A comparison between the amino acid compositions of 2 chymosin-sensitive polypeptides and C-terminal segments of kappa-casein.

Two distinct types of chymosin-sensitive polypeptide were found in the soluble fraction obtained when a solution of crude kappa-casein in 0-4 M-NaCl was adjusted to pH 3. One corresponded to a segment of kappa-casein comprising residues 94-169. The other was very similar to the section Gln77-Val169 in the protein, but contained a phosphate group plus isoleucine and glycine in the region between the N-terminus and the chymosin-sensitive bond. If these polypeptides form as a result of a specific cleavage of kappa-casein, the data indicate that there is a variant of this protein with a number of amino acid replacements on the N-terminal side of the chymosin-sensitive bond. The shorter polypeptide and kappa-casein are both rapidly cleaved by chymosin at pH 6-7 which suggests that the contribution of residues 1-93 to the sensitivity of the Phe-Met bond in kappa-casein is a minor one.