Plasmin Activity in Milk

Abstract

and plasminogen-derived activities were measured in bovine and human skim milk with a chromogenic tripeptide H-D-valyl-L-leucyl-L-lysine-pnitroanilide substrate. One unit of enzyme activity was defined as the amount of plasmin or urokinase-activated plasminogen that produced a change of absorbance at 450 nm of .001 in 1 min at pH 7.4 and 37C under the described reaction conditions. By this definition, about 4 to 5 units of plasmin were in 1 ml of bovine milk at both early (2 to 3 mo) and late (7 to 8 mo) lactation. However, plasminogen-dcrived 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 72C for 15 s, both plasmin and plasminogenderived activities decreased by about 10% whereas a commercial ultrahigh temperature sterilization destroyed plasmin activity to undetectable and plasminogenderived 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. Plasminogenactivator activity was diminished in the skim milk by about 80% compared to activity in the casein micelles, apparently due to naturally occurring inhibitors in Received June 14, 1982. 1 Laboratoire de Physiologie de la Lactation. milk serum. Quality of milk and dairy products may be influenced by amounts of proteolytic enzymes such as plasmin and factors affecting them.