Irreversible heat denaturation of bovine α-lactalbumin

SUMMARY The irreversible heat denaturation of α-lactalbumin (α-la) in 0·1 M-phosphate, pH 7·0, at 100 °C was studied using polyacrylamide-gel electrophoresis (PAGE). PAGE revealed two groups of bands, one moving faster than native α-la and one slower, in addition to some denatured protein which remained at the origin and some residual native α-la. The faster group had unchanged molecular weight, but an increase in charge, partly due to hydrolysis of glutamine and asparagine residues. The slower group was shown by two-dimensional sodium dodecyl sulphate-PAGE to be oligomers of denatured α-la; formation of the smaller oligomers preceded the larger ones. The oligomers reverted to monomers in the presence of dithiothreitol, showing that they were disulphide-linked aggregates of denatured α-la. Immuno-blots of the gels showed that both fast and slow groups of bands had irreversibly lost most of the antigenicity of the native protein.