Binding of Bilirubin to Low-Affinity Sites of Human Serum Albumin in Vitro Followed by Co-Crystallization

Abstract

Human serum albumin has three hydrophobic binding sites for bilirubin, one high affinity and two subordinate sites. At pH 7.4 the two molecules of bilirubin bound at the latter sites are unstable, since the equilibrium concentration of free bilirubin is higher than the solubility. Co-crystallization takes place, resulting in a suspension of micro-crystals containing about 200 molecules of bilirubin for each albumin. The fresh suspension shows a high optical rotation with a negative Cotton effect, indicating that the bilirubin molecules are arranged in an optically active pattern, the asymmetry originating from the albumin.