1. Pyruvate carboxylase from baker's yeast acts with either MgATP2− or MnATP2− as substrate. The optimum pH for the enzyme reaction is 8·0 with MgATP2− and 7·0 with MnATP2−. 2. When the reaction velocity is plotted against MgATP2− (or MnATP2−) concentration slightly sigmoid curves are obtained, either in the presence or in the absence of acetyl-CoA (an allosteric activator). In the presence of excess of free Mg2+ (or Mn2+) the curves turn into hyperbolae, whereas in the presence of excess of free ATP4− the apparent sigmoidicity of the curves increases. 3. The sigmoidicity of the plots of v against MgATP2− (or MnATP2−) concentration can be explained by the inhibitory effect of free ATP4−, the concentration of which, in the experimental conditions employed, is significant and varies according to the total concentration of the ATP–magnesium chloride (or ATP–manganese chloride) mixture. Free ATP4− behaves as a negative modifier of yeast pyruvate carboxylase. 4. The effect of high concentrations of Mg2+ (or Mn2+) on the kinetics of yeast pyruvate carboxylase can be explained as a deinhibition with respect to ATP4−, instead of a direct enzyme activation. 5. At pH6·5 manganese chloride is more effective than magnesium chloride as enzyme activator even in the presence of a great excess (16-fold) of the latter. This is consistent with a significant contribution of the MnATP2− complex to the activity of yeast pyruvate carboxylase, in medium conditions resembling those existing inside the yeast cell (pH6·25–6·75; 12mm-magnesium chloride and 0·75mm-manganese chloride). 6. The physiological significance of the enzyme inhibition by free ATP4− is doubtful since the Mg2+ and Mn2+ concentrations reported to exist inside the yeast cell are sufficient to decrease ATP4− concentrations to ineffective values.
- © 1969 The Biochemical Society