Thr broad objective of this project is to elucidate the mechanisms for biologic and pharmacologic control of active cation transport. Within this framework, we are investigating the structural and catalytic properties of sodium-plus-potassium-activated adenosinetriphosphatase, the enzyme complex associated with the sodium pump in membranes. Ouabain, and other cardioactive steroids, are potent inhibitors of the sodium pump and of this enzyme. Inhibition of the enzyme activity is associated with binding of ouabain to a component of the enzyme complex in membranes. The specific aims of this project are (1) to identify and to isolate the ouabain-binding factor from microsomal membranes enriched in enzyme activity, (2) to establish the relationship between the ouabain-binding factor and catalytic properties of the enzyme, and (3) to isolate the phosphoprotein related to enzyme catalytic activity and to compare its properties and structure with those of the ouabain- binding factor.