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Food Lipids: Chemistry, Nutrition, and Biotechnology

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9-hydroperoxides <strong>and</strong> did not attack the 13-hydroperoxide isomers. The 18 O 2 label<br />

from 18 O 2-labeled 9S-HPOD was inserted into the ether oxygen of the divinyl ether<br />

[86]. These unusual ethers represented major lipids in homogenates of potato tubers<br />

at pH values exceeding 6.5 but are not present in intact tissue. Colneleic <strong>and</strong> colnelenic<br />

acids could be degraded by oxidative cleavage of the ether to form aldehydic<br />

fragments. The degradation was catalyzed by an enzyme in potato tubers but also<br />

occurred in the presence of Fe 2� ions <strong>and</strong> some nonheme proteins.<br />

The formation of divinyl ether is catalyzed by soybean LOX in vitro, <strong>and</strong><br />

knowledge of the mechanism has been used to synthesize divinyl ether from 9S-<br />

HPOD by a biomimetic method [86].<br />

3. Hydroperoxide-Dependent Metabolism: Peroxygenase/<br />

Epoxygenase Pathways<br />

HPPR or HPEP is a membrane-associated oxidase that catalyzes the epoxygenation<br />

of mono- <strong>and</strong> polyunsaturated fatty acids but only in the presence of fatty acid<br />

hydroperoxides. In this pathway (Fig. 12), PUFAs such as 18:2 are first converted<br />

to 13S-HPOD by action of LOX. HPPR or HPEP then catalyzes subsequent intermolecular<br />

transfer of hydroperoxide oxygen from 13S-HPOD to another unsaturated<br />

Figure 12 Biosynthesis of epoxy acids by hydroperoxide-dependent peroxygenase/epoxygenase<br />

pathway of linoleic acid in plant tissues: 13S-HPOD, 13S-hydroperoxy octodecadienoate.<br />

(Adapted from Ref. 79.)<br />

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

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