Apparent co-operative effect of hydrogen carbonate (HCO-3) on pigeon kidney pyruvate carboxylase

Kinetic methods have been used to determine the interrelationship between HCO-3, pyruvate and acetyl-CoA and their effect on pigeon kidney pyruvate carboxylase (pyruvate: CO2 ligase [ADP], EC 6.4.1.1). HCO-3 shows a negative co-operative effect (biphasic kinetics with two different Km values). Pyruvate influences the attachment of HCO-3 to this enzyme. The same has been shown for acetyl-CoA. Contrary to the results of other investigators no co-operative effect was seen with pyruvate even at different concentrations of acetyl-CoA. HCO-3 itself shows hardly any effect on the homotropic positive co-operativity (sigmoidal kinetics) of acetyl-CoA. The negative co-operative effect of HCO-3 could not be removed even at saturating concentrations of pyruvate and/or acetyl-CoA, which is also supported by the n and Rs values. The results of this communication bring out differences between pigeon kidney pyruvate carboxylase and pyruvate carboxylase from other sources. It is also suggested that there may be different allosteric and regulatory sites for acetyl-CoA, HCO-3 and pyruvate.