Effects of calcium ions on pyruvate kinase in pigeon liver.

Abstract

Pigeon liver pyruvate kinase shows allosteric properties similar to those of the yeast and the rat liver enzyme. These properties include cooperative interaction with phosphoenol pyruvate, allosteric inhibition by ATP and allosteric activation by fructose 1,6‐bisphosphate.The Ca2+ effect on pigeon liver pyruvate kinase has been studied on the Mg2+‐activated enzyme at different phosphoenol pyruvate concentrations and in presence of both fructose 1,6‐bisphosphate and ATP. The Ca2+ activation effect is absolutely dependent on the Mg2+ concentration: in the absence of Mg2+, pyruvate kinase has no catalytic activity and it cannot be replaced by Ca2+.Ca2+ has a double and opposite effect on the liver enzyme activity: Ca2+ is an activator at low phosphoenol pyruvate concentrations, however at increased phosphoenolpyruvate concentrations, Ca2+ becomes an inhibitor.The Ca2+ activation effect is present also in the ATP‐inhibited enzyme and at low phosphoenolpyruvate concentrations Ca2+ is able to remove completely the ATP inhibitory effect. On the enzyme activated by fructose 1,6‐bisphosphate, Ca2+ has only an inhibitory effect at all the concentrations of phosphoenolpyruvate tested.The Ca2+ inhibitory effect on the liver pyruvate kinase is observed at phosphoenolpyruvate concentrations that are above the content of phosphoenolpyruvate in the liver, indicating that in vivo Ca2+ does not play a role in the negative control of pigeon liver pyruvate kinase; the different sensitivity to calcium ions showed by hepatic and muscular enzyme suggests that Ca2+ may function differently in the control of the liver and muscle glycolytic pathways.