Role of metal ions in activity and site:site heterogeneity in isocitrate lyase of castor seedling endosperm.

Time-dependent inactivation of isocitrate lyase with EDTA, ATP, and ADP exhibits biphasic kinetics in which half of the initial activity is lost in a fast and the remaining half in a slow phase. Each phase exhibits first-order kinetics. When Mg2+ ions are added to an enzymatically inactive "apo-protein" obtained by EDTA dialysis of isocitrate lyase, the activity regained exhibits anticooperativity with respect to Mg2+ binding. The data suggest the presence of two classes of Mg2+ binding sites of equal specific activity, but having different affinities for Mg2+. It is proposed that the site:site heterogeneity in isocitrate lyase is a consequence of its quaternary structure constraints, and is independent of any of the known ligands.