ENERGETICS OF CYCLODEXTRIN-INDUCED DISSOCIATION OF INSULIN

The energetics of dissociation of bovine insulin oligomers in aqueous solution under various conditions have been investigated by dilution m icrocalorimetry. Addition of cyclodextrins increases dissociation of i nsulin oligomers in solution in a manner consistent with interaction o f these cyclic polysaccharides with protein side chains. For example, assuming monomer-dimer equilibrium, in the absence of cyclodextrins di lution data (25 degrees C, pH 2.5) are consistent with a dimer dissoci ation constant (K-diss) of about 12 mu M and a dimer dissociation enth alpy (Delta H-diss) of +41 kJ mol(-1). Addition of methyl-beta-cyclode xtrin (up to 200 mM) makes dissociation significantly more endothermic (Delta H-diss = 79 kJ mol(-1)) and reduces the apparent dimer dissoci ation constant by more than two orders of magnitude (K-diss approximat e to 1.7 mM). Qualitatively similar results are observed with alpha-cy clodextrin and other beta-cyclodextrin derivatives.