THERMODYNAMIC PARAMETERS FOR 3-STATE THERMAL-DENATURATION OF HUMAN AND BOVINE ALPHA-LACTALBUMIN

Thermodynamic parameters were determined for the thermal denaturation of Ca2+-bound (holo) and Ca2+-free (apo) alpha-lactalbumin from human and bovine milk. Thermal denaturation profiles were determined from ch anges in the intrinsic fluorescence emission intensity (FI) as a funct ion of temperature(T). Human apo alpha-lactalbumin was heat-denatured in a 2-state process with T-m = 25 degrees C, Delta H = 167 kJ mol(-1) , Delta S = 7700 J mol(-1) K-1 and Delta C-p = 15400 J mol(-1) K-1. Th e corresponding values for bovine apo alpha-lactalbumin were: T-m= 20 degrees C, Delta H = 180 kJ mol(-1),Delta S = 9000 J mol(-1) K-1 and D elta C-p = 5100 J mol(-1) K-1. Derivative plots of d(FI)/d(T) versus T revealed that both human and bovine hole cr-lactalbumin were heat-den atured via a 3-state process. Thermal denaturation transitions were as sociated with a T-m value of 67 degrees C or 42 degrees C, based on ch anges in tryptophan or tyrosine FI results, respectively. Apparently C a2+-bound alpha-lactalbumin possesses two regions (domains) with signi ficantly different conformational stability. Based on tryptophan fluor escence measurements, Delta H = 330 kJ mol(-1), Delta S = 4600 J mol(- 1) K-1 and Delta C-p = 8200 J mol(-1) K-1 for human or bovine, hole a- lactalbumin. From tyrosine fluorescence emission changes, Delta H = 54 -103 kJ mol(-1), Delta S = 300-2000 J mol(-1) K-1 and Delta C-p = 3000 -4000 J mol(-1) K-1.