Potassium Ion-dependent Hydrolysis of Adenosine Triphosphate Induced by Nigericin in Mitochondria*

K+ is required for the stimulation by the antibiotic nigericin of adenosine triphosphate hydrolysis in rat liver mitochondria. This action does not occur when K+ is replaced by other monovalent cations. It requires slightly hypotonic conditions and is not related to the swelling phenomena linked to the accumulation of K+ in mitochondria. ATPase is inhibited by small anion molecules to which the mitochondrial membrane is impermeable, less so if the anion is translocated inside the mitochondria. High concentrations cf Rb+ or Na+ and low concentrations cf Ca++ inhibit the ATPase stimulated by nigericin. The ATPase activity of submitochondrial sonic fragments, and of Racker’s coupling factor F1, are not stimulated by nigericin. @-Hydroxybutyrate and succinate partially inhibit the hydrolysis of ATP stimulated by nigericin in K+ media. Neither glutamate, malate, pyruvate, nor cr-ketoglutarate inhibits this enzymatic activity. The inhibition of ATPase by oxidizable substrates is ascribed to the reincorporation of the liberated inorganic orthophosphate into ADP by way of the phosphorylation linked to the oxidation of succinate and /Shydroxybutyrate. Low concentrations of nigericin partially inhibit the ATPase stimulated by uncoupling agents requiring the presence of alkali metal cations for their mechanism of action; the inhibition depends on the ionic environment of the mitochondria. Nigericin does not affect ATPase activities induced by agents with no alkali metal cation requirement. Dianemycin, an antibiotic with properties similar to nigericin regarding induced ion movement and spectra of respiratory inhibition, does not stimulate the hydrolysis of ATP in any cf several media tried. The K+-dependent ATPase stimulated by nigericin appears not to be related to a translocation of alkali metal cations induced by the antibiotic. The possibility that the induction of this activity is related to the participation of a potassium-dependent step in the sequence of reactions leading to the synthesis of ATP in mitochondria is discussed.