Stereochemical aspects of lotaustralin biosynthesis.

The methyl linamarin that occurs naturally in linen flax (Zinum usifafissimum L.) seedlings was shown to have the R configuration, i.e. it is identical with (R)-lotaustralin, (R)-2hydroxy-Z-methylbutyronitrile-P -D glucopyranoside. However, when a mixture of the two enantiomers (R,S)-2-hydroxy-2-methylbutyronitrile, was administered to excised flax shoots, both (R)-lotaustrahn and its epimer (S)-epilotaustralin ((S)-2-hydroxy-2-methylbutyronitrile-S-n-glucopyranoside) were formed. Similarly, both (R)-lotaustralin and (S)-epi-lotaustralin were formed when (R,S)-2-hydroxy 2-methylbutyronitrile served as substrate for the UDPglucose: ketone cyanohydrin-P-n-glucosyltransferase of flax. These results demonstrate that the glucosyltransferase does not distinguish either in vifro or in vivo between the two enantiomeric forms of this glucose acceptor. Since, however, only (R)-lotaustralin occurs in the intact plant, the glucosyltransferase must be presented with only the R enantiomer as a substrate. This form is produced from L-isoleucine ([dS]-La-isoleucine) due to retention of the S configuration at carbon atom 3. When (3R)-Ls-isoleucine (fhreo-Ls-isoleutine) was administered to flax shoots, (S)-epi-lotaustralin was formed due to retention of configuration at carbon atom 3 of the amino acid.