The effect of sulfite on the ATP hydrolysis and synthesis activities in chloroplasts and cyanobacterial membrane vesicles can be explained by competition with phosphate.

The effect of sulfite on ATP synthesis and hydrolysis activities is investigated in spinach chloroplasts and in membrane vesicles from the cyanobacterium Synechococcus 6716. Sulfite inhibits phenazine methosulfate-mediated cyclic photophosphorylation both in thiol-modulated chloroplasts and in cyanobacterial membranes with HSO3- (bisulfite) as the active ionic species. The observed inhibition is not due to inhibition of electron transfer or to uncoupling by sulfite. ATP synthesis in cyanobacterial membranes is more sensitive to sulfite when the inorganic phosphate concentration is decreased. This indicates competition between sulfite and phosphate for the same binding site on the ATP synthase. In cyanobacterial membranes sulfite can replace a proton gradient as activator of ATP hydrolysis in the same way as in reduced chloroplasts. By modeling, competition between sulfite and phosphate can fully explain the findings concerning both inhibition and activation.