title: Potassium-induced changes in phosphorylation and dephosphorylation of (Na+ + K+)-ATPase observed in the transient state.
abstract: 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.
mesh_terms: D000818:Animals; D004557:Electric Organ / Q000201:enzymology*; D004593:Electrophorus; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D010766:Phosphorylation; D011188:Potassium / Q000494:pharmacology*; D012964:Sodium / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
chemical_list: D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium