Genetic variants of bovine β- and κ-casein result in different immunoglobulin E-binding epitopes after in vitro gastrointestinal digestion

Immunoglobulin E-mediated allergy to cow milk is a common allergy in industrialized countries, mainly affecting young children and infants. β-Casein (CN) and κ-CN belong to the major allergens in cow milk. Within these milk proteins, genetic polymorphisms occur, which are characterized by substitutions or deletions of AA, resulting in different variants for each protein. Until now, these variants have not been considered when discussing the allergenic potential of bovine milk. In this study, the focus was placed on the arising peptide pattern after in vitro gastrointestinal digestion of several β- and κ-CN variants to determine resistant fragments containing IgE-binding epitopes and to identify potential differences between these variants. β-Casein A1, A2, and B, as well as κ-CN A, B, and E, were separated and isolated from milk of cows homozygous for these variants and digested with an in vitro gastrointestinal digestion model. The resulting peptides were identified using mass spectrometry and compared with previously determined epitopes. Seven β-CN and 4 κ-CN peptides, common in all β- or κ-CN variants, remained of sufficient size to harbor IgE-binding epitopes. In addition, some peptides and, consequently, epitopes differ from each other due to the AA substitution occurring in the individual variants. The distinct peptides AA 108 to 129 of β-CN A1 and A2, AA 103 to 123 of β-CN B, as well as AA 59 to 72, AA 59 to 80, and AA 58 to 80 of all 3 β-CN variants correspond to the IgE-binding epitopes AA 107 to 120 and AA 55 to 70, respectively. In κ-CN, the 2 variant-specific peptides AA 136 to 149 (κ-CN A, E) and AA 134 to 150 (κ-CN B) are congruent with the IgE-binding epitope AA 137 to 148. The present study shows that genetic polymorphisms affected the arising peptide pattern of the caseins and thus modifications in the IgE-binding epitopes occurred. As a consequence, the casein variants could show differences in their allergenicity. Studies investigating the allergenic potential of these different peptides are currently in progress.