Characterization of Bile Salt/Cyclodextrin Interactions Using Isothermal Titration Calorimetry

The interactions of cholate, deoxycholate, glycocholate, and taurocholate with methyl-β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin were studied by means of isothermal titration calorimetry and molecular modeling. The binding constants, standard molar enthalpy, Gibbs free energy, and entropy changes were determined for the formation of bile salt/cyclodextrin inclusion complexes. We observed a 1:1 stoichiometry for all inclusion complexes and could demonstrate marked differences in binding affinity between the different bile salt and cyclodextrin molecules. The dihydroxy bile salt deoxycholate showed significantly higher affinity toward methyl-β-cyclodextrin (K = 6276 ± 164 M-1) and 2-hydroxypropyl-β-cyclodextrin (K = 4429 ± 34 M-1) compared to the trihydroxy bile salt cholate (K = 2693 ± 25 M-1 and K = 2510 ± 98 M-1, respectively). The conjugation of cholate with glycine or taurine lowered its affinity markedly toward methyl-β-cyclodextrin (K = 1958 ± 178 M-1 and K = 2148 ± 33 M-1, respectively). Our molecular modeling and docking data suggest that the most probable mode of binding would be by insertion of the bile salt A-ring into the rim of the cyclodextrin containing the secondary alcohol moieties. Our results show that bile salt binding to cyclodextrin is influenced both by the degree of bile salt hydroxylation and by bile salt conjugation.