title: Decrease of apparent calmodulin affinity of erythrocyte (Ca2+ + Mg2+)-ATPase at low Ca2+ concentrations.
abstract: The calmodulin activation of the (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied in the range of 1 nM to 40 microM of purified calmodulin. The apparent calmodulin-affinity of the ATPase was strongly dependent on Ca2+ and decreased approx. 1000-times when the Ca2+ concentration was reduced from 112 to 0.5 microM. The data of calmodulin (Z) activation were analyzed by the aid of a kinetic enzyme model which suggests that 1 molecule of calmodulin binds per ATPase unit and that the affinities of the calcium-calmodulin complexes (CaiZ) decreases in the order of Ca3Z greater than Ca4Z greater than Ca2Z greater than or equal to CaZ. Furthermore, calmodulin dissociates from the calmodulin-saturated Ca2+-ATPase in the range of 10(-7)-10(-6) M Ca2+, even at a calmodulin concentration of 5 microM. The apparent concentration of calmodulin in the erythrocyte cytosol was determined to be 3 to 5 microM, corresponding to 50-80-times the cellular concentration of Ca2+-ATPase, estimated to be approx. 10 nmol/h membrane protein. We therefore conclude that most of the calmodulin is dissociated from the Ca2+-transport ATPase in erythrocytes at the prevailing Ca2+ concentration (probably 10(-7)-10(-8) M) in vivo, and that the calmodulin-binding and subsequent activation of the Ca2+-ATPase requires that the Ca2+ concentration rises to 10(-6)-10(-5) M.
mesh_terms: D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000097:blood*; D000252:Calcium-Transporting ATPases / Q000097:blood*; D002147:Calmodulin / Q000097:blood*; D004910:Erythrocyte Membrane / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D007700:Kinetics; D008433:Mathematics; D011485:Protein Binding
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium