THE BIOSYNTHESIS OF ISOLEUCINE AND VALINE. I. ENZYMATIC TRANSFORMATION OF THE DIHYDROXY ACID PRECURSORS TO THE KETO ACID PRECURSORS

Abstract

Cell-free extracts and dried cells of wild-type and isoleucine-valine-requiring mutants of Escherichia coli and Neurospora crassa were assayed for dihydroxy acid dehydrase activity. The E. coli mutant shows no dehydrase activity toward the alpha, beta-dihydroxy acids of isoleucine and valine; the Neurospora mutant shows less than 1/10 the activity of the wild strain, as measured by production of the corresponding keto acids. Added keto acids were not metabolized by any of the preparations. Tryptophane desmolase activity was present in all extracts; threonine deaminase was produced by all strains except the mutant E. coli. Wild-type activity is slightly stimulated by mutant extracts, but mutants are not stimulated by wild-type extracts. The double requirements for isoleucine and valine seems to result from loss of 2 enzyme activities rather than from metabolic interactions. Calculations for E. coli indicate dehydrase activity is great enough to account for isoleucine and valine synthesis by the wild-type and is low enough in the mutant to account for the amino acid growth requirement.