Identification of the site of inhibition by omeprazole of a α-β fusion protein of the H,K-ATPase using site-directed mutagenesis

The alpha subunit of eukaryotic P-type ATPases has ten experimentally defined transmembrane or membrane inserted segments. The fifth and sixth of these are short, not predicted by hydropathy analysis, do not insert independently into microsomal membranes, and are readily removed after tryptic digestion and therefore may be membrane inserted sequences. Acid transport by the gastric H,K-ATPase is covalently inhibited by several substituted pyridyl methylsulfinyl benzimidazoles, such as omeprazole. These act as probes of accessible extracytoplasmic thiols because they are accumulated in the acid transporting gastric vesicles and then convert to thiol reactive, cationic tetracyclic sulfenamides. Inhibition is due mainly to disulfide formation with Cys(813) or Cys(822) in M5/6 and perhaps with a contribution from Cys(892) in the loop between transmembrane segment (TM) 7 and TM8. Identification of the specific cysteine responsible for inhibition should be able to define the turn between M5 and M6. The gastric H,K-ATPase alpha-beta heterodimer was expressed as a fusion protein in HEK 293 cells. Transient transfection resulted in most of the protein being retained in the endoplasmic reticulum with only core glycosylation and minor activity of the ATPase evident. Stable transfection resulted in plasma membrane localization of the protein and complex glycosylation, The transfected but not the control cells displayed cation-stimulated, SCH 28080-inhibited ATPase activity and SCH 28080- and omeprazole-inhibited Rb-86 uptake. The two cysteines in M5/6 and Cys(892) in the TM7/8 loop were mutated to the amino acids found in the Na,K-ATPase in order to determine which of the three cysteine residues were important for benzimidazole inhibition. Mutation of one, two, or all three cysteines did not alter enzyme activity, Rb-86 transport, or SCH 28080 inhibition. Only removal of Cys(822) blocked omeprazole inhibition of Rb-86 transport. These data suggest that Cys(822) is present in a region of the enzyme most easily accessed by the cationic sulfenamide formed by omeprazole, presumably the turn between M5 and M6.