P_i∠ATP exchange in the absence of proton gradient by the H⁺ ‐ATPase from yeast plasma membranes

Purified soluble H⁺ -ATPase from Schizosaccharomyces pombe catalyzes a Pi∠ATP exchange in the absence of a H⁺ gradient. When the pH of the assay medium is raised from 5.5 to 8.0 there is a decrease of the ATPase activity and an increase of the rate of Pi∠ATP exchange. At pH 7.0 the addition of the organic solvent dimethyl sulfoxide (20%, ) promotes a decrease of ATPase activity and an increase of the P_i∠ATP exchange reaction. The effect of the organic solvent on the P_i∠ATP exchange is related to a decrease of the apparent K _m for P_i.