Sodium-Potassium-activated Adenosine Triphosphatase of Electrophorus Electric Organ: II. EFFECTS OF N-ETHYLMALEIMIDE AND OTHER SULFHYDRYL REAGENTS

Abstract

Abstract The sulfhydryl reagents, N-ethylmaleimide (NEM), N-butylmaleimide, p-chloromercuribenzoate, and p-chloromercuriphenylsulfonate, inhibit the Na+-K+-ATPase and Mg++-activated ATP-ADP transphosphorylation catalyzed by electric organ microsomes of Electrophorus. The first two reagents also cause the appearance of a sodium-dependent ATP-ADP transphosphorylation in the presence of 3 mm MgCl2. The sodium-dependent ATP-ADP transphosphorylation catalyzed by the NEM-treated microsomes has several features in common with the sodium-dependent ATP-ADP transphosphorylation catalyzed by the native microsomes (0.3 mm MgCl2). Both reactions specifically require adenosine nucleotides, sodium ions, and a divalent cation (preferably Mg++); both are inhibited by Ca++, ouabain, and K+, but not by oligomycin. With respect to the transphosphorylation, sodium increases the affinity of the microsomes for Mg++, while NEM treatment reduces it. The effects of NEM lend support to a multistep reaction scheme for the Na+-K+-ATPase and indicate that Na+ is required for the initial phosphorylation.