Plasmin Activity in Milk

Abstract

Plasmin and plasminogen-derived activities were measured in bovine and human skim milk with a chromogenic tripeptide H-D-valyl-L-leucyl-L-lysine-p-nitroanilide substrate. One unit of enzyme activity was defined as the amount of plasmin or urokinase-activated plasminogen that produced a change of absorbance at 450nm of .001 in 1min at pH 7.4 and 37°C under the described reaction conditions. By this definition, about 4 to 5 units of plasmin were in 1ml of bovine milk at both early (2 to 3 mo) and late (7 to 8 mo) lactation. However, plasminogen-derived activity was 45.3±13.0 U/ml at the late state of lactation compared to only 26.3±2.7 U/ml at the early stage. Human milk obtained on days 6 to 7-postpartum contained 4.8±4.2 and 12.4±9.9 U/ml of plasmin and plasminogen-derived activity. Upon pasteurization of bovine milk at 72°C for 15s, both plasmin and plasminogen-derived activities decreased by about 10% whereas a commercial ultrahigh temperature sterilization destroyed plasmin activity to undetectable and plasminogen-derived activity by about 90%. By an assay based on fibrinolysis of iodine-125 labeled fibrin, a plasminogen-activator activity was associated with milk casein micelles whereas an inhibitor of a plasminogen-activator, plasmin, or both were localized in milk serum. Plasminogen-activator activity was diminished in the skim milk by about 80% compared to activity in the casein micelles, apparently due to naturally occurring inhibitors in milk serum. Quality of milk and dairy products may be influenced by amounts of proteolytic enzymes such as plasmin and factors affecting them.