Isolation of 2-Methylisocitrate Dehydratase, a New Enzyme Serving in the Methylcitric Acid Cycle for Propionate Metabolism, fromYallowia lipolytica

2-Methylisocitrate dehydratase (EC 4.2.1.-; (2S,3R)-3-hydroxybutane-l,2,3-tricarboxylate hydrolyase), a new enzyme functioning in the methylcitric acid cycle of propionate metabolism, was found in cells of Yallowia lipolytica, separated from the usual aconitase (EC 4.2.1.3), and purified about 120-fold to homogeneity as judged electrophoretically . The new enzyme catalyzed a reversible reaction between 2-methylisocitrate and 2-methyl-cis-aconitate, but was inactive toward 2-methylcitrate, citrate, cis- or trans-aconitate, isocitrate, or the other hydroxy-acids tested. Equilibrium was reached at about 92% of 2-methylisocitrate, with the rest as 2-methyl-cis-aconitate. The isolated aconitase also catalyzed the reversible reaction and its Km for 2-methylisocitrate was 120 μM, but that of the new enzyme was 18 μM.