The competition transport of sodium ions and protons in the cytoplasmic access channel of the Na+,K+-ATPase

The changes in capacitance and conductance of lipid bilayer membranes have been studied with adsorbed membrane fragments containing Na⁺,K⁺-ATPase. These changes have been initiated by fast release of protons from a bound form (“caged H⁺”) induced by an UV flash. The changes of the capacitance in the presence of Na⁺,K⁺-ATPase were affected by the frequency of the applied voltage, pH and the concentration of sodium ions. Addition of sodium ions altered the changes of capacitance caused by a pH jump in the medium due to caged H⁺ photolysis, and the magnitude and sign of this effect depended on the initial pH. These results are explained by competitive binding of sodium ions and protons to the ion-binding sites of the Na⁺,K⁺-ATPase at its cytoplasmic side. The pH at which the sign of the sodium ion effect changed allows the evaluation of the pK of the proton binding site, which is about 7.6.