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Biosynthesis of Cutin -Hydroxylation of Fatty Acids by a Microsomal Preparation from Germinating Vicia faba¹

^a Department of Agricultural Chemistry and Program in Biochemistry and Biophysics, Washington State University, Pullman, Washington 99164

-Hydroxylation of fatty acids, which is a key reaction in the biosynthesis of cutin and suberin, has been demonstrated for the first time in a cell-free preparation from a higher plant. A crude microsomal fraction (105,000g pellet) from germinating embryonic shoots of Vicia faba catalyzed the conversion of palmitic acid to -hydroxypalmitic acid. As the crude cell-free preparation also catalyzes the formation of other hydroxy acids such as - and -hydroxy acids, the -hydroxylation product was identified by gas chromatography on a polyester column and reverse phase, high performance liquid chromatography, two techniques which were shown to resolve the positional isomers. Gas chromatographic analysis of the dicarboxylic acid obtained by CrO₃ oxidation of the enzymic product also confirmed the identity of the enzymic -hydroxylation product. This enzymic hydroxylation required O₂ and NADPH, but substitution of NADH resulted in nearly half the reaction rate obtained with NADPH. Maximal rates of -hydroxylation occurred at pH 8 and the rate increased in a sigmoidal manner with increasing concentrations of palmitic acid. This -hydroxylation was inhibited by the classical mixed function oxidase inhibitors such as metal chelators (o-phenanthroline, 8-hydroxyquinoline, and ,-dipyridyl), NaN₃ and thiol reagents (N-ethylmaleimide and p-chloromercuribenzoate). As expected of a hydroxylase, involving cytochrome P₄₅₀, the present -hydroxylase was inhibited by CO and this enzyme system showed unusually high sensitivity to this inhibition; 10% CO caused inhibition and 30% CO completely inhibited the reaction. Another unusual feature was that the inhibition caused by any level of CO could not be reversed by light (420-460 nm).

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