A fluorometric assay for the potassium-dependent phosphatase activity of the (Na+ + K+)-adenosine triphosphatase.

A fluorometric assay for the K+-dependent phosphatase activity of the (Na+ + K+)-ATPase in both purified and membrane-bound forms is described. The assay utilizes 3-O-methylfluorescein phosphate as substrate and measures the fluorescence of the 3-O-methylfluorescein produced by hydrolysis of the substrate. The assay described is an order of magnitude more sensitive than the assay employing p-nitrophenylphosphate, the substrate most commonly used to measure this activity. The assay is also suitable for the specific measurement of (Na+ + K+)-ATPase activities in membranes which contain high levels of other ATPase activities.