A proenzyme or zymogen is a protein precursor of an enzyme which can give, after activation, an active enzyme. Plasminogen (88 kDa) is the zymogen of plasmin, a key enzyme in the fibrinolysis system. Plasminogen is synthesized mainly by the liver but also the eosinophils, the kidney and the cornea. It exists in 2 molecular forms: glu-plasminogen (native form) and lys-plasminogen (more active form). The main pathways for activating plasminogen to plasmin involve t-PA and u-PA.