Energetics of cyclodextrin-induced dissociation of insulin

Abstract

The energetics of dissociation of bovine insulin in aqueous solution have been investigated by sensitive dilution microcalorimetry. Cyclodextrins increase dissociation of insulin oligomers in a manner consistent with their interaction with protein side chains. For example, assuming monomer-dimer equilibrium, in the absence of cyclo-dextrins the calorimetric dilution data (25 °C, pH 2.5) indicate a dimer dissociation constant (K_diss) of about 12 µM and an endothermic dissociation enthalpy (ΔH_diss) of +41 kJ mol⁻¹. Addition of methyl-β-cyclodextrin (up to 200 mm) makes dissociation significantly more endothermic (ΔH_diss = 79 kJ mol⁻¹) and reduces the apparent dimer dissociation constant by more than two orders of magnitude (K_diss ≈ 1.7 mm). Qualitatively similar results are observed with α-cyclodextrin and other β-cyclodextrin derivatives. Cyclodextrin-induced insulin dissociation is also observed at pH 7.4.