ALDOXIME-FORMING MICROSOMAL-ENZYME SYSTEMS INVOLVED IN THE BIOSYNTHESIS OF GLUCOSINOLATES IN OILSEED RAPE (BRASSICA-NAPUS) LEAVES

Glucosinolates and cyanogenic glucosides are synthesized from amino ac ids via similar intermediates, N-hydroxyamino acids and aldoximes. Mic rosomal preparations from young green leaves of oilseed rape catalyze the NADPH-dependent metabolism of homophenylalanine and dihomomethioni ne to the respective aldoximes, precursors of 2-phenylethyl and 3-bute nyl glucosinolates. Cytochrome P-450-type enzymes are not involved (in contrast to cyanogenic glucoside biosynthesis), because neither activ ity was affected by carbon monoxide or other cytochrome P-450 inhibito rs. Copper ions and diethyl pyrocarbonate were potent inhibitors of th e enzymes, and treatment of microsomes with detergents abolished the o verall activity. Two distinct enzyme systems with similar properties a ppear to be involved, each specific for a particular substrate. One ut ilizes dihomomethionine and is not active with homophenylalanine or an y other amino acid tested, and the other is specific for homophenylala nine. From the characteristics of these enzymes, it seems that these e arly steps in glucosinolate biosynthesis may be catalyzed by flavin-co ntaining monooxygenases comparable to those found in mammalian tissues and elsewhere. The pathways for the biosynthesis of glucosinolates an d cyanogenic glucosides have apparently evolved independently, despite the similar chemical conversions involved.