General Concept and Key Participants
Fibrinolysis is a critical mechanism for restoring vascular patency after a thrombus has fulfilled its hemostatic function. At the center of this biochemical cascade is the conversion of a zymogen into its active form.
Initially, the inactive protein plasminogen circulates in the blood plasma. To initiate clot lysis, it must be transformed into plasmin. This reaction is driven by specific substances: plasma and tissue activators. Once formed, plasmin directly targets the thrombus (networks of insoluble fibrin), cleaving it into degradation products known as fibrin degradation products.
Phase I: Formation of Activators
Fibrinolysis proceeds in three stages. The first phase involves the production and activation of plasminogen stimulators. Notably, the physiological trigger for this phase is the coagulation process itself.
There are three main activation pathways:
- Extrinsic pathway. Represented by tissue activators produced by vascular endothelial cells. A classic example is urokinase (uPA), an activator synthesized by the epithelium of the urinary tract.
- Intrinsic pathway. Based on plasma protein components. The primary stimulator here is activated factor XII (Hageman factor).
- Cellular pathway. Includes activators produced directly by blood cells: erythrocytes, leukocytes, and platelets.
Phase II: Transformation into Plasmin
The second phase involves the key biochemical reaction: the conversion of plasminogen into active plasmin under the influence of accumulated activators.
The mechanism is strictly regulated:
- Activated factor XII (XIIa) acts on prekallikrein, releasing the enzyme kallikrein.
- Kallikrein, working alongside kininogen, catalyzes the conversion of plasminogen to plasmin.
A crucial feature of this phase is its localization. Plasminogen adsorbs onto fibrin strands. Consequently, the entire cascade of plasmin generation unfolds not diffusely in the bloodstream, but directly within the fibrin clot itself.
Phase III: Lysis of the Thrombus
The final stage involves the direct destruction of the thrombus. Under the influence of active plasmin, peptide bonds within the insoluble fibrin molecules are cleaved. As a result, the thrombus breaks down into soluble fragments—individual peptides and amino acids.
In addition to cleaving fibrin, plasmin exhibits broad proteolytic activity. It can also degrade:
- Fibrinogen
- Factor V
- Factor VIII
This combined action ensures both the removal of existing clots and the prevention of excessive new thrombus formation in the affected vascular segment.