A Hybrid between Na+,K+-ATPase and H +,K+-ATPase Is Sensitive to Palytoxin, Ouabain, and SCH 28080

Abstract

Na+,K+-ATPase is inhibited by cardiac glycosides such as ouabain, and palytoxin, which do not inhibit gastric H +,K+-ATPase. Gastric H+,K+-ATPase is inhibited by SCH28080, which has no effect on Na+,K +-ATPase. The goal of the current study was to identify amino acid sequences of the gastric proton-potassium pump that are involved in recognition of the pump-specific inhibitor SCH 28080. A chimeric polypeptide consisting of the rat sodium pump α3 subunit with the peptide Gln905-Val 930 of the gastric proton pump α subunit substituted in place of the original Asn886-Ala911 sequence was expressed together with the gastric β subunit in the yeast Saccharomyces cerevisiae. Yeast cells that express this subunit combination are sensitive to palytoxin, which interacts specifically with the sodium pump, and lose intracellular K + ions. The palytoxin-induced K+ efflux is inhibited by the sodium pump-specific inhibitor ouabain and also by the gastric proton pump-specific inhibitor SCH 28080. The IC50 for SCH 28080 inhibition of palytoxin-induced K+ efflux is 14.3 ± 2.4 μM, which is similar to the Ki for SCH 28080 inhibition of ATP hydrolysis by the gastric H+,K+-ATPase. In contrast, palytoxin-induced K+ efflux from cells expressing either the native α3 and β1 subunits of the sodium pump or the α3 subunit of the sodium pump together with the β subunit of the gastric proton pump is inhibited by ouabain but not by SCH 28080. The acquisition of SCH 28080 sensitivity by the chimera indicates that the Gln905-Val930 peptide of the gastric proton pump is likely to be involved in the interactions of the gastric proton-potassium pump with SCH 28080.