Effect of Bovine Plasmin on αs1 -B and κ-A Caseins

Both alpha-S1- and kappa-caseins were incubated at 37 C in the presence of bovine plasmin (.28 mg/ml) prepared from fresh blood plasma. The electrophoretic pattern of kappa-casein A was unchanged following 60-min incubation with plasmin. However, the electrophoretic band corresponding to alpha-S1-casein B gradually disappeared during the initial 30-min incubation with plasmin. Proteolysis was accompanied by the formation of one polypeptide band with electrophoretic mobility slightly slower than alpha-S1-casein B and several bands with faster electrophoretic mobilities. Two of the faster electrophoretic bands contained phosphorus. Estimates of molecular weights were 20,500, 12,300, and 10,300 daltons for three of these early degradation products of alpha-S1-casein B by plasmin.