Omeprazole and bafilomycin, two proton pump inhibitors: Differentiation of their effects on gastric, kidney and bone H +-translocating ATPases

Abstract

The effects of omeprazole and bafilomycin on processes dependent on two different types of H +-translocating ATPases were compared. A H +-ATPase of the E 1E 2-type, the H +,K +-ATPase, was purified from gastric mucosa. Vacuolar type H +-ATPases were prepared both from kidney medulla and from osteociast-containing medullary bone. H +,K +-ATPase-mediated proton transport in gastric vesicles was selectively inhibited by omeprazole with a high potency (inhibitory concentrations ≥ 3 μM) and in a time- and pH-dependent manner. This result is consistent with the mechanism of action of omeprazole, which is dependent on acid-induced transformation of the drug into an active inhibitor reacting with luminally accessible sulfhydryl groups of the enzyme. Accordingly, the presence of the membrane-impermeable mercaptane glutathione did not affect the inhibitory action of omeprazole on the H +, K +-ATPase. Proton transport in kidney- and bone-derived membrane vesicles was also inhibited by omeprazole, but with a lower potency (inhibitory concentrations ≥ 100 μM). Furthermore, the presence of glutathione totally abolished this inhibition, indicating that cytosolic, rather than luminal, SH-groups of the respective vascuolar H +-ATPase were interacting with omeprazole at high concentrations. In line with these results, it was found that omeprazole was much more potent in inhibiting acid production in isolated gastric glands (IC 50 ≈ 0.25 μM) than in inhibiting osteoclast-mediated 45Ca-release in isolated mouse calvaria (IC 50 ≈ 200 μM). Bafilomycin, on the other hand, was much more effective in inhibiting proton transport mediated by the vacuolar H +-ATPases in the kidney- and bone-derived membrane vesicles (IC 50 ≈ 2 nM) than in inhibiting H +,K +-ATPase-mediated proton transport in gastric membrane vesicles (IC 50 ≈ 50 μM). Thus, approximately 10 4 times higher concentrations of bafilomycin were needed to inhibit the H +, K +-ATPase to the same extent as the vacuolar H +-ATPase. A similar difference in potency of befilomycin was found when its inhibitory effect was determined in isolated mouse calvaria (IC 50 ≈ 2.5 nM) and in isolated gastric glands (IC 50 ≈ 5 μM). Hence, omeprazole was found to be a specific inhibitor of the H +,K +-ATPase under physiological conditions, i.e. in the presence of glutathione, while bafilomycin was found to be selective towards vacuolar H +-ATPases.