To study whether the phosphoserine residue is associated with the antigenicity of bovine α_s1-casein, we examined the antigenic reactivity of dephosphorylated α_s1-casein, peptide 1-25 from bovine β-casein and three chemical reagents with IgG antibody specific to native α_s1-casein by an enzyme-linked immunosorbent assay.
The reaction between native α_s1-casein and its IgG antibody was inhibited more strongly by native α_s1-casein than by dephosphorylated α_s1-casein. Peptide 1-25, having a phosphoserine residue-concentrated region from bovine β-casein, noticeably inhibited the reaction between native α_s1-casein and its antibody. Furthermore, the O-phospho-L-serine residue inhibited the reaction of peptide 61-123 with anti-native α_s1-casein antibody, although L-serine and sodium phosphate showed no measurable inhibition.
These results suggest that the phosphoserine residue associated with part of an antigenic site in bovine α_s1-casein.

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