The Oligomycin-sensitive Adenosine Diphosphate-Adenosine Triphosphate Exchange in an Inner Membrane Matrix Fraction of Rat Liver Mitochondria

Abstract

Abstract In the absence of added Mg++, the inner membrane matrix fraction of rat liver mitochondria catalyzed an oligomycin-sensitive ADP-ATP exchange reaction but failed to catalyze significant transphosphorylations between the following substrate pairs: UDP-UTP, CDP-CTP, GDP-GTP, ADP-UTP, ADP-GTP, and ADP-ADP. The inner membrane matrix ADP-ATP exchange was inhibited by low concentrations of not only oligomycin, but also aurovertin, 2,4-dinitrophenol, sodium oleate, carbonylcyanide p-trifluoromethoxyphenylhydrazone, atractyloside, p-chloromercuribenzoate, Ag+, Ca++, Sr++, and a salicylanilide-uncoupling agent. Phosphate in excess of 4.0 mm inhibited the ADP-ATP exchange, whereas lower concentrations of this anion were without effect. Large decreases of oligomycin-sensitive ADP-ATP exchange activity accompanied loss of acceptor control of respiration in the inner membrane matrix brought about by the following mild procedures: incubation at 37°, freezing and thawing, and titration with Mg++. Lubrol WX completely inhibited the oligomycin-sensitive exchange. In the absence of added Mg++, the inner membrane matrix also catalyzed an inorganic orthophosphate-ATP exchange reaction and oxidative phosphorylation at significantly high rates. Concentrations of atractyloside, oligomycin, and 2,4-dinitrophenol which maximally inhibited the ADP-ATP exchange also provided maximal inhibition of these activities. These findings strengthen the hypothesis that the oligomycin-sensitive ADP-ATP exchange is catalyzed by an enzyme system normally participating in oxidative phosphorylation. It is suggested that this enzyme system is most likely a highly ordered membrane complex containing, at the minimum, the mitochondrial ATPase, an oligomycin sensitivity-conferring factor, and a divalent cation.