Protein S is a cofactor for tissue factor pathway inhibitor.

Protein S is a vitamin K-dependent protein that acts as a cofactor of the anticoagulant protein APC. However, protein S also exhibits anticoagulant activity in the absence of APC. Thrombin generation experiments in normal plasma and in plasma deficient in tissue factor pathway inhibitor (TFPI) and/or protein S demonstrated that protein S stimulates the inhibition of TF by TFPI. Kinetic analysis in model systems containing purified proteins showed that protein S enhances the formation of the binary FXa:TFPI complex by reducing the Ki of TFPI from approximately 4 nM to approximately 0.5 nM. Enhancement of inhibitory activity of TFPI by protein S is only observed with full-length TFPI and in the presence of a negatively charged phospholipid surface. The Ki decrease brings the TFPI concentration necessary for FXa:TFPI complex formation within range of the plasma TFPI concentration which increases FXa:TFPI complex formation and accelerates feedback inhibition of the TF pathway by enhancing the formation of the quaternary TFPI:FXa:TF:FVIIa complex. Thus, protein S is not only a cofactor of APC, but also of TFPI. A reduced TFPI cofactor activity may contribute to the increased risk of venous thrombosis in protein-S deficient individuals. Using calibrated automated thrombography we have developed two assays that enable quantification of the functional activity of the TFPI/protein S system in plasma. These assays show that the activity of the TFPI/protein S system is greatly impaired in oral contraceptive users.

Development of a calibrated automated thrombography based thrombin generation test in mouse plasma.

BACKGROUND: Mouse models have become increasingly important in thrombosis research. However, only a limited number of assays are available for assessment of the coagulation system in mouse plasma. OBJECTIVES: To quantify tissue factor-initiated thrombin generation in murine platelet-rich and platelet-free plasma and to develop a test for measurement of resistance to activated protein C (APC) in mouse plasma. METHODS: Thrombin generation was monitored with calibrated automated thrombography (CAT) using a low-affinity fluorogenic substrate for thrombin. RESULTS: To overcome the higher activity of coagulation inhibitors in mouse plasma as compared with human plasma, the reaction temperature was lowered to 33 degrees C and the assay was carried out at a 2-fold higher final plasma dilution (1:3) than commonly used for CAT in human plasma. This increased the endogenous thrombin potential (ETP) 4- to 5-fold and enabled reliable measurement of thrombin generation in both platelet-free and platelet-rich mouse plasma. For the APC resistance measurement, the reaction conditions were further optimized with respect to tissue factor, phospholipid, APC and CaCl(2) concentrations. The test was validated using plasma of mice with different genetic background with respect to the factor V Leiden mutation (FV Leiden). Mice homozygous for FV Leiden had higher APC sensitivity ratios (mean 5.46; 95% CI 4.88-6.03) than heterozygous FV Leiden mice (mean 4.21; 95% CI 3.53-4.89) and than wild-type mice (mean 2.71; 95%CI 2.15-3.27). CONCLUSIONS: We have established reaction conditions for measurement of thrombin generation and APC resistance in mouse plasma. This assay enables evaluation of the coagulation system and the function of the protein C system in mouse models.

Structural and functional characterization of a prothrombin activator from the venom of Bothrops neuwiedi.

A prothrombin activator from the venom of Bothrops neuwiedi was purified by gel filtration on Sephadex G-100, ion-exchange chromatography on DEAE-Sephacel and affinity chromatography on a Zn2+-chelate column. The overall purification was about 200-fold, which indicates that the prothrombin activator comprises about 0.5% of the crude venom. The venom activator is a single-chain protein with an apparent molecular weight of 60 kDa. It readily activated bovine prothrombin with a Km of 38 microM and a Vmax of 120 mumol prothrombin activated per min per mg of venom activator. Venom-catalyzed prothrombin activation was not accelerated by the so-called accessory components of the prothrombinase complex, phospholipids plus Ca2+ and Factor Va. Gel-electrophoretic analysis of prothrombin activation indicated that the venom activator only cleaved the Arg-323-Ile-324 bond of bovine prothrombin, since meizothrombin was the only product of prothrombin activation. The activator did not hydrolyze commercially available p-nitroanilide substrates and its prothrombin-converting activity was not inhibited by benzamidine, phenylmethylsulfonyl fluoride, dansyl-Glu-Gly-Arg-chloromethyl ketone and soy-bean trypsin inhibitor. However, chelating agents such as EDTA, EGTA and o-phenanthroline rapidly destroyed the enzymatic activity of the venom activator. The activity of chelator-treated venom activator could be partially restored by the addition of an excess CaCl2. These results indicate that the venom activator remarkably differs from Factor Xa and that the enzyme is not a serine proteinase, but likely belongs to the metalloproteinases. The structural and functional properties of the venom prothrombin activator from B. neuwiedi are similar to those reported for the venom activator from Echis carinatus.

Determinants of the APTT- and ETP-based APC sensitivity tests.

BACKGROUND: A reduced sensitivity for activated protein C (APC) is associated with an increased risk of venous thrombosis even in the absence of the factor (F)V Leiden mutation. This risk has been demonstrated with two APC sensitivity tests, which quantify the effects of APC on the activated partial thromboplastin time (APTT) and the endogenous thrombin potential (ETP), respectively. OBJECTIVES: We examined determinants of both APC sensitivity tests in the control group of the Leiden Thrombophilia Study (LETS). METHODS: Multiple linear regression analysis was performed with normalized APC-SR(APTT) or APC-SR(ETP) as dependent variable and putative determinants [levels of FII, FV, FVII, FVIII, FIX, FX, FXI, FXII, FXIII A subunit, FXIII B subunit, protein S total, protein S free, protein C, tissue factor pathway inhibitor (TFPI) total, TFPI free, antithrombin and fibrinogen] as independent variables. RESULTS AND CONCLUSIONS: The major determinant of the APTT-based test was FVIII level, followed by FII level. The ETP-based test was influenced most by free protein S and free TFPI levels. In both tests FXa formation plays a major role, as the effect of FVIII and TFPI on the tests seems to be executed via FXa. The ETP-based test was also strongly influenced by oral contraceptive use, even when we adjusted for all the clotting factors listed above. This means that the effect of oral contraceptives on the ETP-based test is not fully explained by the changes of coagulation factor levels investigated in this study, and that the molecular basis of acquired APC resistance during use of oral contraceptives remains to be established.

Expression of the normal factor V allele modulates the APC resistance phenotype in heterozygous carriers of the factor V Leiden mutation.

BACKGROUND: Functional defects of the protein C pathway, detectable in plasma as activated protein C (APC) resistance, are a prevalent risk factor for venous thrombosis. The factor V (FV) Leiden mutation causes APC resistance by interfering with the APC-mediated inactivation of both FVa and FVIIIa. Co-inheritance of FV Leiden and quantitative FV deficiency on different alleles, a rare condition known as pseudo-homozygous APC resistance, is associated with pronounced APC resistance and 50% reduced FV levels, because of non-expression of the non-Leiden FV allele. OBJECTIVES: The role of normal FV in modulating the APC resistance phenotype in carriers of FV Leiden was investigated in patients with pseudo-homozygous APC resistance and in model systems. PATIENTS/METHODS: Four functional plasma assays probing both components of APC resistance (susceptibility of FVa to APC and cofactor activity of FV in FVIIIa inactivation) were employed to compare seven clinically and genetically characterized FV Leiden pseudo-homozygotes to 30 relatives with different FV genotypes (including 12 FV Leiden heterozygotes and seven carriers of FV deficiency) and to 32 unrelated FV Leiden homozygotes. RESULTS AND CONCLUSIONS: All assays consistently indicated that FV Leiden pseudo-homozygotes are significantly more APC-resistant than heterozygotes and indistinguishable from homozygotes. Thrombin generation measurements in FV-deficient plasma reconstituted with purified normal FV and FV Leiden confirmed these observations and showed that the expression of the normal FV allele is an important modulator of APC resistance in FV Leiden heterozygotes. These findings provide an explanation for the higher thrombotic risk of FV Leiden pseudo-homozygotes when compared with heterozygotes.

Factor V.

Factor V is a single chain glycoprotein that plays an essential role in the regulation of blood coagulation. After initiation of coagulation, factor V is converted into factor Va through limited proteolysis. Factor Va acts as protein cofactor in the prothrombin-activating complex, which is comprised of the serine protease factor Xa, Ca2+ ions and a procoagulant membrane surface. Factor Va accelerates factor Xa-catalysed conversion of prothrombin into thrombin more than 10(4)-fold. The cofactor activity of factor Va in prothrombin activation is down-regulated by activated protein C (APC). The physiological importance of this regulatory pathway is demonstrated by the occurrence of hereditary thrombophilia in individuals with a genetic defect that makes factor Va less sensitive to proteolytic inactivation by APC (APC resistance).

Venous thrombosis and changes of hemostatic variables during cross-sex hormone treatment in transsexual people.

The incidence of venous thrombosis associated with estrogen treatment in male-to-female (M-->F) transsexuals is considerably higher with administration of oral ethinyl estradiol (EE) than with transdermal (td) 17-beta-estradiol (E(2)). To find an explanation for the different thrombotic risks of oral EE and td E(2) use, we compared the effects of treatment of M-->F transsexuals with cyproterone acetate (CPA) only, and with CPA in combination with td E(2), oral EE, or oral E(2) on a number of hemostatic variables [activated protein C (APC) resistance and plasma levels of protein S, protein C, and prothombin], all of which are documented risk factors for venous thrombosis. APC resistance was determined by quantification of the effect of APC on the amount of thrombin generated during tissue factor-initiated coagulation; plasma levels of total and free protein S were determined by standard ELISA; and levels of prothrombin and protein C were determined with functional assays after complete activation of the zymogens with specific snake venom proteases. CPA-only, td-E(2)+CPA, or oral-E(2)+CPA treatment produced rather small effects on hemostatic variables, whereas oral EE treatment resulted in a large increase in APC resistance from 1.2 +/- 0.8 to 4.1 +/- 1 (P < 0.001), a moderate increase in plasma protein C (9%; P = 0.012), and a large decrease in both total and free plasma protein S (30%; P < 0.005). The large differential effect of oral EE and oral E(2) indicates that the prothrombotic effect of EE is due to its molecular structure rather than to a first-pass liver effect (which they share). Moreover, these differences may explain why M-->F transsexuals treated with oral EE are exposed to a higher thrombotic risk than transsexuals treated with td E(2). Testosterone administration to female-to-male transsexuals had an antithrombotic effect.

Regulation of thrombin formation by activated protein C: effect of the factor V Leiden mutation.

Activated Protein C (APC) resistance, one of the most common genetic risk factors for venous thrombosis, is caused by a single base mutation (G1691-->A) in the factor V (FV) gene resulting in the replacement of Arg506 by Gln at a predominant cleavage site for APC. Great progress in understanding the mechanism of downregulation of FVa activity via the protein C pathway has been achieved by studying APC-mediated inactivation of FVa purified from homozygous APC-resistant individuals. This review briefly summarizes the role of FVa in prothrombin activation and the structure-function relationship of FV and FVa. Subsequently, APC-dependent inactivation of FVa and FVa Leiden and its modulation by protein S and factor Xa in model systems containing purified proteins is discussed. FV also has a function in increasing the inactivation of FVIII/VIIIa by APC. This cofactor activity appears diminished in FV Leiden. Thus, an intricate mechanism of regulation of thrombin formation via the protein C pathway is starting to emerge. Extensive studies in plasma milieu will be needed to gain more insight into the relation between the presence of FV Leiden and impaired downregulation of thrombin formation in APC-resistant individuals.

Coagulation factor V: an old star shines again.

Blood coagulation factor V plays an important role in the regulation of thrombin formation. Activation of factor V by traces of activated coagulation factors (thrombin, factor Xa or meizothrombin) yields factor Va, the non-enzymatic cofactor of the prothrombinase complex. Since factor Va accelerates prothrombin activation under physiological conditions more than 10(4)-fold it is not surprising that down-regulation of factor Va cofactor activity by the protein C pathway is a very effective way for maintaining the hemostatic balance. In this paper we have reviewed the present status of structural knowledge of factor V and Va, the molecular changes in factor V that occur during factor V activation, the function of factor Va in prothrombin activation and the molecular mechanism of inactivation of factor Va by APC. Although considerable insight in the structure-function relationship of factor V and Va has been achieved, the study of mutated factor V molecules obtained by recombinant DNA technology will undoubtedly resolve remaining questions. The latter is illustrated by the fact that the discovery of factor VaLeiden has significantly contributed to our present knowledge on the regulation of the cofactor activity of factor Va via the protein C pathway. It appears that modulation of the activity of APC by protein S and factor Xa will strongly affect the in vivo activity of this pathway. Factor V not only plays an important role in the regulation of the activity of the prothrombinase complex but also acts as cofactor in APC-mediated inactivation of factor VIIIa. This gives rise to a rather intricate mechanism of regulation of thrombin formation by APC that thus far has been mainly studied in model systems containing purified proteins. Thus, extensive studies in plasma will be required in order to get more insight in the in vivo regulation of thrombin formation via the protein C pathway.

An assay to quantify the two plasma isoforms of factor V.

Blood coagulation factor V (FV) circulates in the blood in two forms designated FV1 and FV2. In model systems containing purified proteins FV1 appears to be more thrombogenic than FV2. Recently, we reported that in plasma from carriers of the R2 haplotype, a polymorphism which encodes several amino acid changes in FV and which is associated with an increased risk of thrombosis, the FV1/FV2 ratio is shifted in favor of the more thrombogenic form FV1. Here we describe in detail the assay that enables quantification of the plasma levels of FV1 and FV2. FV present in highly diluted plasma samples was activated with thrombin and the FVa generated was subsequently quantified in two prothrombinase-based assay systems. In the first assay, which is performed at saturating amounts of FXa and phospholipid vesicles with a high mole fraction phosphatidylserine, FVa1 and FVa2 express the same cofactor activity in prothrombin activation. Hence, this assay quantifies the total FV level (FV1 + FV2) present in plasma. In the second assay, which is performed at suboptimal amounts of FXa and phospholipid vesicles with a low mole fraction phosphatidylserine, FVa2 has approximately an 8-fold higher cofactor activity than FVa1. Therefore, the response in this assay depends on the relative amounts of FV1 and FV2 in the plasma sample. Calibration curves made with samples containing known concentrations of purified FVa1 and FVa2 subsequently allowed calculation of the amounts of FV1 and FV2 present in plasma.

Amidolytic detection of prothrombin activation products after SDS-gel electrophoresis.

In this paper we report a method via which enzymatically active products formed during prothrombin activation can be detected by simple photographic means after SDS-gel electrophoresis, blotting onto a nitrocellulose membrane and visualization with the chromogenic substrate, S2238. After amidolytic detection the same nitrocellulose membrane can also be used for immunologic detection of prothrombin activation products, thus allowing a complete description of product formation during prothrombin activation. The detection limit of the so-called "amidoblot" is approximately 3 ng thrombin per gel sample which is comparable to the sensitivity of immunoblotting. It is further shown that the amidoblot technique can also be applied to other coagulation factors for which a suitable chromogenic substrate is available (factor XIIa, kallikrein, factor XIa, factor Xa, plasmin and activated protein C).

Functional properties of factor V and factor Va encoded by the R2-gene.

Carriership of the factor V (FV) gene marked by the R2-haplotype, a series of linked polymorphisms encoding several amino acid changes in FV, is associated with mild resistance to activated protein C (APC) and with an increased risk of thrombosis. We compared the functional properties of normal FV(a) and R2-FV(a) in model systems and in plasma. FV and R2-FV were equally well activated by thrombin and expressed identical cofactor activities in prothrombin activation. Rate constants of APC-catalyzed inactivation of FVa and R2-FVa were similar both with and without protein S. However, significant differences were observed between haemostatic parameters determined in plasma from homozygous carriers of the R2-gene (n = 5) and age-matched non-carriers (n = 19). Plasma from R2-carriers contained significantly lower FV levels and the ratio of the two FV isoforms (FV1 and FV2) was shifted in favor of FV1. The FV2/FV1 ratio was 1.4 (95% CI = 1.3-1.5) in homozygous carriers of R2 and 2.8 (95% CI = 2.5-3.1) in controls (p < 0.00001). In an APC resistance test which quantifies the cofactor activity of FV in APC-catalyzed FVIII(a) inactivation, homozygous R2-carriers had significantly lower (p < 0.00001) APC sensitivity ratios (APCsr = 1.54, 95% CI = 1.48-1.60) than controls (APCsr = 2.17, 95% CI = 2.05-2.28). This indicates that R2-FV has reduced cofactor activity in APC-catalyzed FVIII(a) inactivation. The changes of the relative amounts of FV1 and FV2 in carriers of the R2-gene will result in increased thrombin formation in the presence of APC and may provide a mechanistic explanation for the increased thrombotic risk associated with the R2-haplotype.

Factor V enhances the cofactor function of protein S in the APC-mediated inactivation of factor VIII: influence of the factor VR506Q mutation.

Factor V and protein S are cofactors of activated protein C (APC) which accelerate APC-mediated factor VIII inactivation. The effects of factor V and protein S were quantitated in a reaction system in which plasma factor VIII was inactivated by APC and the loss of factor VIII activity was monitored in a factor X-activating system in which a chromogenic substrate was used to probe factor Xa formation. Factor V increased the rate of APC-mediated factor VIII inactivation in a dose-dependent manner in representative plasma samples with protein S or factor V deficiency, abnormal factor V (heterozygous or homozygous for factor VR506Q), or a combination of heterozygous protein S deficiency and heterozygous factor VR506Q. This effect was much less pronounced in the plasma samples with a decreased protein S level, but the impaired response in these plasmas was corrected by addition of protein S, indicating that both factor V and protein S are required for optimal inactivation of factor VIII by APC. The effects of factor V and protein S were also studied in a reaction system with purified proteins. APC-catalysed factor VIII inactivation was enhanced 3.7-fold in the presence of 1.1 nM factor V and 1.5-fold in the presence of 2.4 nM protein S. When both 1.1 nM factor V and 2.4 nM protein were present the rate enhancement was 11-fold. Factor V is a more potent cofactor than protein S, as can be concluded from the fact that 0.04 nM factor V gave the same stimulation as 2.4 nM protein S. Protein S lost its cofactor function after complexation with C4b binding protein, which indicates that it is free protein S that acts as a cofactor. To investigate the effect of the R506Q mutation in factor V on APC-mediated factor VIII inactivation, factor V was purified from the plasma of patients homozygous for factor VR506Q. In the absence of protein S, factor VR506Q did not enhance factor VIII inactivation by APC, but in the presence of 2.4 nM protein S a slight enhancement was observed. The APC cofactor activity of factor V was lost when factor V was activated with thrombin or with the factor V activator from Russell's viper venom. These data indicate that optimal inactivation of factor VIII by APC requires the presence of an intact factor V molecule and free protein S.

Factor XI activation by meizothrombin: stimulation by phospholipid vesicles containing both phosphatidylserine and phosphatidylethanolamine.

The activation of factor XI by meizothrombin was investigated using recombinant meizothrombin (R155A meizothrombin) that is resistant to autocatalytic removal of fragment 1. Meizothrombin was capable of activating factor XI at an activation rate similar to that of thrombin. Dextran sulphate and heparin, known cofactors of thrombin-mediated factor XI activation, did not stimulate the activation of factor XI by meizothrombin. However, the activation of factor XI by meizothrombin was markedly enhanced by vesicles containing phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidylethanolamine (PE), whereas PC/PS or PC/PE vesicles only had a minor effect on the activation. Thrombin-mediated factor XI activation was not influenced by phospholipids. The effect of PC/PS/PE and PC/PS vesicles was studied in a factor XI dependent clot lysis assay. In this assay, factor XI inhibits clot lysis by a feedback loop in the intrinsic pathway via thrombin-mediated factor XI activation. Removal of endogenous phospholipids in plasma by centrifugation resulted in an increased clot lysis, which could be restored to the pre-centrifugation level by the addition of PC/PS/PE vesicles, but not by PC/PS vesicles. When clot lysis was initiated by factor IXa in the presence of a factor XIa blocking antibody, there was no difference in inhibitory effect of PC/PS/PE or PC/PS vesicles. These data suggested that the differences in clot lysis inhibition observed between PC/PS/PE and PC/PS vesicles were caused by factor XI activation by meizothrombin. Meizothrombin-mediated factor XI activation may therefore play an important role in the antifibrinolytic feedback loop in the intrinsic pathway.

Activation of factor IX by factor XIa--a spectrophotometric assay for factor IX in human plasma.

The activation of Factor IX by partially purified Factor XIa was followed by active site titration, gelelectrophoresis and by a spectrophotometric assay. The assay is based on the finding that the rate of Factor X activation in the presence of phospholipid and Ca2+ is linear in time and proportional to the amount of Factor IXa present and can be determined with the chromogenic substrate S2222. Conditions were found that allowed complete activation of Factor IX in human plasma by Factor XIa. The amount of Factor IXa present in the plasma sample can be determined with the spectrophotometric assay and is proportional with the amount of plasma present. In plasma from patients receiving vitamin-K antagonists reduced Factor IX activity is found with the spectrophotometric assay and the new assay method may be useful in monitoring oral anticoagulant therapy.

Increased fibrinolytic activity during use of oral contraceptives is counteracted by an enhanced factor XI-independent down regulation of fibrinolysis: a randomized cross-over study of two low-dose oral contraceptives.

The effect of oral contraceptives (OC) on fibrinolytic parameters was investigated in a cycle-controlled cross-over study in which 28 non-OC using women were randomly prescribed either a representative of the so-called second (30 microg ethinylestradiol, 150 microg levonorgestrel) or third generation OC (30 microg ethinylestradiol, 150 microg desogestrel) and who switched OC after a two month wash out period. During the use of OC, the levels of tissue-type plasminogen activator (tPA) activity, plasminogen, plasmin-alpha2-antiplasmin complexes and D-dimer significantly increased (by 30 to 80%), while the levels of plasminogen activator inhibitor- (PAI-1) antigen, PAI-1 activity and tPA antigen significantly decreased (25 to 50%), suggesting an increase in endogenous fibrinolytic activity. These OC-induced changes were not different between the two contraceptive pills. TAFI (thrombin-activatable fibrinolysis inhibitor) levels increased on levonorgestrel, and even further increased on desogestrel. A clot lysis assay that probes both fibrinolytic activity and the efficacy of the coagulation system to generate thrombin necessary to down regulate fibrinolysis via TAFI showed no change of the clot lysis time during OC use. This finding suggests that the OC-induced increase in endogenous fibrinolytic activity is counteracted by an increased capacity of the coagulation system to down regulate fibrinolysis via TAFI. Indeed we observed that during OC use there was a significant increase of F1+2 generation during clot formation. When these assays were performed in the presence of an antibody against factor XI, we observed that the clot lysis time was significantly increased during OC use and that the increase in F1+2 generation during OC therapy was due to a factor XI-independent process, which was significantly higher on desogestrel than on levonorgestrel. These data indicate that the OC-induced inhibition of endogenous fibrinolysis takes place in a factor XI-independent way and is more pronounced on desogestrel than on levonorgestrel-containing OC.

A randomized cross-over study on the effects of levonorgestrel- and desogestrel-containing oral contraceptives on the anticoagulant pathways.

The use of oral contraceptives (OC) causes disturbances of the procoagulant, anticoagulant and fibrinolytic pathways of blood coagulation which may contribute to the increased risk of venous thrombosis associated with OC therapy. Here we report the results of a cycle-controlled randomized cross-over study, in which we determined the effects of so-called second and third generation OC's on a number of anticoagulant parameters. In this study, 28 non-OC using women were randomly prescribed either a second generation (150 microg levonorgestrel/30 microg ethinylestradiol) or a third generation OC (150 microg desogestrel/30 microg ethinylestradiol) and who switched to the other OC after a two month wash out period. The anticoagulant parameters determined were: antithrombin (AT), alpha2-macroglobulin (alpha2-M), alpha1-antitrypsin, protein C inhibitor (PCI), protein C, total and free protein S and activated protein C sensitivity ratios (APC-sr) measured with two functional APC resistance tests which quantify the effect of APC on either the activated partial thromboplastin time (aPTT) or on the endogenous thrombin potential (ETP). During the use of desogestrel-containing OC the plasma levels of alpha2-M, alpha1-antitrypsin, PCI and protein C significantly increased, whereas AT and protein S significantly decreased. Similar trends were observed with levonorgestrel-containing OC, although on this kind of OC the changes in AT, PCI and protein S (which was even slightly increased) did not reach significance. Compared with levonorgestrel, desogestrel-containing OC caused a significant decrease of total (p <0.005) as well as free protein S (p <0.0001) and more pronounced APC resistance in both the aPTT (p = 0.02) and ETP-based (p <0.0001) APC resistance tests. These observations indicate that the activity of the anticoagulant pathways in plasma from users of desogestrel-containing OC is more extensively impaired than in plasma from users of levonorgestrel-containing OC.

A prothrombinase-based assay for detection of resistance to activated protein C.

In this paper we present a new method for the detection of resistance to activated protein C (APC) that is based on direct measurement of the effect of APC on the cofactor activity of plasma factor Va. The factor V present in a diluted plasma sample was activated with thrombin and its sensitivity towards APC was subsequently determined by incubation with phospholipids and APC. The loss of factor Va cofactor activity was quantified in a prothrombinase system containing purified prothrombin, factor Xa and phospholipid vesicles and using a chromogenic assay for quantitation of thrombin formation. The reaction conditions were optimized in order to distinguish normal, heterozygous and homozygous APC-resistant plasmas. Maximal differences in the response of these plasmas towards APC were observed when factor Va was inactivated by APC in the absence of protein S and when the cofactor activity of factor Va was determined at a low factor Xa concentration (0.3 nM). Addition of 0.2 nM APC and 20 microM phospholipid vesicles to a 1000-fold diluted sample of thrombin-activated normal plasma resulted in loss of more than 85% of the cofactor activity factor Va within 6 min. Under the same conditions, APC inactivated approximately 60% and approximately 20% of the factor Va present in plasma samples from APC-resistant individuals that were heterozygous or homozygous for the mutation Arg506-->Gln in factor V, respectively. Discrimination between the plasma samples from normal and heterozygous and homozygous APC-resistant individuals was facilitated by introduction of the so-called APC-sensitivity ratio (APC-sr). The APC-sr was defined as the ratio of the factor Va cofactor activities determined in thrombin-activated plasma samples after 6 min incubation with or without 0.2 nM APC and was multiplied by 100 to obtain integers (APC-sr = ¿factor Va+APC square root of factor Va-APC¿ x 100). Clear differences were observed between the APC-sr of plasmas from normal healthy volunteers (APC-sr: 8-20, n = 33) and from individuals that were heterozygous (APC-sr: 35-50, n = 17) or homozygous APC resistant (APC-sr: 82-88, n = 7). There was no mutual overlap between the APC-sr of normal plasmas and plasmas from heterozygous or homozygous APC resistant individuals (p < 0.0001). In all cases our test gave the same result at the DNA-based assay. Since the test is performed on a highly diluted plasma sample there is no interference by conditions that affect APC resistance tests that are based on clotting time determinations (e.g. coagulation factor deficiencies, oral anticoagulation, heparin treatment, the presence of lupus anticoagulants, pregnancy or the use of oral contraceptives). Furthermore, we show that part of the factor Va assay can be performed on an autoanalyzer which increases the number of plasma samples that can be handled simultaneously.

Mutations in the R2 FV gene affect the ratio between the two FV isoforms in plasma.

Molecular genetics and biochemical studies were performed in homozygotes for the R2 allele (4070G) in the factor V gene, most of them affected by coronary artery disease. Novel polymorphisms (G642T, 156Ser; T1328C, 385Met/Thr), among which a functional candidate (A6755G, 2194Asp/Gly) located in the C2 domain of FV, were identified in the R2 gene. In chromatographic studies R2 FV appeared qualitatively identical to normal FV. However, a relative increase of the more thrombogenic and more glycosylated FV isoform (FV1) was observed in plasma of 2194Gly homozygotes (mean FV1/FV2 ratio 0.71, 95% CI 0.66-0.77) as compared to R2-free controls (0.37, 95% CI 0.34-0.40). We conclude that carriership of the R2 FV gene is associated with an imbalance between the two functionally different FV isoforms, and propose that genetically determined differential glycosylation of FV could represent a novel mechanism of thrombotic disease.

Effects on coagulation of levonorgestrel- and desogestrel-containing low dose oral contraceptives: a cross-over study.

Combined oral contraceptives (OC) are known to increase the risk of venous thromboembolism. The aim of this randomized, cycle-controlled, cross-over study in 28 healthy volunteers was to assess potential differences between the effects of an OC containing 150 microg levonorgestrel (as representative of the so-called second generation OC) and an OC containing 150 microg desogestrel (as representative of the third generation OC) in combination with 30 microg ethinylestradiol on several coagulation factors and markers of thrombin formation. All participants used each OC for two cycles, and were switched to the other OC after a washout period of two menstrual cycles. The plasma concentrations of factors II, VII, X, and fibrinogen significantly increased during use of both the levonorgestrel- and desogestrel-containing OC's. The plasma concentrations of factor VIII increased, and of factor V decreased, changes which only reached statistical significance during the use of the desogestrel-containing OC. During exposure to the desogestrel-containing OC, as compared with the levonorgestrel-containing OC, both factor VII and factor II showed a greater increase (FVII: 32% and 12% respectively; p <0.0001; FII: 16% and 12% respectively; p = 0.048), whereas factor V showed a greater decrease (-11% and -3% respectively; p = 0.010). Only one of the markers for ongoing coagulation (prothrombin fragment 1+2) showed a significant increase during OC use, whereas concentrations of thrombin-antithrombin complexes and soluble fibrin remained unchanged. For these markers, there was no difference between the tested OC's. We conclude that there are differences between the effects of levonorgestrel and desogestrel-containing OC's on some coagulation factors. Whether these changes provide a biological explanation for the reported differences in venous thromboembolic risk is as yet unclear. The real challenge now becomes to define a pattern of changes in the various systems which, if affected simultaneously, may tip the hemostatic balance towards a prethrombotic state and may lead to overt clinical venous thromboembolism.