Identification of λ-Casein as Plasmin-Derived Fragments of Bovine αs1 -Casein

Crude h-casein, prepared by extraction of whole casein with N,N-di-methylformamide, produced an electrophoretic pattern at pH 9.6 in 4 M urea containing numerous bands with mobilities identical to peptides formed during incubation of %l-casein with plasmin at 37C for 10 min. Peptides from two electrophoretic bands were extracted from X-casein and were radioiodinated as well as two peptides with identical electrophoretic mobilities from the plasmin digest of %lcasein. Autoradiograms of tryptic peptide maps from the two peptides extracted from X-casein matched peptide maps generated by the corresponding fragments of ~l-casein produced by incubation with plasmin. Sodium dodecyl sulfate gel electrophoretic patterns obtained for the two peptides extracted from X-casein were also identical with corresponding peptides extracted after plasmin digestion of 0tst-casein. Molecular weights of 5,500 and 6,000 were obtained for the two peptides. The ?~-casein fraction consists predominantly of fragments of %l-caseins which can be generated in vitro by incubation with bovine plasmin.