Inhibitory effects of cations on the gastric H+, K+ -ATPase. A potential-sensitive step in the K+ limb of the pump cycle.
The presence of a cation inhibitory site on the dephosphoform of the H+, K+ -ATPase was confirmed by comparing the effects of K+ and NH4+ on overall activity and on phosphorylation and dephosphorylation. Inhibition of ATPase activity was pronounced at high cation/ATP ratios, but NH4+ was much less effective. At 60 mM cation, although the ATPase activity was greater in the presence of NH4+ (17.1 mumol/mg.h) as compared to K+ (5.1 mumol/mg.h), dephosphorylation of preformed phosphoenzyme was faster with K+ (2101 min-1) than with NH4+ (1401 min-1). Increasing K+ concentrations at the cytosolic face of the enzyme, at constant ATP, decreased the rate of phosphorylation from 1343 to 360 min-1 at 25 mM K+. Increasing ATP concentrations in the presence of constant K+ concentrations accelerated ATPase activity and increased the steady-state phosphoenzyme level. Therefore, inhibition by cations was due to cation stabilization of a dephospho form of the enzyme at a cytosolically accessible cation-binding site. ATP promoted cation dissociation from this site. In ion-permeable vesicles, increasing K+ concentrations, at constant ATP, activated and then inhibited ATPase activity, with a K0.5(I) of 22 mM. In intact, ion-impermeable inside-out vesicles, in the presence of valinomycin, ATPase activity increased up to 175 mM K+. Collapse of this potential by the addition of the electrogenic protonophore 3,3',4', 5-tetrachlorosalicylanilide restored the K+ inhibition of ATPase activity. Thus, the cation inhibition of the ATPase activity appears to be voltage-sensitive; and hence, its connection to the voltage sensitivity of acid secretion demonstrated in intact gastric mucosa is discussed.
Lorentzon
P
P
Department of Biology, Hässle Gastrointestinal Research Laboratories, Mölndal, Sweden.
Sachs
G
G
Wallmark
B
B
D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors*; D000641:Ammonia / Q000494:pharmacology; D000818:Animals; D002412:Cations; D017506:H(+)-K(+)-Exchanging ATPase; D007700:Kinetics; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D013270:Stomach / Q000201:enzymology; D013552:Swine
D002412:Cations; D000641:Ammonia; D000251:Adenosine Triphosphatases; D017506:H(+)-K(+)-Exchanging ATPase; D011188:Potassium