Mg2+-induced ADP-dependent inhibition of the ATPase activity of beef heart mitochondrial coupling factor F1.

Incubation of F₁ in the presence of Mg²⁺ results in a pronounced lag in its ATPase activity measured with the ATP-regenerating system. A decrease of the initial rate of ATPase induced by Mg²⁺ is also observed when free nucleotides were separated from the enzyme by Sephadex gel filtration. No inhibition is observed when F₁ treated to remove tightly bound nucleotides was preincubated in the presence of Mg²⁺. Mg²⁺-induced inhibition of ATPase activity of nucleotide-depleted F₁ can be restored by an addition of low concentrations of ADP. In all cases the inhibited ATPase can be activated by the ADP-removing system /phosphoenol pyruvate + pyruvate kinase/. It is concluded that i/ Mg²⁺-induced inhibition of the ATPase activity of F₁ is due to the formation of an inactive F₁. ADP complex; and ii/ unusual inhibition of oligomycin-sensitive ATPase by ADP /Fitin et al., Biochem. Biophys. Res. Communs. 1979, $\text{86}$, 434/ is directed to F₁ component of the complete mitochondrial ATPase system.