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Effect of Sodium and Calcium Addition on Thermal Denaturation of Apo--Lactalbumin: a Differential Scanning Calorimetric Study

Ecole Nationale Supérieure des Industries Agro-Alimentaires, Laboratoire de Biophysique du Département Science de l'Aliment, 1, Avenue des Olympiades, 91306 Massy, France

Changes of thermodynamic parameters related to thermal denaturation (5 to 100°C) of commercial -lactalbumin in Ca⁺⁺-depleted form have been determined by differential scanning calorimetry. The thermograms obtained with -lactalbumin dissolved in distilled water show two peaks corresponding to Ca⁺⁺-free (apo--lactalbumin) and bound (holo--lactalbumin) forms, respectively. Because the commercial Ca⁺⁺-free sample contains both forms, the enthalpy change corresponding to pure apo--lactalbumin is calculated from the increase in peak area versus added Ca⁺⁺. The transition temperature and the enthalpy change of the apo--lactalbumin form are also dependent on sodium concentration. The heat capacity change following denaturation is calculated from the linear variation of the apparent enthalpy change versus the transition temperature of the apo--lactalbumin form to which various sodium concentrations had been added. Addition of Na₂-EDTA to the commercial holo--lactalbumin form lowers the apparent enthalpy change, and a glasslike transition is observed during a second heating run.

The calculated thermodynamic parameters are of the same order of magnitude as most published data obtained by different methods (circular dichroism, fluorescence, and nuclear magnetic resonance) at much lower concentrations.