Excess magnesium converts red cell (sodium+potassium) ATPase to the potassium phosphatase.

1. The ATPase and phosphatase activities of red cell membranes were measured simultaneously as a function of the magnesium content of the medium. 2. It was found that when the magnesium concentration was greater than that of ATP, magnesium inhibited the ATPase and simultaneously stimulated the phosphatase. The concentrations of magnesium needed for half-maximal stimulation of the phosphatase and half-maximal inhibition of the ATPase were similar. 3. It is suggested that increasing the concentration of magnesium directly causes a change in the conformation of the enzyme from one which favours ATPase activity to one which favours phosphatase activity.