Heat denaturation of human orosomucoid in water/methanol mixtures

Heat denaturation of orosomucoid in solutions of methanol concentrations ranging from 0 to 70% (v/v) has been studied by using circular dichroism, intrinsic protein fluorescence and thermal difference absorption spectroscopy. Regardless of its high saccharide content (40%), the highly cooperative denaturation transition of orosomucoid is fully reversible in neutral water solution. A two-state model has been successfully applied; the numerical analysis results in thermodynamical parameter values that are in close agreement with previously reported experimental data from calorimetric measurements. However, in solutions containing even minute concentrations of methanol (5%) the heat denaturation is irreversible. After cooling of the denatured protein the refolded molecules exhibit a higher alpha-helical content than the native one. Possibilities of methanol interaction with native and denatured protein molecule are discussed.