Identification of IgE- and IgG-binding epitopes on αs1-casein: Differences in patients with persistent and transient cow’s milk allergy.(1)
Casein is the main component in cow’s milk, constituting 80% of the total protein. Cow’s milk casein is comprised of 4 proteins, αs1-, αs2-, β-, and κ-casein. These proteins have little primary structure homology. αs1-Casein is the most abundant protein in cow’s milk, constituting 34% of total milk proteins. It is a single-chain phosphoprotein of 199 amino acids (AAs) characterized by a high content of proline residues distributed throughout the molecule. αs1-Casein has nearly 70% unordered structure, with only a small amount of secondary structure, such as α-helix or β-sheets. In addition, it lacks disulfide bonds, resulting in a reduction of tertiary interactions.
Densitometric scanning indicated that the highest cumulative intensity of IgE binding was in regions 17-36, 69-78, 109-120, and 173-194, recognized by 75%, 46%, 53%, and 63% of the patients, respectively.
Regions 69-78 and 109-120 had AA sequence homology, both showing a sequence of X1EIVPNSX2EX3, where X1 was glutamic acid (E) versus leucine (L), X2 was valine (V) versus alanine (A), and X3 was glutamine (Q) versus glutamic acid.
2 IgE-binding regions (AA 69-78 and AA 173-194) were recognized by the majority of patients in the older age group (67% and 100%, respectively) but by none of the patients in the younger age group.
The most frequently recognized region was AA 15-36, which was recognized by 92% of patients. This region also had the highest cumulative intensity of IgG binding. Region 69-78, which was the IgE-binding epitope and distinguishes the older and the younger age group, was not found to be an IgG-binding epitope.
In contrast to IgE binding, no striking difference existed for IgG binding between the group of older patients with persistent CMA and the group of younger children likely to outgrow their allergy
The carboxyl terminal end of αs1-casein has been found to be the major allergenic epitope in mice. The absence of IgG binding to this peptide also could be due to the synthetic peptide, lacking a phosphorylated form. Region 64-75 is the multiphosphorylated segment of αs1-casein consisting of 5 phosphoseryl residues.