Adenosine Triphosphatase from Rat Liver Mitochondria

Abstract Homogeneous mitochondrial ATPase from rat liver binds ADP in a rapidly reversible manner. The enzyme-bound ADP can be recovered quantitatively as ADP. In the absence of Mg2+, the enzyme exhibits 0.88 binding sites for ADP per enzyme molecule with an intrinsic dissociation constant of 0.94 µm. In the presence of Mg2+, the enzyme loses 90% of its ATPase activity but does not lose the ability to bind ADP. Short term binding experiments detect 0.65 binding sites per enzyme molecule with an intrinsic dissociation constant of 2.1 µm. The Km (ADP) for oxidative phosphorylation catalyzed by purified inner membrane vesicles, in which mitochondrial ATPase is located on the outer surface of the membrane directly available to added ADP, was found to be 3.8 µm. Aurovertin, a potent inhibitor of oxidative phosphorylation, inhibits binding of ADP by mitochondrial ATPase in the absence of Mg2+ in a manner similar to its inhibition of oxidative phosphorylation. In the presence of Mg2+, however, binding is enhanced by aurovertin. Taken together, the results show that the binding of ADP by soluble mitochondrial ATPase has important properties in common with the interaction of ADP with the functional oxidative phosphorylation system.