LOCALIZATION OF 2 INTERCHAIN DISULFIDE BRIDGES IN DIMERS OF BOVINE ALPHA-S2-CASEIN - PARALLEL AND ANTIPARALLEL ALIGNMENTS OF THE POLYPEPTIDE-CHAINS

Carboxymethylation of bovine skimmed milk with C-14-labelled iodoacetic acid followed by purification of the alpha(s2)-casein dimer showed that all four cysteine residues in the protein are engaged in disulfide linkages. Mass spectrometry and sequence analysis of cystine-containing tryptic peptides revealed the presence of two interchain disulfide bridges in the protein. Sequence analysis of disulfide-linked peptides resulting from an enzymatic cleavage between the bridges demonstrated that the individual chains in the dimers are either aligned in an antiparallel or a parallel orientation. The identity of some of the disulfide-linked peptides was further verified by performic acid oxidation followed by sequence analysis of the resulting peptides.