Differential effects of compound 48/80 on the ATPase and phosphatase activities of the Ca2+ pump of red cells.

The calmodulin antagonist compound 48/80 inhibits the phosphatase activity of the Ca2+-ATPase lowering its maximum velocity and leaving unaltered its apparent affinity for the substrate regardless on whether phosphatase activity is elicited by Ca2+ plus ATP or by calmodulin. Compound 48/80 has no effect on the Ki for ATP as inhibitor of the phosphatase. These results contrast sharply with the large increase that compound 48/80 induces in the apparent affinity of the regulatory site for the nucleotide of the Ca2+-ATPase and suggest that the active site for phosphatase activity is different from the regulatory site for ATP of the Ca2+-ATPase.