Molecular Biological Aspects of Fibrinolysis

The fibrinolytic enzyme system plays a role in the removal of fibrin from the blood vessels or urinary tract, and also in tissue repair (angiogenesis), cell transformation, macrophage function, ovulation and embryo implantation. Growing endothelial cells, malignant cells, macrophages, granulosa cells and trophoblast cells.produce plasminogen activators which activate plasminogen in the blood or interstitial fluids. Local plasmin formation appears to be a generalized mechanism involved in tissue destruction and repair. Fibrinolysis in the blood seems to be regulated by specific molecular interactions between (tissue) plasminogen activator, plasmin(ogen), fibrin and α2-antiplasmin, the physiological plasmin inhibitor. Plasmin(ogen) contains structures - lysine-binding sites - which mediate its interaction with fibrin and with α2-antiplasmin. When fibrin forms in plasma a small amount of plasminogen is bound via these structures. Plasminogen activator present or released in the blood is strongly adsorbed to the fibrin and activates bound plasminogen in situ. The formed plasmin, which remains transiently complexed to fibrin, both by its lysine-binding site(s) and active center, is only slowly inactivated by α2-antiplasmin, but plasmin which is released from digested fibrin is rapidly and irreversibly neutralized by α2-antiplasmin. Many in vitro and in vivo experiments support this hypothesis for the mechanism of fibrinolysis (Nature 272, 549,1978). Which allows speculation on more efficient therapeutic schemes for thrombolysis.