It functions as an inhibitor of coagulation by inhibiting thrombin. Fibrin also binds and reduces the activity of thrombin. These include thrombin, tissue activating plasminogen and plasmin. It can bind to thrombin that decreases the fibrin formation. Examples include thrombin, trypsin, and human neutrophil elastase. Heparin increases the affinity of antithrombin to thrombin (as well as factor Xa). The activated AT then inactivates factor Xa, thrombin, and other coagulation factors. Hirudin is the most potent natural inhibitor of thrombin. This receptor is activated when the platelet is stimulated by ADP, epinephrine, collagen, or thrombin. Argatroban is an anticoagulant that is a small molecule direct thrombin inhibitor. This cleaves prothrombin at two sites, converting it to thrombin. Dabigatran is an oral direct thrombin inhibitor. The compound was found to be active against thrombin and neutrophil elastase. VI was the factor that accelerated the conversion from prothrombin to thrombin. It inhibits proteases like chymotrypsin, kallikrein, plasmin, thrombin, and trypsin.