Peptides generated from milk proteins in the bovine mammary gland during involution.

Proteolytic activity in mammary gland secretions is associated with hydrolysis of secretory proteins during involution. Peptides generated from hydrolysis of milk proteins were characterized in secretions from the bovine mammary gland during involution to understand the fate of the milk proteins better. Mass spectral analysis of mammary secretions showed numerous peptides ranging between 0.7 and 14 kDa during mammary involution, but these peptides were not observed in normal milk. Mass spectral profiles representing discrete peptide fragments were similar on d 7, 14, and 21 of involution, suggesting that milk proteins were only partially hydrolyzed during involution. N-Terminal amino acid sequences of four peptides indicated that they were produced by hydrolysis of beta-casein during involution and probably resulted from plasmin hydrolysis. A 20-kDa peptide was identified as a fragment of a 39-kDa protein that was previously identified in bovine mammary secretions during involution. Mass spectral analysis of lactoferrin isolated from mammary secretions during involution showed major hydrolytic products. Immunoblot analysis confirmed that lactoferrin that was isolated from mammary secretions during involution contained a number of hydrolytic products. Intramammary hydrolysis of milk proteins by plasmin probably leads to the generation of the discrete peptides observed in the mammary secretions and contributes to the fate of these proteins during involution of the bovine mammary gland.