Formation Of Prothrombinase Complex Research Articles

Fibroblasts, glial, and neuronal cells are involved in extravascular prothrombin activation.

A membrane-associated prothrombin activator (MAPA) was found on various cultured cells derived from non-hematopoietic cells [Sekiya, F. et al. (1994) J. Biol. Chem. 269, 32441-32445]. In this study, we investigated the enzymatic properties of this enzyme using protease inhibitors. While the metalloproteinase inhibitor, o-phenanthroline, had no effect, some Kunitz type serine protease inhibitors attenuated MAPA activity. Recombinant tissue factor pathway inhibitor (rTFPI) also markedly reduced the activity (IC(50), 1. 3+/-0.6 x 10(-10) M). MAPA activity is, therefore, most likely to be due to factor Xa. We evaluated the effect of exogenous factor Xa on MAPA activity. Factor Xa-dependent prothrombin activation was observed on fibroblast cells (apparent K(d), 1.47+/-0.72 nM). Activation was also observed on glial and neuronal cells, which expressed MAPA activity. These results imply that membrane-bound factor Xa results in MAPA activity on these cells. Therefore, we considered the involvement of factor Va, a component of prothrombinase, in this activity. We examined whether or not the prothrombinase complex is assembled on these cells. Prothrombin was activated in a manner dependent on both exogenous factor Xa and factor Va (apparent K(d) of 0.51-1.81 nM for factor Va). These results indicate that the prothrombinase complex forms specifically on various extravascular cells. Although the prothrombinase complex can be assembled on monocytes and lymphocytes, it is not known why these cells can activate prothrombin specifically. These cells which have the capacity for prothrombin activator activity could also activate factor X; i.e. cells with factor X activation activity were able to convert prothrombin. These observations suggest that thrombin was generated via two procoagulant activities; factor X activation and subsequent prothrombinase complex formation on the surface of these cells. This mechanism may explain the various pathological states involving or resulting from extravascular thrombin and fibrin formation.

Ancylostoma caninum Anticoagulant Peptide: A novel inhibitor of coagulation factor Xa with potent antimetastatic activity. • 650

Although complications related to activation of the coagulation cascade are common in patients with cancer, the role of individual clotting factors in tumorigenesis and metastasis remains unknown. We have identified a novel anticoagulant with potent antimetastatic activity. This molecule, named Ancylostoma caninum Anticoagulant Peptide (AcAP), is a specific inhibitor of coagulation factor Xa (Ki = 265 pM) and has been previously isolated from bloodfeeding hookworms. Subcutaneous injection of SCID mice with recombinant AcAP (rAcAP) (0.01-0.2 mg/mouse) prior to tail vein injection of 105 highly metastatic LOX human melanoma cells resulted in a dose dependent reduction in pulmonary macro-metastases. In order to investigate the mechanism of this protection, as well as to further understand the role of coagulation proteases in tumor cell biology, LOX cells were assayed for the ability to bind factor Xa in vitro. Using biotinylated factor Xa, we found that the protease binds to LOX monolayers in a specific and saturable fashion, with a KD of 15 nM. Increasing concentrations of rAcAP maximally inhibited labelled factor Xa binding by approximately 90%. The LOX cells also supported formation of the clot-forming prothrombinase complex, whose catalytic activity was effectively blocked in the presence of equimolar concentrations of rAcAP. Competition binding studies revealed that an excess of active site blocked factor Xa (EGR-Xa) successfully inhibited prothrombinase activity by 98%, while causing only a minimal reduction in total biotinylated Xa binding. These data suggest that the majority of factor Xa (>80%) binds to the surface of LOX cells at its active-site, rendering it unavailable for prothrombinase complex formation and thrombin generation. Therefore, in addition to its well recognized function in clot formation, factor Xa may play a role in promoting LOX metastasis via previously uncharacterized active site mediated interactions with the tumor cell membrane. rAcAP, by inhibiting both the binding of factor Xa as well as its catalytic activity, represents a potentially valuable therapeutic agent for the prevention of tumor metastasis.

State-of-the-Art Review: Factor Xa Versus Factor IIa Inhibitors

The development of specific inhibitors of blood coagulation enzymes has led to a number of new anticoagulant/antithrombotic agents that could be useful for prophylaxis and/or treatment of thromboembolic disorders. Because thrombin is the central bioregulatory enzyme in hemostasis, blocking of the active site of the enzyme by a fast reaction with an inhibitor may effectively prevent intravascular coagulation as well as other important biological effects of thrombin. For the direct inactivation of thrombin, several classes of compounds have been developed and characterized in vitro and in vivo as potential antithrombotic agents including naturally occurring as well as synthetic thrombin inhibitors such as hirudin, hirulog, arginine (argatroban), and benzamidine (NAPAP) derivatives as well as tripeptide-type inhibitors (efegatran, D-Phe-Pro-Arg-CH2Cl, boroarginine derivatives). Experimental findings suggest that directly acting thrombin inhibitors may be effective in a wide range of arterial and venous thrombotic events. The important role of factor Xa in the coagulation cascade at the stage of the conversion of extrinsic and intrinsic pathways and the amplification of its procoagulant action by prothrombinase complex formation makes factor Xa a promising target also for antithrombotic drugs. Experimental studies on highly effective and selective factor Xa inhibitors (antistasin, tick anticoagulant peptide, yagin, DX-9065a) showed that inhibition of coagulation at a relatively early stage is a very effective way for the prevention of thrombotic processes. Furthermore, it is assumed that factor Xa inhibitors selectively inhibit thrombus formation without compromising hemostasis. In conclusion, both thrombin and factor Xa inhibitors are promising drugs for the management of thrombotic disorders. However, besides the strong inhibitory potency against the target enzyme, other pharmacological aspects such as pharmacodynamics, pharmacokinetics, and toxic side effects must be included in the evaluation of the potential usefulness of thrombin and factor Xa inhibitors for clinical indications. Key Words: Anticoagulants—Antithrombotics—Factor Xa inhibitors—Thrombin inhibitors.

Tissue factor pathway inhibitor attenuates prothrombinase complex formation on balloon-injured arteries in pigs

Tissue factor pathway inhibitor attenuates prothrombinase complex formation on balloon-injured arteries in pigs

Structure and functions of coagulation factor IX/factor X-binding protein isolated from the venom of Trimeresurus flavoviridis.

Snake venoms contain various anticoagulants that affect blood coagulation system (1, 2). The anticoagulant is classified into two categories: anticoagulant enzymes and non-enzymatic anticoagulants. The examples of the former category are phospholipases, fibrinogenolytic enzymes, protein C activators, and proteolytic enzymes that convert coagulation factors into degraded inactive forms. An interesting example of the non-enzymatic anticoagulants is an inhibitor of the activation of prothrombin. The inhibitors in this category have been first found in the venom of Agkistrodon acutus (3) and Trimeresurus gramineus (4). The anticoagulant protein isolated from A. acutus inhibits the participation of factor Xa in the prothrombinase complex formation (5, 6). In order to understand the anticoagulant mechanism we have isolated and characterized another anticoagulant protein with Mr 27,000 from the venom of the habu snake Trimeresurus flavoviridis (7). This anticoagulant protein forms a complex with either coagulation factor IX or factor X with a stoichiometry of 1 to 1 in a calcium-dependent fashion, and thereby blocks the amplification of the coagulation cascade (7). Thus, we named it IX/X-bp (factor IX/factor X-binding protein). This chapter describes the structural and functional properties of IX/X-bp from the venom of the habu snake (T. flavoviridis).

Kinetics of human factor VII activation.

In this study the activation of human factor VII by a variety of potential activators in the presence and absence of mixed phospholipid vesicles [25% phosphatidylserine (PS), 75% phosphatidylcholine (PC)] is evaluated. At the plasma concentration of factor VII, 10 nM, the activation rate of the zymogen by 0.05 nM factor Xa is anionic phospholipid (PCPS) dependent and achieves a maximum value of 18 pM/s at 5-20 microM PCPS; further increases in the levels of PCPS decrease the activation rate of factor VII. The maximum activation rate of factor VII (10 nM) by the factor VIIa-tissue factor complex (0.1 nM), 0.76 pM/s, is achieved at 200 microM PCPS. No detectable activation of 10 nM factor VII is observed under similar conditions when either thrombin (0.1 nM) or factor IXa (0.1 nM) is used as an activator. Factor VIIa (10 nM) and factor XIa (1 nM) are not observed to activate factor VII at detectable rates. The observed Michaelis-Menten constants (KM) for factor VII activation in the presence of PCPS at optimal concentrations vary from 1.2 microM for factor Xa to 3.2 microM for the factor VIIa-tissue factor complex. The highest catalytic constant (kcat) value (15.2 s-1) is observed for factor Xa-PCPS. The factor VIIa-tissue factor complex, factor IXa, and thrombin kcat values are 1.4, 0.32, and 0.061 s-1, respectively. Tissue factor does not increase the factor VII activation rate by factor Xa, factor IXa, or thrombin. Factor VIIIa in the presence of PCPS has no effect on factor VII activation by factor IXa. In contrast, factor Va decreases the factor VII activation rate by factor Xa, reaching saturation at concentrations consistent with complete prothrombinase complex formation. The formed prothrombinase complex activates factor VII at approximately 30% the rate of factor Xa bound to phospholipids. These data allow us to conclude that the predominant physiological factor VII activator is, most likely, membrane-bound factor Xa.

Effects of nutritional folate deficiency on the adhesive properties of murine melanoma cells

Folate deficient murine B16 melanoma cells adhered more rapidly and in higher percentages to plastic plates or dishes coated with laminin or fibronectin than folate replete cells. These changes in the adhesive properties of murine melanoma cells induced by nutritional folate deficiency were not mediated by changes in cell size, proliferative capacity or cell cycle distribution. While melanoma cells served as a suitable surface for prothrombinase complex formation, folate deficiency did not alter this membrane function, suggesting that the membrane changes associated with folate deficiency are relatively specific.

Prothrombinase complex assembly. Contributions of protein-protein and protein-membrane interactions toward complex formation.

Equilibrium binding studies of prothrombinase complex formation were undertaken using phospholipid vesicles composed of phosphatidylcholine and phosphatidylserine (PCPS), factor Va, and factor Xa modified with dansyl glutamylglycinylarginyl chloromethyl ketone (DEGR.Xa). The interaction between the Va.PCPS and DEGR.Xa.PCPS binary complexes was experimentally isolated using saturating concentrations of PCPS. Fluorescence titrations indicated that the membrane-bound proteins interact tightly (Kd approximately 10(-9) M) with a stoichiometry of 1 mol of Va bound/mol of DEGR.Xa at saturation. Complex formation was also investigated by kinetic studies of prothrombin activation using unmodified factor Xa. The kinetic studies yielded a Kd approximately 10(-9) M, which was independent of the concentration of prothrombin in the range of 0.5-5.0 microM. Fluorescence studies of complex assembly at limiting PCPS concentrations provided evidence for an altered DEGR.Xa-PCPS interaction when the enzyme was assembled into the complex. The data suggest that although both proteins are associated with PCPS when complexed with each other, the presence of factor Va on the membrane surface increases the affinity for the Xa-PCPS interaction by an estimated 100-fold. Prothrombinase complex assembly therefore proceeds independently of the availability of substrate and is stabilized by protein-protein and protein-phospholipid interactions. Linkage between the two protein-membrane combination events leads to the further stabilization of the complex on the vesicle surface.

Differentiation-dependent expression of phosphatidylserine in mammalian plasma membranes: quantitative assessment of outer-leaflet lipid by prothrombinase complex formation.

Phosphatidylserine (PS) is asymmetrically distributed in mammalian cell membranes, being preferentially localized in the inner leaflet. Some studies have suggested that a disturbance in the normal asymmetric distribution of PS--e.g., PS exposure in the outer leaflet of the cell membrane, which can occur upon platelet activation as well as in certain pathologic red cells--serves as a potent procoagulant surface and as a signal for triggering their recognition by macrophages. These studies suggest that the regulation of PS distribution in cell membranes may be critical in controlling coagulation and in determining the survival of pathologic cells in the circulation. In this paper we describe a sensitive technique, based on PS-dependent prothrombinase complex activity, for assessing the amount of PS on the external leaflet of intact viable cells. Our results indicate that tumorigenic, undifferentiated murine erythroleukemic cells express 7- to 8-fold more PS in their outer leaflet than do their differentiated, nontumorigenic counterparts. Increased expression of PS in the tumorigenic cells directly correlated with their ability to be recognized and bound by macrophages.

Enhanced thrombin generation and platelet binding on herpes simplex virus-infected endothelium.

Atherosclerotic lesions have been reported to contain herpes simplex virus 1 (HSV-1) genomic material. This, and other previous evidence, suggests that latent viral infection may be an atherogenic trigger. Moreover, active HSV-1 lesions manifest marked fibrin deposition in microvessels. In this report we show that very early infection of human endothelial cells with HSV-1 appears to alter surface conformation as detected by merocyanine 540 staining. Concomitantly, the efficiency of prothrombinase complex assembly increases, resulting in a 2- to 3-fold accelerated rate of thrombin generation on the cell surface. Increased thrombin generation is probably doubly procoagulant, since we also demonstrate that thrombin-induced platelet accumulation on HSV-infected endothelium (50.7 +/- 9.3%) is increased compared to uninfected endothelium (9.5 +/- 2.1%; P less than 0.002). Associated with HSV infection, prostacyclin secretion in response to thrombin is diminished by a factor of 20, probably explaining the enhanced platelet attachment. We conclude that HSV infection shifts endothelial cell properties from anticoagulant to procoagulant, both by promoting prothrombinase complex formation and function and by increasing platelet binding, well before cell disruption takes place. Virus-induced changes in the endothelial plasma membrane and diminished prostacyclin secretion are suggested as the pathways for this pathophysiologic mechanism, which may be germane to atherosclerotic thrombosis as well as HSV-mediated tissue necrosis.

The inhibition of thrombin-dependent positive-feedback reactions is critical to the expression of the anticoagulant effect of heparin

Heparin catalyses the inhibition of two key enzymes of blood coagulation, namely Factor Xa and thrombin, by enhancing the antiproteinase activities of plasma antithrombin III and heparin cofactor II. In addition, heparin can directly inhibit the activation of Factor X and prothrombin. The contributions of each of these effects to the anticoagulant activity of heparin have not been delineated. We therefore performed experiments to assess how each of these effects of heparin contributes to its anticoagulant activity by comparing the effects of heparin, pentosan polysulphate and D-Phe-Pro-Arg-CH2Cl on the intrinsic pathway of coagulation. Unlike heparin, pentosan polysulphate catalyses only the inhibition of thrombin by plasma. D-Phe-Pro-Arg-CH2Cl is rapid enough an inhibitor of thrombin so that when added to plasma no complexes of thrombin with its inhibitors are formed, whether or not the plasma also contains heparin. Heparin (0.66 microgram/ml) and pentosan polysulphate (6.6 micrograms/ml) completely inhibited the intrinsic-pathway activation of 125I-prothrombin to 125I-prothrombin fragment 1 + 2 and 125I-thrombin. On the addition of thrombin, a good Factor V activator, to the plasma before each sulphated polysaccharide, the inhibition of prothrombin activation was demonstrable only in the presence of higher concentrations of the sulphated polysaccharide. D-Phe-Pro-Arg-CH2Cl also completely inhibited the intrinsic-pathway activation of prothrombin in normal plasma. The inhibitory effect of D-Phe-Pro-Arg-CH2Cl was reversed if thrombin was added to the plasma before D-Phe-Pro-Arg-CH2Cl. The inhibition of the activation of prothrombin by the three agents was also abolished with longer times with re-added Ca2+. Reversal of the inhibitory effects of heparin and pentosan polysulphate was associated with the accelerated formation of 125I-thrombin-antithrombin III and 125I-thrombin-heparin cofactor complexes respectively. These results suggest that the anticoagulant effects of heparin and pentosan polysulphate are mediated primarily by their ability to inhibit the thrombin-dependent activation of Factor V, thereby inhibiting the formation of prothrombinase complex, the physiological activator of prothrombin.