Biosynthesis of Cutin

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Abstract
Long chain dicarboxylic acids are constituents of the protective biopolymers cutin and suberin of plants. Cell-free extracts from the excised epidermis of Vicia faba leaves catalyzed conversion of 16-hydroxy[G-3H]hexadecanoic acid to the corresponding dicarboxylic acid with nicotinamide-adenine dinucleotide phosphate as the preferred cofactor. This enzymatic activity, located largely in the 100,000g supernatant fraction, had a pH optimum near 8. This dehydrogenase showed an apparent Km of 1.25 × 10−5m and 3.6 × 10−4m for 16-hydroxyhexadecanoic acid and NADP, respectively. Modification of the substrate, either by esterification of the carboxyl group or by introduction of another hydroxyl group at C-10, resulted in a substantial (two-thirds) decrease in the rate of reaction, and hexadecanol was not a good substrate. The enzyme was inhibited by thiol reagents such as N-ethylmaleimide and p-chloromercuribenzoate. The aldehyde intermediate was trapped by the inclusion of dinitrophenyl hydrazine in the reaction mixture, and the 16-oxo compound was regenerated and identified. Furthermore, synthetic 16-oxo-[G-3H] hexadecanoic acid was readily converted to the dicarboxylic acid by the cell-free preparation. These results demonstrate that epidermis of Vicia faba contains an ω-hydroxyacid dehydrogenase and an ω-oxoacid dehydrogenase.
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