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Food Lipids: Chemistry, Nutrition, and Biotechnology

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that the member of the enzyme family involved was not the same one participating<br />

in in-chain oxidation or midchain hydroxylation.<br />

The presence of 16-oxo-9- or 16-oxo-10-hydroxypalmitic acid in cutin <strong>and</strong><br />

dicarboxylic acids in suberin has suggested that the �-hydroxyl group is further<br />

oxidized in many plant tissues. Conversion of exogenous labeled 10,16-dihydroxypalmitic<br />

acid to the corresponding 16-oxo acid was demonstrated with slices of<br />

embryonic shoots of V. faba [68]. Experiments to determine how dicarboxylic fatty<br />

acids are synthesized were carried out with cell-free extracts of epidermis of V. faba<br />

leaves. Kolattukudy et al. [69] showed that the �-hydroxy C 16 acid was converted<br />

into both 1,16-dioic <strong>and</strong> 16-oxo-C 16 acids, while feeding the latter resulted only in<br />

synthesis of the 1,16-dioic C 16 acid. The dehydrogenase activity, located mainly in<br />

the 100,000g supernatant, showed an optimum pH near 8 <strong>and</strong> a requirement for<br />

NADP � . Modification of substrate by esterfication of the carboxyl group, replacement<br />

of the carboxyl group by a methyl group, or introduction of another hydroxyl<br />

group at C-10, rendered it a poor substrate. Thiol-directed reagents strongly inhibited<br />

oxidation of the �-hydroxy group. Agrawal <strong>and</strong> Kolattukudy [70] demonstrated that<br />

two different dehydrogenases in extracts of suberizing potato slices were involved<br />

in the oxidation of �-hydroxy acid. One (�-hydroxy acid dehydrogenase) converted<br />

the C 16 �-hydroxy substrate to the �-oxo derivative, <strong>and</strong> the second (�-oxo acid<br />

dehydrogenase) converted the latter to the dioic fatty acid. Only the �-hydroxy acid<br />

dehydrogenase was induced by wounding (16-fold). Its purification identified a dimer<br />

of 31-kDa subunits. A detailed characterization revealed that this enzyme was quite<br />

similar to other dehydrogenases [51,72]. With alkanals as model substrates, the Michaelis<br />

constant K m deceased drastically from 7000 �M to90�M as the chain length<br />

of the substrate increased from C 3 to C 8; a further increase in the chain length from<br />

C 8 to C 20 resulted in only a small further decrease in K m. Aliphatic chains longer<br />

than C 20 show extremely low rates.<br />

B. In-Chain Oxidation/Midchain Hydroxylation<br />

10,16-Dihydroxy C16 acid is a major cutin monomer. The hydroxylation of �-hydroxypalmitic<br />

acid at C-10 was first demonstrated by Walton <strong>and</strong> Kolattukudy [71]<br />

in a cell-free preparation from excised epidermis of exp<strong>and</strong>ing V. faba leaves. The<br />

C-10 hydroxylase appeared to be located in the endoplasmic reticulum [72]. Further<br />

purification of the crude microsomal preparation showed that the enzyme catalyzed<br />

synthesis of both 9- <strong>and</strong> 10-isomers. This midchain hydroxylation required O2 <strong>and</strong><br />

NADPH <strong>and</strong> was inhibited by NaN3, metal ion chelators, as well as by thiol-directed<br />

reagents. The inhibition of midchain hydroxylation caused by CO was photoreversible,<br />

indicating a typical cyt P450 enzyme. Loss of only one hydrogen during<br />

the hydroxylation suggested that an insertion mechanism was involved.<br />

�<br />

CH2OH — (CH 2) n— CH 2— (CH 2) m— COOH � O2 � NADPH � H<br />

cyt P450 monoxygenase<br />

(midchain hydroxylase) CH OH — (CH ) — CHOH — (CH ) —<br />

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2 2 n 2 m COOH<br />

�<br />

� HO� 2 NADP<br />

A microsomal preparation from aged Jerusalem artichoke slices catalyzed hydroxylation<br />

of C 12 acid at the C-8, C-9, <strong>and</strong> C-10 positions with NADPH <strong>and</strong> O 2 as<br />

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

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