Suitability of animals' purified milk caseins and their subunit kappa-caseins as substrates for subtilisin and trypsin.

Acid casein and kappa-casein were purified from different species of animal's milk, such as cow, sheep, goat, and water buffalo. These caseins were used as substrates for commercially available subtilisin and trypsin. It was established that, when acid caseins were used as a substrate for subtilisin, cow acid casein was found to be a better substrate for the enzymes, compared to other animals' milk casein. It was suggested that this acid casein has significantly more aromatic amino acids, as compared to arginine and lysine. K(M) and Vmax values, which were obtained for cow kappa-casein, showed that cow kappa-casein was a better susbstrate for trypsin than the others, suggesting that cow kappa-casein has a rich content of lysine, arginine, and aromatic amino acids by comparison with the others. The calculated C/N ratio also supports this suggestion.