ACETOLACTATE SYNTHETASE FROM BARLEY SEEDLINGS

Acetolactate synthetase has been isolated from barley seedlings. The enzyme requires TPP and either Mn2+ or Mg2+ for maximal activity. It has a broad pH optimum from pH 6·5 to 8·5. The major product of the reaction, with pyruvate as substrate, is acetolactate. The enzyme is inhibited by leucine plus valine acting in a co-operative manner and by the same amino acids, if added singly, at higher concentrations. The pair isoleucine plus leucine also co-operatively inhibit the enzyme, although isoleucine is relatively ineffective on its own. The inhibitions are greater at pH 6·5 than at pH 8·5. The results are consistent with the hypothesis that inhibition of the growth of barley seedlings by valine and leucine is due to their effect on acetolactate synthetase. © 1969.