Observations on thrombin inactivation by human serum.

The inactivation of thrombin by serum follows a first order kinetics with thrombin concentrations of the order of 20 to 40 units per ml. of serum. With higher thrombin concentrations of 300 to 600 units per ml. of serum, the rate slows down and not the logarithm of the clotting time, but the clotting time itself increases proportionally with the incubation time. The antithrombic factor is stable at +4 degrees C. for a period of 2 weeks, or for 3 months at -15 degrees C. Heating over 58 degrees C. destroys it rapidly. The heat inactivation has an energy of activation of 121,000 cal. With low thrombin concentrations the rate of thrombin inactivation increases linearly with increasing serum concentration. The pH optimum of the inactivation is at 8.5. Increasing the temperature increases the rate. The energy of activation of the thrombin destruction by serum is 14,000 cal. Heparin increases the rate considerably with low thrombin concentrations, but does not affect the rate with high thrombin concentrations. The effect of heparin can be abolished with protamine or toluidine blue. Some other reagents were also tested with respect to their effect upon the rate of inactivation of thrombin by serum.