The phosphate-water oxygen exchange reaction of oxidative phosphorylation in submitochondrial preparations.

Work in the laboratories of Cohn (la-3) and Boyer (4, 5) has shown that intact mitochondria catalyze a rapid oxygen exchange reaction between inorganic phosphate and Hz0 of the medium, which can be followed by use of 018 as tracer. The dependence of the reaction on either oxidative phosphorylation or adenosine triphosphate and its sensitivity to 2,4-dinitropheno1 have indicated that the oxygen exchange reaction is a reflection of an intermediate step in the coupling between electron transport and phosphorylation of adenosine diphosphate. The great rapidity of the reaction, in comparison with the rate of oxidative phosphorylation and the adenosine triphosphateinorganic orthophosphate exchange, has posed important problems regarding its site and mechanism. However, Boyer (4, 5) has pointed out that interpretation of the properties of O** exchange is limited by possible enzymatic and morphological “compartmentation” of intact mitochondria. This communication describes the occurrence and properties of the 018 exchange reaction of oxidative phosphorylation in the phosphorylating digitonin fragments derived from the membranes of mitochondria (6, 6a). These preparations carry out electron transport and coupled phosphorylation, but do not catalyze reactions of the Krebs cycle or the fatty acid oxidation cycle. Such preparations are probably relatively free of complicating reactions extraneous to oxidative phosphorylation and are less liiely to show enzyme or metabolite “compartmentation” than intact mitochondria. The possible occurrence of the 018 exchange reaction has also been examined in soluble preparations of the mitochondrial enzyme catalyzing the adenosine triphosphate-adenosine diphosphate exchange reaction which earlier work has associated with the mechanism of oxidative phosphorylation (7).