Origins of the different sensitivities of [formula omitted] adenosinetriphosphatase preparations to ouabain

Abstract

1. 1. Properties of the Na +,K +-ATPase preparations from rat and dog kidney medullae were compared. 2. 2. The two enzymes, with a 1000-fold difference in ouabain sensitivities, had similar subunit compositions and similar K 0.5 values and Hill coefficients for substrates and activators of several catalytic activities; suggesting that the structural differences of the two are limited to the ouabain binding domains. 3. 3. Experiments on the interactions of ouabain, Pi, and Mg 2+ with the enzymes showed that the two enzymes differed (a) in their inherent affinities for ouabain; and (b) in that Mg 2+ binding increased affinity for ouabain to a greater extent in the dog enzyme than in the rat enzyme.