The Heparin Inhibiting Property of Brain Thromboplastin

Antithrombin III is one of the serine proteinase inhibitors of the plasma which has been shown to specifically inhibit thrombin as well as Factor X. Heparin acts via antithrombin III, the heparin cofactor, hence it is difficult to explain the relative insensitivity of the prothrombin time to the presence of heparin in plasma as both thrombin, ana Factox X_a are associated functionally with the prothrombin time. This insensitivity becomes more obvious on appreciating the extreme sensitivity to heparin of the activated partial thromboplastin time as well as the thrombin time. This communication reports the demonstration of heparin inhibiting action of brain thromboplastin. The response of the prothrombin time to heparin under various conditions, and the effect of brain thromboplastin obtained from various sources and by different preparative techniques on the action of heparin in vitvo have been studied. The heparin inhibiting activity was shown to parallel the tissue factor activity. It is heat labile, non-dialysable, destroyed by detergent activity and lies in a high molecular weight fraction of the brain thromboplastin preparation (>300,000). In addition to explaining certain in vitro phenomena, these observations may explain the previously observed heparin resistance in the generalised Schwartzman phenomenon.