Fig. 1.

Na⁺ stimulation of ATP hydrolysis. Washed membrane vesicles (final concentration 0.7 mg/mL) were incubated in a total volume of 1 mL in a 25 mM Pipes-KOH buffer, pH 7, containing 25 mM MgSO₄⋅7H₂O, 0.4 M sucrose, 17 μM valinomycin, 10 mM KHSO₃, 40 mM KCl, and increasing Na⁺ concentrations. The Na⁺ concentration of the assay without additional Na⁺ was 0.75 mM.