Biosynthesis of valine and isoleucine in plants. I. Formation of α-acetolactate in Phaseolus radiatus

Abstract

1. 1. The presence of an enzyme system in plants catalyzing the formation of α-acetolactate from pyruvate has been demonstrated; the system in green gram (Phaseolus radiatus) has been partially purified and its characteristics have been studied. 2. 2. Free acetaldehyde is formed as a product of the reaction and so the reaction is mainly diverted towards the formation of acetoin. 3. 3. The system requires thiamine pyrophosphate and a divalent metal ion (Mn²⁺ or Mg²⁺) for maximum activity. The optimum pH is around 6.0 and the optimum temperature is 60°. 4. 4. The system is very labile in absence of pyruvate, Mn²⁺ and DPT. 5. 5. The K_m values for pyruvate, Mn²⁺, Mg²⁺ and DPT are 3·10^-2 M. 5·10^-5 M, 2·10^-5 M, and e·10^-6 M respectively. The activation energy is 3540 cal/mole. 6. 6. The enzyme is strongly inhibited by p-chloromercuribenzoate and the inhibition can be reversed partially by 2-mercaptoethanol, BAL or cysteine. Heavy metals, such as Hg²⁺ and Ag⁺, are inhibitory but l-valine does not inhibit the reaction.