Investigations on the Inhibitors of the Fibrinolytic System

Summary Anti-plasmin and anti-urokinase activity of plasma was investigated with a method using preformed 131I-labelled clots. The inhibition of plasmin by plasma was found to be produced partly by direct and partly by slow progressive inactivation, while the results did not support the supposition that the anti-plasmin-plasmin complex is reversible in the presence of fibrin. It was found that thrombocytes contain very little anti-plasmin, and that heparin in a concentration as occurs during anticoagulant therapy does not influence the anti-plasmin nor the anti-urokinase activity of the plasma. The results provide evidence that urokinase and blood activator are not identical substances and it is concluded that estimation of the anti-urokinase activity will not produce an index for the presence of an inhibitor of the blood activator.