RELATIVE STRUCTURAL STABILITIES OF BETA-LACTOGLOBULIN-A AND BETA-LACTOGLOBULIN-B AS DETERMINED BY PROTEOLYTIC SUSCEPTIBILITY AND DIFFERENTIAL SCANNING CALORIMETRY

beta-Lactoglobulin (beta-Lg) genetic variants A and B were purified fr om milks of individual cows that were homozygous for each variant, and their structural stabilities were examined by susceptibility to prote olysis by immobilized trypsin and by differential scanning calorimetry (DSC). A 15% increase in the value for V-M/K-M was observed with prot eolysis of beta-Lg A as compared with beta-Lg B. This observation is s ubstantiated by a more rapid disappearance of intact beta-Lg A during proteolysis of equimolar mixtures of the two variants. Lower structura l stability of variant A is also indicated by a 5 degrees C lower ther mal denaturation temperature observed for beta-Lg A by DSC. Thus, the two amino acid residue change differentiating variants A and B (D64G a nd V118A) is manifested in a less stable structure for the A variant.