Physiological effects of the plasminolytic derivatives of fibrinogen

The macromolecular breakdown products of fibrinogen are known mainly for their inhibitory effect on the clotting of fibrinogen by thrombin. This inhibitory effect is due to interference with both the proteolytic action of thrombin and the polymerization of the fibrin monomers. However, the action of these products is not limited to these effects. They can on the one hand inhibit the consumption of Factors II and XIII and promote the inactivation of Factor VIII by thrombin. On the other hand, they can potentiate the activation of prothrombin in purified systems via the intrinsic pathway. The incidental observation was also made that Factor IXa and or Factor Xa inactivate Factor VIII. As substrates of both thrombin and plasmin the large fragments protect these two enzymes from spontaneous inactivation, while at the same time they inhibit their respective proteolytic activities. Contradictory results were obtained regarding their effect on platelets. The micromolecular (dialyzable) breakdown products prolong the thrombin, prothrombin, and partial thromboplastin times of plasma and retard the generation of the intrinsic prothrombin activator. They can also potentiate the effects of biologically active peptides and amines on the smooth muscles and on vascular permeability.