The adenosine triphosphate-adenosine diphosphate exchange reaction of oxidative phosphorylation.

Previous reports from this laboratory have demonstrated that the ATP-Pi32 1 exchange reaction (1) and the dinitrophenolstimulated ATPase activity (2) of mitochondrial subfragments (3-9) are reflections of intermediate enzymatic steps in the complex sequence of reactions (1, 3) in which phosphorylation of ADP is coupled to electron transport in the respiratory chain. This communication concerns another intermediate reaction of osidative phosphorylation, namely the ATP-ADP exchange reaction, which was first observed and briefly described in an earlier paper (1) and has since been discussed in preliminary reports (3, 10). In this reaction, isotopically labeled SDP is reversibly incorporated into ATP in mitochondrial subfragments, in the absence of electron transfer. The relationship of the ATPADP exchange to the ATP-Pi32 exchange reaction has been established by an examination of the requirements of the exchanges and the action of different types of uncoupling agents on these reactions. It is concluded that the ATP-ADP exchange is a reflection of the terminal reaction of oxidative phosphorylation by which ATP is formed from ADP and a “high-energy” phosphate donor: