We have completely purified, in parallel, a low molecular weight, non-peptidic, non-lipidic, Na+,K+-ATPase inhibitory factor from bovine hypothalamic and pituitary tissues. In the final purification step we obtain, from both tissues, a single, homogeneous peak, with a maximal absorbance at 247 nm. This factor, at physiological concentrations of potassium (5–25 mM), inhibits in a dose-response manner Na+,K+-ATPase and displaces ouabain from its receptor at the enzyme structure. The factor isolated from both tissues is identical, being the specific activity per weight of tissue higher in hypophysis. No factor was found in cerebral cortex, used as tissue control.
