Mechanisms of involvement in calmodulin in regulation of affinity for Ca2+ and maximum activity of Ca pump of erythrocyte membranes

The activity of the Ca pump of inside-out vesicles of human erythrocyte membranes was studied using 45Ca and membrane filters. It was found that trifluoperazine completely inhibits the increase in the maximum activity of the Ca pump caused by the addition of calmodulin and has no effect on the calmodulin-stimulated increase in the affinity of the Ca pump for Ca2+. A comparison of characteristic curves of the calmodulin-stimulated components of the activity of the Ca pump, inhibited and not inhibited by trifluoperazine, and the fluorescence intensity of N-phenyl-1-naphthylamine in the presence of calmodulin showed that the mechanisms of action of calmodulin on the maximum activity of the Ca pump and its affinity for Ca2+ differ significantly. In the first case the activation was due to the Ca-calmodulin complex and in the second to the calcium-free form of calmodulin. This conclusion is supported by data on the dependence of the activity of the Ca pump on the calmodulin concentration at low and saturating Ca2+ concentrations as well as by the results obtained in the case of moderate treatment of the membranes with trypsin