Different biochemical modes of action of two irreversible H+,K(+)-ATPase inhibitors, omeprazole and E3810.

Omeprazole and E3810 were found to inhibit gastric H+,K(+)-ATPase following different biochemical mechanisms. Effects of the specific binding of the inhibitors on the conformational state of the enzyme were studied by measuring the fluorescence of the enzyme labeled with fluorescein 5'-isothiocyanate. The absolute fluorescence level of the omeprazole-bound enzyme was lower than that of the control enzyme, and reduction of S-S cross-linking between the enzyme and omeprazole increased the fluorescence. Addition of K+ into the control vesicle solution quenched the fluorescence (E1-->E2K+). The quench was inhibited in the omeprazole-bound enzyme but not in the E3810-bound enzyme. These results suggest that the omeprazole-bound enzyme has a low fluorescence conformation (E2 form). On the other hand, the conformation of the E3810-bound enzyme was the same as that of the control enzyme (E1 form). Phosphoenzyme formation in the absence of K+ was inhibited in both the E3810- and omeprazole-bound enzymes. Binding of 2',3'-o-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate to the enzyme was equally inhibited by E3810 and omeprazole. K(+)-dependent dephosphorylation from the phosphoenzyme was inhibited in the E3810-bound enzyme but not in the omeprazole-bound enzyme. These experimental results have shown that the inhibition mechanism of H+,K(+)-ATPase by omeprazole was different from that by E3810; the partial reaction that was the most differently affected by the inhibitors was the conformational change from the E2 to E1 form for omeprazole and the luminal K(+)-dependent dephosphorylation for E3810.