Pharmacologic targeting of coagulation factors XII and XI by monoclonal antibodies reduces thrombosis in nitinol stents under flow

BACKGROUND

Cardiovascular implantable devices, such as vascular stents, are critical for the treatment of cardiovascular diseases. However, their success is dependent on robust and often long-term antithrombotic therapies. Yet, the current standard-of-care therapies often pose significant bleeding risks to patients. Coagulation factor (F)XI and FXII have emerged as potentially safe and efficacious targets to safely reduce pathologic thrombin generation in medical devices.

OBJECTIVES

To study the efficacy of monoclonal antibody-targeting FXII and FXI of the contact pathway in preventing vascular device-related thrombosis.

METHODS

The effects of inhibition of FXII and FXI using function-blocking monoclonal antibodies were examined in a nonhuman primate model of nitinol stent-related thrombosis under arterial and venous flow conditions.

RESULTS

We found that function-blocking antibodies of FXII and FXI reduced markers of stent-induced thrombosis in vitro and ex vivo. However, FXI inhibition resulted in more effective mitigation of thrombosis markers under varied flow conditions.

CONCLUSION

This work provides further support for the translation of contact pathway of coagulation inhibitors for their adjunctive clinical use with cardiovascular devices.

Identification and characterization of the plasma kallikrein-kinin system inhibitor, haemaphysalin, from hard tick, Haemaphysalis longicornis

The plasma kallikrein-kinin system inhibitor, haemaphysalin, from the hard tick, Haemaphysalis longicornis, was identified. It was found that haemaphysalin inhibited activation of the plasma kallikrein-kinin system by interfering with reciprocal activation between factor XII and prekallikrein. It did not, however, inhibit amidolytic activities of factor XIIa and kallikrein. Direct binding assay indicated that factor XII/XIIa and high molecular weight kininogen (HK) are the target molecules of haemaphysalin, and that Zn2+ ions are involved in the interactions of haemaphysalin with these target molecules. This suggests that haemaphysalin interacts with target molecules by recognizing their conformational changes induced by Zn2+ ions. Furthermore, haemaphysalin interacted with the fibronectin type II domain and domain D5, the cell binding domains of factor XII and HK, respectively. This finding suggests that haemaphysalin interferes with the association of factor XII and the prekallikrein-HK complex with a biologic activating surface by binding to these cell-binding domains, leading to inhibition of the reciprocal activation between factor XII and prekallikrein.

Differential Roles for the Coagulation Factors XI and XII in Regulating the Physical Biology of Fibrin

In the contact activation pathway of the coagulation, zymogen factor XII (FXII) is converted to FXIIa, which triggers activation of FXI leading to the activation of FIX and subsequent thrombin generation and fibrin formation. Feedback activation of FXI by thrombin has been shown to promote thrombin generation in a FXII-independent manner and FXIIa can bypass FXI to directly activate FX and prothrombin in the presence of highly negatively charged molecules, such as long-chain polyphosphates (LC polyP). We sought to determine whether activation of FXII or FXI differentially regulate the physical biology of fibrin formation. Fibrin formation was initiated with tissue factor, ellagic acid (EA), or LC polyP in the presence of inhibitors of FXI and FXII. Our data demonstrated that inhibition of FXI decreased the rate of fibrin formation and fiber network density, and increased the fibrin network strength and rate of fibrinolysis when gelation was initiated via the contact activation pathway with EA. FXII inhibition decreased the fibrin formation and fibrin density, and increased the fibrinolysis rate only when fibrin formation was initiated via the contact activation pathway with LC polyP. Overall, we demonstrate that inhibition of FXI and FXII distinctly alter the biophysical properties of fibrin.

Factor XI and factor XII as targets for new anticoagulants

Although the non-vitamin antagonist oral anticoagulants produce less intracranial bleeding than warfarin, serious bleeding still occurs. Therefore, the search for safer anticoagulants continues. Factor XII and factor XI have emerged as promising targets whose inhibition has the potential to prevent thrombosis with little or no disruption of hemostasis. Thus, thrombosis is attenuated in mice deficient in factor XII or factor XI and patients with congenital factor XII deficiency do not bleed and those with factor XI deficiency rarely have spontaneous bleeding. Strategies targeting factor XII and XI include antisense oligonucleotides to decrease their synthesis, inhibitory antibodies or aptamers, and small molecule inhibitors. These strategies attenuate thrombosis in various animal models and factor XI knockdown with an antisense oligonucleotide in patients undergoing knee replacement surgery reduced postoperative venous thromboembolism to a greater extent than enoxaparin without increasing bleeding. Therefore, current efforts are focused on evaluating the efficacy and safety of factor XII and factor XI directed anticoagulant strategies.

Factor XI and contact activation as targets for antithrombotic therapy

The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.

Platelet surface-associated activation and secretion-mediated inhibition of coagulation factor XII

Coagulation factor XII (fXII) is important for arterial thrombosis, but its physiological activation mechanisms are unclear. In this study, we elucidated the role of platelets and platelet-derived material in fXII activation. FXII activation was only observed upon potent platelet stimulation (with thrombin, collagen-related peptide, or calcium ionophore, but not ADP) accompanied by phosphatidylserine exposure and was localised to the platelet surface. Platelets from three patients with grey platelet syndrome did not activate fXII, which suggests that platelet-associated fXII-activating material might be released from α-granules. FXII was preferentially bound by phosphotidylserine-positive platelets and annexin V abrogated platelet-dependent fXII activation; however, artificial phosphotidylserine/phosphatidylcholine microvesicles did not support fXII activation under the conditions herein. Confocal microscopy using DAPI as a poly-phosphate marker did not reveal poly-phosphates associated with an activated platelet surface. Experimental data for fXII activation indicates an auto-inhibition mechanism (k_i/k_a = 180 molecules/platelet). Unlike surface-associated fXII activation, platelet secretion inhibited activated fXII (fXIIa), particularly due to a released C1-inhibitor. Platelet surface-associated fXIIa formation triggered contact pathway-dependent clotting in recalcified plasma. Computer modelling suggests that fXIIa inactivation was greatly decreased in thrombi under high blood flow due to inhibitor washout. Combined, the surface-associated fXII activation and its inhibition in solution herein may be regarded as a flow-sensitive regulator that can shift the balance between surface-associated clotting and plasma-dependent inhibition, which may explain the role of fXII at high shear and why fXII is important for thrombosis but negligible in haemostasis.

Dynamics of pathologic clot formation: a mathematical model

Recent studies have provided evidence of a significant role of the Hageman factor in pathologic clot formation. Since auto-activation of the Hageman factor triggers the intrinsic coagulation pathway, we study the dynamics of pathologic clot formation considering the intrinsic pathway as the predominant mechanism of this process. Our methodological approach to studying the dynamics of clot formation is based on mathematical modelling. Activation of the blood coagulation cascade, particularly its intrinsic pathway, is known to involve platelets. Therefore, equations accounting for the effects of activated platelets on the intrinsic pathway activation are included in our model. This brings about a considerable increase in the values of kinetic constants involved in the model of the principal biochemical processes resulting in clot formation. The purpose of this study is to elucidate the mechanism of pathologic clot formation. Since the time window of thrombolysis is 3-6h, we hypothesize that in many cases the rate of pathologic clot formation is much lower than that of haemostatic clot. This assumption is used to simplify the mathematical model and to estimate kinetic constants of biochemical reactions that initiate pathologic clot formation. The insights we gained from our mathematical model may lead to new approaches to the prophylaxis of pathologic clot formation. We believe that one of the most efficient ways to prevent pathologic clot formation is simultaneous inhibition of activated factors ХII and ХI.

Inhibiting the intrinsic pathway of coagulation with a factor XII-targeting RNA aptamer

BACKGROUND

Exposure of the plasma protein factor XII (FXII) to an anionic surface generates activated FXII that not only triggers the intrinsic pathway of blood coagulation through the activation of FXI but also mediates various vascular responses through activation of the plasma contact system. While deficiencies of FXII are not associated with excessive bleeding, thrombosis models in factor-deficient animals have suggested that this protein contributes to stable thrombus formation. Therefore, FXII has emerged as an attractive therapeutic target to treat or prevent pathological thrombosis formation without increasing the risk for hemorrhage.

OBJECTIVES

Using an in vitro directed evolution and chemical biology approach, we sought to isolate a nuclease-resistant RNA aptamer that binds specifically to FXII and directly inhibits FXII coagulant function.

METHODS AND RESULTS

We describe the isolation and characterization of a high-affinity RNA aptamer targeting FXII/activated FXII (FXIIa) that dose dependently prolongs fibrin clot formation and thrombin generation in clinical coagulation assays. This aptamer functions as a potent anticoagulant by inhibiting the autoactivation of FXII, as well as inhibiting intrinsic pathway activation (FXI activation). However, the aptamer does not affect the FXIIa-mediated activation of the proinflammatory kallikrein-kinin system (plasma kallikrein activation).

CONCLUSIONS

We have generated a specific and potent FXII/FXIIa aptamer anticoagulant that offers targeted inhibition of discrete macromolecular interactions involved in the activation of the intrinsic pathway of blood coagulation.

Factor XI and XII as antithrombotic targets

PURPOSE OF REVIEW

Arterial and venous thrombosis are major causes of morbidity and mortality, and the incidence of thromboembolic diseases increases as a population ages. Thrombi are formed by activated platelets and fibrin. The latter is a product of the plasma coagulation system. Currently available anticoagulants such as heparins, vitamin K antagonists and inhibitors of thrombin or factor Xa target enzymes of the coagulation cascade that are critical for fibrin formation. However, fibrin is also necessary for terminating blood loss at sites of vascular injury. As a result, anticoagulants currently in clinical use increase the risk of bleeding, partially offsetting the benefits of reduced thrombosis. This review focuses on new targets for anticoagulation that are associated with minimal or no therapy-associated increased bleeding.

RECENT FINDINGS

Data from experimental models using mice and clinical studies of patients with hereditary deficiencies of coagulation factors XI or XII have shown that both of these clotting factors are important for thrombosis, while having minor or no apparent roles in processes that terminate blood loss (hemostasis).

SUMMARY

Hereditary deficiency of factor XII (Hageman factor) or factor XI, plasma proteases that initiate the intrinsic pathway of coagulation, impairs thrombus formation and provides protection from vascular occlusive events, while having a minimal impact on hemostasis. As the factor XII-factor XI pathway contributes to thrombus formation to a greater extent than to normal hemostasis, pharmacological inhibition of these coagulation factors may offer the exciting possibility of anticoagulation therapies with minimal or no bleeding risk.

Identification and characterization of plasma kallikrein-kinin system inhibitors from salivary glands of the blood-sucking insectTriatoma infestans

Two plasma kallikrein-kinin system inhibitors in the salivary glands of the kissing bug Triatoma infestans, designated triafestin-1 and triafestin-2, have been identified and characterized. Reconstitution experiments showed that triafestin-1 and triafestin-2 inhibit the activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII and prekallikrein, and subsequent release of bradykinin. Binding analyses showed that triafestin-1 and triafestin-2 specifically interact with factor XII and high molecular weight kininogen in a Zn2+-dependent manner, suggesting that they specifically recognize Zn2+-induced conformational changes in factor XII and high molecular weight kininogen. Triafestin-1 and triafestin-2 also inhibit factor XII and high molecular weight kininogen binding to negatively charged surfaces. Furthermore, they interact with both the N-terminus of factor XII and domain D5 of high molecular weight kininogen, which are the binding domains for biological activating surfaces. These results suggest that triafestin-1 and triafestin-2 inhibit activation of the kallikrein-kinin system by interfering with the association of factor XII and high molecular weight kininogen with biological activating surfaces, resulting in the inhibition of bradykinin release in an animal host during insect blood-feeding.

Identification and characterization of a new kallikrein-kinin system inhibitor from the salivary glands of the malaria vector mosquito Anopheles stephensi

A new kallikrein-kinin system inhibitor, designated anophensin, was identified in the salivary glands of the malaria vector mosquito, Anopheles stephensi. In vitro reconstitution experiments showed that anophensin inhibits activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII (FXII) and prekallikrein (PK), and subsequent release of bradykinin. Additionally, anophensin inhibits activation of the kallikrein-kinin system on cultured human umbilical vein endothelial cells (HUVECs). Direct binding assays show that this inhibitory effect is due to Zn(2+)-dependent specific binding of anophensin to both FXII and high molecular weight kininogen (HK). Furthermore, anophensin interacts with both the N-terminus of FXII and domain D5 of HK, which are the binding domains for biological activating surfaces. These results suggest that anophensin inhibits activation of the kallikrein-kinin system by interfering with the association of FXII and HK with biological activating surfaces, resulting in the inhibition of bradykinin release in a host animal during insect blood-feeding.

Levels of intrinsic coagulation factors and the risk of myocardial infarction among men: opposite and synergistic effects of factors XI and XII

The role of the intrinsic coagulation system on the risk of myocardial infarction is unclear. In the Study of Myocardial Infarctions Leiden (SMILE) that included 560 men younger than age 70 with a first myocardial infarction and 646 control subjects, we investigated the risk of myocardial infarction for levels of factor XI (factor XIc) and factor XII (factor XIIc). Furthermore, the risks for factor VIII activity (factor VIIIc) and factor IX activity (factor IXc) were assessed. Factor XIc was 113.0% in patients compared with 109.8% in control subjects (difference, 3.2%; 95% CI, 1.1%-5.4%). The risk of myocardial infarction adjusted for age for men in the highest quintile compared with those in the lowest quintile was 1.8-fold increased (ORadj, 1.8; 95% CI, 1.2-2.7). In contrast, factor XIIc among patients with myocardial infarction was lower than in control subjects, respectively, 93.0% and 98.6% (difference, 5.6%; 95% CI, 3.3%-7.9%). The odds ratio of myocardial infarction for men in the highest quintile versus those in the lowest quintile was 0.4 (ORadj, 0.4; 95% CI, 0.2-0.5). The highest risk was found among men with both high factor XIc and low factor XIIc (analyses in tertiles: ORadj, 6.4; 95% CI, 2.2-18.0). Factor VIIIc increased the risk of myocardial infarction although not dose dependently. Factor IXc increased the risk; odds ratio of myocardial infarction for men in the highest quintile versus those in the lowest quintile was 3.2 (ORadj, 3.2; 95% CI, 2.0-5.1). Thus, factors XIc and XIIc have opposite and synergistic effects on the risk of myocardial infarction in men; factor VIIIc and factor IXc increase the risk.

Targeting coagulation factor XII provides protection from pathological thrombosis in cerebral ischemia without interfering with hemostasis

Formation of fibrin is critical for limiting blood loss at a site of blood vessel injury (hemostasis), but may also contribute to vascular thrombosis. Hereditary deficiency of factor XII (FXII), the protease that triggers the intrinsic pathway of coagulation in vitro, is not associated with spontaneous or excessive injury-related bleeding, indicating FXII is not required for hemostasis. We demonstrate that deficiency or inhibition of FXII protects mice from ischemic brain injury. After transient middle cerebral artery occlusion, the volume of infarcted brain in FXII-deficient and FXII inhibitor–treated mice was substantially less than in wild-type controls, without an increase in infarct-associated hemorrhage. Targeting FXII reduced fibrin formation in ischemic vessels, and reconstitution of FXII-deficient mice with human FXII restored fibrin deposition. Mice deficient in the FXII substrate factor XI were similarly protected from vessel-occluding fibrin formation, suggesting that FXII contributes to pathologic clotting through the intrinsic pathway. These data demonstrate that some processes involved in pathologic thrombus formation are distinct from those required for normal hemostasis. As FXII appears to be instrumental in pathologic fibrin formation but dispensable for hemostasis, FXII inhibition may offer a selective and safe strategy for preventing stroke and other thromboembolic diseases.

Are hemostasis and thrombosis two sides of the same coin?

Factor XII (FXII), a clotting enzyme that can initiate coagulation in vitro, has long been considered dispensable for normal blood clotting in vivo because hereditary deficiencies in FXII are not associated with spontaneous or excessive bleeding. However, new studies show that mice lacking FXII are protected against arterial thrombosis (obstructive clot formation) and stroke. Thus, FXII could be a unique drug target that could be blocked to prevent thrombosis without the side effect of increased bleeding.

A mosquito salivary protein inhibits activation of the plasma contact system by binding to factor XII and high molecular weight kininogen

The salivary glands of female mosquitoes contain a variety of bioactive substances that assist their blood-feeding behavior. Here, we report a salivary protein of the malarial vector mosquito, Anopheles stephensi, that inhibits activation of the plasma contact system. This factor, named hamadarin, is a 16-kDa protein and a major component of the saliva of this mosquito. Assays using human plasma showed that hamadarin dose-dependently inhibits activation of the plasma contact system and subsequent release of bradykinin, a primary mediator of inflammatory reactions. Reconstitution experiments showed that hamadarin inhibits activation of the plasma contact system by inhibition of the reciprocal activation of factor XII and kallikrein. Direct binding assays demonstrated that this inhibitory effect is due to hamadarin binding to both factor XII and high molecular weight kininogen and interference in their association with the activating surface. The assays also showed that hamadarin binding to these proteins depends on Zn(2+) ions, suggesting that hamadarin binds to these contact factors by recognizing their conformational change induced by Zn(2+) binding. We propose that hamadarin may attenuate the host's acute inflammatory responses to the mosquito's bites by inhibition of bradykinin release and thus enable mosquitoes to take a blood meal efficiently and safely.

Contradictory Functions of The Sulfatide in the Blood Coagulation System

It has long been controversial whether sulfatide stimulates blood coagulation or inhibits blood coagulation. In this paper, I demonstrated not only anticoagulant activity but also coagulant activity of sulfatide in vivo by using experimental animal models and presented the possibility that sulfatide may function contradictorily under physiological and/or pathological conditions. For instance, I presented possible involvement of sulfatide in thrombotic diseases associated with cancers.

Inhibition of factor XII in septic baboons attenuates the activation of complement and fibrinolytic systems and reduces the release of interleukin-6 and neutrophil elastase

In previous studies, we have shown that administration of monoclonal antibody (MoAb) C6B7 against human factor XII to baboons challenged with a lethal dose of Escherichia coli abrogates activation of the contact system and modulates secondary hypotension. To evaluate the contribution of activated contact proteases to the appearance of other inflammatory mediators in this experimental model of sepsis, we studied the effect of administration of MoAb C6B7 on activation of complement and fibrinolytic cascades, stimulation of neutrophil degranulation, and release of the proinflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6). Activation of the complement system, as reflected by circulating C3b/c and C4b/c levels, was significantly reduced in five animals that had received MoAb C6B7 before a lethal dose of E coli as compared with five control animals that had been given a lethal challenge only. Inhibition of contact activation also modulated the fibrinolytic response, since the release of tissue-type plasminogen activator (t-PA) and the appearance of plasmin-alpha2-antiplasmin (PAP) complexes into the circulation was significantly attenuated upon pretreatment with anti-factor XII MoAb. In contrast, plasma levels of plasminogen activator inhibitor (PAI) were modestly enhanced in the treatment group. Degranulation of neutrophils, as assessed by circulating elastase-alpha1-protease inhibitor complexes, and release of IL-6 but not of TNF-alpha was decreased in anti-factor XII-treated animals. Observed differences in the inflammatory response between treatment and control groups were not likely due to different challenges, since the number of E coli that had been infused, as well as circulating levels of endotoxin after the challenge, were similar for both groups. These data suggest that activation of the contact system modulates directly or indirectly various mediator systems involved in the inflammatory response during severe sepsis in nonhuman primates.

Synchronized inhibition of the phospholipid mediated autoactivation of factor XII in plasma by beta 2-glycoprotein I and anti-beta 2-glycoprotein I

Lupus anticoagulants are a group of antibodies commonly found in patients with autoimmune diseases such as systemic lupus erythematosus. Lupus anticoagulants inhibit phospholipid dependent coagulation and may bind to negatively charged phospholipids. Recent studies have suggested an association between anti-beta 2-glycoprotein I and a lupus anticoagulant, whose activity is frequently dependent on the presence of beta 2-glycoprotein I. Based on these observations, the effect of anti-beta 2-glycoprotein I on the autoactivation of factor XII in plasma was investigated. Autoactivation initiated by the presence of negatively charged phospholipids, but not by sulfatide, was strongly inhibited by immunoaffinity purified anti-beta 2-glycoprotein I. The dose-response curve of anti-beta 2-glycoprotein I was identical with that of a precipitating antibody, showing no inhibition at low and high antibody dilutions and maximal inhibition at an intermediate dilution. At high antibody concentrations, an increased rate of factor XIIa activation was observed. This increase was of the same magnitude as the decreased rate observed in plasma supplemented with the same amount of beta 2-glycoprotein I as in the plasma itself. This confirms the inhibitory effect of beta 2-GP-I on the contact activation and shows that inhibition is effective on the autoactivation of factor XII in plasma. The inhibitory action of beta 2-glycoprotein I was independent of the inhibition caused by the anti-beta 2-glycoprotein I/beta 2 glycoprotein I complex suggesting a synchronized inhibition of factor XII autoactivation by beta 2-glycoprotein I and anti-beta 2-glycoprotein I. The inhibition caused by the antibody is suggested to be caused by a reduced availability of negatively charged phospholipids due to the binding of the anti-beta 2-GP-I/beta 2-GP-I complex. This complex may be a lupus anticoagulant.

Inhibitory action of amyloid precursor protein against human Hageman factor (factor XII)

Amyloid precursor protein forms that contain Kunitz protease inhibitor domains are released from activated platelets, T-lymphocytes, and leukocytes and inhibit trypsin, plasmin, and activated factor XI. We investigated the effects of amyloid precursor protein isoforms on activated Hageman factor (factor XII), activated factor X (Stuart factor), and thrombin. Recombinant amyloid precursor proteins with or without the Kunitz domain, 770 and 695 amino acids, respectively, were produced in insect cells by Baculovirus expression (BAC770 and BAC695). Neither BAC695 nor BAC770 inhibited human alpha-thrombin or activated factor X. The partial thromboplastin time was prolonged by both amyloid precursor proteins, only one of which, BAC770, contains the Kunitz protease inhibitor domain. Both forms of amyloid precursor proteins inhibited ellagic acid-induced activation of Hageman factor but did not inhibit activated Hageman factor. Bismuth subgallate, which is an insoluble analog of ellagic acid, lost its ability to activate Hageman factor on being exposed to BAC770. Inhibition of ellagic acid-induced activation of Hageman factor by both forms of amyloid precursor protein was enhanced by heparin. These findings suggested that the heparin-binding domain of amyloid precursor proteins is not in the Kunitz domain. This heparin-binding domain may block the activation of Hageman factor by negatively charged agents. Thus, amyloid precursor proteins may be involved in the control of hemostasis, properties not all dependent on the Kunitz domain.

Inhibition of expression of monocyte interleukin-1 by inhibitors of Hageman factor (factor XII)

In an earlier study, activated species of Hageman factor (factor XII) induced elaboration of interleukin-1 by human monocytes. These observations did not address whether Hageman factor participated in endotoxin-induced release of interleukin-1. To examine this question, the release of interleukin-1 by endotoxin-stimulated human mononuclear cells was measured in the presence of popcorn inhibitor, a specific inhibitor of Hageman factor. In the experiments herein described, popcorn inhibitor sharply decreased the release of interleukin-1 by human mononuclear cells that were incubated with endotoxin. This observation suggests that Hageman factor may play a role in the elaboration of interleukin-1 by human mononuclear cells. Conforming with this view, the addition of antiserum directed against Hageman factor inhibited the release of interleukin-1 from endotoxin-stimulated mononuclear cells.

Novel plasma kallikrein inhibitors of the benzamidine type

Besides guanidino compounds and amines structurally related to arginine and lysine, compounds with an amidino moiety are inhibitors of trypsin-like enzymes. However, in most cases ordinary benzamidine derivatives are not selective inhibitors. Relatively selective inhibitors of some enzymes were found among amino acids containing a benzamidine moiety at the side chain. For example, derivatives of phenyl-alpha-aminobutyric acid with one or two amidino moieties such as the 4'-amidinoanilide of N alpha-[4-amidinophenylsulfonyl]-phenyl-alpha- aminobutyric acid are potent and selective inhibitors of plasma kallikrein (Ki = 0.15 microM). Using N alpha-arylsulfonylated 3-amidinophenylalanine as the key building block, novel inhibitors of plasma kallikrein were developed. Several esters and amides of the nipecotic acid derivative were synthesized which inhibit plasma kallikrein competitively with Ki values between 0.1 and 1.0 microM. The compounds prolonged aPTT in micromolar concentration, indicating an inhibition of the intrinsic coagulation pathway.

The effect of chemical modification of basic amino acid residues on the activation and amidolytic activity of Hageman factor (factor XII)

Modification of arginyl residues of Hageman factor by phenylglyoxal hydrate inhibits activation of this clotting factor in a plasma-free system, that is, in the absence of the other constituents of the contact activation system. Activation is also inhibited by alteration of the other two basic amino acid residues present, lysine and histidine. Chemical modification of histidine and arginine residues does not inhibit the amidolytic activity of activated Hageman factor. In contrast, modification of amino group(s) in N-terminal and lysine residues inhibits activated Hageman factor. Thus, basic amino acid residues essential to the activation or activity of Hageman factor appear to be variably accessible to chemical modification.

Histidine residues are essential for the surface binding and autoactivation of human coagulation factor XII

The role of histidine residue in the surface binding and autoactivation of human factor XII has been investigated by chemical modification with diethyl pyrocarbonate. It is found that low concentrations of diethyl pyrocarbonate have profound inhibitory effects on the surface binding activity of factor XII. At 2.5-fold molar excess of the reagent, six histidines are modified and 80% of the amidolytic activity is lost. Electrophoretic studies show that the modified protein has lost the capacity to bind to the surface, resulting in diminished proteolytic autoactivation. When modification is performed in the presence of the surface, dextran sulfate, two of the six histidines are protected from modification and the amidolytic activity is completely preserved. It is concluded that histidine residues in factor XII play key role in its surface binding activity.

The contact system contributes to hypotension but not disseminated intravascular coagulation in lethal bacteremia. In vivo use of a monoclonal anti-factor XII antibody to block contact activation in baboons

The hypotension and disseminated intravascular coagulation (DIC) in bacteremia is thought to be mediated by the combined actions of cytokines, prostaglandins, and complement. The contact system, via the release of bradykinin and the activation of Factor XI, has been postulated to be contributing to the observed hypotension and DIC. Using a mAb to Factor XII (C6B7), we blocked the activation of the contact system in an established experimental baboon model in which Escherichia coli was infused to produce lethal bacteremia with hypotension. The untreated group (n = 5) displayed contact activation, manifested by a significant decrease in high molecular weight kininogen (HK) and a significant increase in alpha 2 macroglobulin-kallikrein complexes (alpha 2M-Kal). The C6B7-treated group (n = 5) showed an inactivation of Factor XII and the changes in HK and alpha 2M-Kal complexes were prevented. Both groups developed DIC manifested by a decrease in platelet, fibrinogen, and Factor V levels. The untreated group developed irreversible hypotension. The treated group experienced an initial hypotension that was reversed and extended the life of the animals. This study suggests that irreversible hypotension correlates with prolonged activation of the contact system, and specific antibody therapy can modulate both the pathophysiological and biochemical changes.

Inhibition of the activation of hageman factor (factor XII) by eosinophils and eosinophilic constituents

Several syndromes characterized by striking eosinophilia may be complicated by thrombosis. The experiments described indicate that, paradoxically, eosinophils and certain of their constituents inhibit the activation of Hageman factor (HF, factor XII). In earlier studies, suspensions of mixed types of granulocytes, other nucleated peripheral blood cells, and platelets inhibited activation of Hageman factor by ellagic acid, glass, and sulfatides. After these cells were sedimented by centrifugation, the supernatant fluids were also inhibitory. No attempt had been made earlier to distinguish among different granulocytic species. In the present study, suspensions of eosinophils and the supernatant fluid after eosinophils had been separated by centrifugation inhibited activation of Hageman factor by ellagic acid. The protein concentration of that amount of supernatant fluid that inhibited activation by about half was 16 micrograms/ml, approximately the same as had been described for suspensions of peripheral blood mononuclear cells. Activation of Hageman factor by ellagic acid was also inhibited by certain constituents of eosinophils, including eosinophil peroxidase, eosinophil major basic protein and eosinophil cationic protein. Inhibition was not specific for ellagic acid-induced activation of Hageman factor, as inhibition was also observed with sulfatide-induced activation. Inhibition was presumably related to neutralization of the negative charge of activators of Hageman factor. Thus, bismuth subgallate, a particulate activator of Hageman factor, was no longer effective after it had been exposed to eosinophil cationic protein. The observations reported here raise the question of whether in vivo eosinophils modulate certain of the defense reactions ascribed to Hageman factor.

Factor XII activation and inhibition in inflammation

Biochemical observations during clinical sepsis using functional and immunological measurements of enzymes, cofactors and inhibitors of the kallikrein-kinin system indicate that activation of these proteases occur during hypotensive gram-negative septicemia and adult respiratory distress syndrome. Using animal models of septicemia, we demonstrated that protease inhibitors or neutralizing monoclonal antibodies to proteins of the contact system inhibit or prevent the formation of kallikrein and the decrease in kininogen. In addition, the irreversible phase of hypotension can be prevented and survival prolonged. Thus, bradykinin is one of the important mediators of hypotension. In contrast, the contact system plays little role in the associated DIC. In cardiopulmonary bypass, the formation of kallikrein leads to neutrophil degranulation and release of elastase. Selective inhibitors of kallikrein not only block its activation but play a predominant role in inhibiting elastase release.

Endothelial cells produce a substance that inhibits contact activation of coagulation by blocking the activation of Hageman factor

Human umbilical vein endothelial cells (HUVECs) produce a property that impairs the generation of coagulant and amidolytic activity initiated when normal human plasma is exposed to glass. This inhibitory property blocks the adsorption of Hageman factor (factor XII) to glass, thereby preventing the activation of Hageman factor, but does not impair the coagulant or amidolytic activity of already activated Hageman factor (factor XIIa). This property in HUVEC lysates could be neutralized by a purified preparation of Hageman factor but not by purified prekallikrein or high molecular mass kininogen. A partially purified inhibitory fraction from cell lysates exhibited a single homogeneous band in SDS/PAGE of approximately 22.5 kDa. Inhibitory activity was also found in concentrates of conditioned media from HUVECs, which also impaired the binding of Hageman factor to a surface; it may not be identical with that found in cell lysates.

Inhibition of the activation of Hageman factor (factor XII) by extracts of Schistosoma mansoni

How intravascular helminth parasites evade host hemostatic defense mechanisms and survive within the circulating blood has not been adequately explained. Previous reports have described an inhibitor of the intrinsic clotting pathway in extracts of adult Schistosoma mansoni. Using a purified preparation of Hageman factor, we examined the ability of schistosome extracts and secretory products to inhibit the activation of human Hageman factor (factor XII) in an amidolytic assay. Both schistosome extracts and secretory products inhibited the activation of purified Hageman factor by more than 95%. Schistosome extracts inhibited activation of Hageman factor both by ellagic acid and by bovine sulfatides. In contrast, activated Hageman factor retained full activity in the presence of schistosome extracts as tested both on an amidolytic synthetic substrate and a natural substrate, plasma thromboplastin antecedent (factor XI). Our findings indicate that extracts and secretory products of adult Schistosoma mansoni contain a potent inhibitor of the activation of Hageman factor. Knowledge of a site at which schistosomes inhibit the intrinsic clotting pathway provides added insight into the mechanisms by which the parasites avoid the host hemostatic defense mechanisms.

Prekallikrein activation in human, bovine, and rabbit plasmas: presence of an inhibitor in bovine plasma

Contact of human plasma with a negatively charged surface such as dextran sulfate activates prekallikrein to kallikrein, which releases the vasoactive peptide bradykinin from high-molecular-weight kininogen. The dextran sulfate-induced activation of prekallikrein at 0 degree C (assayed by its amidolytic activity on the chromogenic substate S-2302) could not be observed in either bovine or rabbit plasmas when compared to human plasma. Neither bovine nor rabbit plasma inhibited the amidolytic activity of contact-activated human plasma at 0 degrees C. The activation of prekallikrein in human plasma was significantly inhibited by the addition of bovine plasma but not by rabbit plasma. Bovine plasma (0.025 units, 1 unit = 1 ml of plasma) caused 68.8% inhibition of prekallikrein activation. Eighty percent of the inhibitory property of bovine plasma was present in the greater than 30,000-molecular-weight fraction. These results indicate the presence of an inhibitor(s) of prekallikrein activation in bovine plasma.

Effect of Neurotropin on the activation of the plasma kallikrein-kinin system

Bradykinin (BK), an important mediator of allergic reactions and pain induction, is released by the activation of the plasma kallikrein-kinin (K-K) cascade. Neurotropin is a biological material obtained from inflamed rabbit skin inoculated with vaccinia virus and is widely used clinically in Japan as an effective agent for these disorders. Since its mechanism of action is not clearly known, we have investigated the effects of Neurotropin on the human plasma K-K system. In dextran sulfate-activated plasma, Neurotropin inhibited the formation of BK, the cleavage of high molecular weight kininogen (HK) and the formation of kallikrein-C1 inhibitor and activated coagulation factor XII (FXIIa)-C1 inhibitor complexes. Experiments using purified enzyme of the K-K cascade indicated that Neurotropin inhibited surface-mediated activation of coagulation factor XII (FXII) and the activation of prekallikrein by FXIIa. Neurotropin also inhibited the binding of FXII and HK to the activating surface. These data suggest that the ameliorating effects of Neurotropin in allergic disorders and pain syndromes may be related to this ability to inhibit activation of the K-K cascade and consequently the formation of BK.

Inhibition of the activation of Hageman factor (factor XII) by human vascular endothelial cell culture supernates

The supernatant fluid (conditioned medium) of cultured human vascular endothelial cells inhibits activation of Hageman factor (factor XII), whether by ellagic acid, bovine brain sulfatides, or bismuth subgallate; inhibition appears to be a property of one or more proteins in the culture supernates. This phenomenon may contribute to maintaining the fluidity of circulating blood by inhibiting surface activation of the intrinsic pathway of coagulation.

On the target specificity of plasminogen activator inhibitor 1: the role of heparin, vitronectin, and the reactive site

Plasminogen activator inhibitor 1 (PAI-1) is the fast-acting inhibitor of both tissue-type and urokinase-type plasminogen activators (t-PA, u-PA) and is an essential regulatory protein of the fibrinolytic system. In the presence of either the protein vitronectin or the glycosaminoglycan heparin, PAI-1 is also an efficient inhibitor of thrombin. To assess whether these cofactors turn PAI-1 into a general protease inhibitor or whether their influence is restricted to thrombin, the second-order association rate constants between PAI-1 and the human plasma proteases t-PA, u-PA, plasmin, thrombin, Factor Xa (FXa), and Factor XIIa (FXIIa) in the absence and in the presence of either vitronectin or heparin are determined. In addition, the role of the PAI-1 reactive site P3 to P3' residues for the specificity of inhibition was studied by using PAI-1 reactive site mutants. Our results show that: (1) Heparin exclusively increases the rate of inhibition of thrombin by PAI-1, whereas in the presence of heparin the rate of inhibition of the other proteases is not altered; (2) Vitronectin is an obligatory cofactor for the inhibition of thrombin by PAI-1. In addition, vitronectin moderately increases the rate of inhibition by PAI-1 of u-PA and of plasmin, but does not alter the rate of inhibition of t-PA, FXa, or FXIIa; (3) Apart from the important role of the P1 residue, no consensus can be presented on the nature of other residues within the P3 to P3' region with regard to target protease specificity.

Inhibition of the activation of Hageman factor (factor XII) by soluble human placental collagens types III, IV, and V

The initial step in the formation of thrombin via the intrinsic pathway is the activation of Hageman factor (factor XII). Some, but not all, studies have shown that this activation may be brought about by collagen. We examined the effect of three types of soluble human placental collagen on Hageman factor. Collagen types III, IV, and V did not appear to activate Hageman factor under the conditions tested. To the contrary, these collagen species inhibited activation of Hageman factor by glass or ellagic acid. These studies suggest that some types of collagen may play an inhibitory role in blood coagulation.

Inhibition of the activation of Hageman factor (factor XII) and of platelet aggregation by extracts of Brugia malayi microfilariae

In human filariasis, large numbers of blood-borne microfilariae circulate unimpeded through the blood stream. How intravascular filarial parasites avoid precipitating thrombosis has not been studied in detail. We hypothesized that extracts of Brugia malayi microfilariae would contain factors that inhibit activation of hemostatic mechanisms. Initial studies demonstrated an inhibitor specific for the intrinsic coagulation cascade. The addition of microfilarial extracts to human plasma prolonged the activated partial thromboplastin time in a dose-dependent fashion but did not prolong the prothrombin, thrombin, or Russell's viper venom times. Microfilarial extracts (0.1 mg/ml) completely inhibited activation of Hageman factor (factor XII, at 0.05 U/ml) as measured in an amidolytic assay. Hageman factor previously activated by ellagic acid (factor XIIa) retained full enzymatic activity in the presence of microfilarial extract (0.1 mg/ml). The presence of inhibitory activity in the culture medium of live parasites raises the possibility that microfilariae secrete an inhibitory protein into their local environment. Microfilarial extracts at a final concentration of 0.1 mg/ml also inhibited collagen- and adenosine diphosphate-induced platelet aggregation. Arachidonic acid-induced platelet aggregation was inhibited by microfilarial extracts at a final concentration of 0.6 mg/ml. These results suggest that microfilariae of Brugia malayi, a human filarial parasite, may avoid initiating thrombosis through inhibition of the intrinsic coagulation pathway and platelet aggregation.

A new protein inhibitor of trypsin and activated Hageman factor from pumpkin (Cucurbita maxima) seeds

A protein inhibitor (CMTI-V; Mr 7106) of trypsin and activated Hageman factor (Factor XIIa), a serine protease involved in blood coagulation, has been isolated for the first time from pumpkin (Cucurbita maxima) seeds by means of trypsin-affinity chromatography and reverse phase high performance liquid chromatography (HPLC). The dissociation constants of the inhibitor complexes with trypsin and Factor XIIa have been determined to be 1.6 x 10(-8) and 4.1 x 10(-8) M, respectively. The primary structure of CMTI-V is reported. The protein has 68 amino acid residues and one disulfide bridge and shows a high level of sequence homology to the Potato I inhibitor family. Furthermore, its amino terminus consists of an N-acetylates Ser. The reactive site has been established to be the peptide bond between Lys44-Asp45. The modified inhibitor which has the reactive site peptide bond hydrolyzed inhibits trypsin but not the Hageman factor.

[Polycythemia vera with an inhibitor against factor XII]

A case of polycythemia vera with an inhibitor against factor XII was reported. A 60-year-old female was admitted to Hokkaido University School Hospital because of erythrocytosis and hepatosplenomegaly. The hemoglobin was 22.5 g/dl and white cell count was 9,500/microliters without immature cells. The platelet count was 484,000/microliters. Bone marrow specimens showed marked hypercellularity. Philadelphia chromosome was not found on chromosome analysis. She was diagnosed as polycythemia vera according to the criteria of polycythemia Vera Study Group. Activity of factor XII was found to be decreased on the initial examination, but she had no personal and familial history of bleeding. In order to clarify the cause of decreased activity of factor XII, her plasma was mixed with normal plasma, and then examined PTT using factor XII deficient plasma. Her plasma mixed with equivalent normal plasma did not show the correction of prolonged PTT. It was suggested that an inhibitor of her plasma was included in the IgG fraction using gel chromatography. The patient was treated with phlebotomy and administration of N4-palmitoyl (1- -D-arabinofurasyl) cytosine (derivative of cytosine arabinoside; PLAC) 200 mg/day and Busulfan (1 mg/day). Factor XII was not corrected by phlebotomy, but corrected gradually by administration of PLAC and Busulfan.

Effect of heparin on the inhibition of the contact system enzymes

1. One can accurately predict the contribution of each inhibitor to the total inactivation of an enzyme in plasma once its pseudo-first-order reaction rate constant and concentration are known. 2. Because the mechanism of augmentation of the inactivation rate of an enzyme by ATIII occurs via formation of an ATIII-heparin complex, the degree of potentiation can be predicted by knowing the binding capacity (sites per mole) of the heparin preparation and the concentration of heparin in the reaction (to calculate the concentration of the ATIII-heparin complex). 3. The augmentation by heparin of the inactivation rate of a particular enzyme by ATIII is dependent upon the presence of other enzymes with higher kassoc, since these would strongly compete for the ATIII-heparin complex. 4. In a plasma environment, using therapeutic levels of heparin, there is no augmentation of the inactivation rate of any of the contact enzymes.

A monoclonal antibody recognizing an iscosapeptide sequence in the heavy chain of human factor XII inhibits surface-catalyzed activation

A monoclonal antibody (mAb B7C9) to human factor XII was produced which blocked the surface mediated coagulant activity of factor XII but not the amidolysis of H-D-ProPheArg-pNA. The mAb epitope was localized to a 20 amino acid sequence of 2.5 kDa in the heavy chain of factor XII.

Induction of fibrinolysis by polyanions in human plasma

The fibrinolytic potency of several polyanions was comparatively investigated. Fibrinolytic activity was measured in a whole plasma assay using H-D-Val-Leu-Lys-pNA (S-2251) as chromogenic substrate and by a fibrin plate assay using plasminogen rich fibrin plates. In the chromogenic substrate assay potent fibrinolytic polyanions comprised dextran sulfate, GAGPS, pentosan polysulfate, polyanethol sulfate, l-carrageenan and i-carrageenan. Chondroitin sulfates A, B, C, keratan sulfate, ribonucleic acid, k-carrageenan and heparin were weakly fibrinolytic. Hyaluronic acid and lipopolysaccharide from E. coli were inactive. Similar results were obtained when fibrinolytic activity was measured by a fibrin plate assay. All polyanions except lipopolysaccharide produced lysis zones. Induction of fibrinolytic activity in human plasma was shown to be at least partially dependent on Hageman factor. In factor XII deficient plasma no fibrinolysis was induced by any of the polyanions when measured in the fibrin plate assay. In the chromogenic substrate assay corn Hageman factor inhibitor (CHFI) inhibited the activation of S-2251 cleaving enzyme by GAGPS, pentosan polysulfate, polyanethol sulfate, heparin, and ribonucleic acid near completely. The activation by dextran sulfate was inhibited by 45%. Heparin, pentosan polysulfate and GAGPS, three polyanions of therapeutic interest were separately compared. In both assays GAGPS proved the most potent activator, while pentosan polysulfate exhibited 83% and 44% and heparin 32% and 14% of GAGPS fibrinolytic activity in the chromogenic substrate test and the fibrin plate assay, respectively.

[Activation of the kallikrein-kinin system by intravenous administration of ionic and non-ionic radiographic contrast media]

Conventional ionic contrast media (sodium meglumine and amidotrizoate) were injected intravenously in 22 patients (group A); the nonionic, low-osmolar contrast medium iohexol was similarly administered to 24 patients (group B). This resulted in a significant (group A: P less than 0.05; group B: P less than 0.01) reduction in prekallikrein (93 and 94%, respectively), inhibition of kallikrein (90 and 88%), beta-factor XIIa inhibition (86 and 85%), and C1 inhibition (90 and 91%). Factor XII levels remained unchanged in both groups. These results indicate that the kallikrein-kinin system is activated even during routine, complication-free radiographic procedures involving contrast-media injection. There was no difference between ionic and nonionic contrast media.

Interaction of bovine factor XIIa with an inhibitor from bovine plasma

An inhibitor of factor XIIa has been purified from bovine plasma and characterized (Thornton, R.D. and Kirby, E.P. (1987) J. Biol. Chem. 262, 12714-12721). This inhibitor interacts with XIIa to form a very stable complex with a 1:1 stoichiometry. The active site of XIIa, located on the light chain, is directly involved in the interaction, and complex formation between factor XIIa inhibitor and XIIa can be blocked by diisopropyl fluorophosphate, corn trypsin inhibitor, or the chromogenic substrate S2302. Incubation of the complex with excess XIIa does not result in cleavage of the complex. The complex does not spontaneously dissociate and is stable to boiling, SDS, thiocyanate, acid, and hydroxylamine or Tris at pH 7-10. In addition to complex formation, a cleaved form of factor XIIa inhibitor can be observed. We suggest that the inhibitor is acting as a mechanism-based inactivator, using the criteria of time-dependent inactivation under pseudo-first-order conditions, 1:1 stoichiometry, active site involvement, kinetic protection by substrate or by an active site inhibitor, and partitioning between cleavage of factor XIIa inhibitor and inactivation by complex formation.

Isolation and characterization of an inhibitor of factor XIIa from bovine plasma

An inhibitor of factor XIIa has been purified to homogeneity from bovine plasma. The purification steps included precipitation of contaminating proteins with polyethylene glycol and chromatography on DEAE-cellulose, Affi-Gel blue, and immobilized wheat germ lectin. The apparent molecular weight of the XIIa inhibitor (called INH1) was 85,000, reduced, and 92,000, nonreduced, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The extinction coefficient (E0.1%(280)) of INH1 is 1.3, and the protein contains 17.7% carbohydrate. Purified antibody to INH1 raised in either rabbits or chickens formed a precipitin line of identity with purified INH1 and a component of bovine plasma, but there was no reaction with purified human inhibitors or with any component of human plasma. INH1 inhibits bovine and human XIIa, bovine and human C1-esterase, and human kallikrein, but does not inhibit bovine kallikrein, bovine trypsin, human plasmin, or human thrombin. This activity is similar to that of C1-inhibitor but different from antithrombin III, alpha 2-antiplasmin, or alpha 1-protease inhibitor. INH1 at a physiological concentration (0.47 microM) causes rapid inactivation of XIIa. The two molecules react in a 1:1 stoichiometry with a second-order rate constant of 1.23 X 10(6) M-1 min-1.

Effect of negatively charged activating compounds on inactivation of factor XIIa by Cl inhibitor

Human factor XII, upon exposure to negatively charged surfaces such as kaolin, sulfatides, and heparin, is converted to enzymatic forms, factor XIIa and factor XIIf. Cl inhibitor has been quantitatively demonstrated to be the primary plasma inhibitor of both factor XIIa and factor XIIf. Studies were performed to determine whether the presence of artificial, negatively charged surfaces influenced the ability of Cl inhibitor to inhibit factors XIIa and XIIf. Kaolin and sulfatides slowed the rate of inhibition of factor XIIa by Cl inhibitor 4.8- and 2-fold, respectively, whereas they had no effect on the inhibition of factor XIIf by Cl inhibitor. Heparin in a concentration of 65 U/ml decreased the inhibition rate of factor XIIa by Cl inhibitor, but, at the same concentration, had less of an effect on the ability of Cl inhibitor to inhibit factor XIIf. These studies indicate that negatively charged surfaces protect factor XIIa but not factor XIIf from inhibition from Cl inhibitor. Since the difference between factors XIIa and XIIf consists of the presence of a surface binding region in factor XIIa, the basis of this protection must reside in the surface binding residues of factor XII. These in vitro events suggest that surface-bound factor XIIa may hydrolyze its physiologic substrates, factor XI and prekallikrein, in an environment partially protected from inhibition by Cl inhibitor.

Inhibition of the activation of Hageman factor (factor XII) by complement subcomponent C1q

Hageman factor (HF, Factor XII) is activated by glass, collagen, and ellagic acid, and initiates blood coagulation via the intrinsic pathway. C1q inhibits collagen-induced platelet aggregation and adherence of platelets to glass, effects attributable to the collagen-like region of C1q. We examined the actions of C1q on HF activation. Incubation of C1q with HF before addition of HF-deficient plasma extended the activated partial thromboplastin time. Similarly, when glass tubes were coated with C1q before testing, the partial thromboplastin time of normal plasma was increased. C1q reduced the activation of HF by ellagic acid, as measured by the release of p-nitroaniline from the synthetic substrate H-D-prolyl-L-phenylalanyl-L-arginine-p-nitroanilide dihydrochloride, an effect inhibited by monoclonal anti-human C1q murine IgG and by digestion of C1q by collagenase. Thus, C1q inhibits activation of HF in vitro in clot-promoting and amidolytic assays and suggests a regulatory mechanism for the inhibition of coagulation.

Protection by recombinant alpha 1-antitrypsin Ala357 Arg358 against arterial hypotension induced by factor XII fragment

The specificity of serpin superfamily protease inhibitors such as alpha 1-antitrypsin or C1 inhibitor is determined by the amino acid residues of the inhibitor reactive center. To obtain an inhibitor that would be specific for the plasma kallikrein-kinin system enzymes, we have constructed an antitrypsin mutant having Arg at the reactive center P1 residue (position 358) and Ala at residue P2 (position 357). These modifications were made because C1 inhibitor, the major natural inhibitor of kallikrein and Factor XIIa, contains Arg at P1 and Ala at P2. In vitro, the novel inhibitor, alpha 1-antitrypsin Ala357 Arg358, was more efficient than C1 inhibitor for inhibiting kallikrein. Furthermore, Wistar rats pretreated with alpha 1-antitrypsin Ala357 Arg358 were partially protected from the circulatory collapse caused by the administration of beta-Factor XIIa.