Thermal denaturation of mixtures of alpha-lactalbumin and beta-lactoglobulin: a differential scanning calorimetric study
2000
Boye, J.I. | Alli, I.
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 (Td) Of beta-lg 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 Td of apo-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 alpha-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.
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