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

Abstract

Proteose‐péptone component 5 has ben 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 β‐casein molecule, a finding that was confirmed by peptide mapping. This demonstration that proteose‐peptone components correspond to the N‐terminal portions of the β‐casein molecule while the γ‐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. Copyright © 1978, Wiley Blackwell. All rights reserved