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Food Lipids: Chemistry, Nutrition, and Biotechnology

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of eicosanoids. It is present in all tissues <strong>and</strong> is particularly enriched in phosphatidylcholine,<br />

<strong>and</strong> ether-linked phospholipid membrane pools.<br />

5. Docosatetraenoic Acid (22:4n-6)<br />

Docosatetraenoic acid is the direct elongation product of arachidonic acid <strong>and</strong> is<br />

present in minimal amounts in animal tissues. Docosatetraenoic acid is a substrate<br />

for peroxisomal retroconversion resulting in the formation of arachidonic acid [55].<br />

6. Other n-6 Fatty Acids<br />

The family of n-6 fatty acids is derived exclusively from the production of linoleic<br />

acid but can be converted by elongation, desaturation, � oxidation, <strong>and</strong> so forth.<br />

Other rare but naturally occurring n-6 fatty acids include 16:2, 20:2, 22:2, 22:3,<br />

24:2, 25:2, 26:2, <strong>and</strong> 30:4 [104].<br />

D. n-3 Fatty Acids<br />

1. �-Linolenic Acid (18:3n-3)<br />

�-Linolenic acid is produced de novo by the �12 <strong>and</strong> �15 desaturation of oleic acid<br />

in plants. Along with linoleic acid, �-linolenic acid constitutes one of the two primary<br />

PUFA products of plant fatty acid biosynthesis. It is primarily present in the leaves<br />

of plants but is also a minor component of seed oils. �-Linolenic acid serves as the<br />

metabolic precursor for the production of n-3 fatty acids in animals. The success of<br />

agricultural seed oils has caused a significant shift in the natural balance of linoleic<br />

<strong>and</strong> linolenic acids, <strong>and</strong> over the last 100 years the average dietary content of �linolenic<br />

acid has declined significantly [94].<br />

2. Eicosapentaenoic Acid (20:5n-3)<br />

Eicosapentaenoic acid (EPA) is produced de novo by marine algae <strong>and</strong> in animals<br />

by the desaturation/elongation of �-linolenic acid. EPA is the primary fatty acid of<br />

fish oil (approximately 25–20% by weight) although it is not produced de novo by<br />

fish. It has also been reported that significant EPA production can occur in animals<br />

by the �-oxidation chain shortening of DHA [55]. EPA has been investigated extensively<br />

for its action as a competitive inhibitor of arachidonic acid metabolism. Although<br />

eicosanoids can be produced from EPA, they appear to have either no activity<br />

or an activity that opposes arachidonic acid–derived eicosanoids.<br />

3. Docosapentaenoic Acid (22:5n-3)<br />

Docosapentaenoic acid is the elongation product of EPA <strong>and</strong> is present in most<br />

marine lipids. Docosapentaenoic acid can be converted to DHA via a three-step<br />

process involving a unique �6 desaturation in animals (see above) [8].<br />

4. Docosahexaenoic Acid (22:6n-3)<br />

Docosahexaenoic acid (DHA) is produced de novo by marine algae <strong>and</strong> is a primary<br />

component of fish oil (approximately 8–20% by weight). The production of DHA<br />

in animals from linolenic acid occurs via the desaturation/elongation of �-linolenic<br />

acid to 24:5n-3. This very long chain unsaturated fatty acid is desaturated by a �6<br />

desaturase (possibly a unique �6 desaturase enzyme) <strong>and</strong> the resulting fatty acid<br />

undergoes one cycle of � oxidation to form DHA [8,9]. Animals appear to have a<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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