Studies of the Mechanism of Cation Transport

Abstract

SUMMARY.Membrane-bound cation stimulated adenosine-triphosphatase is thought to be closely associated with active ion transport phenomena. As this enzyme system requires the combination of both sodium and potassium ions for full activation its mechanism of action has been investigated under these conditions. Using adenosine triphosphate labelled with P^ô nly in the terminal position as substrate for the reaction we have been able to show that ATP^2 is hydroiysed to adenosine diphosphate and radioactive orthophosphate via a phosphorylated intermediate. Sodium but not potassium ions are required for the formation of this intermediate and adenosine diphosphate; potassium ions are subsequently required for dephosphorylation of the intermediate complex with tlie release of P-''^ orthophosphate -that is, both sodium and potassium ions arc required for the turnover of the intermediate in a two step reaction.The cardiac glycoside ouabain (Strophanthin-G) is without effect upon the formation of this intermediate complex, but can completely inhibit the K+ -requiring dephosphorylation reaction. Preliminary experiments indicate that sodium ions are bound to the enzyme system during the phosphorylation reaction and are released when this complex is broken down. Solvent extraction procedures designed to remove phospholipid components did not decrease the specific activity of the phosphorylaled intenncdiate which suggests that it is phosphoprotein in nature.

Hydroxylamine and a 32P-Labelled Intermediate in Sodium-plus-Potassium Ion-Activated Adenosine Triphosphatase

“…Formation of this complex is stimulated by Na+ but not K+; when both ions are present the steady-state concentration of 32p_ labelled phosphoprotein complex is greatly decreased whereas the hydrolysis of [y-32P]ATP is enhanced. Most investigators have concluded that the 32P-labelled phosphoprotein complex recovered in these experiments is an acyl-phosphate-enzyme derivative that functions as an intermediate (E P) during the hydrolysis ofATP by (Na+ + K+)activated ATPase (Charnock, Rosenthal & Post, 1963;Nagano et al 1965;Bader, Sen & Post, 1966).…”