SPECIFICITY OF PLANT ACETOHYDROXYACID SYNTHASE - FORMATION OF PRODUCTS AND INHIBITION BY HERBICIDES

Plant acetohydroxyacid synthase (EC 4.1.3.18) or AHAS catalyses either the condensation of two molecules of pyruvate or one molecule of pyru vate with one molecule of 2-ketobutyrate. 2-Aceto-2-hydroxybutyrate an d 2-acetolactate are formed when the pyruvate concentration is ten tim es greater than the 2-ketobutyrate concentration. This result indicate s that plant AHAS has a preference for 2-ketobutyrate in the second su bstrate site. A similar inhibition by sulfometuron methyl was observed for the two reactions catalysed by the enzyme. These studies were dev eloped by using a new chromatographic method based on the formation of 2,4-dinitrophenylhydrazone of acetoin and 2,4-dinitrophenyl-hydrazone of 3-hydroxy-2-pentanone (compounds resulting of decarboxylation of t he enzyme products).