Evidence for Biosynthetical Equivalence of the Epimeric Isoleucine Residues in Angolide

Abstract

Pithomyces sacchari was grown on media containing L-valine. Instead of the metabolite angolide (cyclo-L-α-hydroxyisovaleryl1-erythro-L-isoleucyl2-L-α-hydroxyisovaleryl3-threo-D-isoleucyl4), synthesized on un-supplemented media, it produced a mixture with very similar physical properties. Acid hydrolysis liberated erythro-L- and threo-D-isoleucines (1:1), and DL-valine in amounts proportional to the exogenous valine concentration; L-α-hydroxyisovaleric acid was the only other component. Radioactivity from L-valine-U-14C was incorporated without dilution into valine, and with extensive dilution into the hydroxy acid.Partial fractionation of the mixture was achieved by column chromatography. Mass spectra of two fractions, containing different proportions of valine, were compared with the spectrum of angolide. A simple mathematical treatment showed that the fractions contained angolide, its divaline homologue, and a homologue with only one isoleucine replaced by valine. This must be a 1: 1 mixture of the two possible isomers, since any deviation from this ratio is inconsistent with the analytical results. It follows that the erythro-L- and the threo-D-isoleucine residues of angolide are equivalent with respect to their replacement by valine. It is inferred that angolide biosynthesis involves a cyclic intermediate containing both amino acid residues in the L configuration.