Proteolytic specificity of plasmin on bovine αs1‐Casein

Abstract

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, Lys₁₀₂‐Lys₁₀₃, Lys₁₀₃‐Tyr₁₀₄, Lys₁₀₅‐Val₁₀₆, Lys₁₂₄‐Glu₁₂₅ and 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.