Inhibition of Photophosphorylation in Spinach Chloroplasts by Inorganic Sulfate

Illumination of chloroplasts in the presence of sulfate, ADP, and Mg2+ causes 35 to 50% inhibition of a subsequent phosphorylation reaction; ADP cannot be replaced by other nucleotides. Compounds that normally inhibit photophosphorylation prevent the appearance of the sulfate-induced inhibition. No evidence was found for phosphorylation site specificity of the sulfate inhibition. In chloroplasts inhibited by preliminary illumination with sulfate, the light-induced pH rise and the P/e2 ratio in a phosphorylating Hill reaction are inhibited as much as photophosphorylation is. The pH rise can be restored by dicyclohexylcarbodiimide. Both the trypsin- and dithiothreitol-activated Ca2+-dependent ATPase activities associated with the chloroplast coupling factor (CF1) are decreased in sulfate-inhibited chloroplasts. Removal of CF1 from the membranes followed by recombination with CF1 from noninhibited chloroplasts restores the pH rise completely and phosphorylation to a large extent. From these and other experimental results it is concluded that sulfate, in the presence of ADP and Mg2+, interacts with CF1 on chloroplast membranes under conditions of coupled electron flow so as to cause a modification of CF1 structure.