Human erythrocyte calmodulin. Further chemical characterization and the site of its interaction with the membrane.

Human erythrocyte and bovine brain calmodulins were indistinguishable by tryptic peptide mapping, indicating that the primary sequence of the two proteins is either very similar or identical. Calcium binding determinations of human erythrocyte calmodulin, by equilibrium dialysis and fluorescence titration, were in close agreement with previous studies on other calmodulins. The calcium-activated adenosine triphosphatase which is stimulated by calmodulin was shown to be firmly associated with smooth erythrocyte plasma membranes devoid of spectrin and actin. Kinetic titration demonstrated that there are 4500 calmodulin binding sites per erythrocyte and that the turnover number of this calcium-activated adenosine triphosphatase is 3000 mumol of Pi . (mumol of site)-1 . min-1 which is similar to the turnover numbers of other transport adenosine triphosphatases. Furthermore, calmodulin stimulates calcium-activated adenosine triphosphatase by a simple enzyme-ligand association.