Comparative study of methods for the isolation and purification of bovine κ-casein and its hydrolysis by chymosin

Summary

κ-Casein was purified from a single batch of whole acid casein (κ-A variant) using different methods in order to compare their merits in producing a purified material with a carbohydrate and phosphate heterogeneity representative of the whole κ-casein complement in milk. Ion-exchange methods of purification gave products of higher purity than precipitation techniques involving final purification by ethanol fractionation, but all methods resulted in κ-caseins of apparently similar heterogeneity and chemical composition. The purified κ-caseins were hydrolysed with chymosin and the derived macropeptides isolated. These were all virtually identical as determined by reversed-phase chromatography and gel electrophoresis. Some observations on chymosin hydrolysis of κ-casein were made. In addition to formation of the major para-κ-casein (Glu¹–Phe¹⁰⁵) and macropeptide (Met¹⁰⁶–Val¹⁶⁹), chymosin hydrolysis at pH 6·6 also resulted in two minor para-κ-caseins with N-termini corresponding to Phe¹⁸ and Ser³³ of κ-casein. At pH 5·5 and 4·5 para-κ-casein was rapidly hydrolysed into at least six fragments, one of which had an N-terminus corresponding to Trp⁷⁶ of κ-casein. At pH 6·6, 5·5 and 4·5 the κ-casein macropeptide was stable to chymosin, but at pH 2·3 it was hydrolysed by chymosin into fragments with N-termini corresponding to Met¹⁰⁶, He¹²⁵, Ala¹³⁸, Val¹³⁹, Thr¹⁴⁵ and Glu¹⁴⁷ of κ-casein.