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

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Abstract
Summary: Two distinct types of chymosin-sensitive polypeptide were found in the soluble fraction obtained when a solution of crude κ-casein in 0·4 m-NaCl was adjusted to pH 3. One corresponded to a segment of κ-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 κ-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 κ-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 κ-casein is a minor one.