Sodium-Potassium-activated Adenosine Triphosphatase of l!ZZectrophorus Electric Organ

Microsomes prepared from electric organ of Electrophorus electricus contain, in addition to a sodiumand potassiumactivated adenosine triphosphatase, two different ATP-ADP transphosphorylases; one requires only magnesium, while the other requires Mg++ + Na+. The Mg++-activated nucleotide exchange is nonspecific with respect to substrates and is probably unrelated to the highly specific Na+-K+ATPase. The (Mg++ + Na+)-activated exchange is highly specific with respect to Na+ and the adenine nucleotides and is probably a component of the Na+-K+-ATPase. However, the Na+-activated exchange may be deomonstrated only at low Mg++ concentrations relative to the optimum for the ATPase. Calcium, K+, ouabain, and j-chloromercuribenzoate inhibit both the Na+-activated exchange and the ATPase. Oligomycin and incubation at pH 9 inhibit the ATPase with little effect on the exchange. It is concluded that the initial step of the Na+-K+-ATPase reaction is a reversible phosphorylation of the enzyme which requires both Na+ and Mg++ as activators. The nonspecific nucleotide exchange is considered to be irrelevant to the Na+-K+-ATPase.