Potassium-induced changes in phosphorylation and dephosphorylation of (Na+ + K+)-ATPase observed in the transient state.

Effects of K+ and Na+ on the transient state kinetics of (Na+ + K+)-ATPase from Electrophorus electricus were examined. Exposure of the enzyme to K+ for brief intervals prior to the addition of ATP and Na+ converts the enzyme to a form (E2 . K) which is transiently less reactive than when all three ligands are added simultaneously. Enzyme is reconverted to the rapidly reacting (E1) form if Na+ is added prior to ATP. Exposure of the ATPase to K+ without Na+ for 1 to 2 h partially restores the initial phosphate (Pi) burst but greatly depresses the amount of phosphoprotein intermediate (E-P) observed. Experiments in the presence of valinomycin suggest that much of this depression in E-P is related to the presence of sealed vesicles with K+ sites sequestered in the interior. Although the results are largely consistent with a simple model in which ATP hydrolysis occurs only through the phosphoenzyme intermediate, the partial restoration of the Pi burst following long term exposure to K+ appears to be attributable to a slow change which may allow some ATP hydrolysis to occur within formation of a phosphoenzyme intermediate.