Compound 48/80 and calmodulin modify the interaction of ATP with the ( [formula omitted] of red cell membranes

Abstract

(1) Compound 48/80, an anti-calmodulin agent, reduces the maximum effect of ATP and does not affect the apparent affinity for ATP of the high-affinity site of the Ca 2+-ATPase from calmodulin-bound membranes of human red cells. (2) In the same preparation, 48/80 reduces more than 50-times the apparent affinity for ATP of the low-affinity site with little change in the maximum effect of the nucleotide at this site of the Ca 2+-ATPase. (3) The effects of compound 48/80 are independent of the concentration of Ca 2+ between 30 and 200 μM. (4) The apparent affinity of the low-affinity site of the Ca 2+-ATPase for ATP is almost 100-fold less in calmodulin-stripped membranes than in calmodulin-bound membranes. In calmodulin-stripped membranes, exogenous calmodulin increases the apparent affinity for ATP up to the control values. (5) These results indicate that apart from increasing the apparent affinity of the transport site for Ca 2+, calmodulin also increases the apparent affinity of the regulatory site of the Ca 2+-ATPase for ATP. Since this effect is exerted within the physiological ranges of ATP concentrations, it may participate in the physiological regulation of Ca 2+ pumping by calmodulin.