in Figures 5 and 6 [15]. The prostaglandins are
called eicosanoids as a class and originate from the action of cyclooxygenase on 20:
4�6 to produce PGG 2. The standard nomenclature of prostaglandins allows usage
of the names presented in Figure 5. For a name such as PGG 2, the PG represents
prostaglandin, the next letter (G) refers to its structure (Fig. 7), and the subscript
number refers to the number of double bonds in the molecule.
The parent structure for most of the prostaglandins is prostanoic acid (Fig. 7)
[14]. Thus, the prostaglandins can be named based on this parent structure. As well,
Figure 3 Pathway of 18:2�6 metabolism to 20:4�6.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.