1. Fractions have been obtained from human whole casein closely resembling the alpha(s)- and kappa-fractions of cow casein. 2. The alpha(s)-fraction (human alpha(s)-casein) is calcium-sensitive, heterogeneous in zone analysis and inert towards rennin. 3. The kappa-fraction (human kappa-casein) is calcium-insensitive, heterogeneous in zone analysis, and forms a soluble glycopeptide when acted upon by rennin. 4. Human kappa-casein stabilizes human alpha(s)-casein in the presence of Ca(2+) ions. 5. The glycopeptides released by rennin from human casein and from cow casein have been compared. There are important differences in both the peptide and non-peptide structures of the two compounds. 6. In both human and bovine glycopeptides some of the carbohydrate residues are joined to the peptide by O-glycosidic links with threonine, and possibly with serine.

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