Inhibition of the gastric (H+ + K+)-ATPase by fenoctimine.

The effects of fenoctimine, an inhibitor of gastric acid secretion, on the microsomal (H+ + K+)-ATPase were studied. In the micromolar concentration range, fenoctimine inhibited hydrolysis of ATP and p-nitrophenyl phosphate by the (H+ + K+)-ATPase. Inhibition was reversible and noncompetitive with substrate. The apparent Ki was dependent on the concentration of membranes, being increased by added liposomes or high microsomal membrane concentrations. Over the concentration range that (H+ + K+)-ATPase was inhibited, fenoctimine increased the turbidity of microsomal suspensions. The effects of fenoctimine were not specific for the gastric (H+ + K+)-ATPase, since the hydrolytic activities of the (Na+ + K+)-ATPase and mitochondrial ATPase were also inhibited by the drug. These results suggest that inhibition of hydrolysis may not be the direct result of an interaction between the (H+ + K+)-ATPase and fenoctimine but the secondary effect of a fenoctimine-induced perturbation of the microsomal membrane.