Food Lipids: Chemistry, Nutrition, and Biotechnology

plants [27]. However, the requirement by animals for the n-6 family of fatty acids
is not solely related to thermal tolerance, and it is intriguing that a completely separate
functionality has evolved in animals to take advantage of the n-6 family of
PUFAs as precursors to more unsaturated species. The protein structure of the �12
desaturase at the molecular and sequence level is known, and the ability of the single
gene product to affect �12 desaturation in transfection experiments with Saccharomyces
is known [28]. Although structural studies have not reached the same level of
understanding for the stearoyl CoA desaturase, similar functional themes are apparent,
such as analagous required histidine residues [29].
3. �15 Desaturase
The �15 or n-3 desaturase catalyzes the conversion of linoleic acid to �-linolenic
acid (as the respective acyl CoA). This enzyme activity is widely distributed in plant
tissues and is the basis for the abundance of its products, the n-3 family of fatty
acids, in plants and animals. Although these fatty acids are conspicuously enriched
in thylakoid photosynthetic membranes, the molecular advantage that the n-3 double
bond provides has not yet been determined. At the molecular level, there are two
isoforms of the gene, whose products are located in either the microsomal fraction
or plastid [29]. The plastid form of the enzyme appears to be associated with
photosynthesis because it is induced by light [30]. The gene product may also have
a role in altering membrane composition as a stress response as one of the n-3
desaturase genes is induced by wounding [31].
4. �6 and �5 Desaturases
Although ubiquitous, the �6 and the �5 desaturase enzymes have only recently been
cloned from animals [32,33]. Interestingly, although the �6 desaturase is exceedingly
rare in the plant kingdom, it had previously been cloned and functionally characterized
from borage [34]. Like animal SCD, these desaturases utilize a cytochrome b5 domain as an electron transfer system [32,33]. The expression of the �6 desaturase
was presumed to be limited to hepatic tissue; however, Northern blot analyses now
demonstrate that many tissues—including heart, kidney, lung, skeletal muscle, and,
notably, brain—express the desaturase [33]. There is a broad distribution of �5
desaturase mRNA expression among tissues as well, with Northern analyses identifying
expression in lung, skeletal muscle, placenta, kidney, and pancreas [32]. Like
the �6 desaturase, the �5 desaturase is expressed most abundantly in liver, brain,
and heart [32]. The abundance of �6 and �5 desaturase mRNA in tissues not typically
found to have desaturase activity may indicate a posttranslational regulation of
desaturase activity in these tissues. However, it is still an open question as to whether
tissues such as heart and brain are capable of fatty acid desaturation. Although surprisingly
few functional characterizations of the �5 and �6 desaturase genes have
been reported, there is evidence that the mRNA expression of both the �6 and �5
desaturases is regulated by nutrition. Hepatic �6 desaturase expression was suppressed
in mice fed corn oil (containing high concentrations of linoleic acid) relative
to those fed triolein [33]. Although oleic acid is not a typical substrate for the �6
desaturase and, by contrast, linoleic acid is an excellent substrate, these results may
indicate that the �6 desaturase is regulated by the degree of membrane unsaturation.
�5 desaturase mRNA expression also appears to be regulated by dietary fatty acids.
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