BIOSYNTHESIS OF SINALBIN .3. ROLE OF P-COUMARIC ACID AND P-HYDROXYPHENYLACETALDEHYDE OXIME

The biosynthesis of sinalbin, a phenolic mustard oil glucoside with a C6-C2 skeleton, proceeds via L-tyrosine and p-hydroxyphenylacetaldehyde oxime. The incorporation of the oxime was found to be more than one order of magnitude higher than that of L-tyrosine. This transformation of the p-hydroxyphenylacetaldehyde oxime into sinalbin and S-(β-D-glucopyranosyl)-p-hydroxyphenylacetothiohydroximic acid could be demonstrated in 6 weeks old plants of Sinapis alba. The finding that phenylacetaldehyde oxime was incorporated only into glucotropaeolin but not into sinalbin indicates that either the hydroxylation of the benzene ring occurs at the level of the amino acid or that the hydroxyl group comes from the cyclohexane precursors. The chemical synthesis and the physical properties of the p-hydroxyphenylacetaldehyde oxime are described. p-Hydroxyphenylacetaldehyde oxime-1-14C was obtained by condensation of p-hydroxybenzaldehyde with nitromethane-14C and by reduction of the nitrostyrene with zine-acetic acid. The conversion of L-tyrosine into p-coumaric acid catalysed by the L-tyrosine ammonia lyase was found to be reversible. The transformation of p-coumaric acid into L-tyrosine could be shown in detached leaves of Sinapis alba and also with purified enzyme preparations from that plant and Hordeum vulgare. There are indications that three biosynthetic routes leading to L-tyrosine exist in plants. These biosynthetic pathways and their physiological significance are discussed. © 1969 Springer-Verlag.