Proteolytic specificity of plasmin on bovine α_s1-casein

The proteolytic specificity of plasmin (fibrinolysin, E. C. 3.4.21.7, from bovine plasma) on bovine α_s1-casein was determined in solution in 50 mM ammonium bicarbonate buffer, pH 8.4, at 37°C. Peptides, isolated by reverse-phase high performance liquid chromatography on a C₁₈ column or by electroblotting from urea-polyacrylamide gel electrophoretograms, were identified from their amino acid sequence and mass. The principal plasmin cleavage sites were found at Arg₂₂-Phe₂₃, Arg₉₀-Tyr₉₁, Lysi₁₀₂-Lys₁₀₃, Lys₁₀₃-Tyr₁₀₄, Lys₁₀₅-Val₁₀₆, Lys₁₂₄-Glu₁₂₅and Arg₁₅₁-Gln₁₅₂. The initial cleavage sites and the order of production of small (pH 4.6-soluble) peptides suggest that α_s1-casein was cleaved initially towards the centre of the molecule.