Effect of heat treatment on the activity of inhibitors of plasmin and plasminogen activators in milk

The effect of heat treatment on inhibitors of both plasminogen activator and plasmin was studied. Fractions of milk with inhibitory activities against plasminogen activator and plasmin were isolated. Different approaches were followed to study the thermal stability of the inhibitors. The inhibitors activities were measured after heat treating fractions isolated from raw milk; and after isolating fractions from pasteurized milk. Also plasmin activity was measured in heat-treated milk samples. Thermal inactivation of plasminogen activator inhibitor (81.1%) was considerably different from that of plasmin inhibitor (35.8%) in milk after heat treatment at 75°C for 15 s. The plasmin inhibitor in the isolated fractions was suggested to be α-2-antiplasmin, since it reacted immunochemically with polyclonal goat anti-human α-2-antiplasmin and competitively inhibited plasmin. Results showed that plasminogen activator inhibitor is less heat-stable than plasmin inhibitor, indicating that plasminogen activation could overcome any inhibition of plasmin resulting after milk pasteurization.