Inhibition by valinomycin of atractyloside binding to the membrane-bound ADP/ATP carrier: counteracting effect of cations.
The atractyloside binding capacity of rat heart mitochondria, but not the binding affinity, was markedly decreased by preincubation of the mitochondria with valinomycin in isotonic KCl medium. Maximum inhibition was attained with 5 ng of valinomycin per mg of mitochondrial protein; it corresponded to a 40% decrease of the atractyloside binding capacity. The inhibitory effect of valinomycin was maximal between pH 7.0 and 7.5. It was more marked for heart mitochondria than for liver mitochondria. Valinomycin inhibition of atractyloside binding to heart mitochondria was counteracted by nigericin and FCCP, by sublytic concentrations of cationic surfactants such as cetyltrimethylammonium bromide, and by low concentrations of trivalent and divalent metal ions at acidic pH's still compatible with atractyloside binding, i.e., down to pH 5.5; trivalent metal ions were more effective than divalent metal ions. The effect of valinomycin was also counteracted by exceedingly high concentrations of K+ (more than 300 mM), resulting in a substantial increase in the ionic strength. These results were discussed in terms of the relation between the atractyloside binding capacity of the inner mitochondrial membrane and the surface potential of this membrane.