Biosynthesis of valine and isoleucine in plants. III. Reductoisomerase of Phaseolus radiatus

Abstract

1. The reductoisomerase from Phaseolus radiatus has been partially purified and its properties studied. 2. L-Ascorbic acid was found to enhance the rate of reaction when α-aceto-α-hydroxybutyrate was used as the substrate while it has no effect or is slightly inhibitory when α-acetolactate was used as the substrate. 3. The optimum pH is around pH 8.6 in Tris-HCl buffer when α-aceto-α-hydroxybutyrate was used as the substrate in the presence of L-ascorbic acid. No definite pH optimum was observed when α-acetolactate was the substrate. 4. The K_m values for α-aceto-α-hydroxybutyrate and α-acetolactate are 1.2·10^-4 M and 5.5·10^-3 M respectively. 5. No definite involvement of a metal ion could be demonstrated. 6. The enzyme is not inbihited by p-hydroxymercuribenzoate, cysteine and 2-mercaptoethanol. Arsenite is slightly inhibitory.