The control of NAD specific malic enzyme from cauliflower bud mitochondria by metabolites

NAD malic enzyme (EC. 1.1.1.39) has been purified from cauliflower (Brassica oleracea. var. botrytis) bud mitochondria. The enzyme exhibits complex regulatory properties being activated by a variety of metabolites including glycolytic intermediates, CoA, sulphate and Krebs cycle acids-the tricarboxylic acids with the exception of citrate being more effective than dicarboxylic acids. Fructose diphosphate which is a positive effector of the enzyme increases the affinity of the enzyme for L-malate.The enzyme is inhibited by glutamate, aspartate, phosphate and ATP, in the latter case the inhibition is largely due to chelation of Mg(2+). The plot of rate versus malate concentration is sigmoid at pH 7.0 with Mg(2+) but normal Michaelis-Menten kinetics are observed with Mn(2+). The molecular weight of the enzyme as measured by gel filtration is ca. 400000. The physiological significance of the responses to metabolites is discussed.