Isolation of a Microsomal Enzyme System Involved in Glucosinolate Biosynthesis from Seedlings of Tropaeolum majus L.

An in vitro system that converts phenylalanine to phenylacetaldoxime in the biosynthesis of the glucosinolate glucotropaeolin has been established in seedlings of Tropaeolum majus L. exposed to the combined treatment of jasmonic acid, ethanol, and light. The treatment resulted in a 9-fold induction, compared with untreated, dark-grown seedlings, of de novo biosynthesis measured as incorporation of radioactively labeled phenylalanine into glucotropaeolin. Formation of the inhibitory degradation product benzylisothiocyanate during tissue homogenization was prevented by inactivation of the thioglucosidase myrosinase by addition of 100 mM ascorbic acid to the isolation buffer. This allowed the isolation of a biosynthetically active microsomal preparation from the induced T. majus plant material. The enzyme, which catalyzes the conversion of phenylalanine to the corresponding oxime, was sensitive to cytochrome P450 inhibitors, indicating the involvement of a cytochrome P450 in the biosynthetic pathway. It has previously been shown that the oxime-producing enzyme in the biosynthesis of p-hydroxybenzylglucosinolate in Sinapis alba L. is dependent on cytochrome P450, whereas the oxime-producing enzymes in Brassica species have been suggested to be flavin monooxygenases or peroxidase-type enzymes. The result with T. majus provides additional experimental documentation for a similarity between the enzymes converting amino acids into the corresponding oximes in the biosynthesis of glucosinolates and cyanogenic glucosides.