Food Lipids: Chemistry, Nutrition, and Biotechnology

tems, PTAP is a likely candidate for regulatory control (35), and the 18:1 �9-induced
translocation of PTAP from the cytosol to the endoplasmic reticulum of maturing
safflower seeds was suggested to be a feedward control mechanism (173,174).
DAGAT has also been suggested as being rate-limiting in rapeseed (35) and is a
candidate enzyme for overall control of flux toward triacylglycerol synthesis (14),
since it is the only enzymic step dedicated to triacylglycerol assembly (109). In
species where storage oils are dominated by 18:1 �9/18:2 �9,12/18:3 �9,12,15, CPT is suggested
to control flux of fatty acids and glycerol toward triacylglycerol assembly
(177,178). The acyl-CoA pool may also have regulatory features, since it induces
import of PTAP (174), and acyl-CoA levels and fatty acid profile have impact on
rates of some enzyme activities in the Kennedy pathway (180). Finally, two metabolically
discrete pools of diacylglycerol are suggested to exist (181), potentially
exerting control over flux toward triacylglycerol synthesis. In any case, triacylglycerol
accumulation is a developmentally regulated process that is synchronized with
the maturation of seed or fruit tissue (35,148) and must involve coordinated expression
of seed/fruit-specific genes.
C. Seed Oil Enriched in Unsaturated (�9, �12, �15) 18-Carbon
Fatty Acids
1. General Aspects
In these species [soybean, safflower, sunflower (Helianthus annuus), avocado, cocoa
(Theobroma cacao), maize (Zea mays), palm, and linseed (Linum usitatissimum)],
triacylglycerol assembly results in the occupation of the sn-1,3 positions by 16:0/
18:X, and of the sn-2 position by 18:X. Arabidopsis is a good genetic model for
these species because it accumulates 30% 18:2�9,12, 20% 18:3�9,12,15 (in addition to
22% 20:1�11) in seed oil, and the genetics of this organism are well known (182).
Species accumulating unsaturated fatty acids with double bonds in positions other
than �9, �12, and �15, are discussed as a group in Sec. V.E.
2. Involvement of Kennedy Pathway
Generally, GPAT has broad specificity, as characterized by the safflower enzyme
(170,183), although it best recognizes 16:0/18:X-CoA substrates. The GPAT from
cocoa beans is slightly different in that only saturated 16/18-CoA substrates are
recognized and 18:1-CoA is not very reactive (184). Because GPAT is relatively
nonselective, the fatty acyl residue appearing at the sn-1 site will largely be representative
of the acyl-CoA pool available, which in turn is a reflection of the primary
products of FAS and modification reactions.
In contrast, LPAAT, is characterized from cocoa bean, palm endosperm (kernel),
maize, rape, and safflower, tends to be very selective for unsaturated 18:X-CoA
substrates (171,176,183,184). Thus, the combined selectivities of GPAT and LPAAT
confer the ‘‘eukaryotic’’ configuration of glycerolipids in these species. Furthermore,
LPAAT is selective also for the species of lysophosphatidic acid (LPA) cosubstrate
in that maize and rape LPAAT prefer 18:1�9-LPA as substrate and will react with
12:0-LPA (176). On the other hand, palm endosperm LPAAT will recognize both
12:0- and 18:1�9-LPA, but will tend to couple 12:0-LPA with another 12:0 residue,
whereas no selectivity between 12:0- and 18:1�9-CoA is observed with 18:1�9-LPA as substrate (176). This property helps partition unusual or ‘‘nonfunctional’’ fatty
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.