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Food Lipids: Chemistry, Nutrition, and Biotechnology

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<strong>and</strong> nonphotosynthetic tissues. While the fatty acid hydroperoxide levels were similar<br />

to wild type in the individual LOX antisensed plants with large reductions in LOX,<br />

basal fatty acid hydroperoxide levels, by contrast, were elevated by 38% in transgenic<br />

potato leaves antisense repression of HPLS.<br />

c. Hydroperoxide Isomerase Pathway. HPIS catalyzes the intramolecular oxygenation<br />

of hydroperoxides of fatty acids to form epoxyhydroxy or dihydroxy fatty<br />

acids. This enzyme appears to be totally different from the ‘‘hydroperoxide isomerase’’<br />

that was recently renamed HPDS. HPISs were reported in cereal flours during<br />

1970s [86]. Recently Hamberg [230–232] described an enzyme system, in the fungus<br />

Saprolegnia parasitica, that converts PUFAs into allylic epoxy alcohols. Two enzyme<br />

activities were identified to be responsible for the transformation. The first enzyme,<br />

�6-LOX, catalyzes oxygenation of 18:2 to 13S-HPOD. The other enzyme, HPIS,<br />

converts the 13S-HPOD into a mixture of the two epoxy alcohols: 11R,12R-epoxy-<br />

13S-hydroxy-9Z-octadecadienoic acid <strong>and</strong> 9R,10R-epoxy-13S-hydroxy-11E-octadecadienoic<br />

acid. The enzyme system favored the E,Z-conjugated 9S- <strong>and</strong> 13S-HPOD<br />

over the corresponding E,E-conjugated isomers <strong>and</strong> also used 20:4 <strong>and</strong> 5,8,11,14,17eicosapentaenoic<br />

acids as subtrates. The C20-epoxy alcohols could be subsequently<br />

hydrolyzed to produce four isomeric trihydroxy-eicosatrienoic acids [86]. In addition,<br />

this HPIS catalyzes slow isomerization of unnatural 13R-HPOD.<br />

HPIS is involved in diol fatty acid biosynthesis by red alga (Gracilariopsis<br />

lemaneiformis). Jiang <strong>and</strong> Gerwick [233] found that an acetone powder preparation<br />

of red algae catalyzed the conversion of 20:4 to 12R,13S-dihydroxy-5Z,8Z,10E,14Zeicosatetraenoic<br />

acid. Further studies reveal that conversion of 20:4 occurs by initial<br />

12-LOX-catalyzed oxygenation into 12S-hydroperoxy eicosatetraenoic acid followed<br />

by the HPIS-catalyzed conversion of the hydroperoxide into 12R,13S-dihydroxy-<br />

5Z,8Z,10E,14Z-eicosatetraenoic acid [234]. This HPIS converts C18 fatty acid hydroperoxides<br />

(e.g., 13S-HPOD, 13R-HPOD, 9S-HPOD, 13S-HPOT) into corresponding<br />

diol fatty acids as well.<br />

The fungus Gaeumannomyces graminis also contains HPIS, which catalyzes<br />

formation of another diol fatty acid, 7S,8S-dihydroxy-9Z,12Z-octadecadienoic acid,<br />

following incubation with 18:2 [235]. Linoleic acid is initially oxygenated into 8Rhydroperoxy-9Z,12Z-octadecadienoic<br />

acid in the presence of the dioxygenase called<br />

8R-dioxygenase. The resulting hydroperoxide is further converted to 7S,8S-dihydroxy-9Z,12Z-octadecadienoic<br />

acid [79] by action of a HPIS. Oxygen isotope ( 18 O)<br />

studies showed that the HPIS-catalyzed conversion occurred by an intramolecular<br />

process: that is, the terminal hydroperoxide oxygen was utilized by the isomerase<br />

for hydroxylation of the C-7 methylene group.<br />

Steric analysis shows that the configurations of the epoxide groups of the �,�<strong>and</strong><br />

�,�-epoxy alcohols produced from a number of different hydroperoxides are<br />

invariably R,R <strong>and</strong> S,R, respectively, <strong>and</strong> diol fatty acids produced from several fatty<br />

acid hydroperoxide isomers always have the S configuration. Thus, the stereochemistry<br />

of the intramolecular oxygenation is dictated by the isomerase [232,234].<br />

d. Divinyl Ether Formation. During the early 1970s, Galliard <strong>and</strong> coworkers<br />

reported that a potato extract catalyzed the transformation of either 9S-HPOD <strong>and</strong><br />

HPOT to divinyl ether derivatives [9-(1�E,3�Z-nonadienyloxy)8E-nonenoic acid or<br />

9-(1�E,3�Z,6Z-nonatrienyloxy)8E-nonenoic acid, respectively]. These compounds<br />

were named colneleic <strong>and</strong> colnelenic acids [236]. The enzyme was specific for the<br />

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

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