[Mechanism of involvement of calmodulin in the regulation of affinity to Ca2+ and maximum activity of Ca-pump in the erythrocyte membrane].

The activity of the Ca-pump in inside-out oriented human erythrocyte membrane vesicles was studied with the use of 45Ca and membrane filters. It was found that trifluoroperazine fully inhibits the calmodulin-induced increase in the maximal activity of the Ca-pump without affecting the calmodulin-stimulated increase in the Ca-pump affinity for Ca2+. The dependence of calcium concentrations of calmodulin-stimulated components of the Ca-pump activity, both inhibited and noninhibited by trifluoroperazine, as well as the dependence on calcium concentrations of the fluorescence intensity of N-phenyl-1-naphthylamine were analyzed. This analysis revealed essential differences between the mechanisms of calmodulin action on the maximal activity of the Ca-pump and its affinity for Ca2+. The maximal activity was elevated by addition of the Ca-calmodulin complex, whereas the increase in the affinity for Ca2+ was induced by calmodulin alone. These findings were supported by data on the dependence of the Ca-pump activity on calmodulin concentrations at low and saturating concentrations of Ca2+ as well as by data obtained in the study on moderate treatment of erythrocyte membranes with trypsin.