HUMAN ALPHA(S1)-CASEIN - PURIFICATION AND CHARACTERIZATION

The human counterpart of alpha(s1)-casein has been purified by a combi nation of gel-filtration and ion-exchange chromatography under denatur ing conditions. SDS-PAGE analysis revealed the presence of a diffuse l adder with a high molecular mass which upon reduction was replaced by several closely spaced bands of lower molecular masses and a broad dif fuse band corresponding to K-casein. Amino acid sequence analysis of t he closely spaced bands all resulted in the same N-terminal sequence w hich was found to be homologous with alpha(s1)-casein from other speci es. Sequence analysis of a major radiolabelled tryptic peptide from pu rified C-14-carboxymethylated alpha(s1)-casein demonstrated that the p rotein contains at least two cysteine residues. As judged by SDS-PAGE in the presence or absence of a reducing agent, the molecular structur e of the polymers constituting the ladder is composed of heteropolymer s of alpha(s1)- and kappa-casein cross-linked by disulfide bonds.