Some properties of oxaloacetate-synthesizing enzyme

Abstract

The effect of inhibitors on the formation of oxaloacetate from phosphopyruvate by the oxaloacetate-synthesizing enzyme of pigeon liver has been studied, and data have been obtained on the kinetics of the reaction. Under the conditions studied, the values of the apparent Michaelis constant (Km) were as follows: phosphopyruvate (2 x 10-4 [image]), inosine diphosphate (4 x 10-5 [image]), CO2 (2 x 10-3 [image]) and Mn++ (10-4 [image]). It is confirmed (Utter and Kurahashi, 1954) that for the maximum rate of formation of oxaloacetate added Mn++ were required; Mg++ and Co++ were almost inactive. High concentrations (10 m[image]) bf Mn++ decreased the oxaloacetate yield, probably owing to the non-enzymic decarboxylation of the oxaloacetate. Co++, Ni++ and Cu++ also decreased the yield. KCl(0.1 [image]) inhibited about 90%. NaCl at the same concentration inhibited 14%. Tartronate and oxalate inhibited the reaction about 50% at a concentration twice that of phosphopyruvate. The inhibition was partially competitive and partially non-competitive. A value for Ki of 2 x 10-4 M was obtained graphically for tartronate. Mesoxalate (10 m[image]) inhibited 52% with m[image]-phosphopyruvate and fluoromalate (m[image]) 22% with 0.1 m[image]-phosphopyruvate. There was less inhibition (approximately-15%) by L-malate, mesotartrate, malonate, fluorocitrate and citrate. The ketogenic effect of tartronate, mesoxalate, oxalate and fluoromalate could be accounted for by the assumption that it is due to an inhibition of the formation of oxaloacetate via either malic enzyme or oxaloacetate-synthesizing enzyme. However, the ketogenic effect of fluorocitrate and malonate could possibly be explained by inhibition of malic enzyme but not of oxaloacetate-synthesizing enzyme.