Localization of two interchain disulfide bridges in dimers of bovine α_s2‐casein: Parallel and antiparallel alignments of the polypeptide chains

Carboxymethylation of bovine skimmed milk with ¹⁴C‐labelled iodoacetic acid followed by purification of the α_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.