The composition, structure and origin of proteose-peptone component 5 of bovine milk

Proteose-peptone component 5 has been isolated from bovine milk. Molecular weight values within the range 12000--13500 were obtained by sedimentation equilibrium, dodecylsulphate/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 and 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 favour of a proteolytic mechanism for the formation of these substances in vivo and in vitro.