Studies on the Mechanism of Synthesis of D-valine by Streptomyces Antibioticus.

The actinomycins produced by Xtreptomyces antibioticus contain 2 moles of n-valine per mole of antibiotic (l-3). However, the biochemical mechanism for the synthesis of n-valine is unknown. Recent radioisotopic experiments have revealed that n-valine, but not the D isomer, is utilized effectively for n-valine synthesis (4). These results provide evidence for the hypothesis that L-valine is an obligatory precursor of the n-valine in the actinomycin peptide and that free n-valine is not an intermediate in the reaction. Previous findings support this view, since it has been shown that n-valine inhibits actinomycin formation without affecting growth of the organism and that L-valine reverses this inhibition (5). During the inversion process involving L-valine, an intermediate may be formed with the loss of the amino group to the cell metabolic pool. As a result, the n-valine subsequently produced would possess a carbon skeleton derived largely from L-valine, but would contain only a small amount of L-valine nitrogen owing to an appreciable dilution from the pool nitrogen. Alternatively, the reaction may proceed through an intermediate, but in such a way that the amino group is retained without appreciable dilution in the conversion of Lto n-valine. In order to ascertain whether the amino nitrogen of n-valine is retained during the conversion of Lto o-valine, an experiment was conducted in which L-valine-14C and n-valineJ5N were supplied to S. antibioticus during actinomycin synthesis. LValine was isolated from the cell protein, and n-valine and Nmethyl-L-valine were obtained from actinomycin. Analysis of the 1% and 15N content of these amino acids revealed that the amino group of n-valine is retained during its conversion to the D enantiomorph.