The binding of deoxycholic acid to band 3 protein from human erythrocyte membranes and to bovine serum albumin

The binding of a water-soluble steroid, deoxycholic acid, to solubilized band 3 protein from erythrocyte membranes was studied by equilibrium dialysis. At acid pH, a single high affinity binding site with an association constant K = 4 . 105M-1 was found. In addition, the protein showed a large number of low affinity binding sites. High affinity binding of the steroid was not observed when the pH was made alkaline or when deoxycholic acid was substituted by cholic acid. The results support the suggestion previously derived from monolayer experiments that band 3 possesses a single cholesterol-binding site of high affinity and specificity (Klappauf, E. & Schubert, D. (1979) Hoppe-Seyler's Z. Physiol. Chem. 360, 1225-1235). Bovine serum albumin was also found to possess a single high affinity binding site for deoxycholic acid (K = 4 . 105M-1. In contrast to band 3, this site is observed both at acid and alkaline pH