The gastric [H,K]ATPase:H+/ATP stoichiometry.

An H+/ATP ratio of 2 for H+ transport was determined from initial rate measurements at pH 6.1 in a purified gastric microsomal fraction containing the [H,K]ATPase. This ratio was independent of external KCl, though the apparent K0.5 for ATP was increased from 10.78 +/- 0.51 (n = 3) to 64.6 +/- 11.9 (n = 3) microM ATP and from 5.13 +/- 0.64 (n = 3) to 65.2 +/- 0.64 (n = 3) microM ATP for H+ transport and the K+-stimulated ATPase, respectively, as K+external was increased from 12 to 150 mM. The H+/ATP ratio was also relatively independent of ATP concentration. Maximum initial rates obtained in KCl-equilibrated vesicles were independent of added valinomycin, though net H+ transport was increased 29.3 +/- 1.03% (n = 6) by the addition of ionophore. Maximum net H+ transport in this vesicle preparation was 185 +/- 2.1 (n = 14) nmol mg-1 of protein. Initial rate measurements of ATPase represent a burst of K+-dependent activity of approximately 10-15 s duration. The H+/ATP stoichiometry was calculated based on the K+-stimulated component of hydrolysis. Under most conditions, the Mg2+-dependent component of hydrolysis was less than 10% of the (Mg2+ + K+) component of hydrolysis.