Bile salts-bovine serum albumin binding: spectroscopic and thermodynamic studies

The binding of hydroxyl and keto bile salts to bovine serum albumin was studied by fluorescence and circular dichroism spectroscopies. It was found that the hydroxyl and keto bile salts produced a quenching of the native fluorescence emission of the protein at 350 nm. In the ligand-protein saturation conditions, cholanate-3-one, cholanate-3,6-dione and beta 5-cholanate produced a 100% fluorescence quenching, while hydroxy bile salts produced only a 50% quenching. This demonstrates that the two tryptophan residues of the protein are accessible to the keto bile salts, while only one tryptophan residue is accessible to the hydroxy parent compounds. Keto bile salts produced a change in the circular is related to a microrearrangement of the environment at the albumin-binding sites. All the tested bile salts produced quenching of the fluorescence probe, 1-aniline-8-naphthalene sulfonate, which is not covalently bound to the protein. This effect is due to an energy transfer between the tryptophan residues and the acceptor fluorescence probe. The binding of hydroxyl bile salts was associated with an endothermic process, while keto bile salts-albumin interaction was associated with a negative enthalpic change.