Abstract
In recent years much interesting information about the mechanism of the $Na^+-K^+$ activated ATPase has been obtained from investigation of the $K^+-activated$ phosphatase activity which appears to be catalysed by the same enzyme. Also several studies have indicated that a $K^+-activated p-nitrophenylphosphatase activity is intimately related to the ATPase activity. And then the exact relation of p-nitrophenylphosphatase activity to $Na^+-K^+$ ATPase activity is not known. The effects of some ions and drugs on the p-nitrophenylphosphatase activity of the rat brain were investigated and the results were summarized as follows. 1. The p-nitrophenylphosphatase was stimulated markedly by low concentrations of $K^+$, while the activity was activated slightly in the presence of $Na^+$ and oligomycin. 2. Addition of both ATP and $Na^+$ caused a remarkable increase in the activity of the $K^+-dependent$ phosphatase at low concentrations of $K^+$. 3. In the presence of $Na^+$ and low concentrations of $K^+$, oligomycin activated the p-nitrophenylphosphatase. 4. O1igomycin inhibited the stimulation of the enzyme activity caused by $Na^{+}+ATP$. 5. Ouabain inhibited the $K^+-dependent$ p-nitrophenylphosphatase activity more in the presence of ATP and $Na^+$ than in their absence. 6. Quinidine inhibited both $Na^+-K^+$ ATPase and p-nitrophenylphosphatase. These inhibitory effects of the drug were partially antagonized by increasing $K^+$ concentrations. The sensitivity of the $K^+-dependent$ p-nitrophenylphosphatase to quinidine was greater than the that of $Na^+-K^+$ ATPase.