Calorimetric investigation on the interaction of sodium taurodeoxycholate with human serum albumin

The interaction between sodium taurodeoxycholate and human serum albumin was investigated in aqueous phosphate buffer at pH 7.2 through ITC titrations at 298.15 K and 310.15 K and TM-DSC measurements. The analysis of the ITC data required the previous determination of the thermodynamic properties of the self-associated oligomeric species of the bile salt in the buffer solution, according to a literature chemical model. Use of a computer program which allows the simultaneous determination of the stability constants and formation enthalpies of complexes indicated that albumin forms a strong 1–1 (salt to albumin) complex at low bile salt concentration and at least one larger complex at higher concentrations. In these larger complexes the protein seems to cooperatively bind a number of moles of the bile salt which increases with temperature. Thermodynamic data are provided for the formation of the most probable species which include a 4–1 complex at 298.15 K and an additional 9–1 complex at 310.15 K. TM-DSC data, while evidencing the partially reversible nature of the folding–unfolding process and the stabilization of the protein by interaction with the bile salt, supported the stoichiometry of the complexes found by analysis of ITC data.