The H+/ATP stoichiometry of the (H+ +K+)-ATPase of dog gastric microsomes.

Gastric microsomal vesicles isolated from dog fundic mucosa were shown to be relatively ion tight and have a low level of proton permeability. The H+ translocase, basal ATPase and K+-activated ATPase activities of these vesicles were measured and the H+/ATP stoichiometry calculated using either the total K+-ATPase or the K+-stimulatable component (total K+-ATPase--basal ATPase). The former estimations consistently gave stoichiometric of approximately one, whereas the use of only the K+-stimulatable component gave widely differing values. Measurement of the dephosphorylation of the enzyme under basal conditions revealed both a labile and a stable phosphoenzyme component. The rate of decay of the labile component completely accounted for the basal ATPase activity observed. We conclude that the basal ATPase associated with our preparations is a spontaneous dephosphorylation of the phosphoenzyme occurring in the absence of K+ and that the H+/ATP stoichiometry of the gastric ATPase is one.