Food Lipids: Chemistry, Nutrition, and Biotechnology

erol backbone of triacylglycerols can have a profound influence on the oxidative
stability of an oil (242–244). If a systematic understanding regarding the influence
of triacylglycerol molecular species on oxidative stability is developed (which arguably
is primarily in the domain of food science), then the appropriate targets for
genetic manipulation are easily identifiable. Unfortunately, elucidation of how triacylglycerol
molecular species influence oxidative stability will take years, if not
decades; yet such understanding is necessary before cogent genetic strategies can be
identified to address the problem of oxidative stability within this context. The obvious
advantages of genetically engineering oxidative stability are improved quality
and stability of lipids in food, and potentially, a diminished dependence on exogenous
antioxidants.
D. Modifying Nutritional Functionality
Some approaches to improving nutritional quality of plant-derived lipids involve
genetic interventions just described (Sec. VI.C). Developing plant oils enhanced in
monounsaturated fatty acids (18:1�9) are among these strategies, and the general
health-promoting view of monounsaturates (239) has led to interest in developing
high-18:1�9 lines for sunflower, corn, safflower, canola, and peanut oils. In addition,
genetic manipulation to enhance �12DES and suppress �15DES would enhance
levels of 18:2�9,12, arguably the most essential dietary fatty acid (213), although
18:3�9,12,15 is also essential (245,246).
Some microalgae (lower plants), such as Navicula pelliculosa, Cyclotella cryptica,
and Phaeodactylum tricornatum, are capable of accumulating lipids enriched
(18–26%) in 20:5�5,8,11,14,17 (247,248). Thus, the synthetic potential exists in the plant
kingdom to yield 20:5�5,8,11,14,17 and 22:6�4,7,10,13,16,19, the two fish oil fatty acids (referred
to as n-3, or �3) that are widely believed to promote human health (246).
Starting with 18:2�9,12, all the enzymes representing the sequence of steps involved
in elongation and �6,5,4 desaturation, whether via the n-6 or n-3 pathway (246),
have been demonstrated in higher plants: see Secs. III.B.6 (98,99), IV.C (167,168),
and V.E.1 (193), as well as in algae (247,248) and fungi (4). Whether these traits
can be incorporated in a manner that will permit them to collectively function to
yield 20:5�5,8,11,14,17 and 22:6�4,7,10,13,16,19 in oils of transgenic plants depends on the
abilities of the constitutive and cloned desaturases and elongases to coordinatively
channel the acyl chain, alternatively linked to PC and CoA, toward these fatty acids,
and then recognize them in triacylglycerol assembly steps. At least two immediate
and specific concerns can be identified in this regard. Could the �4DES [of coriander:
see Sec. III.B.6 (98,99)] be cloned in a manner effective at yielding 22:
6�4,7,10,13,16,19, since the �4DES is believed to be plastidic stromal (soluble) enzyme
acting on acyl-ACP, and its participation in 22:6�4,7,10,13,16,19 biosynthesis would be
the terminal step, likely requiring action on a PC-linked acyl chain? Second, would
the elongases, known to exist in Brassica and Crambe spp., as well as in leek, jojoba,
and meadowfoam (see Sec. IV.B), be active with acyl with acyl chains of three to
five double bonds, since only those of zero to two double bonds are elongated in
vivo in wild-type plants?
Finally, opportunities exist to manipulate plants to yield ‘‘structured glycerides,’’
or triacylglycerol species that have specific nutritional functions that promote
health (29,34,231) (see Chapter 28). The impact of dietary lipids on human cardio-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.