A hybrid between Na+,K+-ATPase and H+,K+-ATPase is sensitive to palytoxin,ouabain, and SCH 28080

Na+,K+-ATPase is inhibited by cardiac glycosides such as ouabain, and palyt oxin, which do not inhibit gastric H+,K+-ATPase. Gastric H+,K+-ATPase is in hibited by SCH28080, which has no effect on Na+,K(+-)ATPase, The goal of th e current study was to identify amino acid sequences of the gastric proton- potassium pump that are involved in recognition of the pump-specific inhibi tor SCH 28080, A chimeric polypeptide consisting of the rat sodium pump alp ha3 subunit with the peptide Gln(905)-Val(930) of the gastric proton pump a lpha subunit substituted in place of the original Asn(886)-Ala(911) sequenc e was expressed together with the gastric beta subunit in the yeast Sacchar omyces cerevisiae. Yeast cells that express this subunit combination are se nsitive to palytoxin, which interacts specifically with the sodium pump, an d lose intracellular K+ ions. The palytoxin-induced K+ efflux is inhibited by the sodium pump-specific inhibitor ouabain and also by the gastric proto n pump-specific inhibitor SCH 28080, The IC50 for SCH 28080 inhibition of p alytoxin-induced K+ efflux is 14.3 a 2.4 muM, which is similar to the K-i f or SCH 28080 inhibition of ATP hydrolysis by the gastric H+,K+-ATPase. In c ontrast, palytoxin-induced K+ efflux from cells expressing either the nativ e alpha3 and pi subunits of the sodium pump or the alpha3 subunit of the so dium pump together with the beta subunit of the gastric proton pump is inhi bited by ouabain but not by SCH 28080. The acquisition of SCH 28080 sensiti vity by the chimera indicates that the Gln(905)-Val(930) peptide of the gas tric proton pump is likely to be involved in the interactions of the gastri c proton-potassium pump with SCH 28080.