Stereochemical aspects of the biosynthesis of the epimeric cyanogenic glucosides dhurrin and taxiphyllin.

Dhurrin, I ((S)-p-hydroxymandelonitrile-beta-D-glucopyranoside), and taxiphyllin, II (the (R) epimer), occur in the genera Sorghum and Taxus, respectively. Both derive biosynthetically from L-tyrosine via the hydroxylation of p-hydroxyphenylacetonitrile, III. (3R)- and (3S)-L-[3-3H1]tyrosine, prepared by enzymic hydroxylation of the corresponding phenylalanines, were fed separately to shoots from sorghum seedlings (Sorghum bicolor (Linn) Moench) and cuttings from Japanese Yew (Taxus cuspidata Sieb. and Zucc.) and the appropriate cyanogenic glycoside was isolated (I or II). The fraction of the 3H conserved in I and II was calculated from both parallel feeding and 3H:14C double labeling experiments. The results for II were the reverse of I. Both hydroxylations of III, which give rise to the enantiomeric products (S)- and (R)-p-hydroxymandelonitrile, occurred with retention of configuration. The retention mode is characteristic of hydroxylations at aliphatic carbons catalyzed by mixed function oxidases.