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Food Lipids: Chemistry, Nutrition, and Biotechnology

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2-18:1 �9-PC for the 12-MO. Upon hydroxylation of the 18:1 �9-PC in castor bean,<br />

the ricinoleic acid is released quickly by phospholipase A 2 to form 18:1 �9,12-OH (<strong>and</strong><br />

ultimately its -CoA derivative) <strong>and</strong> lysophosphatidylcholine (LPC) (147). LPC is<br />

reacylated by LPCAT, which is selective for 18:1 �9-CoA. The repetition of this cycle<br />

allows for ricinoleic acid to accumulate in the acyl-CoA pool <strong>and</strong> exclude it from<br />

the PC pool <strong>and</strong> functional lipids. Ricinoleoyl-CoA is formed <strong>and</strong> can be used successively<br />

in GPAT, LPAAT, <strong>and</strong> DAGAT reactions to yield the tri-18:1 �9,12-OHacylglycerol.<br />

4. Unique Features of Species in This Group<br />

A notable exception of the trends of triacylglycerol assembly in these plant species<br />

rests with the LPAAT of meadowfoam. The meadowfoam LPAAT can utilize<br />

22:1�13 <strong>and</strong> is most reactive with mono-22:1�13-acylglycerol (196,197). The potential<br />

to utilize this trait in transgenic rape to favor the formation of trierucin is an opportunity<br />

currently being evaluated (198,199).<br />

This process by which unusual fatty acids are channeled into storage lipid in<br />

cori<strong>and</strong>er <strong>and</strong> carrot endosperm (<strong>and</strong> related species that can accumulate up to 85%<br />

of storage triacylglycerol as 18:1�6: (see Table 2, Refs. 27 <strong>and</strong> 28), appears to be<br />

different from that for other species in this group. In carrot <strong>and</strong> cori<strong>and</strong>er endosperm,<br />

the PC-diacylglycerol exchange pathway (CPT step) constitutes a major route for<br />

triacylglycerol assembly, since newly synthesized (<strong>and</strong> radiolabeled) fatty acids (including<br />

18:1�6) become most immediately concentrated in PC, <strong>and</strong> then in triacylglycerol<br />

(200). This observation implies a role for PC in triacylglycerol assembly<br />

beyond its well-documented role of simply allowing for acyl modification reactions<br />

to occur.<br />

The general enzymic patterns of triacylglycerol assembly in species belonging<br />

to the groups discussed above (Secs. V.C–V.E) is offered as a summary in Table 6.<br />

F. Oil Body Genesis<br />

Some of the earliest in vitro experiments demonstrating triacylglycerol accumulation<br />

mediated by safflower microsomal membranes yielded a milky-white suspension<br />

(201). Upon centrifugation of the reaction mixture, a buoyant ‘‘fat pad’’ was formed<br />

at the surface, <strong>and</strong> electron microscopic examination of this material indicated the<br />

presence of ‘‘naked’’ fat globules. These oil bodies or ‘‘oleosomes’’ contained detectable<br />

activities of some of the enzymes involved in triacylglycerol assembly (202).<br />

Since these initial studies, a reasonable view of genesis of oil bodies in plant<br />

seed (<strong>and</strong> pollen) tissues has evolved (145,146,203,204). Oil bodies (�20 �m diameter)<br />

form as vesicles that ‘‘bud off’’ from the endoplasmic reticulum, <strong>and</strong> this<br />

process may be induced by triacylglycerol accumulation <strong>and</strong> the attendant swelling<br />

or distension of the membrane. This is not a universal view of oil droplet formation,<br />

for there is evidence of a cytoplasmic origin for seed/endosperm tissues of some<br />

species. However, these apparently discordant views collectively provide the basis<br />

for a growing realization that there may be distinct subpopulations of the endoplasmic<br />

reticulum, each with a different profile of enzyme activities with lipid metabolism.<br />

This view is supported by the body of studies in which microsomal fractions<br />

have been prepared from various tissues at varying degrees of maturation, with<br />

findings of varying levels <strong>and</strong> profiles of marker enzymes. This view of endoplasmic<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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