The Binding of Ca2⊕to Solubilized Band 3 Protein of the Human Erythrocyte Membrane

Abstract

The binding of 45Ca2+ to isolated band 3 protein, the anion transport protein of the human erythrocyte membrane, was studied by equilibrium dialysis. The protein was solubilized and purified by either the nonionic detergent Ammonyx-LO or acetic acid. Each preparation of band 3 protein showed a single high-affinity Ca2+ binding site and several Ca2+ binding sites of lower affinity. The association constant of the high-affinity site was 4-13 .times. 104M-1; it was only moderately dependent on ionic strength. Mg2+ effectively competed with Ca2+ for the site. Anion exchange across the human erythrocyte membrane is inhibited by micromolar concentrations of intracellular Ca2+. This inhibition apparently is due to the binding of the cation to a single site on band 3 protein.