Primary structure of bovine beta casein. Linking of tryptic peptides and of peptides gained by the action of cyanide bromide

Abstract

In a previous communication [1] we have reported the isolation and composition of the tryptic and ‘CNBr’ peptides that account for the complete peptide chain of the bovine β‐casein A2. These peptides had been respectively numbered T1 to T14 and CN1 to CN7.The positioning of peptidde T1 at the amino terminus of the protein was elucidated by comparing the results of Edman degradations carried out on this peptide with those obtained by Kalan et al. [2] for the whole protein molecule. Peptide CN1, which gave peptide T1 by tryptic hydrolysis, accounts for the 92 amino‐terminal residues of β‐casein A2.Similarly peptide T4 is carboxyl terminal since it does not contain any lysyl or arginyl residue. Furthermore the amino acids liberated from this peptide by carboxypeptidase A are those obtained from the whole protein by Peterson et al. [3]. Peptide CN7, which has the same carboxyl terminal sequences, accounts for the 24 carboxyl terminal residues of the protein.The comparison of the fragments obtained by tryptic hydrolysis from the intact β‐casein A2 moelcule and from the ‘CNBr’ eptides have allowed to locate all these ‘CNBr’ peptides, and to assign the correct position of the tryptic fragments from CN2, CN4, CN5, CN6 and CN7, which contain two, one or none arginyl or lysyl residues.The alignment of the tryptic fragments from CN1, which contains six arginyl or lysyl residues, was elucidated by taking use of some previously obtained pptides resulting from incomplete tryptic clavages of the whole β‐casein A2 molecule.Finally the tryptic peptides from CN3, which contains three Lys or Arg residues, were positionned after analysis of fragments liberated from this ‘CNBr’ peptide by hydrolysis with thermolysin.The order of the tryptic and ‘CNBr’ peptides in the β‐casein A2 can then be written: H‐T1‐T9‐T11‐T3‐T2‐T10‐T6‐T12‐T7‐T14‐T5‐T8‐T13‐T4‐OH, and H‐CN1‐CN5‐CN6‐CN2‐CN4‐CN3‐CN7‐OH.