Energetics of cyclodextrin-induced dissociation of insulin
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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 (Kdiss) of about 12 µM and an endothermic dissociation enthalpy (ΔHdiss) of +41 kJ mol−1. Addition of methyl-β-cyclodextrin (up to 200 mm) makes dissociation significantly more endothermic (ΔHdiss = 79 kJ mol−1) and reduces the apparent dimer dissociation constant by more than two orders of magnitude (Kdiss ≈ 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.
Key words
Cyclodextrin Insulin Dimer Dissociation Microcalorimetry ThermodynamicsPreview
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