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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 7 Proposed mechanism of �-oxidation in plants, catalyzed by four enzymes: 1,<br />

FAD-flavoprotein enzyme; 2, anhydrodecarboxylase; 3, NAD-specific aldehyde dehydrogenase;<br />

4, peroxidase. (Adapted from Ref. 56.)<br />

The �-oxidation system was first observed in crude extracts of germinated<br />

peanut (Arachis hypogaea L.) cotyledons by Newcomb <strong>and</strong> Stumpf. With this microsomal<br />

system, studies from Stumpf’s laboratory [53] found several interesting<br />

results:<br />

1. No cofactors were required for 14 CO 2 production from 1- 14 C16:0, but<br />

NAD � was specifically required for 14 CO 2 production from internally labeled<br />

16:0 (2- 14 C16:0).<br />

2. Synthetic 16:0-CoA was inactive.<br />

3. The tricarboxylic acid cycle was not responsible for the CO 2 formation<br />

from 16:0 oxidation.<br />

4. The system required an H 2O 2/H 2O 2 generating system <strong>and</strong> a NAD � -linked<br />

long chain fatty aldehyde dehydrogenase.<br />

5. The long chain fatty aldehydes were only intermediates.<br />

6. Addition of long chain fatty aldehydes to peanut extract resulted in the<br />

formation of the corresponding free fatty acids.<br />

Thus, Martin <strong>and</strong> Stumpf [57] proposed the �-oxidation pathway, consisting of two<br />

separate reactions: (1) a peroxidative decarboxylation of the acid to yield CO 2 <strong>and</strong><br />

an aldehyde <strong>and</strong> (2) an NAD-specific dehydrogenation of the aldehyde to the corresponding<br />

acid having one less carbon atom than the original acid.<br />

Hitchcock <strong>and</strong> James [58] observed that 16:0 was also converted to pentadecanoate<br />

by pea leaf extracts with the accumulation of �-hydroxy fatty acids, mainly<br />

the D configuration. The leaf system required O 2 rather than H 2O 2. Addition of a<br />

mixture of LD-�-hydroxy fatty acids (16:0) increased CO 2 release with formation of<br />

C n�1 fatty acids. The plant leaves had a system that appeared to differ from the<br />

peanut system, <strong>and</strong> an amplified version was proposed by Hitchcock <strong>and</strong> James [59].<br />

Apparent, differences between the two earlier proposals were reconciled by<br />

Shine <strong>and</strong> Stumpf [56], who used the same experimental system to compare partially<br />

purified preparations from pea leaves <strong>and</strong> peanut cotyledons. This investigation dem-<br />

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

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