Thrombin physiology

Endothelial cells are the major source of ET-1-synthesis. ET-1 is also produced by astrocytes, neurons, hepatocytes, bronchial epithelial cells, renal epithelial and mesangial cells. Physiological stimuli of ET-1-synthesis in endothelial cells are angiotensin II, catecholamines, thrombin, growth factors, insulin, hypoxia and shear stress. Inhibitors of ET-1 synthesis are atrial natriuretic peptide, prostaglandin E2 and prostacyclin. ET-2 is mainly synthesized in kidney, intestine, myocardium and placenta and ET-3 is predominantely produced by neurons, astrocytes and renal epithelial cells. 

Four naturally occurring thrombin inhibitors exist in normal plasma. The most important is antithrombin III (often called simply antithrombin), which contributes approximately 75% of the antithrombin activity. Antithrombin III can also inhibit the activities of factors IXa, Xa, XIa, Xlla, and Vila complexed with tissue factor. a2-Macroglobulin contributes most of the remainder of the antithrombin activity, with heparin cofactor II and aj-antitrypsin acting as minor inhibitors under physiologic conditions. 

SIN-1 spontaneously releases NO and superoxide under physiological conditions thereby stimulating cGMP production. SIN-1 significantly decreased expression of P-selectin and both total and activated GP Ilb/IIIa and also promoted reversal of activated GP Ilb/IIIa complex in platelets stimulated with thrombin [61]. However, in rats SIN-1 could only partially reduce the degree of platelet activation [62]. SIN-1 stimulated VASP Ser157 phosphorylation and inhibited GP Ilb/IIIa activation. Threshold... 

Platelets can be activated by a variety of agents including the physiologic agonists ADP, thromboxane A2, epinephrine, collagen, and thrombin. Platelet activation is generally associated with a change in platelet shape (except for epinephrine-induced platelet activation) from discs to spiny spheres with pseudopodia. Platelet pseudopod formation is dependent on actin polymerization in the activated platelets. The interaction of actin filaments with myosin, mediated by calcium (9), facilitates platelet contractile activity (e.g., clot retraction). 

Micromedex, lepirudin directly inhibits all actions of thrombin. It inhibits free and clot-bound thrombin without requiring endogenous cofactors. Lepirudin is not inhibited by platelet factor 4 and acts independently of antithrombin III and heparin cofactor II. It has no direct effect on platelet function, except inhibition of thrombin-induced platelet activation. No physiological inhibitor of lepirudin is known. 

Schematic representation of Subsite Utilization in Thrombin Complexes (after Reference 8). Fibrinogen interacts with three thrombin subsites (here thrombin is represented by a large oval and the interconnected subsites by an irregular three-armed shape). Physiological effectors of thrombin and thrombin inhibitors form distinct interactions at these subsites. Additional subsites, such as the heparin-binding site, exist on the thrombin surface and are not indicated here.

It is noteworthy that a qualitatively similar dependence was observed for heparin in solution116>. The complexes of immobilized heparin with fibrinogen and thrombin are rather stable (the corresponding association constants for the complexes with fibrinogen and thrombin are (1.4 0.5) x 105 M and (8 2) x 10s A/-1) and are not subjected to dissociation in physiological solution. 

Profumo, A., Turci, M., Damonte, G., Ferri, F., Magatti, D., Cardinali, B., Cuniberti, C., and Rocco, M. (2003). Kinetics of fibrinopeptide release by thrombin as a function of CaC12 concentration Different susceptibility of FPA and FPB and evidence for a fibrinogen isoform-specific effect at physiological Ca2+ concentration. Biochem. 42, 12335-12348. 

The active site structure of trypsin-like enzymes is considered to be very similar to that of bovine trypsin, yet little is known about them. Refinement of these structures is important also for the purpose of designing physiologically active substances. With a view to comparing the spatial requirements of active sites of these enzymes, dissociation constants of the acyl enzyme-ligand complex, K-, which were defined before, were successfully analyzed By taking advantage of inverse substrates which have an unlimited choice of the acyl component, development of stable acyl enzymes could be possible. These transient inhibitors for trypsin-like enzymes could be candidates for drugs. In this respect, the determination of the deacylation rate constants for the plasmin- and thrombin-catalyzed hydrolyses of various esters were undertaken 77). 

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