Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.4.21.5 (thrombin)
33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Growing evidence suggests that moderately elevated levels of homocysteine are associated not only with arterial thrombosis and atherosclerosis but also with venous thrombosis as well. We have reviewed recent studies that indicate that homocysteine inhibits several different anticoagulant mechanisms that are mediated by the vascular endothelium. The protein C enzyme system appears to be one of the most important anticoagulant pathways in the blood. Homocysteine inhibits the expression and activity of endothelial cell surface thrombomodulin, the thrombin cofactor responsible for protein C activation. Homocysteine inhibits the antithrombin III binding activity of endothelial heparan sulfate proteoglycan, thereby suppressing the anticoagulant effect of antithrombin III. Homocysteine also inhibits the ecto-ADPase activity of human umbilical vein endothelial cells (HUVECS). Because ADP is a potent platelet aggregatory agent, this action of homocysteine is prothrombotic. Homocysteine also interferes with the fibrinolytic properties of the endothelial surface because it inhibits the binding of tissue plasminogen activator. Homocysteine stimulates HUVEC tissue factor activity. We have found that lipoprotein(a) [Lp(a)] also stimulates HUVEC tissue factor activity. The combination of Lp(a) plus homocysteine induced more tissue factor activity than either agent alone. These disruptions in several different vessel wall-related anticoagulant functions provide plausable mechanisms for the occurrence of thrombosis in hyperhomocysteinemia.
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PMID:Homocysteine and hemostasis: pathogenic mechanisms predisposing to thrombosis. 864 72

Folic acid deficiency represents a vitamin deficiency that may be due either to an inadequacy of the dietary supply or to an increased requirement. It leads to a number of abnormalities including hematological, neurological and cardiovascular disorders. In this study, we investigated whether folic acid deficiency would influence platelet and macrophage activities. For 6 weeks, rats were fed a test diet containing a low amount of folic acid (250 mu g/kg) by comparison with a control diet (750 mu g/kg). We found 40 and 32 percent reductions (P < 0.05) of plasma and erythrocyte folates, respectively in the tested group. Peritoneal macrophages of the folic acid deficient animals exhibited greater (20 x) tissue factor (TF) activity than in the controls. We also found that folate depletion significantly enhanced the thrombin- and ADP-induced platelet aggregation (+64 and + 13 percent, respectively). Moreover, the results of incubations with radiolabeled arachidonic acid indicated that platelets of folic acid deficient animals incorporated more labeling than controls did. When stimulated with thrombin, the mobilization of arachidonate from platelet phospholipids and its subsequent formation of cyclooxygenase and lipoxygenase metabolites were enhanced in the deficient animals. In particular, thromboxane biosynthesis was markedly increased. The analysis of the plasma fatty acid composition showed a decrease in the plasma unsaturation index related to a marked fall of long chain (n-3) fatty acids which was also observed in platelets. These data suggested the occurrence of an oxidative stress in folic acid deficient animals which was confirmed by increases in plasma lipid peroxidation products (more than +20 percent) and an enhanced susceptibility of erythrocytes to free radicals (+23 percent). Altogether these data suggested that folic acid deficiency altered the circulating and cellular fatty acid composition and thus influenced the balance of the platelet eicosanoid synthesis. In addition, total homocysteine and glutathione concentrations were highly increased in plasma from folate-depleted rats. From these results, we conclude that folate deficiency can potentiate the coagulation pathway mediated by the macrophage TF as well as the platelet activation process. It is suggested that these dysfunctions might be related to the loss of (n-3) polyunsaturated fatty acids. The latter could result from an increased lipid peroxidation triggered by the folic acid deficiency-induced hyperhomocysteinemia.
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PMID:Pro-thrombotic effects of a folic acid deficient diet in rat platelets and macrophages related to elevated homocysteine and decreased n-3 polyunsaturated fatty acids. 912 97

Congenital homocysteinuria is a rare inherited metabolic disorder with early onset atherosclerosis and arterial and venous trombosis. Moderate hyperhomocysteinemia is more frequently encountered and is recognized as an independent cardiovascular risk factor. Several case-control studies demonstrate an association between venous thromboembolism and moderate hyperhomocysteinemia. A patient with moderate hyperhomocysteinemia has a 2-3 relative risk of developing an episode of venous thromboembolism. The occurrence of mild hyperhomocysteinemia in heterozygotes for the mutation of Leiden factor V involves a 10-fold increase in the risk of venous thromboembolism. The biochemical mechanism by which homocysteine may promote thrombosis is not fully recognized. Homocysteine inhibits the expression of thrombomodulin, the thrombin cofactor responsible for protein C activation, and inhibits antithrombin-III binding. Treatment with folic acid reduces the plasma level of homocysteinemia, but no study has demonstrated its efficacy in reducing the incidence of venous thromboembolism or atherosclerosis. Hyperhomocysteinemia should be included in the screening of abnormalities of hemostasis and thrombosis in patients with idiopathic thromboembolism, and mild hyperhomocysteinemia may justify a trial of folic acid.
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PMID:[Homocysteine and venous thromboembolism]. 930 44

In previous studies conducted in female rats and in women, oral contraceptives (OC) were found to induce a platelet hyperactivity that was related to an oxidative stress. Because cases of megaloblastic anemia have been reported to occur in women taking OC, these treatments are suspected of depleting folate stores. In the study presented herein, which was conducted in rats, we sought to determine the influence of dietary folic acid deficiency (FD) on the thrombogenicity of OC. Animals were fed for 6 weeks with either a folic acid-deficient diet (250 micrograms/kg folic acid) or a control diet (750 micrograms/kg). One-half of the animals in each group were treated with OC (ethinyl estradiol plus lynestrenol). FD and OC individually potentiated platelet aggregation in response to thrombin and ADP and the release and metabolism of arachidonic acid, in particular, the biosynthesis of thromboxane. These platelet activities were further enhanced in animals given both the folic acid-deficient diet and the OC treatment. In addition, FD enhanced the pro-oxidant state in OC-treated rats characterized by (1) a fall in platelet and plasma n-3 fatty acids, (2) an increase in plasma lipid peroxidation products such as conjugated dienes, lipid peroxides, and thiobarbituric reactive substances, (3) a rise in ex vivo erythrocyte susceptibility to free radicals. Moreover, we found that OC treatment led to a reduction of plasma and erythrocyte folate concentrations associated with a moderate hyperhomocysteinemia. Under our experimental conditions, we did not find significant synergistic effects between OC and FD. We propose that, although the untoward effects associated with the OC treatment may not primarily be dependent on FD, the folic acid deficiency magnified OC-induced oxidative stress, which resulted in platelet hyperactivity by elevating the pro-oxidant homocysteine plasma concentration. Despite the limitations of this animal model, the data of the present study suggest that in addition to cigarette smoking, inadequate folic acid intake might predispose those taking OC to vascular thrombosis.
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PMID:Folic acid deficiency enhances oral contraceptive-induced platelet hyperactivity. 935 57

A moderate elevation of plasma homocysteine is a risk factor for atherosclerosis and arterial and veinous thrombosis. However, the mechanisms leading to vascular disorders are poorly understood because studies that have investigated the potential atherothrombogenicity of hyperhomocysteinemia in vivo are scarce. Using a rat model, we were the first to show that dietary folic acid deficiency, a major cause of basal hyperhomocysteinemia, is associated with enhanced macrophage-derived tissue factor and platelet activities. We proposed that an homocysteine-induced oxidative stress may account for this hypercoagulable state. To determine the true thrombogenicity of moderate hyperhomocysteinemia and better understand its etiology, we have carried out an acute methionine load in control and folate-deficient animals. When rats were fed the control diet, a transient fourfold increase in plasma homocysteine levels was observed 2 h after the methionine administration. As with prolonged dietary folic acid deficiency, this methionine load potentiated the platelet aggregation in response to thrombin and ADP as well as the thrombin-induced thromboxane synthesis. It also stimulated the basal and lipopolysaccharide-induced tissue factor activity of peritoneal macrophages. These prothrombotic effects were associated with an increased lipid peroxidation characterized by an elevation of plasma conjugated dienes, lipid hydroperoxides, and thiobarbituric acid-reactive substances. When rats were fed a folic acid-deficient diet, the methionine load did not cause any further increase in plasma homocysteine concentration, platelet activation, macrophage tissue factor-dependent coagulation, or lipoperoxidation. Altogether, our data showed that the prethrombotic state due to both the altered remethylation and transsulfuration pathways resulted from the moderate elevation of circulating homocysteine. We conclude that moderate hyperhomocysteinemia plays a role in the development of a thrombogenic state that might be mediated by the occurrence of oxidative stress.
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PMID:Acute methionine load-induced hyperhomocysteinemia enhances platelet aggregation, thromboxane biosynthesis, and macrophage-derived tissue factor activity in rats. 936 51

Increased thrombin generation occurs in many individuals with inherited defects in the antithrombin or protein C anticoagulant pathways and is also seen in patients with thrombosis without a defined clotting abnormality. Hyperhomocysteinemia (H-HC) is an important risk factor of venous thromboembolism (VTE). We prospectively followed 48 patients with H-HC (median age 62 years, range 26-83; 18 males) and 183 patients (median age 50 years, range 18-85; 83 males) without H-HC for a period of up to one year. Prothrombin fragment F1+2 (F1+2) was determined in the patient's plasma as a measure of thrombin generation during and at several time points after discontinuation of secondary thromboprophylaxis with oral anticoagulants. While on anticoagulants, patients with H-HC had significantly higher F1+2 levels than patients without H-HC (mean 0.52 +/- 0.49 nmol/l, median 0.4, range 0.2-2.8, versus 0.36 +/- 0.2 nmol/l, median 0.3, range 0.1-2.1; p = 0.02). Three weeks and 3, 6, 9 and 12 months after discontinuation of oral anticoagulants, up to 20% of the patients with H-HC and 5 to 6% without H-HC had higher F1+2 levels than a corresponding age- and sex-matched control group. 16% of the patients with H-HC and 4% of the patients without H-HC had either F1+2 levels above the upper limit of normal controls at least at 2 occasions or (an) elevated F1+2 level(s) followed by recurrent VTE. No statistical significant difference in the F1+2 levels was seen between patients with and without H-HC. We conclude that a permanent hemostatic system activation is detectable in a proportion of patients with H-HC after discontinuation of oral anticoagulant therapy following VTE. Furthermore, secondary thromboprophylaxis with conventional doses of oral anticoagulants may not be sufficient to suppress hemostatic system activation in patients with H-HC.
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PMID:Levels of prothrombin fragment F1+2 in patients with hyperhomocysteinemia and a history of venous thromboembolism. 940 13

The effect of homocysteine-lowering treatment on thrombin generation was investigated in 17 subjects with hyperhomocysteinemia (aged 22-60 years), 11 of whom had symptomatic atherosclerotic vascular disease. All subjects had fasting total homocysteine levels above 16 micromol/L. The formation of thrombin was assessed by measuring thrombin-antithrombin III complexes and prothrombin fragment 1+2 in peripheral venous blood and in the bleeding time blood collected at 30-second intervals from skin incisions on a forearm. All the tests were performed before and after an 8-week treatment with folic acid p.o. 5 mg/day, vitamin B6 p.o. 300 mg/day, and vitamin B12 i.m. 1000 microg given on a weekly basis. Following the 8-week therapy, the median plasma homocysteine concentration became significantly reduced from 20 to 10 micromol/L, while plasma levels of fibrinogen, prothrombin, and antithrombin III as well as activity of protein C, S, and factor VII showed no changes. Vitamin treatment was associated with a significant fall in thrombin-antithrombin III complexes and prothrombin fragment 1+2 concentrations in peripheral venous blood. Bleeding time became prolonged by about 60 seconds. At sites of hemostatic plug formation, plasma concentrations of both thrombin markers significantly decreased. Compared with pretreatment values, significantly less thrombin was produced during the first 3 minutes of bleeding after homocysteine-lowering therapy. In subjects with hyperhomocysteinemia a reduction of plasma fasting homocysteine concentration by folic acid and vitamins B12 and B6 administration is associated with attenuation of thrombin generation both in peripheral blood and at sites of hemostatic plug formation.
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PMID:Treatment of hyperhomocysteinemia with folic acid and vitamins B12 and B6 attenuates thrombin generation. 1052 5

Over the last 30 years, a growing body of evidence has documented the role of hyperhomocysteinemia (HHcy) as an independent vascular risk factor. However, the mechanisms through which elevated circulating levels of homocysteine (Hcy) cause vascular injury and promote thrombosis remain elusive. Most findings have been achieved in in vitro studies employing exceedingly high concentrations of Hcy, whereas only a few studies have been carried out in vivo in humans. In homocystinuric patients, homozygotes for mutations of the gene coding for the cystathionine beta-synthase enzyme, abnormalities of coagulation variables reflecting a hypercoagulable state, have been reported. In vitro studies provide a biochemical background for such a state. In homocystinuric patients, an in vivo platelet activation has also been reported. The latter abnormality is not corrected by the bolus infusion of concentrations of hirudin, which determines a long-lasting impairment of the conversion of fibrinogen to fibrin by thrombin; in contrast, it appears at least in part lowered by the administration of the antioxidant drug probucol. During the autooxidation of Hcy in plasma, reactive oxygen species are generated. The latter initiate lipid peroxidation in cell membranes (potentially responsible for endothelial dysfunction) and in circulating lipoproteins. Oxidized low-density lipoproteins (LDL) may trigger platelet activation as well as some of the hemostatic abnormalities reported in such patients. Thus the oxidative stress induced by Hcy may be a key process in the pathogenesis of thrombosis in HHcy. Accumulation of adenosylhomocysteine in cells (a consequence of high circulating levels of homocysteine) inhibits methyltransferase enzymes, in turn preventing repair of aged or damaged cells. This mechanism has been recently documented in patients with renal failure and HHcy and provides an additional direction to be followed to understand the tendency to thrombosis in moderate HHcy.
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PMID:Homocysteine, coagulation, platelet function, and thrombosis. 1101 42

We report the effect of homocysteine on the inactivation of factor Va by activated protein C (APC) using clotting assays, immunoblotting, and radiolabeling experiments. Homocysteine, cysteine, or homocysteine thiolactone have no effect on factor V activation by alpha-thrombin. Factor Va derived from homocysteine-treated factor V was inactivated by APC at a reduced rate. The inactivation impairment increased with increasing homocysteine concentration (pseudo first order rate k = 1.2, 0.9, 0.7, 0.4 min(-1) at 0, 0.03, 0.1, 1 mm homocysteine, respectively). Neither cysteine nor homocysteine thiolactone treatment of factor V affected APC inactivation of derived factor Va. Western blot analyses of APC inactivation of homocysteine-modified factor Va are consistent with the results of clotting assays. Factor Va, derived from factor V treated with 1 mm beta-mercaptoethanol was inactivated more rapidly than the untreated protein sample. Factor V incubated with [(35)S]homocysteine (10-450 micrometer) incorporated label within 5 min, which was found only in those fragments that contained free sulfhydryl groups: the light chain (Cys-1960, Cys-2113), the B region (Cys-1085), and the 26/28-kDa (residues 507-709) APC cleavage products of the heavy chain (Cys-539, Cys-585). Treatment with beta-mercaptoethanol removed all radiolabel. Plasma of patients assessed to be hyperhomocysteinemic showed APC resistance in a clot-based assay. Our results indicate that homocysteine rapidly incorporates into factor V and that the prothrombotic tendency in hyperhomocysteinemia may be related to impaired inactivation of factor Va by APC due to homocysteinylation of the cofactor by modification of free cysteine(s).
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PMID:Homocysteine inhibits inactivation of factor Va by activated protein C. 1108 58

Hyperhomocysteinemia has been proposed to inhibit the protein C anticoagulant system through 2 mechanisms: decreased generation of activated protein C (APC) by thrombin, and resistance to APC caused by decreased inactivation of factor Va (FVa). We tested the hypotheses that generation of APC by thrombin is impaired in hyperhomocysteinemia in monkeys and that hyperhomocysteinemia produces resistance to APC in monkeys, mice, and humans. In a randomized crossover study, cynomolgus monkeys were fed either a control diet or a hyperhomocysteinemic diet for 4 weeks. Plasma total homocysteine (tHcy) was approximately 2-fold higher when monkeys were on the hyperhomocysteinemic diet than when they were on the control diet (9.8 +/- 2.0 microM versus 5.6 +/- 1.0 microM; P <.05). After infusion of human thrombin (25 microg/kg of body weight), the peak level of plasma APC was 136 +/- 16 U/mL in monkeys fed the control diet and 127 +/- 13 U/mL in monkeys fed the hyperhomocysteinemic diet (P >.05). The activated partial thromboplastin time was prolonged to a similar extent by infusion of thrombin in monkeys fed the control diet and in those fed the hyperhomocysteinemic diet. The sensitivity of plasma FV to human APC was identical in monkeys on control diet and those on hyperhomocysteinemic diet. We also did not detect resistance of plasma FV to APC in hyperhomocysteinemic mice deficient in cystathionine beta-synthase (plasma tHcy, 93 +/- 16 microM) or in human volunteers with acute hyperhomocysteinemia (plasma tHcy, 45 +/- 6 microM). Our findings indicate that activation of protein C by thrombin and inactivation of plasma FVa by APC are not impaired during moderate hyperhomocysteinemia in vivo.
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PMID:Effect of hyperhomocysteinemia on protein C activation and activity. 1260 65


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