Food Lipids: Chemistry, Nutrition, and Biotechnology

acids in a manner that induces them to evolve into triacylglycerols (see Sec. V.D for
other aspects of medium chain triaclyglycerol assembly).
The next step in the Kennedy pathway, PTAP, has not been well studied, but
in safflower it shows greater selectivity for phosphatidic acid (PA) with two unsaturated
acyl groups than for PA species with one or no unsaturated acyl groups (185).
This pattern of selectivity probably affords little influence on triacylglycerol composition,
but it may help ensure that saturated fatty acids are not allowed to readily
gain entrance into the PC pool via subsequent CPT action.
Finally, DAGAT from maize, castor, and groundnut, and probably many other
oilseed species, like GPAT, exhibits rather broad specificity among 14:0/16:0/18:0/
18:X-CoA substrates (175,176). Thus, the fatty acyl groups evolving at the sn-3
position are largely defined by the profile of the available acyl-CoA pool. Palm
mesocarp is unusual in that the DAGAT associated with the developing oil bodies
(derived from the endoplasmic reticulum, see Sec. V.F regarding oil body genesis)
is selective for 18:1�9- over 16:0-CoA compared to the broad specificity of endoplasmic
reticular DAGAT activity (176). This implies the presence of two populations
of DAGAT (perhaps conferred by the membrane environment), with that in
developing oil bodies selecting for unsaturated 18:X acyl species at sn-3 for triacylglycerols
being actively deposited in accumulating oil bodies. Some sources of
DAGAT have more pronounced selectivities; that of cocoa beans, for example, is
selective for 16:0-CoA and 18:0-CoA substrates (184), contributing to the characteristic
triacylglycerol species in this species, which is 24–30% 16:0 and 30–36%
18:0 (28). The unusual enrichment of 18:0 in storage traiacylglycerols in cocoa beans
must be accounted for, in part, by the accumulation of 18:0-CoA in the acyl-CoA
pool. Thus, during development or maturation of cocoa beans, some mechanism is
evoked whereby �9DES activity is suppressed (and/or TE action toward 18:0-ACP
is enhanced), allowing 18:0 to be produced at levels much beyond the normal few
percent that typically occurs in plant tissues. This yet-to-be-identified trait may constitute
an alternative approach to manipulating plant species to produce high 18:0
oils, which in turn could circumvent the need for hydrogenation as a means to yield
margarine-type fats (see Sec. VI.B).
3. Involvement of PC Exchange Reactions
Perhaps the most important enzyme activities in these plant species are CPT and
LPCAT, which mediate steps in triaclyglycerol assembly that allow 18:2�9,12/ 18:3�9,12,15 to accumulate (177,181,186). In concert with the Kennedy pathway, and
when 18:1�9 is in abundance (as would be expected from many plant FAS systems),
the collection of these activities gives rise to triacylglycerols with mono- and polyunsaturated
18:X fatty acyl groups occupying all three sites of the triacylglycerols
(instead of 16:0/18:0 residing at one or both of the primary sites). In both sunflower
and safflower seeds, 18:1�9 is esterified at sn-1 and sn-2 positions by GPAT and
LPAAT, respectively. Following LPAAT action, which also favors esterification of
18:1�9, the resulting diacylglycerol can exchange with the PC pool via CPT. The
PC-linked 18:1�9 is now subject to desaturation by �12DES and �15DES, which
are relatively nonspecific for position (sn-1 or -2) of the acyl group (178). The
resulting PC derivatives can undergo a reciprocal exchange (via CPT) to form diacylglycerol,
which is then subject to DAGAT action to yield the final triacylglycerol
product. Alternatively, the presence of LPCAT, which is highly selective for ester-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.