Plasmin and plasminogen in bovine milk: a relationship with involution?

Area of Science:

• Dairy Science
• Biochemistry
• Animal Health

Background:

• Somatic cell count (SCC) is a key indicator of bovine udder health.
• Plasmin and plasminogen are enzymes present in milk with roles in casein proteolysis.
• Understanding factors affecting these enzymes is crucial for milk quality and processing.

Purpose of the Study:

• To investigate the relationship between somatic cell counts (SCC) and milk plasmin/plasminogen levels.
• To explore the influence of lactation stage on plasminogen activation in milk.

Main Methods:

• Collected 774 milk samples from 66 Holstein cows over a lactation period.
• Analyzed samples for plasmin, plasminogen, serum albumin, and SCC.
• Correlated enzyme concentrations and ratios with SCC and lactation stage.

Main Results:

• Increased SCC (from <250,000 to >1,000,000 cells/ml) significantly elevated plasmin (105%), plasminogen (74%), and serum albumin (140%).
• Elevated levels of these components suggest blood influx due to mastitis-induced epithelial damage.
• Plasminogen to plasmin ratio decreased with higher SCC, supporting direct plasmin influx during mastitis.
• Plasmin and plasminogen increased towards the dry period, with a lower plasminogen/plasmin ratio indicating activation during late lactation, independent of epithelial damage.

Conclusions:

• Two primary mechanisms elevate milk plasmin: blood plasmin influx during mastitis and increased plasminogen activation during late lactation.
• Mastitis significantly increases plasmin, plasminogen, and serum albumin in milk.
• Lactation stage influences plasminogen activation, impacting enzyme activity even without compromised mammary epithelium.