Inactivation of bovine α- and β-thrombin by heat defibrinated human plasma and heparin was studied. It has been found that the inactivation rate of the two forms of thrombin were different, i.e. 80 percent of α-thrombin was inactivated in 3 minutes, whereas the same percent of β-thrombin activity was inhibited in 4 minute incubation with plasma. Heparin, however, changed extremely the sensitivity of α-thrombin to plasma inhibitor(s), while β-thrombin inactivation was only slightly affected. Namely, more than 90 percent of α-thrombin was inactivated by plasma in 1 minute incubation at a low heparin level of 0.0025 units per ml, whereas about 50 percent of β-thrombin activity was lost at a 10-fold higher heparin concentration.
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