Aldoxime-Forming Microsomal Enzyme Systems Involved in the Biosynthesis of Glucosinolates in Oilseed Rape (Brassica napus) Leaves

Abstract

Glucosinolates and cyanogenic glucosides are synthesized from amino acids via similar intermediates, N-hydroxyamino acids and aldoximes. Microsomal preparations from young green leaves of oilseed rape catalyze the NADPH-dependent metabolism of homo-phenylalanine and dihomomethionine to the respective aldoximes, precursors of 2-phenylethyl and 3-butenyl glucosinolates. Cytochrome P-450-type enzymes are not involved (in contrast to cyanogenic glucoside biosynthesis), because neither activity was affected by carbon monoxide or other cytochrome P-450 inhibitors. Copper ions and diethyl pyrocarbonate were potent inhibitors of the enzymes, and treatment of microsomes with detergents abolished the overall activity. Two distinct enzyme systems with similar properties appear to be involved, each specific for a particular substrate. One utilizes dihomomethionine and is not active with homophenylalanine or any other amino acid tested, and the other is specific for homophenylalanine. From the characteristics of these enzymes, it seems that these early steps in glucosinolate biosynthesis may be catalyzed by flavin-containing monooxygenases comparable to those found in mammalian tissues and elsewhere. The pathways for the biosynthesis of glucosinolates and cyanogenic glucosides have apparently evolved independently, despite the similar chemical conversions involved.