Analysis of protein S C4b-binding protein interactions by homology modeling and inhibitory antibodies

A monoclonal antibody (mAb 6F6) directed against the beta-chain of C4b-binding protein (C4BP) was previously shown to inhibit the binding of protein S to C4BP. To localize the epitope of this antibody, 23 overlapping synthetic peptides (15-mers) covering the entire sequence (1-235) of the beta-chain of C4BP were used. When the immobilized peptides were screened for their ability to bind mAb 6F6, only peptide beta(51-65) showed high-affinity binding. The apparent affinity of mAb 6F6 for immobilized peptide beta(51-65) was somewhat similar to that for native C4BP with Kd approximately 1 nM for C4BP and approximately 9 nM for peptide beta(51-65). Peptide beta(51-65) inhibited the binding of the mAb 6F6 to immobilized C4BP with half-maximal inhibition at 30 microM peptide. Clotting assays of protein S anticoagulant cofactor activity using a factor Xa-1-stage assay with activated protein C allow measurement of free protein S in solution since only free protein S is active. Studies using such clotting assays showed that preincubation of C4BP with either mAb 6F6 or polyclonal anti-beta(31-45) antibodies inhibited the formation of the complex between C4BP and protein S. Previous studies showed that, although peptide beta(51-65) itself does not inhibit complex formation, peptide beta(31-45) does bind directly to protein S and does inhibit protein S binding to C4BP. The three-dimensional structure of the first SCR (residues 2-60) of the C4BP beta-chain was made on the basis of homology modeling.(ABSTRACT TRUNCATED AT 250 WORDS)

Exonic polymorphisms in the protein C gene: interethnic comparison between Caucasians and Asians

Plasma protein C deficiency is associated with inherited thrombotic disease. Allelic frequencies of five previously reported DNA polymorphisms and a new polymorphic site (C 8480 T) were calculated in Asian and American Caucasian individuals by direct genomic sequencing and compared to previous reports.

Isolation and characterization of the gene encoding 2,3-oxidosqualene-lanosterol cyclase from Saccharomyces cerevisiae

The ERG7 gene encoding oxidosqualene-lanosterol cyclase [(S)-2,3-epoxysqualene mutase (cyclizing, lanosterol forming), EC 5.4.99.7] from Saccharomyces cerevisiae has been cloned by genetic complementation of a cyclase-deficient erg7 strain. The DNA sequence of this gene has been determined and found to contain an open reading frame of 2196 nt (including stop codon) that encodes a predicted protein of 731 amino acids. The predicted molecular mass of the S. cerevisiae cyclase, 83.4 kDa, is similar to the predicted molecular masses of the oxidosqualene-lanosterol cyclase from Candida albicans and the oxidosqualene-cycloartenol cyclase from Arabidopsis thaliana, as well as to the molecular masses assigned to vertebrate oxidosqualene-lanosterol cyclases; however, it is substantially larger than the molecular mass assigned to purified S. cerevisiae cyclase. At the level of DNA and predicted amino acid sequences, the S. cerevisiae and C. albicans cyclases share 56% and 63% identity, respectively. Tryptophan and tyrosine residues are unusually abundant in the predicted amino acid sequences of (oxido)-squalene cyclases, leading to a hypothesis that electron-rich aromatic side chains from these residues are essential features of cyclase active sites.

Blood coagulation factor Va abnormality associated with resistance to activated protein C in venous thrombophilia

A coagulation test abnormality, termed activated protein C (APC) resistance, involving poor anticoagulant response to APC is currently the most common laboratory finding among venous thrombophilic patients. Because the anticoagulant activity of APC involves inactivation of factors Va and VIIIa, studies were made to assess the presence of abnormal factors V or VIII. Diluted aliquots of plasma from two unrelated patients with APC resistance and thrombosis were added to either factor VIII-deficient or factor V-deficient plasma and APC resistance assays were performed. The results suggested that patients' factor V but not factor VIII rendered the substrate plasma APC resistant. When factor V that had been partially purified from normal or APC resistant patients' plasmas using immunoaffinity chromatography was added to factor V-deficient plasma, APC resistance assays showed that patients' factor V or factor Va, but not normal factor V, rendered the substrate plasma resistant to APC. Studies of the inactivation of each partially purified thrombin-activated factor Va by APC suggested that half of the patients' factor Va was resistant to APC. These results support the hypothesis that the APC resistance of some venous thrombophilic plasmas is caused by abnormal factor Va.

Alteration of fibrin network by activated protein C

The antithrombotic plasma enzyme, activated protein C (APC), may play a role in thrombolysis. In vitro, acceleration of clot lysis by APC depends on its ability to inhibit the activation of prothrombin. The effect of APC on the assembly and dispersion of fibrin network was studied using turbidimetry, plasmin digestion of fibrin, and electron microscopy of plasma clots. The addition of APC before clotting but not after clotting accelerated clot lysis. The rate of increase in the turbidity of clotting plasma was reduced by APC. The turbidity of plasma clots containing APC was directly related to the clot lysis time. Fibrin from plasma clots that were formed in the presence of APC yielded less fibrin degradation products than fibrin from clots without added APC. Furthermore, APC reduced the diameter and relative number of fibrin fibers in plasma clots during gel assembly. We propose that APC may enhance the efficacy of thrombolysis by reducing the relative mass of fibrin within maturing thrombi.

Structural basis for type I and type II deficiencies of antithrombotic plasma protein C: patterns revealed by three-dimensional molecular modelling of mutations of the protease domain

Familial deficiency of protein C is associated with inherited thrombophilia. To explore how specific missense mutations might cause observed clinical phenotypes, know protein C missense mutations were mapped onto three-dimensional homology models of the protein C protease domain, and the implications for domain folding and structure were evaluated. Most Type I missense mutations either replaced internal hydrophobic residues (I201T, L223F, A259V, A267T, A346T, A346V, G376D) or nearby interacting residues (I403M, T298M, Q184H), thus disrupting the packing of internal hydrophobic side chains, or changed hydrophilic residues, thus disrupting ion pairs (N256D, R178W). Mutations (P168L, R169W) at the activation site destabilized the region containing the activation peptide structure. Most Type II mutations involved solvent-exposed residues and were clustered either in a positively charged region (R147W, R157Q, R229Q, R352W) or were located in or near the active site region (S252N, D359N, G381S, G391S, H211Q). The cluster of arginines 147, 157, 229, and 352 may identify a functionally important exosite. Identification of the spatial relationships of natural mutations in the protein C model is helpful for understanding manifestations of protein C deficiency and for identification of novel, functionally important molecular features and exosites.

Models of the serine protease domain of the human antithrombotic plasma factor activated protein C and its zymogen

Three-dimensional structural analysis of physiologically important serine proteases is useful in identifying functional features relevant to the expression of their activities and specificities. The human serine protease anticoagulant protein C is currently the object of many genetic site-directed mutagenesis studies. Analyzing relationships between its structure and function and between naturally occurring mutations and their corresponding clinical phenotypes would be greatly assisted by a 3-dimensional structure of the enzyme. To this end, molecular models of the protease domain of protein C have been produced using computational techniques based on known crystal structures of homologous enzymes and on protein C functional information. The resultant models corresponding to different stages along the processing pathway of protein C were analyzed for structural and electrostatic differences arising during the process of protein C maturation and activation. The most satisfactory models included a calcium ion bound to residues homologous to those that ligate calcium in the trypsin structure. Inspection of the surface features of the models allowed identification of residues putatively involved in specific functional interactions. In particular, analysis of the electrostatic potential surface of the model delineated a positively charged region likely to represent a novel substrate recognition exosite. To assist with future mutational studies, binding of an octapeptide representing a protein C cleavage site of its substrate factor Va to the enzyme's active site region was modeled and analyzed.

Protein S binds to and inhibits factor Xa

Although human protein S binds to human factor Va and inhibits prothrombinase activity, this inhibition is not totally dependent on factor Va. Hence, we investigated possible interaction of protein S with human factor Xa. Factor Xa, diisopropylphospho-factor Xa and their biotin derivatives ligand blotted specifically to protein S and protein S ligand blotted specifically to factor X and factor Xa. Biotinylated factors X and Xa bound to immobilized protein S and, reciprocally, protein S bound to immobilized factor Xa with a Kd of approximately 19 nM. In fluid phase, protein S bound to factor Xa with a Kd of approximately 18 nM. Protein S at 33 nM reversibly inhibited 50% of factor Xa amidolytic activity. Protein S inhibition of prothrombin conversion to thrombin by factor Xa was phospholipid-independent and was 1.6 times stimulated by Ca2+ ions. Inhibition of prothrombinase activity by protein S was 2.3-fold more potent in the presence of factor Va, with 50% inhibition at approximately 8 nM protein S. Protein S prolonged the factor Xa one-stage clotting time of protein S-depleted plasma in a dose-dependent manner. These data demonstrate mechanisms of anticoagulant action for protein S that are independent of activated protein C and that involve direct binding to factors Xa and Va and direct inhibition of factor Xa.

A protein S binding site on C4b-binding protein involves beta chain residues 31-45

C4b-binding protein (C4BP) down-regulates the anticoagulant cofactor activity of protein S in the protein C pathway since free protein S but not the protein S:C4BP complex is anticoagulantly active. To identify beta chain residues responsible for binding protein S, synthetic overlapping pentadecapeptides covering the entire 1-235 sequence were tested as inhibitors of complex formation. The peptide comprising residues 31-45 (VCIKGYHLVGKKTLF) from the first short consensus repeat domain inhibited the binding of C4BP to protein S with half-maximal inhibition at 20-45 microM, and studies suggested the sequence of YxLVG was crucial. Peptide beta(31-45) specifically inhibited the APC cofactor activity of purified protein S in Xa-1-stage coagulation assays with 50% inhibition at 15 microM peptide. Peptide beta(31-45) and related peptides such as beta(34-42) inhibited the binding of protein S to an antipeptide monoclonal antibody made against residues 420-434 of protein S (monoclonal antibody LJ-56). Polyclonal anti-beta(31-45) peptide antibodies inhibited complex formation. Dose-dependent binding studies showed that protein S bound directly to immobilized peptide beta(31-45). These results show that residues 31-45 of the C4BP beta chain provide a binding site for protein S, and they suggest that the C4BP beta chain residues 34-42 are located near residues 420-434 of protein S in the protein S:C4BP complex.

Evidence for the regulation of urokinase and tissue type plasminogen activators by the serpin, protein C inhibitor, in semen and blood plasma

Since the serine protease inhibitor, protein C inhibitor (PCI), is present in seminal plasma at approximately 3 microM, complexes of PCI with urokinase (uPA) and tissue type (tPA) plasminogen activator were quantitated using sandwich enzyme-linked immunosorbent assays (ELISA's). Seminal plasma (N = 10) collected in the absence of extrinsic inhibitors had a mean of 25 +/- 5 ng/ml uPA:PCI, 76 +/- 23 ng/ml tPA:PCI, and 4 +/- 2 ng/ml of tPA complexes with plasminogen activator inhibitor-1 (tPA:PAI-1). 93% of the uPA and 17% of the tPA antigen in seminal plasma was in complex with PCI and, when complexation was inhibited by collecting semen into an 1,10-phenanthrolinium solution, 33% of the uPA and 7% of the tPA was complexed to PCI. Urine (N = 10) contained 4 +/- 1 ng/ml uPA:PCI. In purified system, complexation of uPA and tPA to PCI paralleled the inhibition of the enzymes. In vitro studies in blood and seminal plasma showed that heparin stimulated complexation of uPA and tPA with PCI, suggesting that negatively charged glycosaminoglycans in blood vessels and in the reproductive system may regulate PCI reactions with uPA and tPA. These results suggest that PCI is a physiologic regulator of uPA and tPA in male reproductive tissues and raises questions about a potential role of PCI in human fertility and in uPA-dependent cell invasiveness.

A novel exosite in the light chain of human activated protein C essential for interaction with blood coagulation factor Va

Activated protein C (APC) exerts its physiologic anticoagulant role by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. To identify regions on the surface of the light chain of APC that mediate anticoagulant activity, 10 synthetic peptides were prepared and tested for their ability to inhibit APC anticoagulant activity. The synthetic peptide-(142-155) inhibited APC anticoagulant activity in Xa-1-stage coagulation assays in normal and protein S-depleted plasma with 50% inhibition at 5-25 microM peptide. In a system using purified clotting factors, peptide-(142-155) inhibited APC catalyzed inactivation of factor Va in the presence or absence of phospholipids with 50% inhibition at 50 microM peptide. However, peptide-(142-155) had no effect on APC amidolytic activity or on the reaction of APC with the serpin, recombinant [Arg358]-alpha 1-antitrypsin. Moreover, peptide-(142-155) inhibited factor Xa clotting activity in normal plasma as well as in a prothrombinase assay in the presence of factor Va with 50% inhibition at 5 microM and 50 microM peptide, respectively, under the assay conditions. The peptide had no significant effect on factor Xa or thrombin amidolytic activity and no effect on the clotting of purified fibrinogen by thrombin, suggesting that it does not directly inhibit these enzymes' active sites. Peptide-(142-155) was shown to bind directly to immobilized factor Va.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of activated protein C during thrombolysis

The observation that urokinase infusion increases circulating levels of the anticoagulant activated protein C (APC) in baboons implies that APC might be elevated during thrombolytic therapy. Patients undergoing coronary thrombolysis showed an 11-fold increase (means from 6 to 69 micrograms/L) in APC during infusion of streptokinase. Thrombolytic therapy thus generates at least two potent antithrombotic factors in the circulation--the fibrinolytic enzyme, plasmin, and the anticoagulant enzyme, APC. APC may help prevent reocclusion during or after thrombolysis.

Antithrombotic effects of thrombin-induced activation of endogenous protein C in primates

The effects on thrombosis and hemostasis of thrombin-induced activation of endogenous protein C (PC) were evaluated in baboons. Thrombosis was induced by placing into arteriovenous shunts a segment of Dacron vascular graft, which generated arterial platelet-rich thrombus, followed by an expansion region of low-shear blood flow, which in turn accumulated fibrin-rich venous-type thrombus. Thrombosis was quantified by 111In-platelet imaging and 125I-fibrinogen accumulation. Intravenous infusion of alpha-thrombin, 1-2 U/kg-min for 1 h, increased baseline activated PC levels (approximately 5 ng/ml) to 250-500 ng/ml (P < 0.01). The lower thrombin dose, which did not deplete circulating platelets, fibrinogen, or PC, reduced arterial graft platelet deposition by 48% (P < 0.05), and platelet and fibrin incorporation into venous-type thrombus by > 85% (P < 0.01). Thrombin infusion prolonged the activated partial thromboplastin clotting time, elevated fibrinopeptide A (FPA), thrombin-antithrombin III complex (T:AT III), and fibrin D-dimer plasma levels (P < 0.01), but did not affect bleeding times. Thrombin's antithrombotic effects were blocked by infusing a monoclonal antibody (HPC-4) which prevented PC activation in vivo, caused shunt occlusion, increased the consumption of platelets and fibrinogen, elevated plasma FPA and T:AT III levels, and reduced factor VIII (but not factor V) procoagulant activity (P < 0.05). We conclude that activated PC is a physiologic inhibitor of thrombosis, and that activation of endogenous PC may represent a novel and effective antithrombotic strategy.

Genetic mutations in ten unrelated American patients with symptomatic type 1 protein C deficiency

Symptomatic patients with Type 1 protein C deficiency and venous thrombosis were analysed for defects in this gene using polymerase chain reaction amplification and direct sequencing of all nine exons. Ten different heterozygous point mutations were detected in 19 patients from eleven American families. Seven represent novel mutations. Two of these were found in the TATA box or near the transcription initiation site and presumably lead to loss of transcription, and seven missense mutations were found including G103R, P168L, R169W, I201T, P279L, T298M, and C384Y. These may lead to abnormal folding or thermodynamic instability of the protein C molecule, potentially causing abnormal secretion or rapid clearance from the circulation. Two other protein C mutations, a nonsense mutation at codon Trp-145 and a deletion inducing a frameshift at codon 364 resulting in premature termination at codon 378, likely lead to unstable products. The previously published R169W mutation resulted in a Type 1 deficiency. The data show that diverse molecular defects result in similar phenotypes and emphasize that a wide variety of mutations are responsible for Type 1 protein C deficiency in the American setting of a diverse population.

Anticoagulant protein C pathway defective in majority of thrombophilic patients

A defect involving poor anticoagulant response to activated protein C (APC), an anticoagulant serine protease known to inactivate factors Va and VIIIa in plasma, was recently reported and the existence of a novel APC cofactor was suggested. To define the frequency of this defect among 25 venous thrombophilic patients with no identifiable laboratory test abnormality and among 22 patients previously identified with heterozygous protein C or protein S deficiency, the APC-induced prolongation of the activated partial thromboplastin time assay for these patients was compared with results for 35 normal subjects. The results show that this new defect in anticoagulant response to APC is surprisingly present in 52% to 64% of the 25 patients, ie, in the majority of previously undiagnosed thrombophilia cases, but is not present in 20 of 22 heterozygous protein C or protein S deficient patients, suggesting that the new factor is a risk factor independent of protein C or protein S deficiency. The results demonstrate that abnormalities in the anticoagulant protein C pathway are present in the majority of thrombophilic patients.

Interactions and inhibition of blood coagulation factor Va involving residues 311-325 of activated protein C

Activated protein C (APC) exerts its physiologic anticoagulant role by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. The synthetic peptide-(311-325) (KRNRTFVLNFIKIPV), derived from the heavy chain sequence of APC, potently inhibited APC anticoagulant activity in activated partial thromboplastin time (APTT) and Xa-1-stage coagulation assays in normal and in protein S-depleted plasma with 50% inhibition at 13 microM peptide. In a system using purified clotting factors, peptide-(311-325) inhibited APC-catalyzed inactivation of factor Va in the presence or absence of phospholipids with 50% inhibition at 6 microM peptide. However, peptide-(311-325) had no effect on APC amidolytic activity or on the reaction of APC with the serpin, recombinant [Arg358]alpha 1-antitrypsin. Peptide-(311-325) surprisingly inhibited factor Xa clotting activity in normal plasma, and in a purified system it inhibited prothrombinase activity in the presence but not in the absence of factor Va with 50% inhibition at 8 microM peptide. The peptide had no significant effect on factor Xa or thrombin amidolytic activity and no effect on the clotting of purified fibrinogen by thrombin, suggesting it does not directly inhibit these enzymes. Factor Va bound in a dose-dependent manner to immobilized peptide-(311-325). Peptide-(311-315) inhibited the binding of factor Va to immobilized APC or factor Xa.(ABSTRACT TRUNCATED AT 250 WORDS)

Normalization of markers of coagulation activation with a purified protein C concentrate in adults with homozygous protein C deficiency

Homozygous or double heterozygous protein-C deficiency can present at birth with purpura fulminans or later in life with venous thrombosis. Two homozygous patients who had previously sustained thrombotic episodes were investigated at a time when they were asymptomatic and not receiving antithrombotic therapy. The plasma levels of protein-C antigen and activity in both individuals were approximately 20% of normal. We administered a highly purified plasma-derived protein C concentrate to these individuals and monitored levels of several markers of in vivo coagulation activation. Assays for protein-C activation (activated protein C and protein C activation peptide) showed a sustained increase from reduced baseline levels, whereas thrombin generation (as measured by prothrombin fragment F1 + 2) gradually decreased over about 24 hours into the normal range. These investigations provide direct evidence that protein C is converted to activated protein C in vivo, and that the protein-C anticoagulant pathway is a tonically active mechanism in the regulation of hemostatic system activation in humans.

Identification of residues 413-433 of plasma protein S as essential for binding to C4b-binding protein

Reversible association of protein S with C4b-binding protein (C4BP) in plasma down-regulates protein S activity, since free protein S but not the protein S.C4BP complex is an anticoagulant cofactor for activated protein C. To identify regions on the surface of protein S that mediate complex formation with C4BP, a number of nonoverlapping synthetic pentadecapeptides comprising protein S sequences were prepared and tested for their ability to inhibit complex formation. The most potent pentadecapeptide, residues 420-434 (PSP-420) (SGIKEIIQEKQNKHC), gave half-maximal effect at 20 microM. A peptide with the reverse sequence, 434-420, did not inhibit. A peptide containing the sequence of protein S residues 408-434 inhibited complex formation by > 95% with 50% inhibition at 5 microM peptide. Biotinylated C4BP bound specifically to plates coated with PSP-420 but not with the 434-420 peptide; and biotinylated PSP-420 bound to plates coated with C4BP. Rabbit antibodies were raised against several keyhole limpet hemocyanin-conjugated peptides, and each was tested for ability to inhibit complex formation. Anti-PSP-420 antibody potently inhibited complex formation with half-maximal effect at 25 nM IgG. A monoclonal antibody (LJ-56) made against PSP-420 showed high affinity for protein S and inhibited complex formation; this monoclonal antibody specifically recognized free protein S but not the protein S.C4BP complex. These results imply that the PSP-420 sequence is surface-exposed, capable of binding to C4BP, and essential for protein S binding to C4BP.

Protein S, an antithrombotic factor, is synthesized and released by neural tumor cells

Protein S, an anticoagulant factor in the protein C antithrombotic pathway, was found to be synthesized and released by six tumor cell lines of neural origin by western blotting and ELISA. The rate of synthesis ranged from three- to 11-fold higher than that of a microvascular endothelial cell line and 36-144% that of a hepatoma cell line. The secreted protein S displayed specific anticoagulant activity similar to that of purified plasma protein S, implying that it was fully gamma-carboxylated. Ten primary brain tumor tissues also expressed protein S antigen, as shown by western blot analysis. Expression of anticoagulantly active protein S by neural cells raises important questions concerning possible physiologic roles for this multidomain protein beyond its function in control of thrombosis.

Binding of protein S to factor Va associated with inhibition of prothrombinase that is independent of activated protein C

Since plasma protein S serves an anticoagulant function by mechanisms which are not completely understood, its possible interaction with Factor Va was investigated. Human protein S bound to immobilized human Factor Va in a calcium-dependent, saturable, and reversible manner and Factor Va bound similarly to immobilized protein S. Binding of protein S to immobilized Factor V was greatly enhanced by pretreatment of the surface-bound Factor V with increasing doses of thrombin up to 1 unit/ml. Binding of protein S to Factor Va was also demonstrated in fluid phase with a Kd of 33 +/- 9 nM. Biotin-labeled heavy chain of Factor Va bound to immobilized protein S, and this binding was reversed by a 17-fold molar excess of intact unlabeled Factor Va. Protein S competed efficiently with prothrombin for binding to immobilized Factor Va. The prothrombinase activity in a reaction mixture of purified clotting factors was inhibited by protein S and exhibited a pattern of mixed inhibition. The concentration of protein S needed for 50% inhibition of the prothrombinase activity of a mixture containing 1 nM Factor Xa, 20 pM Factor Va, and 50 microM phospholipids was about 16 nM. Since not all protein S preparations exhibited this degree of prothrombinase inhibitory activity, extensive control experiments were performed to verify that the inhibitory activity was associated with protein S during immunoaffinity chromatography and was not caused by traces of activated protein C in the protein S preparations. These data show that protein S has an anticoagulant function which is independent of activated protein C and, at least in part, that this is because of its competition with prothrombin for direct binding to Factor Va.

Assembly and expression of an intrinsic factor IX activator complex on the surface of cultured human endothelial cells

Endothelial cells expose specific receptors for blood clotting factors and, upon perturbation, can initiate and propagate the reactions of the extrinsic pathway of blood coagulation leading to fibrin formation on the cell surface. The existence of an intrinsic mechanism of Factor IX activation on cultured human umbilical vein cells (HUVECs) was investigated by studies of the interaction between HUVECs and two proteins of the contact activation system, the cofactor high molecular weight kininogen (H-kininogen) and the zymogen Factor XI. In the presence of zinc ions (10-300 microM), 125I-labeled H-kininogen bound to HUVECs in a time-dependent, reversible, and saturable manner, with calcium ions exerting an inhibitory effect on the zinc-dependent binding. Analysis of the binding data by the LIGAND computer program indicated that HUVECs, in the presence of 2 mM CaCl2 and 100 microM ZnCl2 at 37 degrees C, bound 1.14 x 10(7) H-kininogen molecules per cell with an apparent dissociation constant of 55 nM. HUVEC-bound H-kininogen functions as the cell surface receptor for both 125I-labeled Factor XI and 125I-labeled Factor XIa, since HUVECs cultured in contact factor-depleted serum do not detectably bind either the zymogen or the enzyme in the absence of H-kininogen and zinc ions. In the presence of saturating concentrations of H-kininogen, 2 mM CaCl2 and 100 microM ZnCl2, the binding of 125I-labeled Factor XI and Factor XIa to HUVECs was time-dependent, reversible, and saturable, with apparent dissociation constants of 4.5 and 1.5 nM, respectively. HUVEC-bound complexes of H-kininogen and Factor XI generated Factor XIa activity only after the addition of purified Factor XIIa, and cell-bound Factor XIa in turn activated Factor IX, as documented by a 3H-labeled activation peptide release assay for 3H-Factor IX activation. The results indicate that cultured HUVECs provide a surface for the assembly and expression of an intrinsic Factor IX activator complex that may participate in the initiation of blood coagulation at sites of vascular injury.

The effect of phospholipids, calcium ions and protein S on rate constants of human factor Va inactivation by activated human protein C

Rate constants for human factor Va inactivation by activated human protein C (APC) were determined in the absence and presence of Ca2+ ions, protein S and varying concentrations of phospholipid vesicles of different lipid composition. APC-catalyzed factor Va inactivation in free solution (in the presence of 2 mM Ca2+) was studied under first-order reaction conditions with respect to both APC and factor Va and was characterized by an apparent second-order rate constant of 6.1 x 10(5) M-1 s-1. Stimulation of APC-catalyzed factor Va inactivation by phospholipids was dependent on the concentration and composition of the phospholipid vesicles. Optimal acceleration (230-fold) of factor Va inactivation was observed with 10 microM phospholipid vesicles composed of 20 mol% dioleoylglycerophosphoserine (Ole2GroPSer) and 80 mol% dioleoylglycerophosphocholine (Ole2GroPCho). At higher vesicle concentrations and at higher molar fractions of Ole2GroPSer some inhibition of APC-catalyzed factor Va inactivation was observed. Membranes that contained anionic phospholipids other than phosphatidylserine also promoted factor Va inactivation. The ability of different anionic lipids to enhance factor Va inactivation increased in the order phosphatidylethanolamine less than oleic acid less than phosphatidic acid less than phosphatidylglycerol less than phosphatidylmethanol less than phosphatidylserine. APC-catalyzed factor Va inactivation in the presence of phospholipid vesicles could be saturated with respect to factor Va and the reaction obeyed Michaelis-Menten kinetics. Both the Km for factor Va and the Vmax of factor Va inactivation were a function of the phospholipid concentration. The Km increased from 1 nM at 2.5 microM phospholipid (Ole2GroPSer/Ole2GroPCho 20:80, mol/mol) to 65 nM at 250 microM phospholipid. The Vmax increased from 20 mol factor Va inactivated.min-1.mol APC-1 at 2.5 microM phospholipid to 62 mol factor Va inactivated.min-1.mol APC-1 at 10 microM phospholipid and remained constant at higher phospholipid concentrations. Protein S appeared to be a rather poor stimulator of APC-catalyzed factor Va inactivation. Protein-S-dependent rate enhancements were only observed in reaction mixtures that contained negatively charged phospholipid vesicles. Independent of the concentration and the lipid composition of the vesicles, protein S caused a twofold stimulation of APC-catalyzed factor Va inactivation. This suggests that, in the human system, enhancement of APC binding to phospholipid vesicles by protein S is of minor importance. Considering that protein S is a physiologically essential antithrombotic agent, it is likely that other factors or phenomena contribute to the in vivo antithrombotic action of protein S.

Reevaluation of total, free, and bound protein S and C4b-binding protein levels in plasma anticoagulated with citrate or hirudin

The levels of total, free, and bound protein S and C4BP were determined using enzyme-linked immunosorbent assays (ELISAs) in plasma samples (8 males and 8 females) that were individually subjected to immunoadsorption studies in which "free protein S" was defined as that not adsorbed by anti-C4BP antibody-Sepharose column and "free C4BP" as that not adsorbed by anti-protein S antibody-Sepharose. Bound species were calculated as the difference between total and free species. Free protein S (131 nmol/L) averaged 38% of total protein S (346 nmol/L) and free C4BP (37 nmol/L) averaged 14% of total C4BP (264 nmol/L) in plasma. There was an excellent correlation between bound protein S and bound C4BP with 1:1 molar stoichiometry and a good correlation between free protein S antigen and protein S anticoagulant activity. It appears that free protein S is a necessary consequence of the molar excess of protein S over C4BP. C4BP beta chain antigen levels, measured using a new ELISA, averaged 218 nmol/L, a value indistinguishable from the molar concentrations of bound protein S (215 nmol/L) and bound C4BP (227 nmol/L). The free C4BP beta chain antigen was only 4 nmol/L compared with 131 nmol/L free protein S. These results suggest that free C4BP in plasma predominantly lacks the beta chain, that almost all C4BP capable of binding protein S is associated with protein S, and that the plasma levels of oligomeric forms of C4BP containing a beta chain (alpha 7 beta and alpha 6 beta) that bind protein S directly and stoichiometrically regulate free protein S levels.

Direct detection of activated protein C in blood from human subjects

The antithrombotic enzyme, activated protein C (APC) was measured in blood using an enzyme capture assay (ECA). The ECA involved (1) collection of blood into anticoagulant containing a reversible inhibitor of the enzyme, (2) specific affinity capture of the enzyme by an immobilized antibody that does not inhibit the enzyme, (3) removal of the reversible inhibitor by washing, and (4) direct assay of the captured enzyme's amidolytic activity. The ECA for APC used benzamidine for inhibition, anti-PC light-chain murine monoclonal antibody for capture, and the oligopeptide substrate S-2366 for enzyme assay. The sensitivity of this assay was 5 pmol/L (0.3 ng/mL) APC. The APC activity in normal pooled plasma corresponded to the amidolytic activity of 38 pmol/L (2.26 +/- 0.2 ng/mL) purified human plasma-derived APC in the ECA. APC levels in 41 normal donors ranged from 64% to 143%, averaging 104.9% +/- 19.6% (SD). Thus, APC is a measurable and normal component of circulating human blood, and this ECA may be useful for identifying APC deficiency. Moreover, similar ECAs for other enzymes in the circulation may be useful.