INHIBITION BY SULPHATE OF OXYGEN EVOLUTION ASSOCIATED WITH PHOTOSYNTHETIC CARBON ASSIMILATION

1. Inorganic sulphate, at 2·10-2 M, completely inhibits the O2 evolution associated with CO2 fixation by entire isolated spinach chloroplasts. 2. Sulphate also inhibits O2 production from chloroplasts provided with substrate concentrations of 3-phosphoglycerate but without added CO2. 3. The sulphate inhibition is reversed by small quantities of orthophosphate but not by inorganic pyrophosphate. With the endogenous phosphate alone, sulphate produces a 50% inhibition at 5·10-4 M. With 2·10-4 M orthophosphate, 2·10-3 M sulphate is required to produce a 50% inhibition. 4. Sulphate, at 2·10-2 M, does not inhibit O2 evolution from envelope-free chloroplasts utilising ferricyanide or added NADP+ as the hydrogen acceptor. 5. It is concluded that sulphate does not interfere with the mechanism of O2 evolution, or electron transport, but that by affecting photophosphorylation, or ATP utilisation, it indirectly inhibits the conversion of 3-phosphoglycerate to 1,3-diphosphoglycerate and hence the final stages of hydrogen transfer. © 1968.