Thermal denaturation of mixtures of α-lactalbumin and β-lactoglobulin:: a differential scanning calorimetric study

The thermal denaturation of alpha-lactalbumin (alpha-lac), beta-lactoglobulin (beta-lg) and a mixture of the two proteins in the presence of several sugars, sodium salts and at various pH values was studied by differential scanning calorimetry. The effects of N-ethylmaleimide (NEM), cysteine, urea and sodium dodecyl sulfate (SDS) were also investigated. The temperature of denaturation (T-d) Of beta- Ig decreased from 71.9 degrees C in the absence of alpha-lac to 69.1 degrees C in its presence. In contrast, an increase of 2.5 degrees C was observed in the T-d Of ayo-alpha-lac when heated in the presence of beta-lg suggesting that alpha-lac was made more thermally stable in the presence of beta-lg. Glucose and galactose had the greatest effect in stabilizing the proteins against thermal denaturation with the effect being greater for beta-lg than for a-lac. A decrease in thermal stability of both proteins was observed in the presence of sodium bicarbonate; sodium ascorbate, however, had a stabilizing effect. Renaturation of alpha-lac was prevented in the presence of cysteine and NEM, but not in urea or SDS. Translucent gels were formed when the alpha-lac/beta-lg mixtures were heated in the presence of all five sugars and in the presence of cysteine, urea and SDS but not in NEM. This suggests that disulfide-sulfhydryl interchange reactions may be primarily responsible for the gelation of alpha-lac/beta-lg mixtures. (C) 2000 Elsevier Science Ltd. All rights reserved.