When one unit of thrombin was added to recalcified diluted plasma, more thrombin activity was shown in the presence of heparin than in its absence, but no difference was shown after two hour incubation. When one unit of thrombin was added to diluted plasma without the addition of Ca++, no difference in thrombin activities was shown in the presence and absence of heparin. When highly purified α2macroglobulin (α2M) and antithrombin III (ATIII) were used, thrombin activity was initially enhanced in the presence of either ATIII or ATIII, and quick inactivation of thrombin by ATIII regardless of the presence of heparin was observed. Electrophoresis shows that migrating patterns of ATIII depended upon amounts of heparin added to plasma, and ATIII migrated more to the anode with larger amounts of heparin. Thrombin-ATIII complex formed quickly in the undiluted recalcified plasma in the presence of heparin, but little complex formation was shown in the absence of heparin. When α2M was mixed with thrombin, and the mixture was added to TLMe at intervals, hydrolysis of TLMe was enhanced initially, then decreased quickly. α2M-thrombin complex seemed to be not as effective as free thrombin in the capacity to hydrolyze TLMe in contrast to α2M-trypsin or α2M-plasmin complex. α2M may be a primary inhibitor of thrombin in the plasma in the absence of heparin. In the presence of heparin, ATIII seems to be a primary inhibitor of thrombin.
