The central fibrinolytic enzyme(serine protease) is plasmin. It is released enzymatically from plasminogen.
It has been shown that contact can trigger a considerable fibrinolytic process in cell-free plasma, so that an intrinsic and an extrinsic pathway is also assumed in fibrinolysis.
Plasmin is an enzyme with a broad proteolytic activity similar to that of trypsin. After its activation from the plasminogen it is present either as bound plasmin in fibrin or as free plasmin in plasma. The free plasmin present in the plasma is normally quickly destroyed by the antiplasmins. It develops its effect after it is bound with high affinity to the lysine binding site of fibrin. Here it is not affected by antiplasmines in the plasma. Plasmin essentially cleaves arginine and lysine bonds, even outside the fibrinogen.
The main regulation of fibrinolysis is achieved by the concentration of available plasminogen activators.