AN NA+ K+-ATPASE INHIBITOR PROTEIN FROM RAT-BRAIN CYTOSOL/

A protein isolated from rat brain cytosol is found to inhibit Na+/K+-A TPase in rat brain and kidney and H+/K+-ATPase from toad gastric mucos a, but has no effect on Ca2+,Mg2+-ATPase and Ca2+-ATPase isolated eith er from rat testis or goat spermatozoa. The inhibitor has been partial ly purified by ammonium sulphate precipitation followed by gel-filtrat ion through Sephadex G-100. The inhibitor seems to bind at or close to the ATP binding site of Na+/K+-ATPase, such that the binding of the i nhibitor to ATPase is reversible and competitive in nature with respec t to the substrate. Optimum inhibition is observed at around the phase transition temperature of brain Na+/K+-ATPase and the inhibitory acti vity is only partially dependent on -SH or -NH2 group(s) of the inhibi tor protein.