Aldoximes as intermediates in the biosynthesis of tyrosol and tyrosol derivatives

Abstract

Aldoximes derived from amino acids by N-hydroxylation are found to be precursors of tyrosol, p-methoxyphenethyl alcohol and their glycosidic derivatives in higher plants. In Sinapis alba p-hydroxyphenylacetaldehyde oxime is converted into tyrosol with a much higher rate than l-tyrosine. Similarly, in Aubrietia species the biosynthesis of p-methoxyphenethyl alcohol from l- p-methoxyphenylalanine proceeds via p-methoxyphenylacetaldehyde oxime. Only 0·05% of the radioactivity of labelled l- p-methoxyphenylalanine was incorporated into p-methoxyphenethyl alcohol while more than 1% of the oxime was converted into the alcohol and its glucoside. As no transformation of p-hydroxyphenylacetaldehyde oxime into p-methoxyphenethyl alcohol was detectable the O-methylation obviously takes place at the level of the amino acid. The structure of a tyrosol derivative formed after feeding of p-hydroxyphenylacetaldehyde oxime-1- 14C was elucidated; methylation and enzymatic cleavage with purified β-glucosidase yielded only p-methoxyphenethyl alcohol. This indicates that the hydroxyl group of the C 2-side chain of tyrosol is bound to d-glucose by a β-glycosidic linkage. p-Hydroxyphenylacetaldehyde oxime as well as p-methoxyphenylacetaldehyde oxime arise from the corresponding amino acids in the plants investigated. The significance of aldoxime formation and metabolism in various plants is discussed.