Inhibition of the Gastric H,K-ATPase by Clotrimazole

The antimycotic drug clotrimazole inhibits the function of the gastric H,K-ATPase in a manner similar to that observed for the Na,K-ATPase. Because of the high hydrophobicity of the compound, the interaction between clotrimazole and the ion pump occurs at the membrane domain in the apolar core of the membrane. The enzymatic activity was inhibited with a half-saturating concentration of 5.2 microM. Various partial reactions of the pump cycle were analyzed with the electrochromic styryl dye RH421 that has been widely used to study the transport mechanism of P-type ATPases. We discovered that the interaction of clotrimazole with the H,K-ATPase introduces a single "dead-end" branch added to the Post-Albers scheme in the E(1) state of the pump. In this inhibiting state, the ion binding sites have a significantly enhanced affinity for protons and bind up to two protons even at pH 8.5. Inhibition of the pump can be reversed by a decreased pH or increased K(+) concentrations. The mechanistic proposal that allows an explanation of all experiments presented is similar to that published for the Na,K-ATPase.