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 cycle dextrins the calorimetric dilution data (25 degrees C, pH 2.5) indicate a dimer dissociation constant (K-diss) of about 12 mu M and an endothermic dissociation enthalpy (Delta H-diss) of +41 kJ mol(-1). Addition of methyl-beta-cyclodextrin (up to 200 mM) makes dissociation significantly more endothermic (Delta H-diss = 79 kJ mol(-1)) and reduces the apparent dimer dissociation constant by more than two orders of magnitude (K(diss)approximate to 1.7 mM). Qualitatively similar results are observed with alpha-cyclodextrin and other beta- cyclodextrin derivatives. Cyclodextrin-induced insulin dissociation is also observed at pH 7.4.