Thermal denaturation of mixtures of alpha-lactalbumin and beta-lactoglobulin: a differential scanning calorimetric study

The thermal denaturation of alpha-lactalbumin (alpha-lac), beta-lactoglobul in (beta-lg) and a mixture of the two proteins in the presence of several s ugars, sodium salts and at various pH values was studied by differential sc anning calorimetry. The effects of N-ethylmaleimide (NEM), cysteine, urea a nd 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 o f 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 stab le in the presence of beta-lg. Glucose and galactose had the greatest effec t in stabilizing the proteins against thermal denaturation with the effect being greater for beta-lg than for a-lac. A decrease in thermal stability o f 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. Tra nslucent gels were formed when the alpha-lac/beta-lg mixtures were heated i n 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 re actions may be primarily responsible for the gelation of alpha-lac/beta-lg mixtures. (C) 2000 Elsevier Science Ltd. All rights reserved.