A 20,000-dalton casein fragment in human milk.

A new peptide of 20,000 daltons was found in human milk as a constituent of the casein micelle. Enzymic digestion with plasmin or trypsin revealed that the peptide was identical with a degradation product of human beta-casein. The amino acid composition of the degradation product and the previously reported sequence in the N-terminal region of human beta-casein suggested that the peptide was a fragment of beta-casein lacking the C-terminal region. The thermal sensitivity of this peptide was higher than that of beta-casein, but the peptide lost the property of calcium-dependent precipitation, which intact beta-casein possesses.