Proteolysis of milk proteins during involution of the bovine mammary gland

The role of proteolytic hydrolysis of milk proteins in the mammary gland during involution is unknown. The objectives of this study were to determine the activities of plasmin, plasminogen, and plasminogen activator in mammary gland secretions collected during involution and to identify peptides generated by proteolysis of casein and lactoferrin in those secretions. Mammary secretions were collected from Holstein cows on d 7, 14, and 21 of involution and on d 7 postcalving. Activities of plasmin, plasminogen, and plasminogen activator were determined on the defatted, filtered aqueous phase of mammary secretions. Activities of plasmin, plasminogen, and plasminogen activator were significantly higher on d 7, 14, and 21 of involution than were those on d 7 postcalving. Protein fragments resulting from hydrolysis were detected by SDS-PAGE in samples collected on d 7, 14, and 21 of involution, but few protein fragments were observed in samples collected on d 7 postcalving when plasmin activity was low. Immunoblot analysis showed that a number of peptides observed during involution were generated from alpha(s)-casein (CN), beta-CN, kappa-CN, or lactoferrin, The appearance of peptides from proteins of mammary secretions during early involution was generally correlated with increased plasmin activity. Elevated plasmin activity during mammary involution may be primarily responsible for the observed concurrent hydrolysis of milk proteins in mammary secretions.