Food Lipids: Chemistry, Nutrition, and Biotechnology

acyltransferase responsible for esterifying acyl chains into the sn-2 position is lysophosphatidate
acyltransferase [64]. Lysophosphatidate acyltransferase has a higher
activity with unsaturated acyl CoA thioesters and is responsible for the virtual absence
of saturated fatty acids at the sn-2 position in membrane phospholipids. As
might be expected, the lysophosphatidate acyltransferase has a higher activity than
the glycerol-3-phosphate acyltransferases [64], as the majority of the fatty acid remodeling
in membranes occurs with unsaturated fatty acids. The combined preferences
and activities of the acyltransferases are largely responsible for the positional
and site-specific acyl compositions observed in phospholipid membranes.
1. Acyl-Specific Incorporation
Several phospholipid species are noteworthy for their high degree of incorporation
of specific fatty acids. The ether-linked phospholipids, for example, are enriched at
the sn-2 position by a CoA-independent transacylase relatively specific for arachidonic
acid [65,66]. The most salient example of acyl-specific incorporation, however,
can be found in the mitochondrial diphospholipid cardiolipin. Cardiolipin is characteristically
enriched with linoleic acid to as much as 85% of its acyl species in
vivo [67]. Cardiolipin is also selectively enriched in DHA to as much as 50% of its
total fatty acid content [58,67,68]. The implications of the high degree of unsaturation
of cardiolipin may involve its function as the membrane solvent for the electron
transport enzymes. Other phospholipids incorporate specific fatty acids to varying
degrees. Spector and Yorek [69] treated Y79 retinoblastoma cells with arachidonic
acid, DHA, and oleic acid to determine the relative affinity of various phospholipid
classes for unsaturated fatty acids. Phosphatidylcholine, phosphatidylethanolamine,
and phosphatidylserine could all be enriched with the monounsaturated fatty acid
oleic acid. Only phosphatidylethanolamine and phosphatidylinositol were substantially
enriched in arachidonic acid, whereas phosphatidylcholine, phosphatidylethanolamine,
and phosphatidylserine all incorporated docosahexaenoic acid. The differences
in acyl incorporation indicate that the synthesis and remodeling mechanisms
described above have different activities depending on both the fatty acid and the
phospholipid substrates.
IV. DIETARY SOURCES OF UNSATURATED FATTY ACIDS
A. Microbes
Most bacteria are fully capable of synthesizing all of the fatty acids required for
their normal growth and reproduction. The fatty acid synthetase enzyme complex
responsible for this activity adds acetate units to a final chain length of 16–18
carbons. These can be further desaturated (normally once to a monounsaturated fatty
acid) after which the saturated and unsaturated fatty acids are esterified to yield
membrane phospholipids. Bacteria in general do not store energy as fats; hence,
triacylglycerols are not abundant forms of lipid in bacteria. They are thus not a
quantitatively important fat source in foods. The unique metabolism of bacteria,
especially the production of branched chain and odd-numbered fatty acids, can occasionally
generate measurable quantities of unusual fatty acids in, for example,
bovine milk, due to incorporation of the products of rumen fermentation. In the
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.