O18 and P32 exchange reactions of mitochondria in relation to oxidative phosphorylation.

The study of oxidative phosphorylation’ by the classical techniques of fractionation and isolation of the purified reactants has not been feasible because of the lability of the reaction systems. Consequently indirect approaches to the problem have been adopted. This paper reports results from one such approach; namely, study by the use of Pa2 and 01* of enzymic exchange reactions of phosphate oxygen and of inorganic orthophosphate (PJ2 which appear to be intimately related to oxidative phosphorylation. Preliminary reports from this laboratory have described the finding that rat liver mitochondria will catalyze a rapid exchange of Pi with the phosphate of adenosine triphosphate (ATP) (Pi-ATP exchange) as well as the exchange of phosphate oxygens with those of water (OPi-Hz0 exchange) in the absence of added substrate and oxygen uptake (1). Swanson has recently reported a similar exchange of Pi with ATP which was catalyzed by mitochondria (2). The occurrence of a rapid exchange of phosphate oxygen in systems which catalyze net oxidative phosphorylation has been previously demonstrated by Cohn (3) and further studied by Cohn and Drysdale (4). A reversible incorporation of Pt2 into ATP catalyzed by brain homogenates and dependent upon net oxygen uptake has been