Heat inactivation of native plasmin, plasminogen and plasminogen activators in bovine milk: a revisited study

Thermal inactivation, at temperatures between 60 degreesC and 140 degreesC, of native plasmin, plasminogen and plasminogen activators were studied in bovine milk using improved enzymatic assays. While measured heat inactivati on kinetic of plasmin and plasminogen were in line with previously reported values, plasminogen activators were, surprisingly, found to be as heat sen sitive as plasmin and plasminogen in a milk system containing proteins with free SH groups. Activation energies (Ea) for the heat denaturation of plas min, plasminogen and plasminogen activators were 29, 35 and 24 kJ.mol(-1), respectively, in the temperature range 95-140 degreesC, and 244, 230 and 24 1 kJ.mol(-1), respectively, in the temperature range 70-90 degreesC. The he at inactivation of the whole plasmin system in milk appeared to be directly influenced by the presence of beta -lactoglobulin. The rate of plasmin ina ctivation during long heat treatments decreased rapidly, probably because o f the disappearance of available beta -lactoglobulin for S-S linking.