The Composition, Structure and Origin of Proteose-peptone Component 5 of Bovine Milk

Abstract

Proteose-peptone component 5 was isolated from bovine milk. MW values within the range 12000-13500 were obtained by sedimentation equilibrium, dodecylsulfate/polyacrylamide gel electrophoresis and gel filtration in urea-containing buffers. A dansylation procedure showed that the sequence Arg-Glu occupied the N-terminal position while hydrazinolysis revealed C-terminal lysine. The latter was confirmed by experiments with carboxypeptidases B anc C which indicated that a mixture of molecules was present, about 80% of which had a C-terminal sequence -(Ala-Met)-Ala-Pro-Lys while about 20% had an additional -His-Lys in the terminal position. These results, together with data on the overall composition, showed that this component of the proteose-peptone fraction of milk corresponded to a mixture of molecules representing residues 1-105 and 1-107 of the .beta.-casein molecule, a finding that was confirmed by peptide mapping. This demonstration that proteose-peptone components correspond to the N-terminal portions of the .beta.-casein molecule while the .gamma.-caseins represent the matching C-terminal portions provides strong evidence in favor of a proteolytic mechanism for the formation of these substances in vivo and in vitro.