Calmodulin and ATP dependence of the high and low calcium affinity p-nitrophenylphosphatase from human erythrocyte membranes.

In calmodulin depleted membranes from human erythrocytes, the Ca2+-dependent phosphatase showed different sensitivity to calmodulin and ATP with variable affinity towards free calcium concentrations: a calmodulin-dependent activity with high calcium affinity, K1/2 = 1.2 X 10(-7) mol/l calcium, that was fully activated at submicromolar calcium concentrations, higher concentrations being rather inhibitory; an ATP-dependent activity with lower calcium affinity, K1/2 = 10(-6) mol/l calcium, that was fully activated at 10(-5) mol/l calcium in the presence of 50-200 mumol/l ATP and was insensitive to calmodulin, and a calcium dependent phosphatase that was active at a wider ranger of free calcium, 10(-8)-10(-5) mol/l, and required the presence of both calmodulin and ATP.