Prothrombin

Pathophysiological Role of Microglial Activation Induced by Blood-Borne Proteins in Alzheimer's Disease

The blood-brain barrier (BBB) restricts entry of neurotoxic plasma components, blood cells, and pathogens into the brain, leading to proper neuronal functioning. BBB impairment leads to blood-borne protein infiltration such as prothrombin, thrombin, prothrombin kringle-2, fibrinogen, fibrin, and other harmful substances. Thus, microglial activation and release of pro-inflammatory mediators commence, resulting in neuronal damage and leading to impaired cognition via neuroinflammatory responses, which are important features observed in the brain of Alzheimer's disease (AD) patients. Moreover, these blood-borne proteins cluster with the amyloid beta plaque in the brain, exacerbating microglial activation, neuroinflammation, tau phosphorylation, and oxidative stress. These mechanisms work in concert and reinforce each other, contributing to the typical pathological changes in AD in the brain. Therefore, the identification of blood-borne proteins and the mechanisms involved in microglial activation and neuroinflammatory damage can be a promising therapeutic strategy for AD prevention. In this article, we review the current knowledge regarding the mechanisms of microglial activation-mediated neuroinflammation caused by the influx of blood-borne proteins into the brain via BBB disruption. Subsequently, the mechanisms of drugs that inhibit blood-borne proteins, as a potential therapeutic approach for AD, along with the limitations and potential challenges of these approaches, are also summarized.

pKr-2 induces neurodegeneration via upregulation of microglial TLR4 in the hippocampus of AD brain

We recently demonstrated that prothrombin kringle-2 (pKr-2) derived from blood-brain barrier (BBB) disruption could induce hippocampal neurodegeneration and object recognition impairment through neurotoxic inflammatory responses in the five familial Alzheimer's disease mutation (5XFAD) mice. In the present study, we aimed to determine whether pKr-2 induces microglial activation by stimulating toll-like receptor 4 (TLR4) upregulation and examine whether this response contributes to pKr-2-induced neuroinflammatory damage in the hippocampi of mice models. We observed that inflammatory responses induced by pKr-2 administration in the hippocampi of wild-type mice were significantly abrogated in TLR4-deficient mice (TLR4^-/-), and caffeine supply or rivaroxaban treatment that inhibits the overexpression of hippocampal pKr-2 reduced TLR4 upregulation in 5XFAD mice, resulting in the inhibition of neuroinflammatory responses. Similar to the expression patterns of pKr-2, TLR4, and the TLR4 transcription factors, PU.1 and p-c-Jun, seen in the postmortem hippocampal tissues of Alzheimer's disease (AD) patients, our results additionally showed the influence of transcriptional regulation on TLR4 expression following pKr-2 expression in triggering the production of neurotoxic inflammatory mediators. Therefore, we conclude that pKr-2 may play a role in initiating upregulation of microglial TLR4, consequently inducing hippocampal neurodegeneration. Furthermore, the control of pKr-2-induced microglial TLR4 could be a useful therapeutic strategy against hippocampal neurodegeneration in AD.

Control of hippocampal prothrombin kringle-2 (pKr-2) expression reduces neurotoxic symptoms in five familial Alzheimer's disease mice

Background and Purpose

There is a scarcity of information regarding the role of prothrombin kringle‐2 (pKr‐2), which can be generated by active thrombin, in hippocampal neurodegeneration and Alzheimer's disease (AD).

Experimental Approach

To assess the role of pKr‐2 in association with the neurotoxic symptoms of AD, we determined pKr‐2 protein levels in post‐mortem hippocampal tissues of patients with AD and the hippocampi of five familial AD (5XFAD) mice compared with those of age‐matched controls and wild‐type (WT) mice, respectively. In addition, we investigated whether the hippocampal neurodegeneration and object memory impairments shown in 5XFAD mice were mediated by changes to pKr‐2 up‐regulation.

Key Results

Our results demonstrated that pKr‐2 was up‐regulated in the hippocampi of patients with AD and 5XFAD mice, but was not associated with amyloid‐β aggregation in 5XFAD mice. The up‐regulation of pKr‐2 expression was inhibited by preservation of the blood–brain barrier (BBB) via addition of caffeine to their water supply or by treatment with rivaroxaban, an inhibitor of factor Xa that is associated with thrombin production. Moreover, the prevention of up‐regulation of pKr‐2 expression reduced neurotoxic symptoms, such as hippocampal neurodegeneration and object recognition decline due to neurotoxic inflammatory responses in 5XFAD mice.

Conclusion and Implications

We identified a novel pathological mechanism of AD mediated by abnormal accumulation of pKr‐2, which functions as an important pathogenic factor in the adult brain via blood brain barrier (BBB) breakdown. Thus, pKr‐2 represents a novel target for AD therapeutic strategies and those for related conditions.

Discovery and characterization of 2 novel subpopulations of aPS/PT antibodies in patients at high risk of thrombosis

Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies are often detected in patients with antiphospholipid syndrome (APS), but how aPS/PT engage prothrombin at the molecular level remains unknown. Here, the antigenic determinants of immunoglobulin G aPS/PT were investigated in 24 triple-positive APS patients at high risk of thrombosis by using prothrombin mutants biochemically trapped in closed and open conformations, and relevant fragments spanning the entire length of prothrombin. Two novel unexpected findings emerged from these studies. First, we discovered that some aPS/PT are unique among other anti-prothrombin antibodies insofar as they efficiently recognize prothrombin in solution after a conformational change requiring exposure of fragment-1 to the solvent. Second, we identified and characterized 2 previously unknown subpopulations of aPS/PT, namely type I and type II, which engage fragment-1 of prothrombin at different epitopes and with different mechanisms. Type I target a discontinuous density-dependent epitope, whereas type II engage the C-terminal portion of the Gla-domain, which remains available for binding even when prothrombin is bound to the phospholipids. Based on these findings, APS patients positive for aPS/PT were classified into 2 groups, group A and group B, according to their autoantibody profile. Group A contains mostly type I antibodies whereas group B contains both type I and type II antibodies. In conclusion, this study offers a first encouraging step toward unveiling the heterogeneity of anti-prothrombin antibodies in correlation with thrombosis, shedding new light on the mechanisms of antigen-autoantibody recognition in APS.

Interleukin-4 and Interleukin-13 Exacerbate Neurotoxicity of Prothrombin Kringle-2 in Cortex In Vivo via Oxidative Stress

The present study investigated the effects of activated microglia-derived interleukin-4 (IL-4) and IL-13 on neurodegeneration in prothrombin kringle-2 (pKr-2)-treated rat cortex. pKr-2 was unilaterally injected into the Sprague–Dawley rat cerebral cortex and IL-4 and IL-13 neutralizing antibody was used to block the function of IL-4 and IL-13. Immunohistochemical analysis showed a significant loss of NeuN⁺ and Nissl⁺ cells and an increase of OX-42⁺ cells in the cortex at seven days post pKr-2. The levels of IL-4 and IL-13 expression were upregulated in the activated microglia as early as 12 hours post pKr-2 and sustained up to seven days post pKr-2. Neutralization by IL-4 or IL-13 antibodies (NA) significantly increased neuronal survival in pKr-2-treated rat cortex in vivo by suppressing microglial activation and the production of reactive oxygen species, as analyzed by immunohisotochemistry and hydroethidine histochemistry. These results suggest that IL-4 and IL-13 that were endogenously expressed from reactive microglia may play a critical role on neuronal death by regulating oxidative stress during the neurodegenerative diseases, such as Alzheimer’s disease and dementia.

Prothrombin Kringle-2: A Potential Inflammatory Pathogen in the Parkinsonian Dopaminergic System

Although accumulating evidence suggests that microglia-mediated neuroinflammation may be crucial for the initiation and progression of Parkinson's disease (PD), and that the control of neuroinflammation may be a useful strategy for preventing the degeneration of nigrostriatal dopaminergic (DA) projections in the adult brain, it is still unclear what kinds of endogenous biomolecules initiate microglial activation, consequently resulting in neurodegeneration. Recently, we reported that the increase in the levels of prothrombin kringle-2 (pKr-2), which is a domain of prothrombin that is generated by active thrombin, can lead to disruption of the nigrostriatal DA projection. This disruption is mediated by neurotoxic inflammatory events via the induction of microglial Toll-like receptor 4 (TLR4) in vivo , thereby resulting in less neurotoxicity in TLR4-deficient mice. Moreover, inhibition of microglial activation following minocycline treatment, which has anti-inflammatory activity, protects DA neurons from pKr-2-induced neurotoxicity in the substantia nigra (SN) in vivo. We also found that the levels of pKr-2 and microglial TLR4 were significantly increased in the SN of PD patients compared to those of age-matched controls. These observations suggest that there may be a correlation between pKr-2 and microglial TLR4 in the initiation and progression of PD, and that inhibition of pKr-2-induced microglial activation may be protective against the degeneration of the nigrostriatal DA system in vivo. To describe the significance of pKr-2 overexpression, which may have a role in the pathogenesis of PD, we have reviewed the mechanisms of pKr-2-induced microglial activation, which results in neurodegeneration in the SN of the adult brain.

Induction of microglial toll-like receptor 4 by prothrombin kringle-2: a potential pathogenic mechanism in Parkinson's disease

Microglia-mediated neuroinflammation may play an important role in the initiation and progression of dopaminergic (DA) neurodegeneration in Parkinson’s disease (PD), and toll-like receptor 4 (TLR4) is essential for the activation of microglia in the adult brain. However, it is still unclear whether patients with PD exhibit an increase in TLR4 expression in the brain, and whether there is a correlation between the levels of prothrombin kringle-2 (pKr-2) and microglial TLR4. In the present study, we first observed that the levels of pKr-2 and microglial TLR4 were increased in the substantia nigra (SN) of patients with PD. In rat and mouse brains, intranigral injection of pKr-2, which is not directly toxic to neurons, led to the disruption of nigrostriatal DA projections. Moreover, microglial TLR4 was upregulated in the rat SN and in cultures of the BV-2 microglial cell line after pKr-2 treatment. In TLR4-deficient mice, pKr-2-induced microglial activation was suppressed compared with wild-type mice, resulting in attenuated neurotoxicity. Therefore, our results suggest that pKr-2 may be a pathogenic factor in PD, and that the inhibition of pKr-2-induced microglial TLR4 may be protective against degeneration of the nigrostriatal DA system in vivo.

High-yield preparation of recombinant human α-thrombin for therapeutic use

In this study, we established stable cell lines producing 1.5 mg/mL recombinant human prothrombin in 400-L fed-batch culture, using CHO DG44 cells as a host cell line. And we also established a recombinant human α-thrombin purification process that produces a purified product suitable for use as a biopharmaceutical, by using recombinant ecarin from CHO DG44 cells, achieving a total yield of approximately 27% of prothrombin in culture medium. The establishment of stable cell lines with high expression levels, long-term passage stability and satisfactory scale-up are essential to ensure the stable supply of biopharmaceuticals. Furthermore, biopharmaceuticals must be of high quality to assure safety and effectiveness in target applications. We had previously reported that recombinant human prethrombin-2 expression level in a stable cell line established using the mouse myeloma cells, Sp2/0-Ag14, reached 200 μg/mL using animal-free materials in 50-L fed-batch culture. However, the productivity was insufficient to completely replace α-thrombin in human plasma preparations. By employing CHO DG44 cells as a host cell line, we had established a stable cell line and achieved significant improvements in quality, productivity of recombinant human α-thrombin manufacture suitable for use as a biopharmaceutical.

Factor II activity is similarly increased in patients with elevated apolipoprotein CIII and in carriers of the factor II 20210A allele

Background

Few studies have so far investigated the relationship between apolipoprotein CIII (Apo CIII) and coagulation pathway in subjects with or without coronary artery disease (CAD).

Methods and Results

Serum Apo CIII concentrations and plasma coagulant activities of factor II (FII:c), factor V (FV:c), and factor VIII (FVIII:c), and activated factor VII (FVIIa) were analyzed in a total of 933 subjects, with (n=687) or without (n=246) angiographically demonstrated CAD and not taking anticoagulant drugs. Activated factor X (FXa) generation assay was performed on plasma from subgroups of subjects with low and high levels of Apo CIII. A statistical incremental concentration of FII:c, FV:c, and FVIIa levels was observed through the quartiles of Apo CIII distribution in the population considered as a whole. Significant results were confirmed for FII:c in CAD and CAD‐free subgroup when separately considered. Subjects within the highest Apo CIII quartile (>12.6 mg/dL) had high FII:c levels not statistically different from those of carriers of 20210A allele (n=40; 4.28%). In a multiple linear model, Apo CIII was the best predictor of FII:c variability, after adjustment for age, gender, plasma lipids, CRP, creatinine, diagnosis, and carriership of 20210A allele. FXa generation was increased and its lag time shortened in plasmas with high Apo CIII levels. However, after thrombin inhibition by hirudin, differences between low and high Apo C‐III samples disappeared.

Conclusions

Elevated concentrations of Apo CIII are associated with an increase of thrombin activity to an extent comparable with the carriership of G20210A gene variant and mainly modulating the thrombin generation.

Restoring the procofactor state of factor Va-like variants by complementation with B-domain peptides

Coagulation factor V (FV) circulates as an inactive procofactor and is activated to FVa by proteolytic removal of a large inhibitory B-domain. Conserved basic and acidic sequences within the B-domain appear to play an important role in keeping FV as an inactive procofactor. Here, we utilized recombinant B-domain fragments to elucidate the mechanism of this FV autoinhibition. We show that a fragment encoding the basic region (BR) of the B-domain binds with high affinity to cofactor-like FV(a) variants that harbor an intact acidic region. Furthermore, the BR inhibits procoagulant function of the variants, thereby restoring the procofactor state. The BR competes with FXa for binding to FV(a), and limited proteolysis of the B-domain, specifically at Arg(1545), ablates BR binding to promote high affinity association between FVa and FXa. These results provide new insight into the mechanism by which the B-domain stabilizes FV as an inactive procofactor and reveal how limited proteolysis of FV progressively destabilizes key regulatory regions of the B-domain to produce an active form of the molecule.

Association between human prothrombin variant (T165M) and kidney stone disease

We previously reported the association between prothrombin (F2), encoding a stone inhibitor protein - urinary prothrombin fragment 1 (UPTF1), and the risk of kidney stone disease in Northeastern Thai patients. To identify specific F2 variation responsible for the kidney stone risk, we conducted sequencing analysis of this gene in a group of the patients with kidney stone disease. Five intronic SNPs (rs2070850, rs2070852, rs1799867, rs2282687, and rs3136516) and one exonic non-synonymous single nucleotide polymorphism (nsSNP; rs5896) were found. The five intronic SNPs have no functional change as predicted by computer programs while the nsSNP rs5896 (c.494 C>T) located in exon 6 results in a substitution of threonine (T) by methionine (M) at the position 165 (T165M). The nsSNP rs5896 was subsequently genotyped in 209 patients and 216 control subjects. Genotypic and allelic frequencies of this nsSNP were analyzed for their association with kidney stone disease. The frequency of CC genotype of rs5896 was significantly lower in the patient group (13.4%) than that in the control group (22.2%) (P = 0.017, OR 0.54, 95% CI 0.32–0.90), and the frequency of C allele was significantly lower in the patient group (36.1%) than that in the control group (45.6%) (P = 0.005, OR 0.68, 95% CI 0.51–0.89). The significant differences of genotype and allele frequencies were maintained only in the female group (P = 0.033 and 0.003, respectively). The effect of amino-acid change on UPTF1 structure was also examined by homologous modeling and in silico mutagenesis. T165 is conserved and T165M substitution will affect hydrogen bond formation with E180. In conclusion, our results indicate that prothrombin variant (T165M) is associated with kidney stone risk in the Northeastern Thai female patients.

A bipartite autoinhibitory region within the B-domain suppresses function in factor V

Activation of blood coagulation factor V (FV) is a key reaction of hemostasis. FV circulates in plasma as an inactive procofactor, and proteolytic removal of a large central B-domain converts it to an active cofactor (FVa) for factor Xa (FXa). Here we show that two short evolutionary conserved segments of the B-domain, together termed the procofactor regulatory region, serve an essential autoinhibitory function. This newly identified motif consists of a basic (963-1008) and an acidic (1493-1537) region and defines the minimal sequence requirements to maintain FV as a procofactor. Our data suggest that dismantling this autoinhibitory region via deletion or proteolysis is the driving force to unveil a high affinity binding site(s) for FXa. These findings document an unexpected sequence-specific role for the B-domain by negatively regulating FV function and preventing activity of the procofactor. These new mechanistic insights point to new ways in which the FV procofactor to cofactor transition could be modulated to alter hemostasis.

Molecular determinants of phospholipid synergy in blood clotting

Many regulatory processes in biology involve reversible association of proteins with membranes. Clotting proteins bind to phosphatidylserine (PS) on cell surfaces, but a clear picture of this interaction has yet to emerge. We present a novel explanation for membrane binding by GLA domains of clotting proteins, supported by biochemical studies, solid-state NMR analyses, and molecular dynamics simulations. The model invokes a single “phospho-l-serine-specific” interaction and multiple “phosphate-specific” interactions. In the latter, the phosphates in phospholipids interact with tightly bound Ca²⁺ in GLA domains. We show that phospholipids with any headgroup other than choline strongly synergize with PS to enhance factor X activation. We propose that phosphatidylcholine and sphingomyelin (the major external phospholipids of healthy cells) are anticoagulant primarily because their bulky choline headgroups sterically hinder access to their phosphates. Following cell damage or activation, exposed PS and phosphatidylethanolamine collaborate to bind GLA domains by providing phospho-l-serine-specific and phosphate-specific interactions, respectively.

Localization and expression of prothrombin in rodent osteoclasts and long bones

The serum protein prothrombin (PT) is proteolytically converted to thrombin during the coagulation cascade by the cell-associated prothrombinase complex. In vitro, RANKL-differentiated osteoclasts express tissue factor and coagulation factor Xa, which convert PT to thrombin (Karlstrom et al. Biochem Biophys Res Commun 394:593-599, 2010). The present study investigated the localization of PT in bone as well as the expression of PT mRNA in bone and osteoclasts. Herein, immunoblot analysis detected PT and smaller proteolytically cleaved fragments with sizes consistent with the action of prothrombinase in a protein fraction extracted with guanidine-HCl EDTA from mouse tibia. Light microscopic and ultrastructural immunohistochemistry demonstrated the presence of PT in the newly formed bone matrix of the metaphysis. Furthermore, fluorescent immunohistochemistry on metaphyseal trabecular bone showed that PT colocalized with MMP-9-expressing subepiphyseal osteoclasts, whereas cathepsin K-expressing osteoclasts were closely associated with PT of the bone matrix. RT-qPCR analysis revealed that PT mRNA was detected in tibia. Expression of PT mRNA in the tibia was 0.2% of the level in the liver. In addition, PT mRNA expression was increased by RANKL-induced differentiation of bone marrow macrophages to osteoclasts. The results demonstrate that PT is synthesized and proteolytically processed in bone. Furthermore, PT is present mainly in the newly formed bone matrix of the metaphyseal trabecular bone compartment in close association to osteoclasts. In addition, MMP-9-positive osteoclasts contain PT, and PT expression is increased during osteoclastogenesis.

Zymogen-like factor Xa variants restore thrombin generation and effectively bypass the intrinsic pathway in vitro

Inhibitory antibodies to factors VIII or IX represent a serious complication for hemophilia patients. Treatment involves products that bypass the intrinsic pathway and promote thrombin generation. Direct infusion of factor Xa should also restore hemostasis; however, it has a short half-life in plasma and could activate systemic coagulation in an uncontrolled fashion. Here we show that factor Xa mutants with zymogen-like properties (FXa(I16L) and FXa(V17A)) circumvent these limitations. In the absence of factor Va, the FXa variants are poor enzymes for a range of physiological ligands and are resistant to inactivation by antithrombin III and tissue factor pathway inhibitor. Notably, assembly of FXa(I16L) and FXa(V17A) on activated platelets with factor Va to form prothrombinase completely restores biologic activity. In hemophilic plasma, FXa(I16L) and FXa(V17A) have prolonged half-lives compared with wild-type factor Xa (approximately 60 minutes vs approximately 1 minute) and promote robust thrombin generation that bypasses the intrinsic pathway. The variants require factor Va generated in situ for procoagulant function, and cofactor inactivation by the protein C pathway regulates their activity. The efficacy, extended half-life, and mechanism of action suggest that novel zymogen-like forms of factor Xa might prove useful as new therapeutic procoagulants to treat deficiencies upstream of the common pathway.

Clinical and molecular insights into the hepatocellular carcinoma tumour marker des-γ-carboxyprothrombin

Des-γ-carboxyprothrombin (DCP) is known as a tumour marker for hepatocellular carcinoma (HCC). Various tumour markers have been developed for serological diagnosis of cancers, including HCC, in order to increase the survival rate of cancer patients. The currently recommended combined testing of DCP and α-fetoprotein (AFP) or Lens culinaris agglutinin-reactive fraction of α-fetoprotein has been established to diagnose HCC. This combined testing using several tumour markers helps to increase the sensitivity of diagnosis of HCC, thus significantly increasing the clinical usefulness of DCP. The excessive production of DCP may be related to worse tumour behaviour, such as the presence of vascular invasion and intrahepatic metastasis of HCC cells. A high level of DCP was suggested to be useful as one of the factors in new recipient selection criteria of liver transplantation. The clinical use of DCP, therefore, might play a vital role in predicting tumour behaviour in patients with HCC. That said, the basic mechanism of DCP production has not been fully clarified. Various factors such as vitamin K(2) and γ-glutamyl carboxylase may contribute to the production of DCP and have a complex relationship. Moreover, recent studies have revealed that DCP functions as a growth factor and might play significant roles in cancer progression. Thus, DCP represents a potential target of drug discovery to establish new chemotherapeutic strategy for HCC. However, various issues have to be resolved to construct a novel therapy for HCC-targeting DCP. Innovation is required to make further progress in examining DCP.

Prothrombin kringle-2 induces death of mesencephalic dopaminergic neurons in vivo and in vitro via microglial activation

We have shown that prothrombin kringle-2 (pKr-2), a domain of human prothrombin distinct from thrombin could activate cultured rat brain microglia in vitro. However, little is known whether pKr-2-induced microglial activation could cause neurotoxicity on dopaminergic (DA) neurons in vivo. To address this question, pKr-2 was injected into the rat substantia nigra (SN). Tyrosine hydroxylase (TH) immunohistochemistry experiments demonstrate significant loss of DA neurons seven days after injection of pKr-2. In parallel, pKr-2-activated microglia were detected in the SN with OX-42 and OX-6 immunohistochemistry. Reverse transcription PCR and double-label immunohistochemistry revealed that activated microglia in vivo exhibit early and transient expression of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2) and several proinflammatory cytokines. The pKr-2-induced loss of SN DA neurons was partially inhibited by the NOS inhibitor N(G)-nitro-L-arginine methyl ester hydrochloride, and the COX-2 inhibitor DuP-697. Extracellular signal-regulated kinase 1/2, c-Jun N-terminal kinase and p38 mitogen-activated protein kinase were activated in the SN as early as 1 hr after pKr-2 injection, and localized within microglia. Inhibition of these kinases led to attenuation of mRNA expression of iNOS, COX-2 and several proinflammatory cytokines, and rescue of DA neurons in the SN. Intriguingly, following treatment with pKr-2 in vitro, neurotoxicity was detected exclusively in co-cultures of mesencephalic neurons and microglia, but not microglia-free neuron-enriched mesencephalic cultures, indicating that microglia are required for pKr-2 neurotoxicity. Our results strongly suggest that microglia activated by endogenous compound(s), such as pKr-2, are implicated in the DA neuronal cell death in the SN.

Differences in prethrombin-1 activation with human or bovine factor Va can be attributed to the heavy chain

Human and bovine factor Va (FVa) function similarly in the activation of prothrombin but differently in the activation of prethrombin-1 (Pre-1). Pre-1 activation with human FVa proceeds at about 22 percent of the rate with bovine FVa. The dependencies of initial rates on the FVa and Pre-1 concentrations indicate that the differential activity is expressed in kcat differences, rather than differences in the assembly of prothrombinase or the K(m) value of the substrate. The heavy and light chains of both species of FVa were separated and interspecies hybrids were constructed in the presence of Ca(++). Studies of the activation of Pre-1 with these hybrids indicate that the species difference can be attributed specifically to the heavy chain of FVa. Analyses of the reactions by SDS-PAGE indicated that cleavage at Arg271 occurs at about the same rate with both species of FVa, but cleavage at Arg320 with human FVa is specifically retarded. A major difference in primary structure between the human and bovine FVa heavy chains comprises 10 residues at COOH-terminus, adjacent to the negatively charged hirudin-like DYDYQ sequence. These residues have pI values of 12.5 and 4.26 in human and bovine FVa, respectively. The lower value would complement the negatively charged DYDYQ sequence but the higher value would counteract it. Thus, we suggest that the differences in the COOH-terminus of the heavy chain are responsible for the differences in Pre-1 activation, and that it specifically influences cleavage at Arg320 in Pre-1.

Venom factor V from the common brown snake escapes hemostatic regulation through procoagulant adaptations

Venomous snakes produce an array of toxic compounds, including procoagulants to defend themselves and incapacitate prey. The Australian snake Pseudonaja textilis has a venom-derived prothrombin activator homologous to coagulation factors V (FV) and Xa (FXa). Here we show that the FV component (pt-FV) has unique biologic properties that subvert the normal regulatory restraints intended to restrict an unregulated procoagulant response. Unlike human FV, recombinant pt-FV is constitutively active and does not require proteolytic processing to function. Sequence comparisons show that it has shed a large portion of the central B-domain, including residues that stabilize the inactive procofactor state. Remarkably, pt-FV functions in the absence of anionic membranes as it binds snake-FXa with high affinity in solution. Furthermore, despite cleavage in the heavy chain, pt-FV is functionally resistant to activated protein C, an anticoagulant. We speculate this stability is the result of noncovalent interactions and/or a unique disulfide bond in pt-FV linking the heavy and light chains. Taken together, these findings provide a biochemical rationale for the strong procoagulant nature of venom prothrombinase. Furthermore, they illustrate how regulatory mechanisms designed to limit the hemostatic response can be uncoupled to provide a sustained, disseminated procoagulant stimulus for use as a biologic toxin.

The conformational switch from the factor X zymogen to protease state mediates exosite expression and prothrombinase assembly

Zymogens of the chymotrypsin-like serine protease family are converted to the protease state following insertion of a newly formed, highly conserved N terminus. This transition is accompanied by active site formation and ordering of several surface loops in the catalytic domain. Here we show that disruption of this transition in factor X through mutagenesis (FXa(I16L) and FXa(V17A)) not only alters active site function, but also significantly impairs Na(+) and factor Va binding. Active site binding was improved in the presence of high NaCl or with saturating amounts of factor Va membranes, suggesting that allosteric linkage exists between these sites. In line with this, irreversible stabilization of FXa(I16L) with Glu-Gly-Arg-chloromethyl ketone fully rescued FVa binding. Furthermore, the K(m) for prothrombin conversion with the factor Xa variants assembled into prothrombinase was unaltered, whereas the k(cat) was modestly reduced (3- to 4-fold). These findings show that intramolecular activation of factor X following the zymogen to protease transition not only drives catalytic site activation but also contributes to the formation of the Na(+) and factor Va binding sites. This structural plasticity of the catalytic domain plays a key role in the regulation of exosite expression and prothrombinase assembly.

Peptide correlation: A means to identify high quality quantitative information in large-scale proteomic studies

A major challenge of proteomic studies is the accurate quantitation of proteins. LC-MS/MS-based methods are especially suited for profiling proteins in large sample sets. In this setup, the measurement of relative protein abundance relies on the correct quantitation of tryptic peptides. However, peptide intensities often do not unequivocally reflect the abundance of the native proteins in the sample. In this study, we show that peptides that accurately reflect relative protein abundances in large-scale sample sets can be selected based on the correlation to each other. This strategy was tested in a well-controlled experiment using a set of spiked serum samples as well as 55 clinical serum samples from schizophrenia patients and healthy volunteers. The peptide correlation analysis we present here provides an intuitive and simple procedure to obtain a high quality quantitative information from proteomics data.

Inhibitory sequences within the B-domain stabilize circulating factor V in an inactive state

Blood coagulation factor V circulates as a procofactor with little or no procoagulant activity. It is activated to factor Va by thrombin following proteolytic removal of a large central B-domain. Although this reaction is well studied, the mechanism by which bond cleavage and B-domain release facilitate the transition to the active cofactor state has not been defined. Here we show that deletion or substitution of specific B-domain sequences drives the expression of procoagulant function without the need for proteolytic processing. Conversion to the constitutively active cofactor state is related, at least in part, to a cluster of amino acids that is highly basic and well conserved across the vertebrate lineage. Our findings demonstrate that discrete sequences in the B-domain serve to stabilize the inactive procofactor state, with proteolysis primarily functioning to remove these inhibitory constraints. These unexpected results provide new insight into the mechanism of factor V activation.

HD1, a thrombin-directed aptamer, binds exosite 1 on prothrombin with high affinity and inhibits its activation by prothrombinase

Incorporation of prothrombin into the prothrombinase complex is essential for rapid thrombin generation at sites of vascular injury. Prothrombin binds directly to anionic phospholipid membrane surfaces where it interacts with the enzyme, factor Xa, and its cofactor, factor Va. We demonstrate that HD1, a thrombin-directed aptamer, binds prothrombin and thrombin with similar affinities (K(d) values of 86 and 34 nm, respectively) and attenuates prothrombin activation by prothrombinase by over 90% without altering the activation pathway. HD1-mediated inhibition of prothrombin activation by prothrombinase is factor Va-dependent because (a) the inhibitory activity of HD1 is lost if factor Va is omitted from the prothrombinase complex and (b) prothrombin binding to immobilized HD1 is reduced by factor Va. These data suggest that HD1 competes with factor Va for prothrombin binding. Kinetic analyses reveal that HD1 produces a 2-fold reduction in the k(cat) for prothrombin activation by prothrombinase and a 6-fold increase in the K(m), highlighting the contribution of the factor Va-prothrombin interaction to prothrombin activation. As a high affinity, prothrombin exosite 1-directed ligand, HD1 inhibits prothrombin activation more efficiently than Hir(54-65)(SO(3)(-)). These findings suggest that exosite 1 on prothrombin exists as a proexosite only for ligands whose primary target is thrombin rather than prothrombin.

Role of Hirudin-like factor Va heavy chain sequences in prothrombinase function

Proexosite I on prothrombin has been implicated in providing a recognition site for factor Va within prothrombinase. To examine whether hirudin-like sequences (659-698) on the cofactor contribute to this interaction, we expressed and purified two-chain FVa derivatives that were intracellularly truncated at the C terminus of the heavy chain: FVa709 (des710-1545), FVa699 (des700-1545), FVa(692 (des693-1545), FVa678 (des679-1545), and FVa658 (des659-1545). We found that FVa709, FVa699, FVa692, and FVa678 exhibited specific clotting activities that were comparable with plasma-derived and recombinant FVa. Additionally, kinetic studies using prothrombin revealed that the Km and kcat values for these derivatives were unaltered. Fluorescent measurements and chromatography studies indicated that FVa709, FVa699, FVa692, and FVa678 bound to FXa membranes and thrombin-agarose in a manner that was comparable with the wild-type cofactors. In contrast, FVa658 had an approximately 1% clotting activity and reduced affinity for FXa membranes (approximately 20-fold) and did not bind to thrombin-agarose. Surprisingly, however, FVa(658) exhibited essentially normal kinetic parameters for prothrombin when the variant was fully saturated with FXa membranes. Overall our results are consistent with the interpretation that any possible binding interactions between prothrombin and the C-terminal region of the FVa heavy chain do not contribute in a detectable way to the enhanced function of prothrombinase.

Acid-induced denaturation and refolding of prothrombin

Structural transitions of the blood coagulation factor prothrombin (extracted from goat blood) in response to reduction of pH were investigated by fluorescence, circular dichroism and light scattering measurements. The study revealed the presence of a partially unfolded state at around pH 3.5, characterized by marked enhancement of fluorescence from ANS bound to the protein, increase of bimolecular rate constant for tryptophan fluorescence quenching and a sharp peak in the light scattering intensity. Further lowering of the pH caused reversal of the trend of variation of these parameters, suggesting that prothrombin folds back to a compact state containing native-like secondary structural elements. The refolded state at low pH (<pH 3) fits the description of the A-state, the end-point of acid-induced denaturation process of several other monomeric proteins, and is a possible candidate for the class of folding intermediates known as molten globules.

The association of different urinary proteins with calcium oxalate hydromorphs. Evidence for non-specific interactions

It has been proposed that various urinary proteins interact specifically with different calcium oxalate hydromorphs and these interactions have important implications regarding the understanding of the onset and progress of kidney stone disease. Calcium oxalate monohydrate and dihydrate crystals were grown and characterised thoroughly to establish sample purity. These crystals were then incubated in artificial urine samples containing isolated urinary macromolecules. Crystal growth was prevented by saturating the incubation mix with calcium oxalate, and this was confirmed through electron microscopy and calcium measurements of the incubation mix. The surface interactions between the different calcium oxalate hydrates and urinary proteins were investigated by the use of Western blots and immunoassays. The same proteins, notably albumin, Tamm-Horsfall protein, osteopontin and prothrombin fragment 1, associated with both hydrates. There was a trend for more protein to associate with calcium oxalate dihydrate, and greater quantities of different proteins associated with both hydrates when Tamm-Horsfall protein was removed from the incubation mix. There is no evidence from this study to indicate that particular proteins interact with specific calcium oxalate hydrates, which in turn suggests that these protein-mineral interactions are likely to be mediated through non-specific charge interactions.

Active Site-independent Recognition of Substrates and Product by Bovine Prothrombinase

The conversion of prothrombin to thrombin is catalyzed by prothrombinase, an enzyme complex composed of the serine proteinase factor Xa and a cofactor protein, factor Va, assembled on membranes. Kinetic studies indicate that interactions with extended macromolecular recognition sites (exosites) rather than the active site of prothrombinase are the principal determinants of binding affinity for substrate or product. We now provide a model-independent evaluation of such ideas by physical studies of the interaction of substrate derivatives and product with prothrombinase. The enzyme complex was assembled using Xa modified with a fluorescent peptidyl chloromethyl ketone to irreversibly occlude the active site. Binding was inferred by prethrombin 2-dependent perturbations in the fluorescence of Oregon Green(488) at the active site of prothrombinase. Active site-independent binding was also unequivocally established by fluorescence resonance energy transfer between 2,6-dansyl tethered to the active site of Xa and eosin tethered to the active sites of either thrombin or meizothrombin des fragment 1. Comparable interprobe distances obtained from these measurements suggest that substrate and product interact equivalently with the enzyme. Competition established the ability of a range of substrate or product derivatives to bind in a mutually exclusive fashion to prothrombinase. Equilibrium dissociation constants obtained for the active site-independent binding of prothrombin, prethrombin 2, meizothrombin des fragment 1 and thrombin to prothrombinase were comparable with their affinities inferred from kinetic studies using active enzyme. Our findings directly establish that binding affinity is principally determined by the exosite-mediated interaction of either the substrate, both possible intermediates, or product with prothrombinase. A single type of exosite binding interaction evidently drives affinity and binding specificity through the stepwise reactions necessary for the two cleavage reactions of prothrombin activation and product release.

Removal of B-domain sequences from factor V rather than specific proteolysis underlies the mechanism by which cofactor function is realized

Factor V, the precursor of factor Va, circulates in plasma with little or no procoagulant activity. Activity is generated following limited proteolysis indicating that the conversion of factor V to factor Va results in appropriate structural changes, which impart cofactor function. We have produced recombinant partial B-domain-truncated derivatives of factor V (FV(des811-1491) and FV(des811-1491) with Arg(709) and Arg(1545) mutated to Gln) to investigate whether discrete proteolysis within the B-domain followed by a conformational transition is responsible for activation. Direct binding fluorescence measurements as well as steady-state kinetic assays were employed to assess the ability of these factor V derivatives to assemble and function in prothrombinase. In contrast to human factor V, single-chain B-domain-truncated factor V bound to FXa membranes with an affinity that was identical to factor Va. Additionally, it was found that, once this modified derivative was assembled in prothrombinase, it functioned in an equivalent manner to factor Va. Taken together these data support the hypothesis that proteolysis within the B-domain of factor V, although necessary, is incidental to the mechanism by which cofactor function is realized. Instead, our results are more consistent with the interpretation that proteolytic activation of factor V simply eliminates steric and/or conformational constraints contributed by the B-domain that otherwise interfere with discrete binding interactions that govern the eventual function of factor Va.

Unique in vivo modifications of coagulation factor V produce a physically and functionally distinct platelet-derived cofactor: characterization of purified platelet-derived factor V/Va

Platelet- and plasma-derived factor Va (FVa) serve essential cofactor roles in prothrombinase-catalyzed thrombin generation. Platelet-derived FV/Va, purified from Triton X-100 platelet lysates was composed of a mixture of polypeptides ranging from approximately 40 to 330 kDa, mimicking those visualized by Western blotting of platelet lysates and releasates with anti-FV antibodies. The purified, platelet-derived protein expressed significant cofactor activity such that thrombin activation led to only a 2-3-fold increase in cofactor activity yet expression of a specific activity identical to that of purified, plasma-derived FVa. Physical and functional differences between the two cofactors were identified. Purified, platelet-derived FVa was 2-3-fold more resistant to activated protein C-catalyzed inactivation than purified plasma-derived FVa on the thrombin-activated platelet surface. The heavy chain subunit of purified, platelet-derived FVa contained only a fraction ( approximately 10-15%) of the intrinsic phosphoserine present in the plasma-derived FVa heavy chain and was resistant to phosphorylation at Ser(692) catalyzed by either casein kinase II or thrombin-activated platelets. MALDI-TOF mass spectrometric analyses of tryptic digests of platelet-derived FV peptides detected an intact heavy chain uniquely modified on Thr(402) with an N-acetylglucosamine or N-acetylgalactosamine, whereas Ser(692) remained unmodified. N-terminal sequencing and MALDI-TOF analyses of platelet-derived FV/Va peptides identified the presence of a full-length heavy chain subunit, as well as a light chain subunit formed by cleavage at Tyr(1543) rather than Arg(1545) accounting for the intrinsic levels of cofactor activity exhibited by native platelet-derived FVa. These collective data are the first to demonstrate physical differences between the two FV cofactor pools and support the hypothesis that, subsequent to its endocytosis by megakaryocytes, FV is modified to yield a platelet-derived cofactor distinct from its plasma counterpart.

The function of factor XI in tissue factor-initiated thrombin generation

The influence of plasma and platelet factor (F)XI on thrombin generation initiated with 10 pm tissue factor (TF) in a synthetic coagulation model was evaluated in the presence of either 2 x 108 mL-1 platelets or the equivalent (2 microm) phospholipids. In either system, with all proteins present at physiological concentrations, FXI (30 nm) had no effect on thrombin generation. With phospholipids in the absence of FXI, an increase in vitamin K-dependent proteins (VKDP) (up to 500%) significantly prolonged the initiation phase of thrombin generation and decreased maximum thrombin levels. The inhibition was principally caused by the elevated prothrombin and FIX concentrations. When 30 nm FXI was added with elevated VKDP and phospholipids, the initiation phase was decreased and the maximum thrombin levels generated substantially increased. In experiments with platelets (with and without plasma FXI), an increase in VKDP had little effect on the initiation phase of thrombin generation. These data indicate that (i) FXI has no effect on thrombin generation at 10 pm TF and physiological concentrations of VKDP; (ii) platelets and plasma FXI are able to compensate for the inhibitory effects of elevated VKDP.

Effects of water soluble phosphotidylserine on bovine factor Xa: functional and structural changes plus dimerization

Previous work has shown that two molecules of a soluble form of phosphatidylserine, C6PS, bind to human and bovine factor X(a). Activity measurements along with the fluorescence of active-site-labeled human factor X(a) showed that two linked sites specifically regulate the active site conformation and proteolytic activity of the human enzyme. These results imply, but cannot demonstrate, a C6PS-induced factor X(a) conformational change. The purpose of this paper is to extend these observations to bovine factor X(a) and to demonstrate that they do reflect conformational changes. We report that the fluorescence of active-site-labeled bovine factor X(a) also varied with C6PS concentration in a sigmoidal manner, whereas amidolytic activity of unlabeled enzyme varied in a simple hyperbolic fashion, also as seen for human factor X(a). C6PS induced a 70-fold increase in bovine factor X(a)'s autolytic activity, consistent with the 60-fold increase in proteolytic activity reported for human factor X(a). In addition, circular dichroism spectroscopy clearly demonstrated that C6PS binding to bovine factor X(a) induces secondary structural changes. In addition, C6PS binding to the tighter of the two sites triggered structural changes that lead to Ca(2+)-dependent dimer formation, as demonstrated by changes in intrinsic fluorescence and quantitative native gel electrophoresis. Dimerization produced further change in secondary structure, either inter- or intramolecularly. These results, along with results presented previously, support a model in which C6PS binds in a roughly sequential fashion to two linked sites whose occupancy in both human and bovine factor X(a) elicits different structural and functional responses.

Prothrombinase assembly and S1 site occupation restore the catalytic activity of FXa impaired by mutation at the sodium-binding site

Two loop segments (183-189 and 221-225) in the protease domain of factor Xa contribute to the formation of a Na(+)-binding site. Studies with factor Xa indicate that binding of a single Na(+) ion to this site influences its activity by altering the S1 specificity site, and substitution of Tyr(225) with Pro diminishes sensitivity to Na(+). Using full-length factor Xa(Y225P), the allosteric relationship between the Na(+) site and other structural determinants in factor Xa and prothrombinase was investigated. Direct binding and kinetic measurements with probes that target the S1 specificity pocket indicate that assembly of the mutant in prothrombinase corrected the impaired binding of these probes observed with free factor Xa(Y225P). This appears to result from the apparent allosteric linkage between the factor Va, S1, and Na(+)-binding sites, since binding of the cofactor to membrane-bound factor Xa(Y225P) enhances binding at the S1 site and vice versa. Additional studies revealed that the internal salt bridge (Ile(16)-Asp(194)) of factor Xa(Y225P) is partially destabilized, a process that is reversible upon occupation of the S1 site. The data establish that alterations at the factor Xa Na(+)-binding site shift the zymogen-protease equilibrium to a more zymogen-like state, and as a consequence binding of S1-directed probes and factor Va are adversely affected. Therefore, the zymogen-like characteristics of factor Xa(Y225P) have allowed for the apparent allosteric linkage between the S1, factor Va, and Na(+) sites to become evident and has provided insight into the structural transitions which accompany the conversion of factor X to factor Xa.

A peptide derived from human prothrombin fragment 2 inhibits prothrombinase and angiogenesis

We constructed the synthetic peptide library representing human prothrombin fragment 2 (F2) sequence and explored the inhibitory sequence for prothrombinase, which was reconstituted in vitro by adding factor Xa, factor Va, and calcium into phospholipids. The nonapeptide NSAVLQVEN (NSA9) suppressed prothrombinase reconstituted not only on phospholipid vesicles but also on the bovine capillary endothelial (BCE) cell surface. Kinetic analyses demonstrated that NSA9 is a mixed-type inhibitor of Xa. Furthermore, the nonapeptide inhibited the proliferation of BCE cells and also suppressed angiogenesis in chicken embryos. The inhibitory activities of NSA9 were abrogated by pre-incubation with anti-F2 monoclonal antibody, 4E7. These data demonstrate that anti-angiogenic activity of F2 may be related to its ability to inhibit prothrombinase.

Localization of phosphatidylserine binding sites to structural domains of factor Xa

Binding of short chain phosphatidylserine (C6PS) enhances the proteolytic activity of factor X(a) by 60-fold (Koppaka, V., Wang, J., Banerjee, M., and Lentz, B. R. (1996) Biochemistry 35, 7482-7491). In the present study, we locate three C6PS binding sites to different domains of factor X(a) using a combination of activity, circular dichroism, fluorescence, and equilibrium dialysis measurements on proteolytic and biosynthetic fragments of factor X(a). Our results demonstrate that the structural responses of human and bovine factor X(a) to C6PS binding are somewhat different. Despite this difference, data obtained with fragments from both human and bovine factor X(a) are consistent with a common hypothesis for the location of C6PS binding sites to different structural domains. First, the gamma-carboxyglutamic acid (Gla) domain binds C6PS only in the absence of Ca(2+) (k(d) approximately 1 mm), although this PS site does not influence the functional response of factor X(a). Second, a Ca(2+)-dependent binding site is in the epidermal growth factor domains (EGF(NC)) that are linked by Ca(2+) and C6PS binding to the Gla domain. This site appears to be the lipid regulatory site of factor X(a). Third, a Ca(2+)-requiring site seems to be in the EGF(C)-catalytic domain. This site appears not to be a lipid regulatory site but rather to share residues with the substrate recognition site. Finally, the full functional response to C6PS requires linkage of the Gla, EGF(NC), and catalytic domains in the presence of Ca(2+), meaning that PS regulation of factor X(a) involves linkage between widely separated parts of the protein.

Channeling during prothrombin activation

The plasma zymogen prothrombin (II) is converted to the clotting enzyme thrombin (IIa) by two prothrombinase-catalyzed proteolytic cleavages. Thus, two intermediates, meizothrombin (mIIa) and prethrombin-2 (P2), are possible on the reaction pathway. Measurements of the time courses of II, mIIa, P2, and IIa suggested a channeling phenomenon, whereby a portion of the II is converted directly to IIa without free mIIa and P2 as obligatory intermediates. Evidence for this was that the maximum rate of IIa formation preceded the maximum in the level of either intermediate. In addition, analysis of the data according to a model that included two parallel pathways through mIIa and P2 indicated that about 40% of the II consumed did not yield free mIIa or P2. Further studies were carried out in which II was continuously infused in a reactor at a constant rate. Under these conditions II, mIIa, and P2 reached constant steady-state levels, and IIa was produced at a constant rate, equal to that of II infusion. During the steady state, traces of II, mIIa, and P2 were introduced as radiolabels. Time courses of isotope consumption were first order, thus allowing the rates of consumption of II, mIIa, and P2 to be calculated. Under these conditions the rate of II consumption equaled the rate of IIa formation. Rates of consumption of the free intermediates, however, were only 22 (mIIa) and 15% (P2), respectively, of the rate of thrombin formation. Thus, both the time course experiments and the steady-state experiments indicate that an appreciable fraction of II is channeled directly to IIa without proceeding through the free intermediates mIIa and P2.

Exosite binding tethers the macromolecular substrate to the prothrombinase complex and directs cleavage at two spatially distinct sites

The prothrombinase complex, composed of the proteinase, factor Xa, bound to factor Va on membranes, catalyzes thrombin formation by the specific and ordered proteolysis of prothrombin at Arg(323)-Ile(324), followed by cleavage at Arg(274)-Thr(275). We have used a fluorescent derivative of meizothrombin des fragment 1 (mIIaDeltaF1) as a substrate analog to assess the mechanism of substrate recognition in the second half-reaction of bovine prothrombin activation. Cleavage of mIIaDeltaF1 exhibits pseudo-first order kinetics regardless of the substrate concentration relative to K(m). This phenomenon arises from competitive product inhibition by thrombin, which binds to prothrombinase with exactly the same affinity as mIIaDeltaF1. As thrombin is known to bind to an exosite on prothrombinase, initial interactions at an exosite likely play a role in the enzyme-substrate interaction. Occupation of the active site of prothrombinase by a reversible inhibitor does not exclude the binding of mIIaDeltaF1 to the enzyme. Specific recognition of mIIaDeltaF1 is achieved through an initial bimolecular reaction with an enzymic exosite, followed by an active site docking step in an intramolecular reaction prior to bond cleavage. By alternate substrate studies, we have resolved the contributions of the individual binding steps to substrate affinity and catalysis. This pathway for substrate binding is identical to that previously determined with a substrate analog for the first half-reaction of prothrombin activation. We show that differences in the observed kinetic constants for the two cleavage reactions arise entirely from differences in the inferred equilibrium constant for the intramolecular binding step that permits elements surrounding the scissile bond to dock at the active site of prothrombinase. Therefore, substrate specificity is achieved by binding interactions with an enzymic exosite that tethers the protein substrate to prothrombinase and directs cleavage at two spatially distinct scissile bonds.

Purification and characterization of ostrich prothrombin

The work focused on the penultimate enzyme, prothrombin, in the coagulation cascade. Prothrombin was purified and characterized from ostrich plasma. The results obtained contribute to a better understanding of blood coagulation in the ostrich and the evolution of prothrombin and the coagulation cascade. Prothrombin was purified from ostrich plasma by barium chloride precipitation, ammonium sulfate fractionation, and DEAE-cellulose and Cu(2+)-chelate Sepharose chromatography. Ostrich prothrombin exhibited a M(r) of 72,800 and a pI of 6.9 using SDS-PAGE and PAG-isoelectrofocusing, respectively. The N-terminal sequence of ostrich prothrombin showed 78 and 87% identity with human and bovine, respectively. The cDNA was isolated from ostrich liver and the predicted amino acid sequence compared with those from other species. Ostrich prothrombin shares sequence identity with chicken (84%), human (60%), bovine (59%), rat (60%), mouse (59%) and hagfish (50%) prothrombin, suggesting a common function of prothrombin in these vertebrates. Amino acid sequence identities indicate that the thrombin beta-chain (62%) and the propeptide-Gla (75%) domains are the regions most constrained for the common functions of vertebrate prothrombins. Ostrich prothrombin, therefore, shows similarity in structure to other vertebrate prothrombins.

Inhibition of calcium oxalate crystal growth and aggregation by prothrombin and its fragments in vitro: relationship between protein structure and inhibitory activity

During blood coagulation, prothrombin (PT) is ultimately degraded to three fragments, thrombin, fragment 1 (F1) and fragment 2 (F2), which, collectively, contain all of the structural features of PT. One of these fragments, F1, is excreted in human urine and is the principal protein occluded into calcium oxalate (CaOx) crystals precipitated from it. This urinary form of F1, which we have named urinary prothrombin fragment 1 is present in calcium stones and is a potent inhibitor of CaOx crystallization in urine in vitro. The aim of this study was to determine whether PT itself and its other activation products, namely, thrombin, F1 and F2 also inhibit CaOx crystallization, by comparing their effects in a seeded, inorganic crystallization system. A secondary objective was to assess the relationship between the structures of the proteins and their inhibitory activities. PT was isolated from a human blood concentrate rich in vitamin K-dependent proteins. Following initial cleavage by thrombin, the resulting fragments, F1 and F2, were purified by a combination of reversed phase HPLC and low pressure column chromatography. The purity of the proteins was confirmed by SDS/PAGE and their individual effects on CaOx crystallization were determined at the same concentration (16.13 nM) in a seeded, metastable solution of CaOx using a Coulter Counter. [14C]Oxalate was used to assess deposition of CaOx and crystals were visualized using scanning electron microscopy. The Coulter Counter data revealed that the proteins reduced the size of precipitated crystals in the order F1 > PT > F2 > thrombin. These findings were confirmed by scanning electron microscopy which showed that the reduction in particle size resulted from a decrease in the degree of crystal aggregation. [14C]Oxalate analysis demonstrated that all proteins inhibited mineral deposition, in the order F1 (44%) > PT (27.4%) > thrombin (10.2%) > F2 (6.5%). It was concluded that the gamma-carboxyglutamic acid domain of PT and F1, which is absent from thrombin and F2, is the region of the molecules which determines their potent inhibitory effects. The superior potency of F1, in comparison with PT, probably results from the molecule's greater charge to mass ratio.

The platelet high affinity binding site for thrombin mimics hirudin, modulates thrombin-induced platelet activation, and is distinct from the glycoprotein Ib-IX-V complex

The platelet high affinity binding site for thrombin appears to be described by a classical receptor-ligand interaction that is distinct from the platelet thrombin receptor/substrate, PAR-1. However, the identification and function of the high affinity binding site with respect to its physiological importance have continued to elude investigators. Prior studies using two mutant thrombins suggested that thrombin interaction with the platelet high affinity binding site is mediated through an extensive portion of the thrombin molecule involving residues within the substrate binding pocket and the anion binding exosite (Leong, L., Henriksen, R. A., Kermode, J. C., Rittenhouse, S. E., and Tracy, P. B. (1992) Biochemistry 31, 2567-2575) and may mimic a thrombin-hirudin interaction. To test this hypothesis, an anti-hirudin peptide antibody (anti-hirpeptide Ab) was raised against a peptide mimicking the COOH terminus of hirudin. The Ab recognized adherent platelets and those in suspension as determined by enzyme-linked immunosorbent assay and immunofluorescence microscopy, respectively. 125I-Thrombin binding to platelets was inhibited in the presence of the anti-hirpeptide Ab in a dose-dependent manner with maximal inhibition >90%. Analyses of data from binding studies of 125I-thrombin to platelets at a fixed Ab concentration indicated that the anti-hirpeptide Ab inhibited the high affinity binding interaction exclusively. In addition, thrombin-induced increases in platelet [Ca2+]i were enhanced by blocking the high affinity binding site with the Ab due to redistribution of the agonist to PAR-1. Thrombin Quick I-induced platelet calcium mobilization was unaffected by the presence of the Ab, consistent with the inability of thrombin Quick I to bind to the high affinity site. Even though glycoprotein (GP) Ib contains a hirudin-like region within the alpha subunit, the postulated high affinity binding site, direct binding of 125I-thrombin could not be demonstrated to transfected Chinese hamster ovary and L cells expressing the GP Ib-IX-V complex. Furthermore, an anti-GP Ib Ab, raised to the peptide region proposed as the thrombin high affinity site, did not enhance thrombin-induced platelet calcium mobilization. The anti-hirpeptide Ab recognized a population of platelet membrane proteins distinct from PAR-1 and GP Ib by three-color immunofluorescence using confocal microscopy. These combined studies demonstrate that the high affinity binding site for thrombin is a unique platelet protein distinct from GP Ib which modulates the effective thrombin concentration localized at the human platelet surface.

Human prothrombin fragment 1 and 2 inhibit bFGF-induced BCE cell growth

Previously, we reported that the rabbit prothrombin fragment 2 (kringle 2 domain) has an anti-endothelial cell proliferative effect (Lee et al., J. Biol. Chem., in press). In this report, we show that not only rabbit prothrombin fragment 2 but also human prothrombin fragment 1 and 2 have an inhibitory effect on bFGF-stimulated BCE cell growth. Human prothrombin fragment 1 and 2 obtained as proteolytic fragments of human prothrombin display potent inhibitory effects on bovine capillary endothelial cells with a half-maximal concentration (ED50) of approximately 100 nM and 120 nM, respectively. As rabbit prothrombin fragment 2, the human prothrombin fragment 1 and 2 also inhibit angiogenesis in the chorioallantoic membrane (CAM) of chick embryos.

Picosecond tryptophan fluorescence of membrane-bound prothrombin fragment 1

The wavelength-dependent tryptophan (Trp) fluorescence decays of Ca-prothrombin fragment 1 (Ca-BF1), which contains three tryptophan residues, in the presence of pure phosphatidylcholine (PC) small unilamellar vesicles (SUV) and PC-SUV containing either 25% phosphatidyl-l-serine (PS), and 25% or 40% phosphatidylglycerol (PG) are characterized, using fluorescence lifetime distribution, conventional multiexponential, and global analysis. In analogy to previous investigations on apo- and Ca-BF1 (M. Hof, G.R. Fleming, V. Fidler, Proteins Struct. Func. Genet. 24 (1996) 485-494), the analysis resulted in a five exponential decay model in all investigated systems, where the five fluorescence lifetimes (e.g. 0. 04+/-0.02 ns (component A), 0.24+/-0.02 ns (B), 0.66+/-0.03 ns (C), 2.4+/-0.3 ns (D), and 5.4+/-0.4 ns (E) for Ca-BF1 in the presence of PC-SUV) are wavelength-independent. The fluorescence lifetimes and the corresponding amplitudes of the 'Gla-Trp' (components D and E) and of the two 'kringle-Trp' (components B, C, and D) remain unchanged when bound to the PS-containing vesicles. Saturation binding to PG-containing membranes leads to a prolongation of the Gla component E from 5.3 in solution to 7.5 ns, indicating a change in the Gla-domain conformation. The results represent the first experimental evidence of a lipid-specific conformational change in the N-terminal 'Gla domain' of a vitamin K-dependent protein.

Incomplete embryonic lethality and fatal neonatal hemorrhage caused by prothrombin deficiency in mice

Deficiency of blood coagulation factor V or tissue factor causes the death of mouse embryos by 10.5 days of gestation, suggesting that part of the blood coagulation system is necessary for development. This function is proposed to require either generation of the serine protease thrombin and cell signaling through protease-activated receptors or an activity of tissue factor that is distinct from blood clotting. We find that murine deficiency of prothrombin clotting factor 2 (Cf2) was associated with the death of approximately 50% of Cf2(-/-) embryos by embryonic day 10.5 (E10.5), and surviving embryos had characteristic defects in yolk sac vasculature. Most of the remaining Cf2(-/-) embryos died by E15.5, but those surviving to E18.5 appeared normal. The rare Cf2(-/-) neonates died of hemorrhage on the first postnatal day. These studies suggest that a part of the blood coagulation system is adapted to perform a developmental function. Other mouse models show that the absence of platelets or of fibrinogen does not cause fetal wastage. Therefore, the role of thrombin in development may be independent of its effects on blood coagulation and instead may involve signal transduction on cells other than platelets.

Protein C inhibitor secreted from activated platelets efficiently inhibits activated protein C on phosphatidylethanolamine of platelet membrane and microvesicles

Protein C inhibitor (PCI) was detected in human platelets (2.9 ng/10(9) cells) and megakaryocytic cells (1.5 ng/10(6) cells). PCI mRNA was also detected in both platelets and megakaryocytic cells using nested polymerase chain reaction. PCI was found to be located in the alpha-granules of resting platelets. Approximately 30% of the total amount of PCI in platelets was released after stimulation with ADP, collagen, adrenalin, thrombin, or thrombin receptor-activating peptide. Secreted PCI was detected on the surface of activated platelets and phospholipid microvesicles. PCI secreted from thrombin receptor-activating peptide-stimulated platelets inhibited activated protein C (APC) efficiently. PCI significantly inhibited APC in the presence of phospholipid vesicles prepared using rabbit brain cephalin (RBC) or a mixture of 40% phosphatidylethanolamine (PE), 20% phosphatidylserine (PS), and 40% phosphatidylcholine (PC) with a second order rate constant of 1.0 x 10(6) M-1.min-1. Of these phospholipids, PE was critical for this inhibition. The dissociation constants of the binding of APC or PCI to solid phase phospholipids showed that APC binds more preferably to PE than to RBC or PS, and PCI to PE or RBC than to PS or PC. PCI binding to solid phase phospholipids depended on the presence of PE. RBC- or PE-bound PCI inhibited APC significantly but only weakly the gamma-carboxyglutamic acid domainless APC. The gamma-carboxyglutamic acid fragment of protein C suppressed the PCI-mediated inhibition of APC on solid phase RBC or PE. Most of the APC.PCI complex formed on solid phase RBC or PE was released into the soluble phase. These findings suggest that PCI secreted from activated platelets binds preferably to PE of platelet membrane and microvesicles and that it inhibits phospholipid-bound APC efficiently.