Regulation by calmodulin of the calcium affinity of the calcium-transport ATPase in human erythrocytes.
The Ca2+ affinity of (Mg2+ + Ca2+)-ATPase in human red blood cells is regulated by a number of intracellular factors, including the association of the enzyme with the cytosolic Ca2+ binding protein, calmodulin. Ghosts prepared by hypotonic lysis in the presence of 0.1 mM CaCl2, or by a gradual stepwise hemolysis procedure, contain an EDTA-extractable protein whose effects are mimicked by calmodulin, whereas ghosts prepared by extensive washes in the absence of added CaCl2 lack calmodulin and contain only a high molecular weight heat stable activator. Purified calmodulin from human red cells or bovine brain shifts the apparent Ca2+ affinity of (Mg2+ + Ca2+)-ATPase activity in extensively washed ghosts to a high Ca2+ affinity state. The shift was most apparent in ghosts in which the Ca2+ affinity was decreased by EDTA treatment. Calmodulin increased the velocity of (Mg2+ + Ca2+)-ATPase in the EDTA-treated ghosts about 36-fold at a low (1.4 microM) Ca2+ concentration, compared with 6-fold before EDTA treatment. The maximum shift in apparent Ca2+ affinity occurred only in the presence of saturating concentrations of calmodulin. It is concluded that red cell calmodulin confers to the Ca2+ transport ATPase the ability to increase its apparent Ca2+ affinity, as well as its maximum velocity, in response to increases in intracellular Ca2+.
D000818:Animals; D001921:Brain; D017301:Ca(2+) Mg(2+)-ATPase; D002135:Calcium-Binding Proteins / Q000097:blood*; D000252:Calcium-Transporting ATPases / Q000097:blood* / Q000302:isolation & purification; D002147:Calmodulin / Q000097:blood* / Q000494:pharmacology; D002417:Cattle; D002458:Cell Fractionation / Q000379:methods; D004492:Edetic Acid / Q000494:pharmacology; D004789:Enzyme Activation; D004910:Erythrocyte Membrane / Q000201:enzymology* / Q000648:ultrastructure; D004912:Erythrocytes / Q000201:enzymology*; D006801:Humans; D007700:Kinetics
D002135:Calcium-Binding Proteins; D002147:Calmodulin; D004492:Edetic Acid; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases