Thrombin activity

Jaffer, F. A., Tung, C. H., Gerszten, R. E. and Weissleder, R. (2002). In vivo imaging of thrombin activity in experimental thrombi with thrombin-sensitive near-infrared molecular probe. Arterioscler. Thromb. Vase. Biol. 22,1929-1935. 

Figure 17.2 The intrinsic and extrinsic pathways involved in blood clotting. Both pathways converge to activate thrombin. Solid arrows represent biochemical conversions whereas dotted arrows represent either catalytic or activating actions. Fibrin is formed as monomers which polymerise to form fibrils. Within the fibrils, the fibrin monomers associate laterally which is facilitated by active XIII (ie Xllla). Thrombin activates XIII.

Mechanism of action - Argatroban is a synthetic, direct thrombin inhibitor that reversibly binds to the thrombin active site. It inhibits thrombin-catalyzed or induced reactions, including fibrin formation activation of coagulation factors V, VIII, and XIII protein C and platelet aggregation. 

Fondaparinux does not inactivate thrombin (activated Factor II) and has no known effect on platelet function, fibrinolytic activity, or bleeding time. 

K. Muller, F.A. Diedrich, Fluorine interactions at the thrombin active site Protein backbone fragments H-C -C = 0 comprise a favorable C-F environment and interactions of C-F with electrophiles, ChemBioChem 5 (2004) 666-675. 

For the inhibition of AT Ill-mediated thrombin activity, derivatives of 8 were prepared containing a flexible spacer of around 50 atoms plus a sulfated maltooligosaccharide or another AT III binding pentasaccharide [35]. 

The effect with which PPT modulate the response of platelets is also pertinent to vascular disease, in particular, thrombosis. Resting platelets inhibit the respiratory burst of neutrophils whereas thrombin-activated platelets increase the respiratory burst. Quercetin and resveratrol at picomolar concentrations attenuate this response by preserving endothelial CD39/ATP-dase," " and on present evidence (see above) such concentrations might be achieved locally following deglucuronidation at a site of inflammation. 

The direct thrombin inhibitors (DTIs) exert their anticoagulant effect by directly binding to the active site of thrombin, thereby inhibiting thrombin s downstream effects. This is in contrast to indirect thrombin inhibitors such as heparin and LMWH (see above), which act through antithrombin. Hirudin and bivalirudin are bivalent DTIs in that they bind at both the catalytic or active site of thrombin as well as at a substrate recognition site. Argatroban and melagatran are small molecules that bind only at the thrombin active site. 

Iwanowicz EJ, Lau WF, Lin J, Roberts DGM, Seiler SM. Retro-binding tripeptide thrombin active inhibitors discovery, synthesis and molecular modeling. J Med Chem 1994 37 2122-2124. 

Stereo view of the three lowest energy minima of benzene obtained with the modified force field and the lowest energy minimum of benzamidine (thick lines for heavy atoms and thin lines for polar hydrogens) in the thrombin active site (thin lines). 

A third underlying mechanism seems to involve a reduction in concentrations of free protein S, again more pronounced with third-generation products. When protein S falls, the antifibrinolytic effect of the so-called thrombin-activated fibrinolysis inhibitor is increased in other words, fibrinolysis is impeded, with an increased risk of clotting problems (104). Again, however, these are recent methods, which were not available when the third-generation products were launched. 

Studies of plasma clots made under a variety of conditions suggest that the network is established early as a result of the activation process (Blomback et al., 1994) and that the thrombin activation pathway modulates clot structure (Torbet, 1995). Light scattering studies, combined with other physical chemical techniques, have substantiated this conclusion for purified fibrinogen and provided more information about the structure and properties of the assembling clot (Bernocco et al., 2000 Ferri et al., 2001, 2002 Profumo et al., 2003). 

Thrombin activable fibrinolysis inhibitor (TAFI) is a plasma protein that is activated by thrombin in the presence of thrombomodulin to a labile carboxypeptidase-B-like enzyme that inhibits fibrinolysis. When TAFIa is included in a clot undergoing lysis induced by tPA and plasminogen, the time to achieve lysis is prolonged and free lysine and arginine are released (Wang et al., 1998). TAFIa retards the fibrin-enhanced activation of plasminogen by tPA and inhibits the accumulation of plasminogen at the lysis front (Sakharov et al., 1997). 

Torbet, J. (1995). The thrombin activation pathway modulates the assembly, structure, and lysis of human plasma clots in vitro. Thromb. Haemost. 73, 785-792. 

Wang, W., Boffa, M. B., Bajzar, L., Walker, J. B., and Nesheim, M. E. (1998). A study of the mechanism of inhibition of fibrinolysis by activated thrombin-activable fibrinolysis inhibitor./. Biol. Chem. 273, 27176-27181. 

Thrombin activates platelets, converts fibrinogen to fibrin, activates factor XIII, which stabilizes fibrin, and activates factors V and VIE, which accelerate the generation of prothrombinase. Therefore, the inhibition of thrombin is essential in preventing and treating thromboembolic disorders. 

Immler, D. Gremm, D. Kirsch, D. Spengler, B. Presek, R Meyer, H. E. 1998. Identification of phosphorylated proteins from thrombin-activated human platelets isolated by two-dimensional gel electrophoresis by electrospray ionization-tandem mass spectrometry (ESI-MS/MS) and liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). Electrophoresis, 19,1015-1023. 

Pipe, S. W., Saenko, E. L., Eickhorst, A. N., Kemball-Cook, G. and Kaufman, R. J. (2001). Hemophilia A mutations associated with 1-stage/ 2-stage activity discrepancy disrupt protein-protein interactions within the triplicated A domains of thrombin-activated factor Villa. Blood 97, 685-691. 

Boffa MB, Hamill JD, Maret D, et al. Acute phase mediators modulate thrombin-activable fibrinolysis inhibitor (TAFI) gene expression in HepG2 cells. J Biol Chem 2003 278(1 l) 9250-9257. 

Current evidence suggests that thrombin activates human platelets by cleaving and activating PAR-1 and PAR-4 (20). Cleavage of human PAR-4 requires a higher concentration of thrombin than those for the cleavage of PAR-1. 

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