Food Lipids: Chemistry, Nutrition, and Biotechnology

oils (rich in 18:3 �6,9,12), and coriander (and other Umbelliferae, rich in 18:1 �6) (24–
26,28); �6DES and �4DES activities account for the presence of these latter two
groupings of species. These and other structurally related fatty acids are strictly
excluded from functional glycerolipids, and in the case of seed oils from rape, mustard,
Crambe spp., groudnuts, and honesty, the VLCFAs are virtually confined to the
sn-1,3 positions in triacylglycerols (28,194); this restricts the present capability of
trying to develop for industrial purposes high-erucic rape exceeding the theoretical
67% yield of erucic acid (145,146,148).
2. Involvement of Kennedy Pathway
By all accounts the Kennedy pathway is utilized for triacylglycerol assembly in these
species (194,195). GPAT has broad selectivity toward acyl-CoA substrates (180,196)
and will readily act on 22:1�13-CoA but shows no preference for it. Thus, the profile
of the acyl-CoA pool largely dictates which acyl group is attached to the sn-1 position
(180). As expected, LPAAT is selective toward unsaturated 18:X-CoA substrates and
is responsible for the exclusion of 22:1�13 (and like fatty acids) from the sn-2 position
in the transient PA pool in rape (180,196,197). This same specificity applies to palm,
castor, soy, and maize LPAAT (197). Although the GPAT and LPAAT of castor
bean have not been studied in detail, it could be hypothesized, based on other sources
of these enzymes, that they are rather broad in selectivity and that the tendency for
castor bean to yield triricinolein is controlled to a large degree by the abundance of
18:1�9,12-OH in the acyl-CoA pool.
DAGAT has pronounced selectivity for 22:1�13-CoA as substrate [even in lowerucic
rapeseed oil, since it is the elongase that is dysfunctional and not the DAGAT
(35,180)], but it also prefers to react with di-18:1�19-, or di-18:2�9,12,-acylglycerols over mixed 18:X/22:1�13-diacylglycerols (181). Again, the acyl-CoA pool would apparently
have substantial influence on the acyl-CoA and diacylglycerol species available
for reaction with the final DAGAT step. DAGAT from borage was inferred to
be highly selective for 18:3�6,9,12 appears at sn-1, whereas it comprises 50% sn-3 acyl
groups in mature seed oil; this composition helps to ensure that 18:3�6,9,12 is dedicated
to triacylglycerol and not functional lipid assembly (168). DAGAT of castor bean
is highly selective for di-18:1�9,12-OH-acylglycerol over other 18:X-diacylglycerol species
and completes the assembly of triacylglycerols rich in 18:1�9,12-OH (147).
Flux of acyl-CoA and glycerol through the Kennedy pathway toward triacylglycerol
assembly in these plant species is rapid, inasmuch as intermediates with
22:1�13 residues do not appreciably accumulate but are rapidly labeled with [ 14 C]acyl-
CoA substrates (194,195). The PC pool is also involved in triacylglycerol assembly,
likely where 18:2�9,12/18:3�9,12,15 residues appear at the sn-2 position of oils from
these species.
3. Involvement of PC Exchange Reactions
Utilization of oxygenated fatty acids in triacylglycerol assembly has been best studied
in the beans of castor, which accumulate over 90% as ricinoleic acid. The utilization
(indeed, genesis) of oxygenated fatty acids requires access of the parent fatty acid
to the PC pool. The 12-MO that forms ricinoleic acid in castor bean (147), and the
12-EPOX system to form vernolic acid (18:1 �9,12/13-epoxy), which accumulates to levels
of 70% in Euphorbia lagascae (149), require PC-linked 18:1 �9, and specifically, sn-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.