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Food Lipids: Chemistry, Nutrition, and Biotechnology

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characteristic processes in seeds or fruit of plant species that collectively accumulate<br />

triacylglycerols composed of (a) unsaturated (�9,�12,�15) 18-carbon fatty acids, (b)<br />

medium chain (18 acyl carbons)<br />

or otherwise unusual fatty acids. It is hoped that the reader will emerge with a clear<br />

picture of the nature of the nonr<strong>and</strong>om distribution of fatty acids in plant storage<br />

triacylglycerols.<br />

B. Control of Triacylglycerol Assembly<br />

Regulation of triacylglycerol assembly is vested in the individual enzymes involved<br />

(see Table 5 <strong>and</strong> Fig. 5). The Kennedy pathway comprises,* in sequence, GPAT<br />

(170), LPAAT (171,172), PTAP (173,174), <strong>and</strong> DAGAT (175,176). These steps<br />

sequentially mediate the addition of acyl groups from the acyl-CoA pool to the<br />

glycerol backbone. The involvement of PC-linked acyl groups involves principally<br />

CPT (177,178) <strong>and</strong> LPCAT (179) [or in some cases, phospholipase A2 (147)]. The<br />

latter three enzymes provide access of 18:1�9 from the CoA pool, via PC, to �12DES<br />

<strong>and</strong> �15DES activities to yield PC-linked 18:2�9,12 <strong>and</strong> 18:3�9,12,15. These polyunsaturated<br />

fatty acids can be used in triacylglycerol assembly either via return to the<br />

acyl-CoA pool (by LPCAT or phospholipase A2) or PC-diacylglycerol exchange<br />

(CPT) followed by DAGAT action. PC is originally assembled by the Kennedy<br />

pathway involving GPAT, LPAAT, PTAP, <strong>and</strong> CPT (15). [The author has not located<br />

any reports of discrete isoforms or pathways exclusive for diacylglycerol or<br />

PC assembly, although the presence of divergent pathways has been implied (20)].<br />

In any event, the salient features of involvement of PC in triacylglycerol assembly<br />

revolve around the role of PC in mediating the desaturation <strong>and</strong> oxygenation modifying<br />

steps.<br />

The features that control channeling of fatty acids toward triacylglycerol assembly<br />

are (a) the level <strong>and</strong> specificities of the enzymes mediating each acylation<br />

step, (b) the nature of the acyl-CoA pool as imported from the plastid or modified<br />

within the endoplasmic reticulum, (c) the exchange of PC-linked fatty acids with the<br />

acyl-CoA pool, <strong>and</strong> (d) the extent to which exchange takes place between the PC<br />

<strong>and</strong> diacylglycerol pools. Features a <strong>and</strong> b are interdependent such that for plant<br />

species where the Kennedy pathway enzymes are relatively nonselective, the profile<br />

of the acyl-CoA pool dictates the types of the fatty acyl residue evolving as components<br />

of triacylglycerols. On the other h<strong>and</strong>, for species where Kennedy pathway<br />

enzymes exhibit pronounced selectivity, the acyl-CoA pool profile has less influence<br />

on which fatty acyl groups evolve as components of triacylglycerols. When Kennedy<br />

pathway enzyme selectivity <strong>and</strong> the acyl-CoA pool are matched, up to 90% of the<br />

fatty acyl groups evolving as components of triacylglycerols can be a single species.<br />

Less attention has been paid to which enzyme(s) regulate triacylglycerol assembly<br />

than to a characterization of the enzyme reactions composing each step.<br />

Based on a comparison of relative magnitudes of in vitro activities in microsomal<br />

membranes of safflower (14), the most limiting enzyme activities are DAGAT, CPT<br />

(PC ⇒ diacylglycerol direction), <strong>and</strong> �12DES, <strong>and</strong> the next most limiting are CPT<br />

(diacylglycerol ⇒ PC direction), PTAP, <strong>and</strong> GPAT. Extrapolating from animal sys-<br />

*See Table 5 for abbreviations.<br />

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

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