Effects of pH on the interaction of ligands with the (H+ + K+)-ATPase purified from pig gastric mucosa.

The effects of K+, Na+ and ATP on the gastric (H+ + K+)-ATPase were investigated at various pH. The enzyme was phosphorylated by ATP with a pseudo-first-order rate constant of 3650 min-1 at pH 7.4. This rate constant increased to a maximal value of about 7900 min-1 when pH was decreased to 6.0. Alkalinization decreased the rate constant. At pH 8.0 it was 1290 min-1. Additions of 5 mM K+ or Na+, did not change the rate constant at acidic pH, while at neutral or alkaline pH a decrease was observed. Dephosphorylation of phosphoenzyme in lyophilized vesicles was dependent on K+, but not on Na+. Alkaline pH increased the rate of dephosphorylation. K+ stimulated the ATPase and p-nitrophenylphosphatase activities. At high concentrations K+ was inhibitory. Below pH 7.0 Na+ had little or no effect on the ATPase and p-nitrophenylphosphatase, while at alkaline pH, Na+ inhibited both activities. The effect of extravesicular pH on transport of H+ was investigated. At pH 6.5 the apparent Km for ATP was 2.7 microM and increased little when K+ was added extravesicularly. At pH 7.5, millimolar concentrations of K+ increased the apparent Km for ATP. Extravesicular K+ and Na+ inhibited the transport of H+. The inhibition was strongest at alkaline pH and only slight at neutral or acidic pH, suggesting a competition between the alkali metal ions and hydrogen ions at a common binding site on the cytoplasmic side of the membrane. Two H+-producing reactions as possible candidates as physiological regulators of (H+ + K+)-ATPase were investigated. Firstly, the hydrolysis of ATP per se, and secondly, the hydration of CO2 and the subsequent formation of H+ and HCO3-. The amount of hydrogen ions formed in the ATPase reaction was highest at alkaline pH. The H+/ATP ratio was about 1 at pH 8.0. When CO2 was added to the reaction medium there was no change in the rate of hydrogen ion transport at pH 7.0, but at pH 8.0 the rate increased 4-times upon the addition of 0.4 mM CO2. The results indicate a possible co-operation in the production of acid between the H+ + K+-ATPase and a carbonic anhydrase associated with the vesicular membrane.