Hydrolysis of Caprine β-Casein by Plasmin

The proteolytic activity of plasmin on soluble caprine β-casein (CN) was studied in 50 mM Tris·HCl buffer, pH 8.0, at 37°C. Electrophoretic studies showed that hydrolysis of this protein results in an electrophoretic pattern that is similar to the pattern obtained from plasmin hydrolysis of bovine β-CN (γ-CN and complementary N-terminal fragments), suggesting that plasmin probably attacks the same regions that are susceptible to cleavage in bovine β-CN. As determined by SDS-PAGE, the γ-like components of caprine milk consisted of two fragments with relative molecular mass of 9200 and two with relative molecular mass of 21,400 that could differ in the level of phosphorylation. Apparently, the high molecular mass components are homologous to bovine β-CN (f 29-209) (γ1-CN), and the low molecular mass components are homologous to bovine β-CN (f 106-209) and β-CN (f 108-209) (γ2- and γ3-CN). Complementary N-terminal fragments had values for molecular masses in the range 13,600 to 8500 and urea-PAGE patterns that were more complex than those obtained in bovine casein because of the different phosphorylation levels in caprine β-CN. These fragments were also present in the hydrolysate of whole caprine casein that had been treated with plasmin.