Proteolytic specificity of plasmin on bovine αs1‐Casein

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Abstract
The proteolytic specificity of plasmin (fibrinolysin, E. C. 3.4.21.7, from bovine plasma) on bovine αs1‐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 αs1‐casein was cleaved initially towards the centre of the molecule.