Studies on leucine biosynthesis in yeast.

Studies with microorganisms grown on labeled substrates have yielded a substantial amount of information on the mechanism of biosynthesis of various amino acids. Little attention has, however, been paid to the origin of the amino acid leucine and hence its origin has remained largely obscure. It was earlier reported from this laboratory (1,2) that, in yeast cultured on glucose and supplied with W-acetate, the amino acids of the cellular proteins contain isot,opic carbon at three different levels and may be grouped accordingly: phenylalanine and tyrosine with insignificant concentrations of acetate carbon, a large number of the amino acids which contain intermediate levels of isotope, and a group consisting of lysine, proline, arginine, glutamic acid, and leucine with Cl4 levels approaching those of the yeast lipides. It was suggested that only for the synthesis of this last group of amino acids, which includes leucine, was acetate utilized as a direct carbon source. The present report deals in greater detail with the contribution of the 2 carbon atoms of acetate carbon to leucine biosynthesis, and also with the utilization of l-C14-glucose in the same process. It includes data on the distribution of acetate and glucose carbon which is obtained on degradation of the leucine molecule.