Calcium ion-dependent p-nitrophenyl phosphate phosphatase activity and calcium ion-dependent adenosine triphosphatase activity from human erythrocyte membranes

In the presence of ATP and of Mg²⁺, human erythrocyte membranes show a phosphatase activity towards p-nitrophenyl phosphate which is activated by low concentrations of Ca²⁺. The effect of Ca²⁺ is strongly enhanced if either K⁺ or Na⁺ is also present. Activation of the p-nitrophenyl phosphate phosphatase by Ca²⁺ reaches a half-maximum at about 8μm-Ca²⁺ and is apparent only when the ion has access to the inner surface of the cell membrane. Ca²⁺-dependent phosphatase activity can only be observed if ATP is at the inner surface of the cell membrane, and the presence of ATP seems to be absolutely necessary, since either its removal or its replacement by other nucleoside triphosphates abolishes the activating effect of Ca²⁺. The properties of the (ATP+Ca²⁺)-dependent phosphatase are very similar to those of the Ca²⁺-dependent ATPase (adenosine triphosphatase), also present in erythrocyte membranes, which probably is involved in Ca²⁺ transport in erythrocytes. The similarities suggest that both activities may be properties of the same molecular system. This view is further supported by the fact that p-nitrophenyl phosphate inhibits to a similar extent Ca²⁺-dependent ATPase activity and ATP-dependent Ca²⁺ extrusion from erythrocytes.