Thermodynamic characterization of calcium-milk protein interaction by isothermal titration calorimetry.

摘要

In milk, casein micelles are the natural vectors of Ca and constitute an ideal vehicle for the delivery of additional Ca, through Ca-protein interactions. Isothermal titration calorimetry (ITC) is the technique of choice in the study of biomolecular interactions and especially mineral-protein interactions. Ca-milk-protein interactions were thermodynamically characterized by titration of milk protein with CaCl2 using ITC. Experiments similar to that of ITC were reproduced using granulometry and electrophoretic mobility measurement to better understand the nature of interactions. Binding titration curves were fitted with the 'one set of sites model' to determine the thermodynamic parameters (N, 6 mol Ca/mol milk protein; K, 1070 mol/l; DeltaH, 1910 cal/mol CaCl2; and DeltaS, 20.3 cal/mol/K). Milk proteins bound up to 8 mg Ca/g protein at saturation. The global thermodynamic signal obtained upon titration was endothermic. The electrophoretic mobility variations of casein micelles that occurred on CaCl2 titration were indicative of Ca-protein interactions of an electrostatic nature. Hence, the usual exothermic signal involved in electrostatic interactions was completely hidden by the strong endothermic signal coming from the release of water molecules, either from the hydration shell of the Ca2+ and/or dehydration of hydrophobic core of proteins. Particle size variations were indicative of casein micelle retraction upon CaCl2 titration.