ANTITHROMBIN AS PROTEINASE INHIBITOR *

During the short time it takes for blood to clot, there are substances that become activated and appear in the clotting mixture and possess powerful procoagulant properties. Subsequently the serum forms, and it contains practically none of these in an active form. When the procoagulants are in their precursor form in the platelets, plasma or tissues, they are not neutralized by the inhibitors found in plasma or serum. The inhibitor mechanisms which operate are very important feedback systems which tend to make clotting a local phenomenon. Certainly, it can also happen that the whole plasma medium becomes involved in what is known as disseminated intravascular coagulation, but that requires the participation of fluid dynamics and special conditions. In disseminated intravascular coagulation, the procoagulants are simultaneously in the process of being formed, neutralized, and distributed in the bloodstream. Whether the procoagulant arise from platelets, plasma or tissues, there is a mechanism for its inactivation. TO a certain extent, each mechanism is understood; however, I shall limit my discussion to thrombin and autoprothrombin c, each one of which arises from prothrombin and becomes neutralized by antithrombin.