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PubMed1992

Inactivation of factor Va by activated protein C on selected human tumor cell lines.

Previous studies have demonstrated that platelets or aortic endothelial cells provide an appropriate surface that augments the proteolytic inactivation of factor Va by activated protein C (APC). We have examined the ability of three human tumor cell lines (HepG2, CAPAN-2 and J82) to support the inactivation of human factor Va by human APC in the presence and absence of human protein S. APC-mediated factor Va inactivation on these tumor cell lines was assessed by measuring the ability of residual cell-bound factor Va to augment the proteolytic activation of prothrombin by factor Xa. Each of the tumor cell lines studied supported factor Va inactivation by APC in the presence of calcium ions. HepG2 cell monolayers supported this reaction most effectively, with CAPAN-2 and J82 cell monolayers exhibiting moderate and weak effectiveness, respectively. Although not essential for this reaction, protein S moderately enhanced the rate of factor Va inactivation by APC on these tumor cell lines. In addition, pretreatment of each tumor cell line with rabbit antihuman protein S IgG had little, if any, effect on its ability to support factor Va inactivation by APC. Our data suggest that these, and perhaps other, tumor cells can provide an appropriate phospholipid surface for promoting factor Va binding and rapid inactivation by APC.

Calcium↗
PubMed1984

Structure of bovine blood coagulation factor Va. Determination of the subunit associations, molecular weights, and asymmetries by analytical ultracentrifugation.

Thrombin-activated bovine factor V (factor Va), an essential component of the blood clotting cascade pro thrombinase complex, is composed of two nonidentical subunits (Vl and Vh) and Ca2+ in tight association. We have examined Vl, Vh, and factor Va using analytical ultracentrifugation. At pH 7.65 in 50 mM tris(hydroxymethyl)aminomethane, 0.1 M NaCl, 1 mM benzamidine, and 10 mM Ca2+, the Vl subunit has a molecular weight (Mr) of 82 500, an S0(20) ,w = 5. 0(2)S , and, assuming a model of a prolate ellipsoid with 0.3 g of H2O/g of protein, an axial ratio of 5:1. The corresponding values for the Vh subunit are an Mr of 92 300, an S0(20) ,w = 5.2(9) S, and an axial ratio of 5:1. We found these same values for Vl and for Vh in a buffer that contained 2 mM ethylenediaminetetraacetate (EDTA) rather than the 10 mM Ca2+. The Vl subunit undergoes a weak, reversible self-association at 9 degrees C with an apparent monomer-dimer association constant of 5.6 X 10(3) M-1 in the presence of 2 mM EDTA and 2.3 X 10(3) M-1 in the presence of 10 mM Ca2+. Our data indicate that the Vl self-association includes dimer and higher oligomers. Factor Va, examined in the presence of 10 mM Ca2+ and at 20 degrees C, has an Mr of 174 000, and S0(20) ,w = 8.1(8)S, an axial ratio of 5:1, and an apparent Vl-Vh association constant of at least 2.7 X 10(8) M-1. Our results suggest that factor Va self-associates to form higher multimers. When solutions of Va are dialyzed against a buffer that contains no Ca2+ and 2 mM EDTA, the apparent Vl-Vh subunit association constant is reduced to 9.4 X 10(3) M-1. Our hydrodynamic data indicate that there is a substantial decrease in molecular asymmetry when factor V is proteolytically activated by thrombin to form factor Va and that Vl and Vh are arranged "side by side" rather than "end to end" in factor Va.

Animals↗
PubMed2003

Amino acids Glu323, Tyr324, Glu330, and Val331 of factor Va heavy chain are essential for expression of cofactor activity.

We have recently demonstrated that amino acid region 323-331 of factor Va heavy chain (9 amino acids, AP4') contains a binding site for factor Xa (Kalafatis, M., and Beck, D. O. (2002) Biochemistry 41, 12715-12728). To ascertain which amino acids within this region are important for the effector and receptor properties of the cofactor with respect to factor Xa, we have synthesized three overlapping peptides (5 amino acids each) spanning the amino acid region 323-331 and tested them for their effect on prothrombinase complex assembly and function. Peptide containing amino acids 323EYFIA327 alone was found to increase the catalytic efficiency of factor Xa but had no effect on the fluorescent anisotropy of active site-labeled factor Xa (human factor Xa labeled in the active site with Oregon Green 488; [OG488]-EGR-hXa). In contrast, peptide containing the sequence 327AAEEV331 was found to interact with [OG488]-EGR-hXa with half-maximal saturation reached at approximately 150 microm, but it was unable to produce a cofactor effect on factor Xa. Peptide 325FIAAE329 inhibited prothrombinase activity and was able to partially decrease the fluorescent anisotropy of [OG488]-EGR-hXa but could not increase the catalytic efficiency of factor Xa with respect to prothrombin. A control peptide with the sequence FFFIA did not increase the catalytic efficiency of factor Xa, whereas a peptide with the sequence AAEMI was impaired in its capability to interact with [OG488]-EGR-hXa. Two mutant recombinant factor Va molecules (Glu323 --> Phe/Tyr324 --> Phe, factor VaFF; Glu330 --> Met/Val331 --> Ile, factor VaMI) showed impaired cofactor activity when used at limiting cofactor concentration, whereas the quadruple mutant (Glu323 --> Phe/Tyr324 --> Phe and Glu330 --> Met/Val331 --> Ile, factor VaFF/MI) had no cofactor activity under similar experimental conditions. Our data demonstrate that amino acid residues Glu323, Tyr324, Glu330, and Val331 of factor Va heavy chain are critical for expression of factor Va cofactor activity.

Amino Acids↗
PubMed2001

Simvastatin depresses blood clotting by inhibiting activation of prothrombin, factor V, and factor XIII and by enhancing factor Va inactivation.

BACKGROUND: The mechanism of the antithrombotic action of statins is unclear. The aim of this study was to evaluate the effects of simvastatin on the coagulation process at sites of microvascular injury. METHODS AND RESULTS: Tissue factor-initiated coagulation was assessed in blood samples collected every 30 seconds from bleeding-time wounds of 17 patients who had advanced coronary artery disease and total cholesterol levels of 224.6+/-11.8 mg/dL (mean+/-SEM). Quantitative Western blotting for time courses of fibrinogen depletion and activation of prothrombin, factor V, and factor XIII was performed before and after 3 months of simvastatin treatment (20 mg/d). Simvastatin induced reductions in total cholesterol (23%) and LDL-cholesterol (36%), which were accompanied by significant decreases in the rates of prothrombin activation (16.2+/-2.1%; P=0.004), formation of alpha-thrombin B-chain (27.4+/-1.8%; P=0.001), generation of factor Va heavy chain (29.7+/-3.1%; P=0.007) and factor Va light chain (18.9+/-1.2%; P=0.02), factor XIII activation (19.8+/-1.3%; P=0.001), and fibrinogen conversion to fibrin (72.2+/-3%; P=0.002). Posttreatment fibrinopeptides A and B concentrations, determined by using high-performance liquid chromatography, were reduced within the last 30 seconds of bleeding. The 30-kDa fragment of the factor Va heavy chain (residues 307 to 506), produced by activated protein C, and the 97-kDa fragment of the factor Va heavy chain (residues 1 to 643) were released more rapidly after simvastatin treatment. The antithrombotic actions of simvastatin showed no relationship to its cholesterol-lowering action. CONCLUSIONS: Simvastatin treatment depresses blood clotting, which leads to reduced rates of prothrombin activation, factor Va generation, fibrinogen cleavage, factor XIII activation, and an increased rate of factor Va inactivation. These effects are not related to cholesterol reduction.

Adult↗
PubMed1993

Determination of the disulfide bridges in factor Va light chain.

The 74-kDa light chain of bovine factor Va is composed of three domains: the NH2-terminal A3 domain and the COOH-terminal C1 and C2 domains. In total, the light chain has eight cysteines: two in the A3 domain and three in each C domain. To determine the locations of the disulfide bridges, peptides were obtained from factor Va and iodo[1-14C]acetamide-labeled factor Va light chains by digestion with trypsin, activated protein C, lysylendopeptidase, and V8 protease. After HPLC purification, amino acid sequence and composition analyses showed that each domain of bovine Va light chain possesses a disulfide bond. The sites are Cys1684-Cys1710 (A3), Cys1866-Cys2020 (C1), and Cys2025-Cys2180 (C2). One free cysteine is located in each C domain, i.e., Cys1953 and Cys2100. The locations of the disulfide bonds in human Va and VIIIa light chains are anticipated to be similar to those of bovine Va light chain, because the cysteines involved are conserved.

Amino Acid Sequence↗
PubMed1996

Binding sites for blood coagulation factor Xa and protein S involving residues 493-506 in factor Va.

Inactivation due to cleavage of Factor Va (FVa) at Arg 506 by activated protein C (APC) helps to downregulate blood coagulation. To identify potential functional roles of amino acids near Arg 506, synthetic overlapping pentadecapeptides comprising FVa heavy chain residues 481-525 were tested for their ability to inhibit prothrombin activation by prothrombinase complexes [Factor Xa (FXa):FVa:phospholipids:Ca2+]. The most potent inhibition was observed for peptide VP493 (residues 493-506), with 50% inhibition at 2.5 microM. VP493 also inhibited FXa in plasma in FXa-1-stage clotting assays by 50% at 3 microM. When the C-terminal carboxamide group of VP493 was replaced by a carboxyl group, most prothrombinase inhibitory activity was lost. VP493 preincubated with FXa inhibited prothrombinase with a pattern of mixed inhibition. Homologous peptides from Factor VIII sequences did not inhibit prothrombinase. Affinity-purified antibodies to VP493 inhibited prothrombinase activity and prolonged FXa-1-stage clotting times. VP493 also blocked the ability of protein S to inhibit prothrombinase independently of APC. Immobilized VP493 bound specifically with similar affinity to both FXa and protein S (Kd approximately 40 nM), but did not measurably bind prothrombin or APC. These studies suggest that FVa residues 493-506 contribute to binding sites for both FXa and protein S, providing a rationale for the ability of protein S to negate the protective effect of FXa toward APC cleavage of FVa. Possible loss of this FVa binding site for FXa due to cleavage at Arg 506 by APC may help explain why this cleavage causes 40% decrease in FVa activity and facilitates inactivation of FVa.

Amino Acid Sequence↗
PubMed2002

Interdomain engineered disulfide bond permitting elucidation of mechanisms of inactivation of coagulation factor Va by activated protein C.

Procoagulant factor Va (FVa) is inactivated via limited proteolysis at three Arg residues in the A2 domain by the anticoagulant serine protease, activated protein C (APC). Cleavage by APC at Arg306 in FVa causes dissociation of the A2 domain from the heterotrimeric A1:A2:A3 structure and complete loss of procoagulant activity. To help distinguish inactivation mechanisms involving A2 domain dissociation from inactivation mechanisms involving unfavorable changes in factor Xa (FXa) affinity, we used our FVa homology model to engineer recombinant FVa mutants containing an interdomain disulfide bond (Cys609-Cys1691) between the A2 and A3 domains (A2-SS-A3 mutants) in addition to cleavage site mutations, Arg506Gln and Arg679Gln. SDS-PAGE analysis showed that the disulfide bond in A2-SS-A3 mutants prevented dissociation of the A2 domain. In the absence of A2 domain dissociation from the A1:A2:A3 trimer, APC cleavage at Arg306 alone caused a sevenfold decrease in affinity for FXa, whereas APC cleavages at Arg306, Arg506, and Arg679 caused a 70-fold decrease in affinity for FXa and a 10-fold decrease in the k(cat) of the prothrombinase complex for prothrombin without any effect on the apparent K(m) for prothrombin. Therefore, for FVa inactivation by APC, dissociation of the A2 domain may provide only a modest final step, whereas the critical events are the cleavages at Arg506 and Arg306, which effectively inactivate FVa before A2 dissociation can take place. Nonetheless, for FVa Leiden (Gln506-FVa) inactivation by APC, A2 domain dissociation may become mechanistically important, depending on the ambient FXa concentration.

Anticoagulants↗
PubMed1987

The effect of factor Va on lipid dynamics in mixed phospholipid vesicles as detected by steady-state and time-resolved fluorescence depolarization of diphenylhexatriene.

We have monitored the thermotropic behavior of mixed dimyristoylglycerophosphoserine (Myr2GroPSer)/dimyristoylglycerophosphocholine (Myr2GroPCho) and Myr2GroPSer/dipalmitoylglycerophosphocholine (Pam2GroPCho) vesicles in the presence of blood-clotting factor Va, using 1,6-diphenyl-1,3, 5-hexatriene as a lipid probe. The Ca2+-independent interaction of factor Va with these vesicles caused a small increase (1-2 degrees C) in the phase transition temperature, regardless of whether Myr2GroPChe was the lower or higher-melting component of the mixed vesicles. The major effect of factor Va was to increase the polarization of diphenylhexatriene when the mixed vesicles were in the liquid crystalline phase. The protein did not change the anisotropy in the bilayer gel state. The increase in the polarization value above the transition temperature closely correlated with the amount of phospholipid-bound factor Va, as verified by a direct binding technique. In addition, we found that the affinity of factor Va for Myr2GroPSer/Myr2GroPCho and Myr2GroPSer/Pam2GroPCho greatly increased at temperatures above the transition temperatures. Time-dependent fluorescence anisotropy measurements of diphenylhexatriene embedded in vesicles in the liquid crystalline state give fluorescence decay curves which can best be fitted by two exponential functions with two rotational correlation times and a constant term. Vesicles composed of Myr2GroPSer exhibit more ordering than Myr2GroPCho vesicles. However, the order parameter of mixed vesicles composed of 40% Myr2GroPSer and 60% Myr2GroPCho (mol/mol) approached that of Myr2GroPCho. Factor Va dramatically increased the longer rotational correlation time of diphenylhexatriene embedded in mixed vesicles in the liquid crystalline state from 3.7 ns to about 17 ns. The second rank-order parameter increased only slightly, but the calculated steady-state anisotropy increased by twofold. These results indicate that the acidic phospholipid-dependent binding of factor Va to mixed vesicles has an ordering effect on the acyl chains of the acidic phospholipids in the outer layer, but leaves the bulk of the phospholipids, mainly phosphatidylcholine, unaltered. None of the factor-Va-induced alterations in the anisotropy parameters point to the occurrence of lateral phase separation.

Diphenylhexatriene↗
PubMed1994

Loss of membrane-dependent factor Va cleavage: a mechanistic interpretation of the pathology of protein CVermont.

Clinical manifestations of arterial and venous thrombosis in a family with protein C deficiency was associated with two mutations in the light chain of protein C: Glu20-->Ala and Val34-->Met. Further studies showed that the mutation Glu20-->Ala which eliminated a gamma-carboxylation site was exclusively responsible for the anticoagulant defect of activated protein C (APC). Membrane-bound human factor Va is inactivated by APC after two sequential cleavages of the heavy chain at Arg506 and Arg306. Human factor Va inactivation by human recombinant APC (rAPC) and a mutant molecule with an alanine instead of a glutamic acid at position 20 (rAPC(gamma 20A)) was investigated in the presence and absence of phospholipid vesicles. During a 2-hour incubation period of the cofactor with either rAPC or rAPC(gamma 20A). In the absence of a membrane surface, factor Va is cleaved quantitatively at Arg506 and retains approximately 60% of its initial cofactor activity. After a 2-hour incubation period with rAPC membrane-bound factor Va has no cofactor activity, whereas in the presence of a membrane surface and rAPC(gamma 20A) factor Va retains 60% of its initial cofactor activity. The completed loss in factor Va cofactor activity upon incubation of the membrane-bound cofactor with phospholipid vesicles and rAPC is associated with cleavages at Arg506 and Arg306, whereas membrane-bound factor Va cleavage at Arg306 by rAPC(gamma 20A) is impaired, resulting in a cofactor that is cleaved at Arg506. Slow cleavage at Arg306 occurs when membrane-bound factor Va is incubated with rAPC(gamma 20A) and only small amounts of fragments of M(r) = 45,000 and 30,000 are noticed. Our data show that the genetic defect which leads to the absence of a gamma-carboxylation site at Glu20 impairs membrane binding of human APC, which in turn is required for cleavage of factor Va at Arg306 and inactivation of the cofactor. The consequence of impaired membrane-dependent cleavage at Arg306 is manifested in vivo by venous and arterial thrombosis.

1-Carboxyglutamic Acid↗
PubMed1992

Immunoglobulin fractions isolated from patients with antiphospholipid antibodies prevent the inactivation of factor Va by activated protein C on human endothelial cells.

We studied the effect of purified immunoglobulins (Ig) from 21 patients with antiphospholipid antibodies (aPL) on factor Va degradation by activated protein C (aPC) on cultured human umbilical vein endothelial cells (HUVEC). Sera from patients were tested on an ELISA aPL assay to determine the isotype with aPL activity. HUVEC were incubated with purified IgG or IgM fraction from controls or patients. Activated PC and factor Va were then added and factor Va degradation was measured after several reaction times. 13 of 14 IgM and 8 of 10 IgG from patients showed an inhibitory effect on factor Va degradation by aPC when compared with control Ig. We also observed the same inhibitory effect with patients' Ig on studying the degradation of factor Va by aPC in a purified system containing aPC, protein S and phospholipids. These results suggest that aPL antibodies disturb the anticoagulant activity of aPC, which may contribute to the thrombotic tendency of these patients.

Antibodies↗
PubMed1994

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.

Adult↗
PubMed1990

Binding of human factors X and Xa to HepG2 and J82 human tumor cell lines. Evidence that factor Xa binds to tumor cells independent of factor Va.

Previous studies demonstrated that several cultured human tumor cell lines potentiate the conversion of prothrombin to thrombin by factor Xa and calcium in the absence of exogenous factor Va. In the present study, the specific binding of radioiodinated preparations of human factor X and factor Xa to a human hepatocellular carcinoma cell line (HepG2) that constitutively synthesizes a factor V/Va molecule, and a human bladder carcinoma cell line (J82) that does not synthesize factor V/Va, was examined. Radioiodinated factor Xa bound specifically to J82 and HepG2 cells, whereas no significant specific binding of 125I-factor X to either cell was observed. The binding isotherm of 125I-factor Xa to each tumor cell line exhibited a hyperbolic profile, and Scatchard analysis demonstrated a single class of binding site for factor Xa on each cell surface with Kd values of 1.66 +/- 0.39 and 1.64 +/- 0.52 nM and 566,000 +/- 71,000 and 28,000 +/- 6,000 binding sites/cell for HepG2 and J82 cells, respectively. Thrombin formation by cell-bound factor Xa was hyperbolic and saturable at 5 nM factor Xa on each cell line. Hanes-Woolf plots of the prothrombin activation data indicated that half-maximal rates of thrombin formation occurred at factor Xa concentrations of 1.50 +/- 0.43 nM and 1.42 +/- 0.48 nM on HepG2 and J82 cells, respectively. Pretreatment of J82 cells with polyclonal anti-human factor V IgG had no measurable effect on either the binding of 125I-factor Xa or prothrombin activation. However, pretreatment of HepG2 cells with anti-human factor V IgG inhibited prothrombin activation in a dose-dependent manner, but did not inhibit the binding of factor Xa to this cell. When both cell lines were preincubated with exogenous human factor Va, the binding of factor Xa to either HepG2 or J82 cells was marginally affected. These data indicate that HepG2 and J82 cells have cell surface factor Xa binding sites proximal to, but independent of, cell surface factor Va of either exogenous or endogenous origin.

Binding Sites↗
PubMed1994

Determination of the disulfide bridges in factor Va heavy chain.

The M(r) = 94,000 heavy chain of bovine factor Va contains 10 cysteine residues which are distributed in the 2 A domains which make up this portion of the factor V molecule. The A1 domain contains four cysteines while the A2 domain contains six cysteines. The locations of disulfide bridges and free cysteines in bovine factor Va heavy chain were analyzed using iodo[14C]acetamide-labeled factor Va heavy chain digested with trypsin, plasmin, V-8 protease, and cyanogen bromide. Following HPLC separation of the resulting peptides, free cysteines were identified by the incorporation of radioactivity while disulfide-containing peptides were detected using an SBD-F fluorometric assay after reduction. All cysteine-containing peptides were analyzed by amino acid sequence analysis. The four cysteines in the A1 domain are associated with two disulfide bonds, Cys139-Cys165 and Cys220-Cys301. One disulfide bond was explicitly identified in the A2 domain; Cys471-Cys497, and a free cysteine was found in the A2 domain at Cys538. Significant difficulties were encountered in preparing identifiable or soluble peptides which would permit the explicit identification of the three remaining cysteines in the A2 domain. On the basis of homology, it is likely that Cys589 is a free SH while a disulfide bridge exists between Cys579 and Cys660. Thus, three major disulfide bonding patterns, characterized as "alpha", "beta", and "gamma" loops, are found in factor V. Each A domain contains a 26 residue "alpha loop at positions 139-165, 471-497, and 1684-1710. The A1 and A2 domains each contain 81 amino acid residue "beta" loops at 220-301 and 579-660.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed2000

Deficient APC-cofactor activity of protein S Heerlen in degradation of factor Va Leiden: a possible mechanism of synergism between thrombophilic risk factors.

In protein S Heerlen, an S-to-P (single-letter amino acid codes) mutation at position 460 results in the loss of glycosylation of N458. This polymorphism has been found to be slightly more prevalent in thrombophilic populations than in normal controls, particularly in cohorts of patients having free protein S deficiency. This suggests that carriers of the Heerlen allele may have an increased risk of thrombosis. We have now characterized the expression in cell cultures of recombinant protein S Heerlen and investigated the anticoagulant functions of the purified recombinant protein in vitro. Protein S Heerlen was synthesized and secreted equally well as wild-type protein S by transiently transfected COS-1 cells. The recombinant protein S Heerlen interacted with conformation-dependent monoclonal antibodies and bound C4b-binding protein to the same extent as wild-type protein S. Protein S Heerlen displayed reduced anticoagulant activity as cofactor to activated protein C (APC) in plasma-based assays, as well as in a factor VIIIa-degradation system. In contrast, protein S Heerlen functioned equally well as an APC cofactor in the degradation of factor Va as wild-type protein S did. However, when recombinant activated factor V Leiden (FVa:Q506) was used as APC substrate, protein S Heerlen was found to be a poor APC cofactor as compared with wild-type protein S. These in vitro results suggest a possible mechanism of synergy between protein S Heerlen and factor V Leiden that might be involved in the pathogenesis of thrombosis in individuals carrying both genetic traits. (Blood. 2000;96:523-531)

Animals↗
PubMed1995

Factor Xa-factor Va complex assembles in two dimensions with unexpectedly high affinity: an experimental and theoretical approach.

The influence of phospholipid vesicle concentration and size on the affinity and the kinetics of assembly of the prothrombin activation complex are examined. Activation of prethrombin 1 was used to monitor complex formation between factors Va and Xa. When activation rates were measured immediately after the addition of the reactants, the rate of activation increased, and subsequently decreased, as a function of increasing vesicle concentration. Larger vesicles did not inhibit the reaction to a comparable extent until much higher phospholipid concentrations were present. The inhibition by high vesicle concentrations was significantly reduced by a prolonged incubation period. These results are interpreted as an initial step of factors Va and Xa binding independently to separate phospholipid vesicles, followed by a slow redistribution between vesicles to maximize complex formation. These experiments indicated that the Kd < or = 25 pM, much tighter than previously reported. Two-dimensional binding on the membrane surface was investigated under conditions where all of the proteins were membrane bound. The complex formation was independent of the surface density of the reactions, indicating a near complete complex formation at the lowest surface density of the reactants. Thus, we conclude that (i) the overall affinity of factor Va-factor Xa interaction in the presence of vesicles is higher than previously appreciated, and (ii) factor Va and factor Xa complex once they bind to the same vesicle.

Animals↗
PubMed2001

Anticoagulant dysfunction of human Arg352Trp-activated protein C caused by defective factor Va inactivation.

The dysfunctional mutant R352W-protein C was found in two patients with venous thrombosis. The mutant R352A-protein C was constructed to define the contribution of charge/size of the residue at 352 on protein C (chymotrypsin numbering 187). Compared with wild type-protein C, R352W-protein C showed no difference in activation by thrombin-thrombomodulin or alpha-thrombin. However, R352W-activated protein C (APC) anticoagulant activity (aPTT assay) was reduced to approximately 65%. Although the catalytic efficiency of R352W-APC towards the oligopeptide substrate S-2366 was unperturbed, factor Va and R506Q-factor Va were not efficiently inactivated by R352W-APC compared with wild type-APC. R352A-APC showed reduced anticoagulant activity and reduced efficiency in factor Va inactivation and in factor VIIIa-inactivation in the presence of protein S. These observations suggest that the dysfunction of R352W-APC in factor Va inactivation may be one of the mechanisms leading to venous thrombosis in affected patients and that R352 plays an important role in the physiological functioning of APC.

Amino Acid Substitution↗
PubMed1994

Phosphatidylethanolamine incorporation into vesicles selectively enhances factor Va inactivation by activated protein C.

Membrane surfaces accelerate the proteolytic inactivation of factor Va by activated protein C. In most coagulation complexes, the most active membrane phospholipid is believed to be phosphatidylserine. In this study, we demonstrate that with phosphatidylserine-containing vesicles, incorporation of phosphatidylethanolamine increased the rate of factor Va inactivation approximately 10-fold at all concentrations of factor Va studied and at all vesicle concentrations at or below the optimum for prothrombin activation. In contrast, phosphatidylethanolamine had very little influence on prothrombin activation. Phosphatidylethanolamine was a critical component of vesicles for the vesicles to support activated protein C anticoagulant activity optimally in plasma. These results demonstrate that the membrane requirements for the different coagulation/anticoagulation complexes differ much more than previously appreciated.

Animals↗
PubMed2000

The Gla domain of human prothrombin has a binding site for factor Va.

The role of the Gla domain of human prothrombin in interaction with the prothrombinase complex was studied using a peptide with the sequence of the first 46 residues of human prothrombin, PT-(1-46). Intrinsic fluorescence measurements showed that PT-(1-46) undergoes a conformational alteration upon binding calcium; this conclusion is supported by one-dimensional (1)H NMR spectroscopy, which identifies a change in the chemical environment of tryptophan 41. PT-(1-46) binds phospholipid membranes in a calcium-dependent manner with a K(d) of 0.5 microm and inhibits thrombin generation by the prothrombinase complex with a K(i) of 0.8 microm. In the absence of phospholipid membranes, PT-(1-46) inhibits thrombin generation by factor Xa in the presence but not absence of factor Va, suggesting that PT-(1-46) inhibits prothrombin-factor Va binding. The addition of factor Va to PT-(1-46) labeled with the fluorophore sulfosuccinimidyl-7-amino-4-methylcoumarin-3-acetic acid (PT-(1-46)AMCA) caused a concentration-dependent quenching of AMCA fluorescence, providing direct evidence of a PT-(1-46)-factor Va interaction. The K(d) for this interaction was 1.3 microm. These results indicate that the N-terminal Gla domain of human prothrombin is a functional unit that has a binding site for factor Va. The prothrombin Gla domain is important for interaction of the substrate with the prothrombinase complex.

Amino Acid Sequence↗