An ATP dependence of mitochondrial F1-ATPase inactivation by the natural inhibitor protein agrees with the alternating-site binding-change mechanism.

The rate of inactivation of F1-ATPase, isolated from beef heart mitochondria, by the active acidic form of the natural inhibitor protein depends on the ATP concentration. An increase in concentration of ATP to approximately 20 microM leads to a decrease in that of the inhibitor protein inducing 50% inhibition of the F1-ATPase during 5 s preincubation (C50); further increase in ATP concentration to 1 mM causes little, if any, change in C50. However, the C50 values show a rise at ATP concentrations higher than 1 mM. This ATP dependence of the inhibitor action may be in agreement with a version of the alternating-site binding-change mechanism, which assumes that the two-site catalytic cycle intermediates possessing (i) the products (ADP + Pi) bound in the low-affinity state at one of the active sites and (ii) an ATP molecule at the other active site are the targets for the acidic form of the inhibitor protein.