Sodium-Potassium-activated Adenosine Triphosphatase of Electrophorus Electric Organ

Abstract Electric organ microsomes from Electrophorus electricus can be phosphorylated by ATP. The properties of this reaction have been studied and compared with the properties of the sodium-potassium-activated adenosine triphosphatase and the Na+-dependent ATP-ADP exchange reaction which occur in the same microsomal preparation. Only sodium ion, among the monovalent cations tested, will increase the extent of microsomal phosphorylation. The phosphorylation also appears to be specific for ATP. The Na+-dependent, ATP-specific phosphorylation is not prevented by treatment with N-ethylmaleimide or oligomycin. Under some conditions, ouabain inhibits the phosphorylation. Following phosphorylation in the presence of sodium and ATP, potassium and most monovalent cations decrease the extent of phosphorylation. This reduction is prevented by N-ethylmaleimide or oligomycin, which, in an earlier report, were shown to inhibit Na+-K+-ATPase and to activate a Na+-dependent ATP-ADP exchange. Other workers had reported that hydroxylamine dephosphorylates brain microsomes without inhibiting Na+-K+-ATPase, thus questioning the relevance of the Na+-dependent phosphorylation reaction. With electric organ microsomes, hydroxylamine will replace K+ to some extent in the activation of ATP hydrolysis. Hydroxylamine will also reduce the extent of phosphorylation of native microsomes. However, under comparable conditions, hydroxylamine does not dephosphorylate acid-denatured microsomes. Thus any hydroxylaminolysis of the microsomal phosphate which may occur is slow relative to the turnover rate of the intact enzyme. These observations are consistent with the hypothesis that the Na+-dependent phosphorylation and exchange reactions are manifestations of the Na+-K+-ATPase and represent the initial step in a sequence of reactions to achieve hydrolysis of ATP. This sequence of reactions is probably related to the mechanism of active cation transport across cell membranes.