The Fate of Phosphate Oxygen in Photophosphorylation.

Photophosphorylation, the light dependent formation of adenosine triphosphate from adenosine diphosphate and inorganic phosphate, has been shown to be essentially an irreversible reaction (1). This conclusion was based mainly on the absence of exchange between adenosine triphosphate and inorganic phosphate, or between adenosine diphosphate and adenosine triphosphate (2, 3). In these respects, photophosphorylation differs from oxidative phosphorylation. This irreversibility of the reaction made it particularly suitable for an analysis of the mechanism of adenosine triphosphate formation, with the use of 180~labeled compounds. Many of the reactions which have complicated the interpretation of the results of 180 experiments conducted in oxidative phosphorylation (4-6) would not be espected to occur in the photophosphorylative system. Experiments with 180-labeled compounds could provide information on two basic problems. (a) What is the source of the oxygen atom which bridges the newly attached y-phosphate group of the ATP molecule to the P-phosphate? (b) Does any exchange of oxygen atoms occur during the reaction? Answers to these questions were sought by experiments conducted with Swiss chard chloroplasts with IsO-labeled water or inorganic phosphate. Preliminary communications describing parts of this work have appeared previously (7, 8).