Fig. 3.

Effect of pH on Na⁺-dependence of ATP hydrolysis (A) and Na⁺ transport (B). Washed membrane vesicles [final concentration 0.73 mg/mL (A) or 0.13 mg/mL (B)] were incubated in a 25 mM Pipes-KOH/Mes-HCl buffer containing 25 mM MgSO₄⋅7H₂O and 0.4 M sucrose. One milliliter of assay was supplemented with 17 μM valinomycin, 10 mM KHSO₃, 40 mM KCl, and increasing Na⁺ concentrations. The pH was adjusted to 7 (■) or 5 (▲). The Na⁺ concentration of the assay without additional Na⁺ was 0.95 mM. The Na⁺ concentration in B was 10 mM.