ATP inhibition competes with activating cations in modulating the NAD(P)(+)-malic enzyme activity in the mitochondrial matrix of Xenopus laevis oocytes

1. ATP inhibits NAD(P)(+)-dependent malic enzyme activity by competing with the essential activators Mn2+ and Mg2+. 2. The kinetics fit an equation of co-operative kind with Ki of 26 microM and KA of 11.3 microM for ATP/Mn2+ competition; with Ki of 1.1 mM and KA of 0.96 mM for ATP/Mg2+ competition. 3. In the absence of the inhibitor, the co-operativity index increases from 1.77 to greater than 4 in the presence of ATP, in the case of ATP/Mn2+ competition, while it increases from 1.88 to greater than 9 for ATP/Mg2+ competition.