Biosynthesis of mustard oil glucosides: N-Hydroxyphenylalanine, a precursor of glucotropaeolin and a substrate for the enzymatic and nonenzymatic formation of phenylacetaldehyde oxime

Abstract

The incorporation of DL- N-hydroxyphenylalanine-2- 14C into the mustard oil glucoside glucotropaeolin was demonstrated by plant feeding experiments. The efficiency of conversion of 14C from this acid into the aglycone moiety of glucotropaeolin was higher than that from DL-phenylalanine-2- 14C or -3- 14C, and was comparable with that observed previously from phenylacetaldehyde oxime-1- 14C. These results support the reaction sequence: phenylalanine → N-hydroxyphcnylalanine → phenylacetaldehyde oxime → glucotropaeolin. Enzyme preparations were obtained from Sinapis alba L., Tropoeolum majus L. and Nasturtium officinale R.Br. which catalyze the transformation of N-hydroxyphenylalanine to phenylacetaldehyde oxime. Although no absolute requirement for cofactors could be found, an increase in the rate of formation of the aldehyde oxime and consumption of oxygen was observed using FMN and enzyme preparations from T. majus and S. alba. Using the enzyme from T. majus it was demonstrated that the μmoles of phenylacetaldehyde oxime formed and of oxygen consumed were equivalent. In this case FMN appeared to be the prosthetic group and oxygen the physiological hydrogen acceptor. In addition to this enzymatic conversion, an unspecific and nonenzymatic oxidation of the N-hydroxyamino acid to phenylacetaldehyde oxime and phenylacetonitrile was observed.