Abstract
The isolation is described of a microsomal fraction from rat brain possessing an adenosine triphosphatase which is stimulated 2.5-fold by sodium plus potassium. The presence of both ions is required for maximal activity. The effect of varying the conditions of isolation of the microsomal fraction and the determination of enzymic activity has been studied. The influence of other univalent ions (Li+, Rb+, Cs+, NH4+ and choline) has been determined. The possible connection between this adenosine triphosphatase and sodium transport is discussed.