A circular dichroism (CD) study of the consecutive binding of serum albumin to bilirubin. Possible implications for the bilirubin level

Abstract

Binding studies on bovine serum albumin (BSA) with bilirubin (BR) have been performed under physiological conditions using circular dichroism (CD) and electronic absorption spectroscopy (UV–VIS). Analysis of data obtained from solutions at varying BSA/BR ratios (R), overall concentrations and pH suggests the coexistence of two similarly strong complexes with 1∶1 stoichiometry (K_ass,Mca. 1 × 10⁷ dm³ mol^–1). Further binding to BSA affords associates for which a 2∶1 stoichiometry (K_ass,Bca. 5 × 10⁴ dm³ mol^–1, pH = 7.4) is proposed. Results indicate that these aggregates comprise the main transport form of BR under physiological conditions. The role of the differently compounded BSA–BR complexes for controlling the concentration of unbound BR in the plasma is discussed. Preliminary experiments performed on human serum albumin (HSA) are in line with an analogous complexing behaviour.