VALINE INHIBITION OF BETA-ISOPROPYLMALATE DEHYDROGENASE TAKES PART IN THE REGULATION OF LEUCINE BIOSYNTHESIS IN CANDIDA-MALTOSA

The beta-isopropylmalate (IPM) dehydrogenase (EC 1.1.1.85) of Candida maltosa, the third pathway-specific enzyme of leucine biosynthesis, was purified, some properties of the enzyme were studied and a novel regulatory pattern was found. The K(m) values of the enzyme were estimated to be 0.42 mM for beta-IPM and 0.34 mM for NAD+. It is demonstrated that the enzyme can be regulated by L-valine. The inhibition was competitive with respect to beta-IPM (K(i) = 1.84 mM) and non-competitive with respect to NAD+ (K(i) = 5.67 mM). Exogenous addition of L-valine to C. maltosa cells increased the intracellular pool of some intermediates of leucine biosynthesis (alpha-ketoisovalerate, alpha-IPM, beta-IPM), but has hardly influence on the leucine pool.