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PubMed1997

Replacing the first epidermal growth factor-like domain of factor IX with that of factor VII enhances activity in vitro and in canine hemophilia B.

Using the techniques of molecular biology, we made a chimeric Factor IX by replacing the first epidermal growth factor-like domain with that of Factor VII. The resulting recombinant chimeric molecule, Factor IXVIIEGF1, had at least a twofold increase in functional activity in the one-stage clotting assay when compared to recombinant wild-type Factor IX. The increased activity was not due to contamination with activated Factor IX, nor was it due to an increased rate of activation by Factor VIIa-tissue factor or by Factor XIa. Rather, the increased activity was due to a higher affinity of Factor IXVIIEGF1 for Factor VIIIa with a Kd for Factor VIIIa about one order of magnitude lower than that of recombinant wild-type Factor IXa. In addition, results from animal studies show that this chimeric Factor IX, when infused into a dog with hemophilia B, exhibits a greater than threefold increase in clotting activity, and has a biological half-life equivalent to recombinant wild-type Factor IX.

Amino Acid Chloromethyl Ketones↗
PubMed1994

Platelet procoagulant complex assembly in a tissue factor-initiated system.

The aim of this study was to examine the assembly of the factor IXa/VIIIa (Xase) and factor Xa/Va (IIase) complexes on the platelet surface in a system designed to mimic tissue factor-initiated coagulation. The experimental system contained tissue factor-bearing monocytes, unactivated platelets, and plasma concentrations of factors V, VIII, IX, X, prothrombin, tissue factor pathway inhibitor (TFPI), antithrombin III (ATIII), and small amounts of factor VIIa. The time courses of platelet activation, coagulation factor binding and thrombin generation were compared. In this system, thrombin generation by the combination of monocytes and platelets was synergistic compared to each cell type alone. Platelet activation and thrombin generation were minimal in the absence of prothrombin or factor X. After a lag period, platelet activation began, followed by progressive binding of factors Va and VIIIa. This was followed by factor IXa and Xa binding and the onset of thrombin generation. Unexpectedly, a transient early increase in platelet-associated factor IX and X was also seen, that was due to release from platelets. The amount of factor IX bound to isolated activated platelets was increased by addition of factor VIIIa, or by activation of factor IX to IXa. In contrast, factor VIIIa binding was not altered by the presence of factor IX or IXa. We conclude that in a tissue factor-initiated system, assembly of the procoagulant complexes on the platelet surface begins after platelet activation occurs. Platelet activation requires thrombin generation in the vicinity of the tissue factor bearing cells. The cofactors Va and VIIIa bind to the platelets and facilitate subsequent binding of factors IXa and Xa to form functional procoagulant complexes.

Blood Coagulation Factors↗
PubMed1984

Functional characterization of single-chain factor X from rat liver.

14C-Labeled single-chain factor X prepared by vitamin K-dependent carboxylation in vitro was partially purified by adsorption to BaSO4 and chromatography on DEAE-Sephacel. Known activators of factor X were analyzed for their effect on the single-chain molecule. 14C-Labeled factor X antigens were recovered immunochemically from incubation mixtures and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Incubation with trypsin resulted in the generation of factor Xa clotting activity, and the 14C-labeled product migrated after reduction with an apparent molecular weight of 22,500 +/- 1500 (mean +/- 1 SD). The light chain produced by factor Xa was similar to that produced by trypsin (Mr 24,500 +/- 1500; mean +/- 1 SD). Incubation of single-chain factor X with factor VII and thromboplastin, factor IXa, or the factor X activating enzyme from Russell's viper venom gave a reducible product with a light chain of higher apparent molecular weight (Mr 37,000-38,000). Incubation with factor VII and thromboplastin also resulted in the generation of factor Xa clotting activity. Incubation of single-chain factor X with platelets resulted in the binding of about 20% of the 14C. The bound 14C-labeled factor X antigen released by freezing and thawing in the presence of EDTA was reduced to give a 14C-labeled polypeptide with Mr 31,000. Walker 256 tumor cells bound about 30% of the 14C. The bound material, after reduction, gave a 14C-labeled polypeptide with Mr 23,000.

Animals↗
PubMed2001

Anticoagulant activities of a monoclonal antibody that binds to exosite II of thrombin.

A monoclonal IgG isolated from a patient with multiple myeloma has been shown to bind to exosite II of thrombin, prolong both the thrombin time and the activated partial thromboplastin time (aPTT) when added to normal plasma, and alter the kinetics of hydrolysis of synthetic peptide substrates. Although the IgG does not affect cleavage of fibrinogen by thrombin, it increases the rate of inhibition of thrombin by purified antithrombin approximately 3-fold. Experiments with plasma immunodepleted of antithrombin or heparin cofactor II confirm that prolongation of the thrombin time requires antithrombin. By contrast, prolongation of the aPTT requires neither antithrombin nor heparin cofactor II. The IgG delays clotting of plasma initiated by purified factor IXa but has much less of an effect on clotting initiated by factor Xa. In a purified system, the IgG decreases the rate of activation of factor VIII by thrombin. These studies indicate that binding of a monoclonal IgG to exosite II prolongs the thrombin time indirectly by accelerating the thrombin-antithrombin reaction and may prolong the aPTT by interfering with activation of factor VIII, thereby diminishing the catalytic activity of the factor IXa/VIIIa complex.

Anions↗
PubMed2002

Selective inhibition of coagulation factors: advances in antithrombotic therapy.

Heparin and coumarin derivatives have long been used for the prophylaxis and treatment of venous thromboembolism (VTE). Although they have demonstrated efficacy and safety, they act at multiple targets within the coagulation cascade and their efficacy is influenced by many patient variables. Because of the need to improve the benefit-to-risk ratio of antithrombotic drugs, newer agents that target single coagulation factors have been developed. These include selective factor Xa inhibitors, direct thrombin inhibitors (DTIs), and inhibitors of factor IXa and the factor VII-tissue factor complex. Three DTIs-hirudin, bivalirudin, and argatroban-have been approved for clinical use. Fondaparinux, a novel pentasaccharide and the first selective factor Xa inhibitor to be approved by the U.S. Food and Drug Administration and the European Agency for the Evaluation of Medicinal Products, is indicated for the prevention of VTE after major orthopedic surgery. Fondaparinux has predictable pharmacokinetics, is almost 100% bioavailable, and has a half-life that allows once-daily dosing in all indications. In addition, the routine monitoring of standard indicators of hemostasis is not required. The availability of these agents and the continued development of investigational selective coagulation inhibitors have the potential to improve efficacy and decrease adverse events in patients at risk for VTE.

Anticoagulants↗
PubMed2002

New characteristics of anti-factor VIII inhibitor antibody epitopes and unusual immune responses to Factor VIII.

Treatment of individuals with severe hemophilia A by plasma-derived or recombinant factor VIII leads to the production of anti-factor VIII antibodies in approximately 30% of such patients. Because some of these antibodies inactivate factor VIII, they are considered a major factor in preventing optimal therapeutic treatment. Factor VIII is a cofactor that must bind to factors IX and X and phospholipids in order for normal blood coagulation to occur. The inhibition of factor VIII activity is due to binding by anti- factor VIII antibodies in the patient plasma to the same sites required for factors IX and X and phospholipid binding. Previously, inhibitor epitopes were localized to the A2, A3, and C2 domains and to a region of acidic amino acids between the A1 and A2 domains. Inhibitor binding to these domains prevented factor VIII binding to factor IXa (A2, A3), factor Xa (C2), and phospholipids (C2), and binding to the acidic region interfered with factor X binding. Antibody binding to a minor C2 domain epitope slowed activated factor VIII release from von Willebrand factor (vWF) and interfered with factor Xa binding to factor VIII.

Amino Acid Sequence↗
PubMed1995

Kinetic characterization of the proteinase binding defect in a reactive site variant of the serpin, antithrombin. Role of the P1' residue in transition-state stabilization of antithrombin-proteinase complex formation.

To elucidate the role of the P1' residue of the serpin, antithrombin (AT), in proteinase inhibition, the source of the functional defect in a natural Ser-394-->Leu variant, AT-Denver, was investigated. AT-Denver inhibited thrombin, Factor IXa, plasmin, and Factor Xa with second order rate constants that were 430-, 120-, 40-, and 7-fold slower, respectively, than those of native AT, consistent with an altered specificity of the variant inhibitor for its target proteinases. AT-Denver inhibited thrombin and Factor Xa with nearly equimolar stoichiometries and formed SDS-stable complexes with these proteinases, indicating that the diminished inhibitor activity was not due to an enhanced turnover of the inhibitor as a substrate. Binding and kinetic studies showed that heparin binding to AT-Denver as well as heparin accelerations of AT-Denver-proteinase reactions were normal, consistent with the P1' mutation not affecting the heparin activation mechanism. Resolution of the two-step reaction of AT-Denver with thrombin revealed that the majority of the defective function was localized in the second reaction step and resulted from a 190-fold decreased rate constant for conversion of a noncovalent proteinase-inhibitor encounter complex to a stable, covalent complex. Little or no effects of the mutation on the binding constant for encounter complex formation or on the rate constant for stable complex dissociation were evident. These results support a role for the P1' residue of antithrombin in transition-state stabilization of a substrate-like attack of the proteinase on the inhibitor-reactive bond following the formation of a proteinase-inhibitor encounter complex but prior to the conformational change leading to the trapping of proteinase in a stable, covalent complex. Such a role indicates that the P1' residue does not contribute to thermodynamic stabilization of AT-proteinase complexes and instead favors a kinetic stabilization of these complexes by a suicide substrate reaction mechanism.

Antithrombins↗
PubMed1996

Transmission of a procoagulant signal from tissue factor-bearing cell to platelets.

The goal of the current study was to examine the mechanism by which factor VIIa/tissue factor (TF) activity leads to platelet activation as the first step in initiation of coagulation. Adherent, endotoxin-treated monocytes were used as a cellular source of TF. The processes that led to platelet activation were rapid, since incubation of coagulation factors and platelets with TF for as little as 15 s initiated platelet activation. Further, direct contact between the TF source and platelets was not required since incubation of plasma levels of coagulation zymogens and inhibitors with TF generated the initiating signal for platelet activation. We hypothesized that thrombin generation on the cells that contained TF was the initiating signal for platelet activation. To test this hypothesis, factor VIIa, inhibitors, and different combinations of coagulation zymogens were incubated with TF-bearing cells. The supernatants were then transferred to a suspension of unactivated platelets with plasma concentrations of zymogen factors and inhibitors. Platelet activation was much more efficient when all the elements of the IIase complex (factors II, V and X) were preincubated with factor VIIa/TF than when only factor X was incubated with factor VIIa/TF. Finally, TF was incorporated into lipid vesicles containing phosphatidyl choline either with or without phosphatidyl serine. Vesicles without phosphatidyl serine have no IIase activity. Platelets were incubated with TF, coagulation zymogens and inhibitors. Platelet activation only occurred when the lipid vesicles could support IIase activity. We conclude that sufficient thrombin generation occurs on the TF-bearing cell (or TF-bearing vesicle) in the absence of platelets, to provide the procoagulant signal that leads to platelet activation. The activated platelet surface then provides sites for TF-activated factor IXa to recruit factor Xa to bind and assemble into functional Xase and IIase complexes.

Blood Coagulation↗
PubMed2001

Factor Xa is a fibroblast mitogen via binding to effector-cell protease receptor-1 and autocrine release of PDGF.

The coagulation cascade protease thrombin is a fibroblast mitogen, but the proliferative potential of other coagulation proteases is not known. In this study we show that factor Xa stimulated human fetal lung fibroblast DNA synthesis in a concentration-dependent manner from 1 nM onward with a fourfold increase at 200 nM. The mitogenic effect of factor Xa was confirmed using a colorimetric proliferation assay and direct cell counting. Factor Xa and thrombin had equivalent potencies, and their stimulatory effects followed a similar time course. Comparable results were also obtained with primary human adult fibroblasts derived from lung, kidney, heart, skin, and liver. Factor VIIa also stimulated fibroblast proliferation, but only at concentrations >10 nM, whereas factor IXa had no effect. To begin to address the mechanism by which factor Xa is acting, we show that human fibroblasts express effector-cell protease receptor-1 and that blocking antibodies to this receptor and the catalytic site of factor Xa inhibited its mitogenic effect. Furthermore, factor Xa upregulated platelet-derived growth factor-A (PDGF-A) mRNA expression, whereas PDGF-B could not be detected, and a blocking antibody to PDGF inhibited the mitogenic effect of factor Xa. We conclude that factor Xa acts as a fibroblast mitogen via binding to effector-cell protease receptor-1 and the autocrine release of PDGF.

Adult↗
PubMed2000

Does inflammation contribute to thrombotic events?

Recent studies have focused on a myriad of mechanisms by which inflammation can potentiate blood clotting. Inflammatory mediators like endotoxin and tissue necrosis factor (TNF)-alpha can cause the expression of tissue factor on monocytes and, possibly, endothelium, thereby initiating the coagulation cascade. Activation of the complement system can lead to exposure of membrane surfaces capable of amplifying the initial tissue factor stimulus by facilitating the assembly of the factor VIIIa-factor IXa and the factor Xa-factor Va complexes. Inflammatory mediators, particularly interleukin-6, can also increase the levels of fibrinogen, an acute-phase reactant. In addition, the inflammatory mediators can elevate the levels of plasminogen activator inhibitor, thus suppressing the fibrinolytic system. These studies alone, however, do not prove that inflammation can trigger clinically relevant thrombus formation in vivo. For instance, TNF-alpha has been studied in cancer patients as a potential cure for cancer, and even though these patients are hypercoaguable, thrombosis was not commonly observed as a side effect of the near-lethal doses of TNF-alpha that were administered. Based on primate studies, inflammatory mediators like TNF-alpha can promote clot deposition effectively only if there is reduced flow and inhibition of the natural anticoagulant pathways. The requirement for multiple simultaneous injurious events probably explains why inflammation alone is not observed as a major cause of thrombosis.

Animals↗
PubMed1999

Vasoflux, a new anticoagulant with a novel mechanism of action.

BACKGROUND: Heparin and direct thrombin inhibitors, such as hirudin, have limitations in the treatment of acute coronary syndromes. Heparin does not inactivate fibrin-bound thrombin, whereas hirudin fails to block thrombin generation. In contrast, Vasoflux is a novel anticoagulant that inactivates fibrin-bound thrombin and attenuates factor Xa generation. METHODS AND RESULTS: Vasoflux is prepared by depolymerization of heparin, restricting molecular size to between 3000 and 8000 Da, and reducing antithrombin affinity by periodate oxidation. Vasoflux catalyzes fibrin-bound thrombin inactivation by heparin cofactor II (HCII) and inhibits factor IXa activation of factor X independently of antithrombin and HCII. Compared with other anticoagulants in a thrombogenic extracorporeal circuit, Vasoflux maintains filter patency at concentrations that produce an activated clotting time (ACT) of 220 seconds. In contrast, to maintain filter patency, heparin, low-molecular-weight heparin (LMWH), and hirudin require concentrations that produced an ACT of 720, 415, and >1500 seconds, respectively, whereas dermatan sulfate was ineffective at concentrations that produced an ACT of 360 seconds. CONCLUSIONS: Vasoflux is more effective than heparin and LMWH because it inactivates fibrin-bound thrombin and is superior to hirudin and dermatan sulfate because it also blocks factor Xa generation.

Anticoagulants↗
PubMed2000

[New anticoagulants].

The quest to develop new antithrombotic agents has been stimulated by clinical needs and by advances in biotechnology that have made it possible to produce drugs that target specific steps in thrombogenesis. Established anticoagulants such as unfractionated heparin and the coumarins are effective, but have two major limitations: narrow therapeutic windows and highly unpredictable dose-response relationships. Consequently, these drugs often cause complications such as serious bleeding that require close monitoring of their use by laboratory tests to balance safety and effect. These limitations provided the impetus for the development of new anticoagulants that inactivate thrombin, factor Xa, factor IXa or the factor VIIa/tissue factor complex. Similarly, agents that enhance the protein C anticoagulant pathway have also been developed. Of these, direct thrombin inhibitors, soluble thrombomodulin, protein C, and activated protein C have been evaluated clinically for parenteral administration. However, there is enormous interest in the development of safer and more effective oral anticoagulants. In the future, such orally active direct inhibitors of thrombin and factor Xa, if they can be given safely without the need for laboratory monitoring, may replace the coumarins for the long-term treatment of thromboembolic disorders. To achieve these goals, these compounds need high, consistent oral bioavailability.

Animals↗
PubMed1992

Deletion analysis of recombinant human factor V. Evidence for a phosphatidylserine binding site in the second C-type domain.

Human coagulation factor V is an integral component of the prothrombinase complex. Rapid activation of prothrombin is dependent on the interactions of this nonenzymatic cofactor with factor Xa and prothrombin in the presence of calcium ions and a phospholipid or platelet surface. Factor V is similar structurally and functionally to the homologous cofactor, factor VIII, which interacts with factor IXa to accelerate factor X activation in the presence of calcium and phospholipids. Both of these cofactors, when activated, possess homologous heavy and light chains. Binding to anionic phospholipids is mediated by the light chains of these two cofactors. In bovine factor Va, a phosphatidylserine-specific binding site has been localized to the amino-terminal A3 domain of the light chain. In human factor VIII, on the other hand, a region within the carboxyl-terminal C2 domain of the light chain has been shown to interact with anionic phospholipids. We have constructed a series of recombinant deletion mutants lacking domain-size fragments of the light chain of human factor V (rHFV). These mutants are expressed and secreted as single-chain proteins by COS cells. Thrombin and the factor V activator from Russell's viper venom process these deletion mutants as expected. The light chain deletion mutants possess essentially no procoagulant activity, nor are they activated by treatment with factor V activator from Russell's viper venom. Deletion of the second C-type domain results in essentially complete loss of phosphatidylserine-specific binding whereas the presence of the C2 domain alone (rHFV des-A3C1, which lacks the A3 and C1 domains of the light chain) results in significant phosphatidylserine-specific binding. The presence of the A3 domain alone (rHFV des-C1C2) does not mediate binding to immobilized phosphatidylserine. Increasing calcium ion concentrations result in decreased binding of recombinant human factor V and the mutant rHFV des-A3C1 to phosphatidylserine, similar to previous studies with purified plasma factor V and phospholipid vesicles. These results indicate that human factor V, similar to human factor VIII, possesses a phosphatidylserine-specific binding site within the C2 domain of the light chain.

Base Sequence↗
PubMed1990

Factor IX is activated in vivo by the tissue factor mechanism.

Despite significant progress in elucidating the biochemistry of the hemostatic mechanism, the process of blood coagulation in vivo remains poorly understood. Factor IX is a vitamin K-dependent glycoprotein that can be activated by factor XIa or the factor VII-tissue factor complex in vitro. To investigate the role of these two pathways in factor IX activation in humans, we have developed a sensitive procedure for quantifying the peptide that is liberated with the generation of factor IXa. The antibody population used for the immunoassay was raised in rabbits and chromatographed on a factor IX-agarose immunoadsorbent to obtain antibody populations with minimal intrinsic reactivity toward factor IX. We determined that the mean level of the factor IX activation peptide (FIXP) in normal individuals under the age of 40 years was 203 pmol/L and that levels increased significantly with advancing age. The mean concentration of FIXP was markedly reduced to 22.7 pmol/L in nine patients with hereditary factor VII deficiency (factor VII coagulant activity less than 7%) but was not significantly different from normal controls in nine subjects with factor XI deficiency (factor XI coagulant activity less than 8%). These data indicate that factor IXa generation in vivo results mainly from the activity of the tissue factor mechanism rather than the contact system (factor XII, prekallikrein, high molecular-weight kininogen, factor XI). Our results may also help to explain the absence of a bleeding diathesis in many patients with deficiencies of the contact factors of coagulation.

Adult↗
PubMed1986

Analysis of the generation and inhibition of activated coagulation factor X in pure systems and in human plasma.

The overall generation and inhibition of human factor Xa have been studied in pure systems and plasma to determine the kinetic characteristics of inhibition during factor Xa generation. Generation curves were measured amidolytically in a pure system containing factor X and antithrombin, which was activated with the factor X-activating enzyme of Russell's viper venom (RVV-X). The measured change in factor Xa level with time was fitted to a 3-parameter 2-exponential model to determine apparent first-order rates of inhibition. With antithrombin at 4.5 microM, the inhibition rate constant thus obtained was very close to the known rate of inhibition of exogenous enzyme. Factor Xa generation curves were also analyzed in plasma; however, to reduce interference in the assay of thrombin, congenitally prothrombin-deficient plasma was used containing 0.5 microM D-Phe-Pro-Arg-chloromethylketone. In plasma, factor Xa generated in the presence of phospholipid and Ca2+ ions by RVV-X, factor IXa, or tissue factor was inhibited more slowly than exogenous enzyme. The reduction was particularly severe with tissue factor activation, where the rate was 0.04-0.06 min-1. This protection by tissue factor was also observed in pure systems and apparently required factor VII.

Antithrombins↗
PubMed1994

Membrane binding properties of the factor IX gamma-carboxyglutamic acid-rich domain prepared by chemical synthesis.

The fully gamma-carboxylated peptides based upon the complete and truncated Gla/aromatic amino acid stack domains of human Factor IX were prepared by solid phase peptide synthesis using Fmoc (N-(9-fluorenyl)methoxycarbonyl) chemistry. A 47-residue peptide Factor IX-(1-47) and a 42-residue peptide Factor IX-(1-42), both containing 12 residues of L-gamma-carboxyglutamic acid, were purified by high performance liquid chromatography and oxidized to form the disulfide bond. Quantitative gamma-carboxyglutamic acid analysis of Factor IX-(1-47) and Factor IX-(1-42) indicated the presence of 12.1 and 11.2 gamma-carboxyglutamic acid residues/mol of peptide, respectively; no glutamic acid was detected. As monitored by fluorescence quenching, calcium ions induced the prototypical conformational transition in Factor IX-(1-47), but not in Factor IX-(1-42), that is observed with Factor IX. Half-maximal quenching of the intrinsic fluorescence of Factor IX-(1-47) was observed at Ca(II) concentrations of about 50 microM. Factor IX-(1-47) bound to the conformation-specific antibodies, anti-Factor IX:Mg(II) and anti-Factor IX:Ca(II)-specific in the presence of metal ions. Factor IX-(1-47) bound to phospholipid membranes, as monitored by energy transfer from intrinsic fluorophores to dansyl (5-dimethylaminonaphthalene-1-sulfonyl)-phosphatidylethanolamine incorporated into a lipid bilayer composed of phosphatidylserine:phosphatidylcholine. In contrast, Factor IX-(1-42) bound poorly to these same membranes. Factor IX-(1-47) did not inhibit Factor XIa activation of Factor IX but did inhibit the activation of Factor X by Factor IXa bound to Factor VIII in the presence of calcium ions and phospholipid. These results show that phospholipid membrane binding is a property of the Gla/aromatic amino acid stack domain and that the Factor IX-(1-47) peptide, prepared by chemical synthesis, preserves the membrane binding properties and the metal-induced conformational transitions observed in native Factor IX. These results indicate that Factor IX-(1-47) but not Factor IX-(1-42) is a suitable model for structural studies of Factor IX-membrane interaction.

1-Carboxyglutamic Acid↗
PubMed1976

Physiological effects of the plasminolytic derivatives of fibrinogen.

The macromolecular breakdown products of fibrinogen are known mainly for their inhibitory effect on the clotting of fibrinogen by thrombin. This inhibitory effect is due to interference with both the proteolytic action of thrombin and the polymerization of the fibrin monomers. However, the action of these products is not limited to these effects. They can on the one hand inhibit the consumption of Factors II and XIII and promote the inactivation of Factor VIII by thrombin. On the other hand, they can potentiate the activation of prothrombin in purified systems via the intrinsic pathway. The incidental observation was also made that Factor IXa and or Factor Xa inactivate Factor VIII. As substrates of both thrombin and plasmin the large fragments protect these two enzymes from spontaneous inactivation, while at the same time they inhibit their respective proteolytic activities. Contradictory results were obtained regarding their effect on platelets. The micromolecular (dialyzable) breakdown products prolong the thrombin, prothrombin, and partial thromboplastin times of plasma and retard the generation of the intrinsic prothrombin activator. They can also potentiate the effects of biologically active peptides and amines on the smooth muscles and on vascular permeability.

Blood Coagulation↗
PubMed1985

Formation of the fibrin clot: the balance of procoagulant and inhibitory factors.

Let us now briefly summarize some major known regulating mechanisms, most of which have already been discussed. A general regulating feature of the coagulation system is provided by the cofactors HMW-kininogen, tissue factor, factor V(a), factor VIII:C(a), protein S and thrombomodulin. Tissue factor and thrombomodulin, as cell membrane constituents, and the other cofactors, thanks to their affinity for certain surface sites, localize coagulation reactions and thus avoid generalized intravascular thrombosis when the clotting system is triggered. Thrombin activates factors V and VIII:C and activated protein C inactivates factors Va and VIII:Ca. Thrombin is regulated by AT III, alpha 2M and possibly heparin-cofactor II, whereby endothelial-cell-bound heparin-like molecules enhance thrombin neutralization. Moreover, binding of thrombin to thrombomodulin abolishes its clotting activity, at least in the case of rabbit thrombomodulin. Thrombin is able to cleave PT-fragment 1 from prothrombin, thus generating prethrombin 1, which lacks the gla-region and does not bind to phospholipids. The hypothesis that thrombin may regulate its own formation by this negative feedback, however, must probably be discarded, because no corresponding fragments are found after blood clotting in vitro (Aronson et al, 1977). Factor Xa and factor IXa are inhibited by AT III and endogenous heparin probably enhances their inactivation. However, phospholipid-bound factor Xa in the presence of factor Va (Marciniak, 1973) and phospholipid-bound factor IXa (Varadi and Elödi, 1982) are relatively protected from inhibition. Platelet-bound factor Xa is completely protected from AT III, even in the presence of heparin (Miletich et al, 1978). Thus, specific cell surface sites modulate the inhibition of proteases in situ. Factor XIa is inhibited by several protease inhibitors, the most important being alpha 1-AT. beta-factor XIIa is inhibited mainly by C1-inhibitor and kallikrein by both C1-inhibitor and alpha 2M. No serine protease inhibitor for factor VIIa is as yet known. However, after initial rapid activation by factor Xa, factor VIIa is subsequently proteolytically inactivated by factor Xa, resulting in a transient burst of factor Xa generation by factor VIIa (Morrison and Jesty, 1984). This proteolytic regulation of factor VIIa by factor Xa dampens factor IX or factor X activation via tissue factor-factor VIIa by feedback proteolytic inhibition and this may constitute a major regulatory mechanism for factor VIIa.(ABSTRACT TRUNCATED AT 400 WORDS)

Antithrombin III↗