Cell-free synthesis of rat and rabbit gastric proton pump

Effects of ranitidine on gastric vesicles containing H+,K(+)-adenosine triphosphatase in rats

BACKGROUND

To ascertain the mechanism for rebound acid hypersecretion after treatment with an H2-receptor blocker, we investigated the effects of ranitidine on gastric H+,K(+)-adenosine triphosphatase (ATPase) in rats.

METHODS

Male Wistar rats received ranitidine (1-50 mg/kg body weight intraperitoneally twice a day for 5 days). The rats were starved for 15 h after the last treatment and then killed, and gastric vesicles containing H+,K(+)-ATPase were prepared.

RESULTS

Treatment with ranitidine dose-dependently increased protein content in the gastric vesicular fraction purified from the gastric mucosa without changing total protein content. Ranitidine also increased the content of a 94,000-dalton protein, the catalytic subunit of H+,K(+)-ATPase. On the other hand, ranitidine did not affect the specific activity of the enzyme (mumol/min/mg of the gastric vesicular protein). Since gastric vesicles in the fasting state mainly consist of the tubulovesicular membrane, these results suggest that ranidine administration increases total tubulovesicular H+,K(+)-ATPase content (mumol/min/rat) by increasing the number of tubulovesicles per parietal cell. The ranitidine-induced increase in total tubulovesicular H+,K(+)-ATPase activity was still evident 1 week after treatment and returned to control level 1 month later.

CONCLUSIONS

All these findings suggest that the increased content and total activity of tubulovesicular H+,K(+)-ATPase after ranitidine treatment may contribute to the mechanism for acid rebound after H2-blocker therapy.

Site-directed antibodies as topographical probes of the gastric H,K-ATPase alpha-subunit

Gastric acid is secreted by an ATP-driven H+ and K+ exchanger (H,K-ATPase), an integral apical membrane protein of parietal cells. Although the primary structure of the enzyme is known, its higher order structure is uncertain. In order to acquire topographical probes of native, microsomal H,K-ATPase, synthetic peptides corresponding to the 17 amino-terminal (N-peptide) and 16 carboxyl-terminal (C-peptide) residues of pig gastric H,K-ATPase alpha-subunit were coupled to keyhole limpet hemocyanin (KLH). Rabbits were immunized with peptide-KLH conjugates and their sera were tested for specificity by enzyme-linked immunosorbent assay (ELISA), immunoblotting, and immunocytochemistry. All sera showed high ELISA reactivities with synthetic peptides, peptide-BSA conjugates, and microsomal H,K-ATPase adsorbed to microtiter wells (some titers greater than 1:10(4)). Immunoblots of H,K-ATPase resolved by SDS-PAGE showed both N-peptide and C-peptide antibodies reacting with a single 94 kDa band. All sera selectively stained parietal cells in pig gastric mucosal sections. Preimmune sera gave negative or weak signals in all assays. In competition ELISAs, N-peptide antibodies, but not C-peptide antibodies, were displaced from the corresponding bound synthetic peptides by added microsomal H,K-ATPase. One of the N-peptide antibodies inhibited H,K-ATPase activity by more than 50%; binding of this antibody was decreased when ATP or K+ were bound to the enzyme. These results indicate a cytoplasmically-oriented alpha-subunit N-terminus which may participate conformationally in the H,K-ATPase catalytic cycle, and suggest that antibodies against synthetic H,K-ATPase peptides are potentially useful probes of native microsomal H,K-ATPase topography.

Gastric H,K-ATPase topography: amino acids 888-907 are cytoplasmic

Gastric acidification is mediated by H,K-ATPase, an integral protein of apical membranes of gastric parietal cells. Hydropathy analysis of H,K-ATPase alpha subunit primary structure predicts eight transmembrane (TM) domains, while omeprazole-binding data were interpreted in terms of ten TM domains (Mercier et al. (1991) FASEB J. 5, A749). In the present study, tryptic hydrolysis of gastric mucosal microsomes gave a set of peptides which bound the monoclonal antibody HK 12.18, a highly specific probe of the H,K-ATPase. An antiserum against the C-terminus of H,K-ATPase alpha subunit bound the same peptides, and one smaller peptide. The binding data suggested a putative epitope for HK 12.18, and a 20-mer peptide encompassing this site was synthesized. This peptide bound directly to HK 12.18, displaced HK 12.18 from microsomal H,K-ATPase, and blocked HK 12.18 immunostaining of gastric parietal cells. In addition, intact gastric microsomes competitively inhibited binding of HK 12.18 to peptide-BSA conjugate. Taken together, these data place the HK 12.18 epitope between amino acids 888-907 and identify this domain as cytosolic. This result specifically excludes a pair of TM domains between the sixth and seventh TM alpha helices of the H,K-ATPase and supports a secondary structure model with eight TM domains.