Food Lipids: Chemistry, Nutrition, and Biotechnology

usual NMIFA structures with potential biological actions include conjugated linoleic
acid that has exhibited potent anticancer properties in animal models of carcinogenesis
[111,112]. The true therapeutic value, as well as the potential toxicities, of these
fatty acids will await the development of commercial sources in food grade quantities
for larger studies. Nevertheless, the potential for modifying the biosynethetic capabilities
of crop plants is already being exploited for many nutritional and functional
targets, and the possibility of producing fatty acids with unusual structures is likely
to be limited only by the documented value of producing them.
VII. SUMMARY
The fatty acid nomenclature system based on n designation was developed to describe
fatty acids in terms of their nutritional functions. Whereas this approach is of
some use, it may lead to confusion over the true basis by which fatty acids modulate
physiology. There is mounting evidence that each fatty acid has its own role in
nutrition that is not dictated by its n designation. Chemically similar fatty acids often
have widely ranging functionalities. This phenomenon is exemplified by the antagonistic
relationship between arachidonic acid and EPA metabolism despite the fact
that the fatty acids differ only by the additional double bond in EPA. The fundamental
relation between members of the same n family is one of interconvertibility. Families
are grouped only by their potential to be converted to longer chain members of the
same n designation or, in a more practical sense, by their ability to act as precursors
for the production of a particular fatty acid of interest. The enzymatic activities
required for the conversion of fatty acids to longer, more unsaturated chain members
of the same n family are redundant. Consequently, if cells were not capable of
recognizing fatty acids based on their n designation and responding by modulating
desaturase and elongase activities appropriately, animals would be completely at the
mercy of their diets.
The regulation of acyl content appears to be critical to the function of a cell,
as mammalian cells expend a substantial amount of energy in maintaining distinct
and heterogeneous membrane fatty acid compositions. Cells are capable of preserving
these compositional identities even when confronted with phospholipid diffusion and
vesicular transport, suggesting that acyl composition is tightly regulated. In addition
to overall membrane acyl content, phospholipid acyl composition is rigorously maintained
in a positionally specific manner. The complex framework of enzymatic activities
that upholds these compositions makes large-scale changes in membrane
composition rare and, as a result, the effects specific fatty acids have on membrane
structure and physiology are not well understood. However, the extraordinary selectivity
of certain tissues for individual PUFAs strongly suggests a specific role for
those fatty acids in cell function. For instance, brain and neural tissues are enriched
in DHA, whereas adrenal glands have a high content of docosatetraenoic acid. Neither
of these PUFAs can be produced de novo in humans and so their accumulation
in cell membranes must be mediated by preferential absorption or specific desaturation
reactions within the given tissue. PUFA specificity is also prevalent in subcellular
organelles. Two examples include the inordinate enrichment of vaccenic acid
in mitochondria and the conspicuous accumulation of arachidonic acid in the sn-2
position of nuclear membrane ether-linked phospholipids. Selective incorporation of
PUFAs implies that fatty acids have some degree of functionality in the cell. In
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.