Human IgE binding capacity of tryptic peptides from bovine alpha-lactalbumin

The specific IgE binding capacity of native bovine alpha-lactalbumin (alpha-La), a globular whey protein, and tryptic peptides was investigated using 19 sera from patients with cow's milk protein allergy. The specific anti-bovine alpha-La IgE titers ranged from 0.6 to 125 IU/ml. Highly purified tryptic peptides from native and disulfide-bond-reduced alpha-La were obtained by reverse phase chromatography. By ELISA technique using immobilized native protein or peptides, 11 of the 19 sera reacted exclusively with intact protein while 8 of them also presented a specific IgE response to different tryptic peptides. Polyclonal IgE population specificity was not related to anti-bovine alpha-La IgE levels. Sequence (17G-K58) and larger peptides sharing this sequence were most strongly and frequently recognized. Competitive ELISA inhibition tests confirmed this IgE-specific response and gave also clear evidence for IgE binding to smaller peptides corresponding to sequences (6C-R10):S-S(115L-L123) and (109A-L123). IgE binding to native alpha-La and large peptides confirmed the importance of conformational epitope(s). However, in some sera reduced and S-alkylated peptide (59I-K94) exhibited a similar or higher IgE binding capacity than the native corresponding fragment, suggesting the existence of sequential epitope(s) exposed through protein denaturation. Moreover, IgE binding sequences were also located within hydrophobic regions of alpha-La and/or within parts with high sequence homology to human alpha-La.