EFFECT OF PROTEOLYTIC ENZYMES ON BOVINE FACTOR V .I. KINETICS OF ACTIVATION AND INACTIVATION BY BOVINE THROMBIN

This study focuses on the interaction between bovine factor V, purified 5000-fold over the starting plasma and free of contaminating coagulation factors, and bovine thrombin free of other proteolytic enzymes. Thrombin increases the activity of factor V more than twofold, a process blocked by the thrombin inhibitor hirudin. The reaction follows pseudo-firstorder kinetics. Both the rate and extent of activation were proportional to thrombin concentration. The loss of factor V activity is 2.5 times more rapid following thrombin proteolysis than observed during incubation of native factor V. Acceleration of inactivation appears to be due to the formation of an unstable species rather than to further digestion since the decay is not blocked by hirudin. Thermal inactivation of factor V was unlikely since the activation energy of the decay process was 12,000 cal/mole. Both the rate and extent of the increase in activity induced by thrombin were inversely proportional to factor V concentration. This is attributed to substrate or product inhibition. Since factor V is necessary for the conversion of prothrombin into thrombin, these studies indicate both a positive and negative feedback mechanism which may help to regulate the rate and extent of blood coagulation. © 1969, American Chemical Society. All rights reserved.