Structural and functional differences between fetal and adult serum albumin

The binding of bilirubin with adult of fetal human serum albumin has been studied by steady-state fluorescence emission spectroscopy. The 1:1 complex between bilirubin and the two albumin samples shows very similar fluorescence properties, as well as essentially identical accessibility of the protein-bound bilirubin to fluorescence quenchers added to the aqueous medium. The intramolecular distance between bilirubin and the single tryptophyl residue can be estimated to be 2.4 +/- 0.2 nm for both proteins by singlet-singlet energy transfer. These findings suggest that fetal and adult human serum albumin have a very similar three-dimensional structure; the different binding capacity for bilirubin displayed by the two proteins is likely to be the consequence of small differences in the physico-chemical properties of some amino acid residues close to the bilirubin binding site, as indicated by pH-titration experiments of the intrinsic albumin fluorescence.