Specificity of disulfide bond formation during thermal aggregation in solutions of beta-lactoglobulin B and kappa-casein A.
Heat-induced (90 degrees C, 10 min, pH 6.7) intermolecular disulfide bond formation in 1:1 mixtures of beta-lactoglobulin B (beta-Lg) and kappa-casein A (kappa-CN) was studied by enzymatic digestion with trypsin or glu-C, reverse-phase HPLC, and MALDI-TOF-MS. Observed masses were compared to theoretically calculated masses of disulfide-bonded peptide dimers and trimers, and the number of different masses matching peptide combinations involving each bond was used as a measure of confidence of identification. The beta-Lg cysteine residues 121 or 119 were involved in bonds with both cysteines of kappa-CN and all cysteines of beta-Lg. This agrees with the supposed initiatory role of beta-C121 in heat-induced SH/SS interchange. The largest numbers of matches corresponded to bonds linking beta-C119/C121 with kappa-C11 or with beta-C66. Multiple matches were recorded for beta-C119/C121 bonding with beta-C119/C121, with beta-C160, or with kappa-C88. However, beta-C106 was observed only in bonds with beta-C119/C121 and did not appear to bond to kappa-CN, suggesting it remains buried in the core of the protein.
Livney
Yoav D
YD
Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
Dalgleish
Douglas G
DG
D002364:Caseins / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D004220:Disulfides / Q000737:chemistry*; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry*; D012996:Solutions; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
D002364:Caseins; D004220:Disulfides; D007782:Lactoglobulins; D012996:Solutions