Na+/K+-ATPase from Xenopus laevis (Daudin) kidney and epidermis: high sensitivity towards regulatory compounds

Na+/K+-ATPase was prepared from Xenopus laevis epidermis. Purification was obtained by ultracentrifugation on sucrose discontinuous gradient. The maximum of enzyme-containing membranes was concentrated in the denser sucrose layer, exhibiting a good and long-lasting activity (specific activity about 55 mumoles of ATP hydrolyzed/mg of protein/hour). The Kd for the ouabain of kidney and epidermis enzymes were very low (in purified preparations respectively 16 nM for kidney enzyme and 4 nM for skin enzyme), indicating a very high sensitivity toward the cardioglycoside. The dose-response graphs of kidney and skin Na+/K+-ATPase vs ouabain concentrations show that at ouabain concentrations ranging from 1 nM and 1 pM the inhibition elicited by the cardioglycoside disappears and is replaced by an activatory effect. At cardioglycoside concentrations higher than 1 nM, the graphs show the typical inhibition curve.