Sodium ions as substitutes for protons in the gastric H,K-ATPase.
In view of the striking homology among various ion-translocating ATPases including Na,K-ATPase, Ca-ATPase, and H,K-ATPase, and the recent evidence that protons can replace cytoplasmic sodium as well as potassium in the reaction mechanism of the Na,K-ATPase (Polvani, C., and Blostein, R. (1988) J. Biol. Chem. 263, 16757-16763), we studied the role of sodium as a substitute for protons in the H,K-ATPase reaction. Using hog gastric H,K-ATPase-rich inside-out membrane vesicles we observed 22Na+ influx which was stimulated by intravesicular potassium ions (K+i) at pH 8.5 but not at pH 7.1. This sodium influx was observed in medium containing ATP and was inhibited by vanadate and SCH28080, a selective inhibitor of the gastric H,K-ATPase. At least 2-fold accumulation of sodium was observed at pH 8.5. Experiments aimed to determine the sidedness of the alkaline pH requirement for K+i-dependent sodium influx showed that K+i-activated sodium influx depends on pHout and is unaffected by changes in pHin. These results support the conclusion that sodium ions substitute for protons in the H,K-ATPase reaction mechanism and provide evidence for a similarity in ion selectivity and/or binding domains of the Na,K-ATPase and the gastric H,K-ATPase enzymes.
Polvani
C
C
Department of Medicine, McGill University, Montreal, Quebec, Canada.
Sachs
G
G
Blostein
R
R
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000897:Anti-Ulcer Agents / Q000494:pharmacology; D005753:Gastric Mucosa / Q000201:enzymology*; D017506:H(+)-K(+)-Exchanging ATPase; D007093:Imidazoles / Q000494:pharmacology; D007700:Kinetics; D011522:Protons; D012964:Sodium / Q000378:metabolism*; D013552:Swine; D014638:Vanadates / Q000494:pharmacology
D000897:Anti-Ulcer Agents; D007093:Imidazoles; D011522:Protons; C035235:Sch 28080; D014638:Vanadates; D000255:Adenosine Triphosphate; D012964:Sodium; D000251:Adenosine Triphosphatases; D017506:H(+)-K(+)-Exchanging ATPase