Multifunctional roles of thrombin

Thrombin is an unique molecule that functions both as a procoagulant and an ticoagulant. In its procoagulant role it activates platelets through its re ceptor on the platelets. It regulates its own generation by activating coag ulation factors V, VIII and even XI resulting in a burst of thrombin format ion. It activates factor XI, thus preventing fibrin clots from undergoing f ibrinolysis. Thrombin not only cleaves fibrinogen to fibrin, but also throu gh the activation of factor XIII effects the crosslinking of fibrin monomer s to produce a firm fibrin clot. Thrombin's role as an anticoagulant is med iated through binding to thrombomodulin, a receptor protein on the endothel ial membrane of the blood vessel, initiating a series of reactions that lea ds to fibrinolysis. Thrombin has chemotactic properties enabling it to exer t its effects during inflammation and vascular injury. It has a mitogenic e ffect stimulating growth Of mammalian cells, fibroblasts and macrophage-lik e tumor cell lines. It has also been implicated in brain development. A mol ecule with multifunctional roles such as thrombin has its activity in vivo modulated by only a few endogenous inhibitors.