New insights into the functions and N-glycan structures of factor X activator from Russell's viper venom

The coagulation factor X activator from Russell's viper venom (RVV-X) is a heterotrimeric glycoprotein. In this study, its three subunits were cloned and sequenced from the venom gland cDNAs of Daboia siamensis. The deduced heavy chain sequence contained a C-terminal extension with four additional residues to that published previously. Both light chains showed 77-81% identity to those of a homologous factor X activator from Vipera lebetina venom. Far-western analyses revealed that RVV-X could strongly bind protein S, in addition to factors X and IX. This might inactivate protein S and potentiate the disseminated intravascular coagulation syndrome elicited by Russell's viper envenomation. The N-glycans released from each subunit were profiled and sequenced by MALDI-MS and MS/MS analyses of the permethyl derivatives. All the glycans, one on each light chain and four on the heavy chain, showed a heterogeneous pattern, with a combination of variable terminal fucosylation and sialylation on multiantennary complex-type sugars. Amongst the notable features were the presence of terminal Lewis and sialyl-Lewis epitopes, as confirmed by western blotting analyses. As these glyco-epitopes have specific receptors in the vascular system, they possibly contribute to the rapid homing of RVV-X to the vascular system, as supported by the observation that slower and fewer fibrinogen degradation products are released by desialylated RVV-X than by native RVV-X.     

烙铁头蛇毒对体外血凝及纤溶的影响

行体外血凝及纤溶观察结果表明：①烙铁头蛇毒能明显延长凝血活酶时间（ＰＴＴ）、凝血酶原时间（ＰＴ）及蝰蛇毒磷脂时间（ＲＶＶＣＴ）,显示出较强的抗凝活性;②对人纤维蛋白原及纤维蛋白有较强的溶解作用,这种作用不完全是单一的活化素样作用,还有脆浆素样作用     

Coagulation factor X activating enzyme from Russell's viper venom (RVV-X). A novel metalloproteinase with disintegrin (platelet aggregation inhibitor)-like and C-type lectin-like domains.

We determined the complete amino acid sequence of RVV-X, the blood coagulation factor X activating enzyme, isolated from Russell's viper venom and studied structure-function relationships. RVV-X (M(r) 79,000) consists of a disulfide-bonded two-chain glycoprotein with a heavy chain of M(r) 59,000 and a light chain of heterogeneous M(r) 18,000 (LC1) and 21,000 (LC2). These chains were separated after reduction and S-pyridylethylation, and the isolated major component LC1 was used for sequence analysis. The heavy chain consists of 427 residues containing four asparagine-linked oligosaccharides, and its entire sequence was similar to that of the high molecular mass hemorrhagic protein, HR1B, isolated from the venom of Trimere-surus flavoviridis. The heavy chain contains three distinct domains, metalloproteinase, disintegrin (platelet aggregation inhibitor)-like and unknown cysteine-rich domains. On the other hand, light chain LC1 consists of 123 amino acid residues containing one asparagine-linked oligosaccharide and shows sequence homology similar to that found in the so-called C-type (Ca(2+)-dependent) lectins. Therefore, RVV-X is a novel metalloproteinase containing a mosaic structure with distintegrin-like, cysteine-rich, and C-type lectin-like domains. RVV-X potently inhibits collagen- and ADP-stimulated platelet aggregations, probably via its distintegrin-like domain, although this domain does not contain the Arg-Gly-Asp sequence which is conserved in various venom distintegrins and which is thought to be one of the interaction sites for platelet integrins. Our findings also indicate that snake venom factor IX/factor X-binding protein with a C-type lectin structure (Atoda, H., Hyuga, M., and Morita, T. (1991) J. Biol. Chem. 266, 14903-14911) inhibits RVV-X-catalyzed factor X activation; hence, the light chain of RVV-X probably participates in recognizing some portion of the zymogen factor X.     

Crystal structure of RVV-X: an example of evolutionary gain of specificity by ADAM proteinases

Russell's viper venom factor X activator (RVV-X) is a heterotrimeric metalloproteinase with a mammalian ADAM-like heavy chain and two lectin-like light chains. The crystal structure of RVV-X has been determined at 2.9 A resolution and shows a hook-spanner-wrench-like architecture, in which the metalloproteinase/disintegrin region constitutes a hook, and the lectin-like domains constitute a handle. A 6.5nm separation between the catalytic site and a putative exosite suggests a docking model for factor X. The structure provides a typical example of the molecular evolution of multi-subunit proteins and insights into the molecular basis of target recognition and proteolysis by ADAM/adamalysin/reprolysin proteinases.     

Coagulation of plasma from the chicken (Gallus domesticus): phospholipids influence clotting rates induced by components from Russell's viper venom

Added phospholipid failed to accelerate chicken-plasma coagulation, induced by high concentrations of crude Russell's viper venom; however, similarly induced coagulation of canine and human plasma proceeded more rapidly when phospholipid was added. Phospholipid reduced clotting times of canine, human and also chicken plasma when partially purified factor X-activating enzyme from Russell's viper venom was the inducing agent. In the absence of added phospholipid, preincubation of chicken plasma with factor V-activating enzyme from Russell's viper venom accelerated factor X-activating-enzyme-induced coagulation. Preincubation of chicken plasma with the factor V-activating enzyme slowed factor X-activating-enzyme-induced coagulation in the presence of added phospholipid.     

Allosteric aptamers controlling a signal amplification cascade allow visual detection of molecules at picomolar concentrations

A broadly applicable homogeneous detection system has been developed. It utilizes components of the blood coagulation cascade in the presence of polystyrene microspheres (MS) as a signal amplifier. Russell's viper venom factor X activator (RVV-X) triggers the cascade, which results in an eye-visible phase transition (precipitation) of MS bound to clotted fibrin. An allosteric RNA aptamer, RNA132, with affinity for RVV-X and human vascular endothelial growth factor (VEGF(165)) was created. RNA132 inhibits enzymatic activity of RVV-X. The effector molecule, VEGF(165), reverses the inhibitory activity of RNA132 on RVV-X and restores its enzymatic activity, thus, triggering the cascade and enabling the phase transition. As few as 5 fmol of VEGF(165) could be detected by the naked eye within an hour. Similar results were obtained for another allosteric aptamer modulated by a protein tyrosine phosphatase. The assay is instrumentation-free for both processing and readout and can be modified to detect molecules to which aptamers can be obtained.     

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)     

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

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

Structural requirements for expression of factor Va activity

Thrombin activated factor Va (factor VIIa, residues 1-709 and 1546-2196) has an apparent dissociation constant (Kd,app) for factor Xa within prothrombinase of approximately 0.5 nM. A protease (NN) purified from the venom of the snake Naja nigricollis nigricollis, cleaves human factor V at Asp697, Asp1509, and Asp1514 to produce a molecule (factor VNN) that is composed of a Mr 100,000 heavy chain (amino acid residues 1-696) and a Mr 80,000 light chain (amino acid residues 1509/1514-2196). Factor VNN, has a Kd,app for factor Xa of 4 nm and reduced clotting activity. Cleavage of factor VIIa by NN at Asp697 results in a cofactor that loses approximately 60-80% of its clotting activity. An enzyme from Russell's viper venom (RVV) cleaves human factor V at Arg1018 and Arg1545 to produce a Mr 150,000 heavy chain and Mr 74,000 light chain (factor VRVV, residues 1-1018 and 1546-2196). The RVV species has affinity for factor Xa and clotting activity similar to the thrombin-activated factor Va. Cleavage of factor VNN at Arg1545 by alpha-thrombin (factor VNN/IIa) or RVV (factor VNN/RVV) leads to enhanced affinity of the cofactor for factor Xa (Kd,app approximately 0.5 nM). A synthetic peptide containing the last 13 residues from the heavy chain of factor Va (amino acid sequence 697-709, D13R) was found to be a competitive inhibitor of prothrombinase with respect to prothrombin. The peptide was also found to specifically interact with thrombin-agarose. These data demonstrate that 1) cleavage at Arg1545 and formation of the light chain of factor VIIa is essential for high affinity binding and function of factor Xa within prothrombinase and 2) a binding site for prothrombin is contributed by amino acid residues 697-709 of the heavy chain of the cofactor.     

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

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

Bovine coagulation factor V visualized with electron microscopy. Ultrastructure of the isolated activated forms and of the activation fragments

Single chain bovine factor V (Mr = 330,000) was isolated and visualized by means of high resolution transmission electron microscopy of negatively stained samples. Both factor Va, activated by thrombin or by the factor V activator from Russell's viper venom, and the isolated fragments, D (Mr = 105,000), C1 (Mr = 150,000), and F1F2 (Mr = 72,000), were studied. Single chain factor V appeared as a multidomain structure with three globular domains of similar size (diameter approximately 80 A), and oriented around a somewhat larger central domain (diameter approximately 140 A). The distance between the center of the molecule and the center of each of the peripheral domains was 120 A and the maximum length of factor V was 300 A. The structure was essentially identical with that recently shown for human single chain factor V (Dahlb?ck, B. (1985) J. Biol. Chem. 260, 1347-1349). Isolated thrombin-activated factor Va (containing fragments D and F1F2) was composed of two domains of similar size, each of which was approximately 80 A in diameter and corresponded in size and shape to the peripheral domains seen in intact factor V. The isolated activation fragment C1 appeared as an irregular structure with an approximate diameter of 140 A and corresponded in size and shape to the larger central domain in intact factor V. The activator from Russell's viper venom only cleaves the bond(s) between C1 and F1F2, which results in two fragments, a larger fragment (Mr = 220,000) bearing the D, E, and C1 region and a smaller one corresponding to the F1F2 fragment. The venom-activated factor Va in the electron microscope demonstrated a multidomain structure similar in size and shape to that obtained with intact factor V. A model for factor V and the molecular events involved in activation is proposed.     

Nonenzymatic anticoagulant activity of the mutant serine protease Ser360Ala-activated protein C mediated by factor Va.

The human plasma serine protease, activated protein C (APC), primarily exerts its anticoagulant function by proteolytic inactivation of the blood coagulation cofactors Va and VIIIa. A recombinant active site Ser 360 to Ala mutation of protein C was prepared, and the mutant protein was expressed in human 293 kidney cells and purified. The activation peptide of the mutant protein C zymogen was cleaved by a snake venom activator, Protac C, but the "activated" S360A APC did not have amidolytic activity. However, it did exhibit significant anticoagulant activity both in clotting assays and in a purified protein assay system that measured prothrombinase activity. The S360A APC was compared to plasma-derived and wild-type recombinant APC. The anticoagulant activity of the mutant, but not native APC, was resistant to diisopropyl fluorophosphate, whereas all APCs were inhibited by monoclonal antibodies against APC. In contrast to native APC, S360A APC was not inactivated by serine protease inhibitors in plasma and did not bind to the highly reactive mutant protease inhibitor M358R alpha 1 antitrypsin. Since plasma serpins provide the major mechanism for inactivating APC in vivo, this suggests that S360A APC would have a long half-life in vivo, with potential therapeutic advantages. S360A APC rapidly inhibited factor Va in a nonenzymatic manner since it apparently did not proteolyze factor Va. These data suggest that native APC may exhibit rapid nonenzymatic anticoagulant activity followed by enzymatic irreversible proteolysis of factor Va. The results of clotting assays and prothrombinase assays showed that S360A APC could not inhibit the variant Gln 506-FVa compared with normal Arg 506-FVa, suggesting that the active site of S360A APC binds to FVa at or near Arg 506.     

Molecular recognition in the activation of human blood coagulation factor X

Factor X can be activated by the extrinsic activation complex (factor VIIa:tissue factor), the intrinsic activation complex (factor IXa:factor VIIIa) and by an enzyme from Russell's viper venom (RVV-X). To identify the regions on the surface of factor X that mediate its association with these three activators, we have prepared 21 synthetic peptides representing 65% of the primary structure of factor X. Only 3 of the 21 peptides inhibited the rate of factor X activation, indicating the regions represented by these three peptides are involved in factor X association. Using purified components, the rate of factor Xa formation was inhibited in a dose-dependent manner by these three peptides with the same relative potency of inhibition in each of the activation systems. The observed relative potencies were: peptide 267-283 greater than or equal to peptide 284-303 greater than peptide 417-431. Kinetic analyses indicated that the three peptides inhibited factor X activation in a non-competitive manner, and in mixed inhibitor assays the peptides were shown to be mutually exclusive of one another. In coagulation-based assays, the potency of inhibition by each peptide was decreased. However, in Russell's viper venom-X-initiated assays peptide 417-431 was the best inhibitor. Fab fragments of antibodies raised to these peptides and affinity purified on factor X-agarose columns inhibited both the purified and coagulation-based assays in a dose-dependent manner. Using the x-ray crystal structure of chymotrypsinogen as a model, these three peptides were found to be located spatially close to one another on the surface of factor X and opposite to the region where factor X is cleaved for activation. These data are consistent with a model in which the three activators combine with factor X through a recognition site composed of multiple loci that is distal to the potential cleavage site. This interaction aligns the active sites of these three enzymes in the correct orientation to cleave factor X at the same arginyl-isoleucyl bond.     

Molecular characterization of an extended binding site for coagulation factor Va in the positive exosite of activated protein C

The anticoagulant human plasma serine protease, activated protein C (APC), inhibits blood coagulation by specific inactivation of the coagulation cofactors factor Va (FVa) and factor VIIIa. Site-directed mutagenesis of residues in three surface loops of a positive exosite located on APC was used to identify residues that play a significant role in binding to FVa. Eighteen different residues were mutated to alanine singly, in pairs, or in triple mutation combinations. Mutant APC proteins were purified and characterized for their inactivation of FVa. Three APC residues were identified that provide major contributions to FVa interactions: Lys(193), Arg(229), and Arg(230). In addition, four residues made significant minor contributions to FVa interactions: Lys(191), Lys(192), Asp(214), and Glu(215). All of these residues primarily contribute to APC cleavage at Arg(506) in FVa and play a small role in the interaction of APC with the Arg(306) cleavage site. In conjunction with previously published work, these results define an extensive FVa binding site in the positive exosite of APC that is primarily involved in binding and cleaving at Arg(506) on FVa.     

A model describing the inactivation of factor Va by APC: bond cleavage, fragment dissociation, and product inhibition.

The inactivation of factor Va is a complex process which includes bond cleavage (at three sites) and dissociation of the A2N.A2C peptides, with intermediate activity in each species. Quantitation of the functional consequences of each step in the reaction has allowed for understanding of the presentation of disease in individuals possessing the factor V polymorphism factor VLEIDEN. APC cleavage of membrane-bound bovine factor Va (Arg306, Arg505, Arg662) leads to the dissociation of fragments of the A2 domain, residues 307-713 (A2N.A2C + A2C-peptide), leaving behind the membrane-bound A1.LC species. Evaluation of the dissociation process by light scattering yields invariant mass loss estimates as a function of APC concentration. The rate constant for A2 fragment dissociation varies with [APC], reaching a maximal value of k = 0.028 s-1, the unimolecular rate constant for A2 domain fragment dissociation. The APC binding site resides in the factor Va light chain (LC) (Kd = 7 nM), suggesting that the membrane-bound LC.A1 product would act to sequester APC. This inhibitory interaction (LC.A1.APC) is demonstrated to exist with either purified factor Va LC or the products of factor Va inactivation. Utilizing these experimental data and the reported rates of bond cleavage, binding constants, and product activity values for factor Va partial inactivation products, a model is developed which describes factor Va inactivation and accounts for the defect in factor VLEIDEN. The model accurately predicts the rates of inactivation of factor Va and factor VaLEIDEN, and the effect of product inhibition. Modeled reaction progress diagrams and activity profiles (from either factor Va or factor VaLEIDEN) are coincident with experimentally derived data, providing a mechanistic and kinetic explanation for all steps in the inactivation of normal factor Va and the pathology associated with factor VLEIDEN.     

Effects of prolonged poisoning by Russell's viper venom on blood coagulation, platelets and fibrinolysis

The effects of Russell's viper venom on blood coagulation, platelets and the fibrinolytic enzyme system were studied in rabbits after injecting repeated doses of 0.05 MLD of the venom. Thrombocytopenia was the earliest change to appear. It was followed by rise in serum fibrinogen degradation products and prolongation of prothrombin time, activated partial thromboplastin time and thrombin time indicating a progressive consumption coagulopathy and activation of fibrinolysis. Red blood cell morphology was unchanged during the first three weeks; whereas the fragmentation appeared after the fourth week and it increased in severity with further envenomations, i.e. when chronic DIC was established.     

Expression and characterization of recombinant human factor V and a mutant lacking a major portion of the connecting region

Human coagulation factor V is a protein cofactor that is an essential component of the prothrombinase complex. A full-length factor V cDNA has been subcloned into the mammalian expression vector pDX and used to transfect COS cells. Approximately 95 +/- 4% of the recombinant human factor V (rHFV) synthesized in COS cells is secreted into the culture medium. Forty-eight hours after transfection rHFV antigen levels in the conditioned medium were 70 +/- 15 ng/mL. Factor V activity determined by fibrometer assay increased approximately 5-fold from 0.027 +/- 0.012 to 0.124 +/- 0.044 unit/mL following activation by the factor V activating enzyme from Russell's viper venom (RVV-V). A chromogenic assay specific for factor Va indicated that recombinant factor V had 3.8 +/- 1.3% of the activity of the activated protein. The estimated specific activity of the recombinant factor Va was approximately 1800 +/- 500 units/mg, which is similar to the specific activity of purified plasma factor Va of 1700-2000 units/mg. Immunoprecipitation of [35S]methionine-labeled rHFV revealed a single high molecular mass component (approximately 330 kDa). Treatment of rHFV with thrombin or RVV-V resulted in the formation of proteolytic products that were similar to those seen with plasma factor V. We have also expressed a mutant, rHFV-des-B811-1441, that lacks a large portion of the highly glycosylated connecting region that is present in factor V. Immunoprecipitation of [35S]methionine-labeled rHFV-des-B811-1441 revealed a single-chain polypeptide with Mr approximately 230 kDa. This mutant constitutively expressed 38 +/- 7% of the activity of the RVV-V-activated protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

The anticoagulant protein C pathway

The anticoagulant protein C system regulates the activity of coagulation factors VIIIa and Va, cofactors in the activation of factor X and prothrombin, respectively. Protein C is activated on endothelium by the thrombin-thrombomodulin-EPCR (endothelial protein C receptor) complex. Activated protein C (APC)-mediated cleavages of factors VIIIa and Va occur on negatively charged phospholipid membranes and involve protein cofactors, protein S and factor V. APC also has anti-inflammatory and anti-apoptotic activities that involve binding of APC to EPCR and cleavage of PAR-1 (protease-activated receptor-1). Genetic defects affecting the protein C system are the most common risk factors of venous thrombosis. The protein C system contains multi-domain proteins, the molecular recognition of which will be reviewed.     

The effect of Arg306-->Ala and Arg506-->Gln substitutions in the inactivation of recombinant human factor Va by activated protein C and protein S.

Factor Va (fVa) is inactivated by activated protein C (APC) by cleavage of the heavy chain at Arg306, Arg506, and Arg679. Site-directed mutagenesis of human factor V cDNA was used to substitute Arg306-->Ala (rfVa306A) and Arg506-->Gln (rfVa506Q). Both the single and double mutants (rfVa306A/506Q) were constructed. The activation of these procofactors by alpha-thrombin and their inactivation by APC were assessed in coagulation assays using factor V-deficient plasma. All recombinant and wild-type proteins had similar initial cofactor activity and identical activation products (a factor Va molecule composed of light and heavy chains). Inactivation of factor Va purified from human plasma (fVaPLASMA) in HBS Ca2+ +0.5% BSA or in conditioned media by APC in the presence of phospholipid vesicles resulted in identical inactivation profiles and displayed identical cleavage patterns. Recombinant wild-type factor Va (rfVaWT) was inactivated by APC in the presence of phospholipid vesicles at an overall rate slower than fVaPLASMA. The rfVa306A and rfVa506Q mutants were each inactivated at rates slower than rfVaWT and fVaPLASMA. Following a 90-min incubation with APC, rfVa306A and rfVa506Q retain approximately 30-40% of the initial cofactor activity. The double mutant, rfVa306A/506Q, was completely resistant to cleavage and inactivation by APC retaining 100% of the initial cofactor activity following a 90-min incubation in the presence of APC. Recombinant fVaWT, rfVa306A, rfVa506Q, and rfVa306A/506Q were also used to evaluate the effect of protein S on the individual cleavage sites of the cofactor by APC. The initial rates of rfVaWT and rfVa306A inactivation in the presence of protein S were unchanged, indicating cleavage at Arg506 is not affected by protein S. The initial rate of rfVa506Q inactivation was increased, suggesting protein S slightly accelerates the cleavage at Arg306. Overall, the data demonstrate high specificity with respect to cleavage sites for APC on factor Va and demonstrate that cleavages of the cofactor at both Arg306 and Arg506 are required for efficient factor Va inactivation.     

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.