Food Lipids: Chemistry, Nutrition, and Biotechnology

1. �9 Desaturase
The stearoyl CoA desaturase enzyme (SCD) introduces a double bond at the �9
position of stearoyl CoA, forming oleic acid and the n-9 family of fatty acids. This
enzyme also adds a double bond at the n-7 position of palmitic acid, forming palmitoleic
acid and the n-7 family of fatty acids. The enzyme appears to be a fundamental
gene product in the regulation of a host of cellular processes. Its structure
has been resolved to 2.6 Aº and various aspects of the functional domains were
identified in the plant enzyme [12]. The sequences of various SCD genes were reported,
and more recent work has focused on the upstream regulatory regions of the
genes from both microbial and animal sources [13–15]. From this work, a host of
interesting regulatory sites have been identified, implying that the SCD genes are
multiply regulated in all cellular systems in which they are expressed. Primitive,
single-cell organisms are known to alter SCD transcription in response to temperature,
environmental shocks (pathogenic and osmotic), and substrate modification
[16–19].
Not surprisingly, in addition to its many cellular actions, SCD appears to play
multiple roles in higher plants and animals. Many reports have associated SCD with
whole-tissue functions such as adipose accretion, lipid secretion, and tissue responses
to stress. For example, plants induce SCD on thermal shock and senescence [20].
The most intriguing aspects of SCD in lipid regulation in animals are found in
the apparent role of the SCD genes in lipogenesis. Much of this information was
summarized by Ntambi [14]. Two highly homologous genes, described in mice, that
code for SCD are termed, logically, scd1 and scd2. scd1 is one of the first genes
induced during adipocyte differentiation and this induction is responsive to insulin,
carbohydrate, and elevated cAMP [15]. Furthermore, inhibition of SCD prevents
adipocyte differentiation and, in mature cells, reduces lipogenesis in adipocytes and
in hepatocytes of avian [21] and mammalian cells. Genetically obese animals exhibit
greater amounts of SCD, consistent with a pivotal role in adipocyte function [22].
The striking ability of PUFA to down-regulate the enzyme activity is well described
biochemically and recently was shown to be due to a substantial decrease in the
stability of scd1 mRNA [23]. It is not yet clear precisely what advantage such a
regulatory control serves. However, various suggestions have been advanced that this
could be a partial basis for reduced hepatic lipoprotein secretion during PUFA feeding
and could possibly reduce adiposity in PUFA-fed animals. Thus, there is abundant
evidence that unsaturated families of fatty acids interact with each other in highly
complex ways.
The SCD gene is known to be sensitive to a variety of hormonal signals. For
example, SCD is differentially regulated in females relative to males and its higher
activity in females may be part of the spectrum of lipid metabolic changes that are
a consequence of sex differences in the activity of growth hormone [24,25].
2. �12 Desaturase
The �12 desaturase of plants is responsible for the conversion of oleic acid to linoleic
acid and is thus the molecular basis for the n-6 family of fatty acids. Although it is
present solely in plants, its functional requirement in plants is not completely understood
[26]. At the present time, the most apparent action of the enzyme activity
is to improve thermal tolerance, especially of specific membrane compartments in
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.