Food Lipids: Chemistry, Nutrition, and Biotechnology

ifying 18:1 �9 and deacylating 18:2 �9,12/18:3 �9,12,15, allows for reversible exchange between
the acyl-CoA pool and PC-linked acyl groups (179). Thus, after desaturation,
18:1 �9,12 and 18:3 �9,12,15 may be released to the acyl-CoA pool to be utilized in other
assembly steps of the Kennedy pathway recognizing 18:2 �9,12- and 18:3 �9,12,15-CoA
as substrates. In safflower, sunflower, and linseed, these fatty acids appear to be
utilized by GPAT and LPAAT (187). Perhaps, more importantly, 18:2 �9,12- and
18:3 �9,12,15-CoA can be used by DAGAT to complete the acylation of triacylglycerols
exclusively with (poly)unsaturated 18:X residues, a trait characteristic of seed oils
of many of these species (28). For example, linseed oil has 50–60% of each acyl
site occupied by 18:3 �9,12,15.
4. Unique Features of Species in This Group
In keeping with the triacylglycerol profile of cocoa butter and avocado oil, the activities
of LPCAT and CPT are not very involved in triacylglycerol assembly: there
is little 18:2 �9,12 in these oils, and it occurs primarily in the sn-2 position (28). It
should be noted that diacylglycerol and PC formation share a common (Kennedy)
pathway, and PC with polyunsaturated 18:X residues is required for assembly of
functional (membrane) lipids in these plant species. Since it is so vital for the plant
cell to segregate fatty acids into storage and functional lipid, it is somewhat perplexing
that CPT (such as in rape and sunflower) exhibits little selectivity (181) and
therefore cannot serve as a ‘‘gatekeeper’’ to exclude unusual fatty acids from the PC
pool. In view of the combined broad specificity of DAGAT, it has been argued that
two pools of diacylglycerol exist, and these are spatially (or metabolically) segregated
to satisfy the divergent needs of storage and functional lipid assembly (181).
Considering the complexity of all the acyl modification and glycerolipid assembly
reactions that take place in the endoplasmic reticulum, it would be surprising if no
metabolic compartmentation existed for some of these processes.
Finally, the importance of the composition of the acyl-CoA pool, as a contributor
to the resulting profile of assembled triacylglycerols, cannot be overstressed.
Even though safflower seed oil is about 90% 18:1 �9 and 18:2 �9,12 (28), when presented
with equimolar levels of 16:0/18:0/18:1 �9 in vitro, the (non)selectivity features
of the enzymes involved in the Kennedy pathway can give rise to cocoa butter–type
fats (notably rich in 16:0 and 18:0) (183). This is the type of biosynthetic potential
that presents opportunities for exploitation in manipulating plant species for specific
end uses in food.
D. Oils Enriched in Medium Chain Fatty Acids
1. General Aspects
Species important in this category are coconut, palm (kernel), and the genus Cuphea
(27,28). In some of these species, more than 70% of the acyl groups in seed oil can
be a single fatty acid, indicating that these ‘‘unusual’’ fatty acids occupy all three
sites of the triacylglycerol. Perhaps the most comprehensive study on triacylglycerol
assembly in this group of oil-bearing species was derived for Cuphea lanceolata
(188), which accumulates up to 83% 10:0 (27). It should be remembered that in
these species, the existence of TE isoforms allows premature termination of FAS
and export of these fatty acids (acyl chain lengths of