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

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Abstract
Crude .lambda.-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 .alpha.s1-casein with plasmin at 37.degree. C for 10 min. Peptides from 2 electrophoretic bands were extracted from .lambda.-casein and were radioiodinated as well as 2 peptides with identical electrophoretic mobilities from the plasmin digest of .alpha.s1-casein. Autoradiograms of tryptic peptide maps from the 2 peptides extracted from .lambda.-casein matched peptide maps generated by the corresponding fragments of .alpha.s1-casein produced by incubation with plasmin. Sodium dodecyl sulfate gel electrophoretic patterns obtained for the 2 peptides extracted from .lambda.-casein were also identical with corresponding peptides extracted after plasmin digestion of .alpha.s1-casein. MW of 5500 and 6000 were obtained for the 2 peptides. The .lambda.-casein fraction consists predominantly of fragments of .alpha.s1-caseins which can be generated in vitro by incubation with bovine plasmin.