Interaction of sodium and potassium with a cation-dependent adenosine triphosphatase system from rat brain

The kinetics of the hydrolysis of ATP by a rat-brain lipoprotein fraction have been investigated. The activity is greatly increased in the presence of Na⁺ and K⁺, but reaches a maximum value and then suffers a diminution in the presence of an excess of either ion. Kinetic evidence indicates that these ions combine with the enzyme protein according to Michaelis-Menten kinetics, that 2 Na⁺ are involved in the activation, and that the interaction of Na⁺ and K⁺ at each site is of a competitive nature. Inhibition of the (Na⁺K⁺)-stimulated ATPase by ouabain, appears to result from a competition between ouabain and both K⁺ and Na⁺ while addition of oligomycin leads to kinetics which suggest competition of oligomycin with Na⁺. NH₄⁺ may substitute for K⁺ and, when present in excess, may also give rise to inhibitory effects.