The kinetics of activation of normal and gamma-carboxyglutamic acid-deficient prothrombins

Characterization of Gla proteoforms and non-Gla peptides of gamma carboxylated proteins: Application to quantification of prothrombin proteoforms in human plasma

Gamma (γ) carboxylation is an essential post-translational modification in vitamin K-dependent proteins (VKDPs), involved in maintaining critical biological homeostasis. Alterations in the abundance or activity of these proteins have pharmacological and pathological consequences. Importantly, low levels of fully γ-carboxylated clotting factors increase plasma des-γ-carboxy precursors resulting in little or no biological activity. Therefore, it is important to characterize the levels of γ-carboxylation that reflect the active state of these proteins. The conventional enzyme-linked immunosorbent assay for protein induced by vitamin K absence or antagonist II (PIVKA-II) quantification uses an antibody that is not applicable to distinguish different γ-carboxylation states. Liquid chromatography-mass spectrometry (LC-MS) approaches have been utilized to distinguish different γ-carboxylated proteoforms, however, these attempts were impeded by poor sensitivity due to spontaneous neutral loss of CO2 and simultaneous cleavage of the backbone bond in the collision cell. In this study, we utilized an alkaline mobile phase in combination with polarity switching (positive and negative ionization modes) to simultaneously identify and quantify γ-carboxylated VKDPs. The method was applied to compare Gla proteomics of prothrombin (FII) in 10 μL plasma samples of healthy control and warfarin-treated adults. We also identified surrogate non-Gla peptides for seven other VKDPs to quantify total (active plus inactive) protein levels. The total protein approach (TPA) was used to quantify absolute levels of the VKDPs in human plasma.

Research progress of nephrotic syndrome accompanied by thromboembolism

Thromboembolism (TE) is a common and serious complication of nephrotic syndrome (NS). NS is associated with hypercoagulability, which may be induced by changes in coagulation, anticoagulant, and fibrinolytic factors. Moreover, accumulating evidence supports the hypothesis that the complex interactions between genetic and acquired risk factors in TE should be considered and that genetic susceptibility should not be ignored. Extracellular vesicles (EVs) also play unique roles. Further research on EVs may provide new insights into the discovery and treatment of TE associated with NS. The occurrence of NS accompanied by TE may be associated with various risk factors. Preventive anticoagulant therapy can not only reduce the risk of TE in patients but also aggravate the risk of bleeding. Heparin and vitamin K antagonists (VKAs), traditional anticoagulant drugs, have been extensively applied in the prevention and treatment of thromboembolic diseases, and emerging direct oral anticoagulants (DOACs) also provide an alternative choice. Owing to the particularity of NS, the safe application of DOACs still needs to be addressed. This review aimed to comprehensively describe the pathophysiology of TE in NS, as well as analyze the associated risk factors, the opportunity for preventive anticoagulation, and current anticoagulant information.

Cryo-EM structures of coagulation factors

A State of the Art lecture titled "Cryo-EM structures of coagulation factors" was presented at the ISTH Congress in 2022. Cryogenic electron microscopy (cryo-EM) is a revolutionary technique capable of solving the structure of high molecular weight proteins and their complexes, unlike nuclear magnetic resonance (NMR), and under conditions not biased by crystal contacts, unlike X-ray crystallography. These features are particularly relevant to the analysis of coagulation factors that are too big for NMR and often recalcitrant to X-ray investigation. Using cryo-EM, we have solved the structures of coagulation factors V and Va, prothrombinase on nanodiscs, and the prothrombin-prothrombinase complex. These structures have advanced basic knowledge in the field of thrombosis and hemostasis, especially on the function of factor V and the molecular mechanism for prothrombin activation, and set the stage for exciting new lines of investigation. Finally, we summarize relevant new data on this topic presented during the 2022 ISTH Congress.

Human endothelial cells and fibroblasts express and produce the coagulation proteins necessary for thrombin generation

In a previous study, we reported that human endothelial cells (ECs) express and produce their own coagulation factors (F) that can activate cell surface FX without the additions of external proteins or phospholipids. We now describe experiments that detail the expression and production in ECs and fibroblasts of the clotting proteins necessary for formation of active prothrombinase (FV-FX) complexes to produce thrombin on EC and fibroblast surfaces. EC and fibroblast thrombin generation was identified by measuring: thrombin activity; thrombin-antithrombin complexes; and the prothrombin fragment 1.2 (PF1.2), which is produced by the prothrombinase cleavage of prothrombin (FII) to thrombin. In ECs, the prothrombinase complex uses surface-attached FV and γ-carboxyl-glutamate residues of FX and FII to attach to EC surfaces. FV is also on fibroblast surfaces; however, lower fibroblast expression of the gene for γ-glutamyl carboxylase (GGCX) results in production of vitamin K-dependent coagulation proteins (FII and FX) with reduced surface binding. This is evident by the minimal surface binding of PF1.2, following FII activation, of fibroblasts compared to ECs. We conclude that human ECs and fibroblasts both generate thrombin without exogenous addition of coagulation proteins or phospholipids. The two cell types assemble distinct forms of prothrombinase to generate thrombin.

Inclusive Quantification Assay of Serum Des-γ-Carboxyprothrombin Proteoforms for Hepatocellular Carcinoma Surveillance by Targeted Mass Spectrometry

Hepatocellular carcinoma (HCC) is a malignant cancer with one of the highest mortality rates. Des-γ-carboxyprothrombin (DCP) is an HCC serologic surveillance marker that can complement the low sensitivity of alpha-fetoprotein (AFP). DCP exists in the blood as a mixture of proteoforms from an impaired carboxylation process at glutamic acid (Glu) residues within the N-terminal domain. The heterogeneity of DCP may affect the accuracy of measurements because DCP levels are commonly determined using an immunoassay that relies on antibody reactivity to an epitope in the DCP molecule. In this study, we aimed to improve the DCP measurement assay by applying a mass spectrometry (MS)-based approach for a more inclusive quantification of various DCP proteoforms. We developed a multiple-reaction monitoring-MS (MRM-MS) assay to quantify multiple noncarboxylated peptides included in the various des-carboxylation states of DCP. We performed the MRM-MS assay in 300 patients and constructed a robust diagnostic model that simultaneously monitored three noncarboxylated peptides. The MS-based quantitative assay for DCP had reliable surveillance power, which was evident from the area under the receiver operating characteristic curve (AUROC) values of 0.874 and 0.844 for the training and test sets, respectively. It was equivalent to conventional antibody-based quantification, which had AUROC values at the optimal cutoff (40 mAU/mL) of 0.743 and 0.704 for the training and test sets, respectively. The surveillance performance of the MS-based DCP assay was validated using an independent validation set consisting of 318 patients from an external cohort, resulting in an AUROC value of 0.793. Conclusion: Due to cost effectiveness and high reproducibility, the quantitative DCP assay using the MRM-MS method is superior to antibody-based quantification and has equivalent performance.

Mechanisms and biomarkers of cancer-associated thrombosis

Cancer-associated thrombosis is a leading cause of non-cancer death in cancer patients and is comprised of both arterial and venous thromboembolism (VTE). There are multiple risk factors for developing VTE, including cancer type, stage, treatment, and other medical comorbidities, which suggests that the etiology of thrombosis is multifactorial. While cancer-associated thrombosis can be treated with anticoagulation, benefits of therapy must be balanced with the increased bleeding risks seen in patients with cancer. Although risk models exist for primary and recurrent VTE, additional predictors are needed to improve model performance and discrimination of high-risk patients. This review will outline the diverse mechanisms driving thrombosis in cancer patients, as well as provide an overview of biomarkers studied in thrombosis risk and important considerations when selecting candidate biomarkers.

Vitamin K for reversal of excessive vitamin K antagonist anticoagulation: a systematic review and meta-analysis

Patients receiving vitamin K antagonists (VKAs) with an international normalized ratio (INR) between 4.5 and 10 are at increased risk of bleeding. We systematically reviewed the literature to evaluate the effectiveness and safety of administering vitamin K in patients receiving VKA therapy with INR between 4.5 and 10 and without bleeding. Medline, Embase, and Cochrane databases were searched for relevant randomized controlled trials in April 2018. Search strategy included terms vitamin K administration and VKA-related terms. Reference lists of relevant studies were reviewed, and experts in the field were contacted for relevant papers. Two investigators independently screened and collected data. Risk ratios (RRs) were calculated, and certainty of the evidence was assessed using Grading of Recommendations Assessment, Development and Evaluation. Six studies (1074 participants) were included in the review and meta-analyses. Pooled estimates indicate a nonsignificant increased risk of mortality (RR = 1.42; 95% confidence interval [CI], 0.62-2.47), bleeding (RR = 2.24; 95% CI, 0.81-7.27), and thromboembolism (RR = 1.29; 95% CI, 0.35-4.78) for vitamin K administration, with moderate certainty of the evidence resulting from serious imprecision as CIs included potential for benefit and harm. Patients receiving vitamin K had a nonsignificant increase in the likelihood of reaching goal INR (1.95; 95% CI, 0.88-4.33), with very low certainty of the evidence resulting from serious risk of bias, inconsistency, and imprecision. Our findings indicate that patients on VKA therapy who have an INR between 4.5 and 10.0 without bleeding are not likely to benefit from vitamin K administration in addition to temporary VKA cessation.

A Novel LC-MS/MS Assay for Quantification of Des-carboxy Prothrombin and Characterization of Warfarin-Induced Changes

Warfarin is a narrow therapeutic index anticoagulant drug and its use is associated with infrequent but significant adverse bleeding events. The international normalized ratio (INR) is the most commonly used biomarker to monitor and titrate warfarin therapy. However, INR is derived from a functional assay, which determines clotting efficiency at the time of measurement and is susceptible to technical variability. Protein induced by vitamin K antagonist-II (PIVKA-II) has been suggested as a biomarker of long-term vitamin K status, providing mechanistic insights about variation in the functional assay. However, the currently available antibody-based PIVKA-II assay does not inform on the position and number of des-carboxylation sites in prothrombin. The assay presented in this paper provides simultaneous quantification of carboxy and des-carboxy prothrombin that are essential for monitoring early changes in INR and, thus, serves as the superior tool for managing warfarin therapy. Additionally, this assay permits the quantification of total prothrombin level, which is affected by warfarin treatment. Prothrombin recovery from plasma was 95% and the liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay was linear (r²  = 0.98) with a dynamic range of 1-100 µg/mL. The assay interday precision was within 20%. A des-carboxy peptide of prothrombin (GNLER) was negatively correlated with active prothrombin (Pearson r = 0.99, P < 0.0001), whereas its association was positively linked with INR values (Pearson r = 0.75, P < 0.015). This novel LC-MS/MS assay for active and inactive prothrombin quantification can be applied to titrate anticoagulant therapy and to monitor the impact of diseases, such as hepatocellular carcinoma on clotting physiology.

Factor Xa inhibition by rivaroxaban in the trough steady state can significantly reduce thrombin generation

AIMS

The aim of the present study was to demonstrate evidence of reduced thrombin generation at the trough plasma rivaroxaban concentration.

METHODS

A single-centre, prospective, nonrandomized, drug-intervention, self-controlled study was conducted in 51 anticoagulation therapy-naïve patients with nonvalvular atrial fibrillation. Plasma rivaroxaban concentration was measured by liquid chromatography tandem mass spectrometry (LC-MS/MS) and the anti-factor Xa chromogenic assay. Partial thrombin time (PT), protein C activity, and protein S antigen, prothrombin fragment 1 + 2 (F1 + 2), D-dimer, thrombomodulin (TM), thrombin-antithrombin complex (TAT), plasminogen activator inhibitor-1 (PAI-1) and tissue factor pathway inhibitor (TFPI) levels were also measured at the trough steady state after 4 weeks of rivaroxaban treatment and compared with baseline.

RESULTS

Plasma concentrations obtained by the LC-MS/MS and anti-Xa assays were correlated (r = 0.841, P < 0.001). The mean concentration of rivaroxaban at the trough steady state was 23.6 ng ml^-1 , at which F1 + 2, TAT and D-dimer had decreased from the baseline values (P < 0.0001, P = 0.029 and P < 0.005, respectively). PT was prolonged (+0.59 s, P < 0.0001). TFPI increased from baseline to the trough steady state in the first to third quartile groups (+0.79 pg ml^-1 , P = 0.048). By contrast, PAI-1, protein C activity, protein S antigen and TM remained within the normal range at the trough steady state.

CONCLUSIONS

Residual plasma rivaroxaban at the trough steady state may explain the antithrombin effect of rivaroxaban in patients with nonvalvular atrial fibrillation.

Observations on Possible Effects of Daily Vitamin K Replacement, Especially Upon Warfarin Therapy

Daily parenteral vitamin K supplement is now recommended by the U.S. Food and Drug Administration (FDA) for patients receiving IV hyperalimentation. This is considered as preferable to the previous recommendations of weekly parenteral or oral supplement, or as in some cases no supplement at all. Supplemental vitamin K1 will ensure adequate supplies for hepatic saturation and thus the production of clotting factors II, VII, IX, and X, plus the anticoagulants protein C, protein S, and protein Z. But this is not the entire story. This recommended supplement will affect other physiologic systems that also use vitamin K-dependent gamma-carboxylation. Vitamin K is not 1 molecule but rather 2 natural substances, vitamin K1 and K2, and the synthetic K3's. It is not understood, what, if any, effect may occur because of the saturation or competition from the vitamin K1 upon the functioning of vitamins K2 and the derivatives of K3 in vivo upon bone mineralization, cell growth, and blood vessel health, all known to be influenced by the vitamins K. There are probably other physiologic systems yet to be studied relative to vitamins K and gamma-carboxylation. This review also considers the available research upon warfarin when given to patients receiving hyperalimentation and what effects the vitamin K supplements may have. Because studies to date have not controlled for vitamin K intake, consideration is given to whether one should expect any change in previously reported outcomes when using low-dose warfarin for prophylaxis against central vein thrombosis. Also considered are possible positive or negative effects that chronic warfarin therapy may have upon the other vitamin K-dependent systems under discussion. This review offers a platform for further discussion and derived clinical research provoked by this new FDA recommendation.

Getting to the Outer Leaflet: Physiology of Phosphatidylserine Exposure at the Plasma Membrane

Phosphatidylserine (PS) is a major component of membrane bilayers whose change in distribution between inner and outer leaflets is an important physiological signal. Normally, members of the type IV P-type ATPases spend metabolic energy to create an asymmetric distribution of phospholipids between the two leaflets, with PS confined to the cytoplasmic membrane leaflet. On occasion, membrane enzymes, known as scramblases, are activated to facilitate transbilayer migration of lipids, including PS. Recently, two proteins required for such randomization have been identified: TMEM16F, a scramblase regulated by elevated intracellular Ca(2+), and XKR8, a caspase-sensitive protein required for PS exposure in apoptotic cells. Once exposed at the cell surface, PS regulates biochemical reactions involved in blood coagulation, and bone mineralization, and also regulates a variety of cell-cell interactions. Exposed on the surface of apoptotic cells, PS controls their recognition and engulfment by other cells. This process is exploited by parasites to invade their host, and in specialized form is used to maintain photoreceptors in the eye and modify synaptic connections in the brain. This review discusses what is known about the mechanism of PS exposure at the surface of the plasma membrane of cells, how actors in the extracellular milieu sense surface exposed PS, and how this recognition is translated to downstream consequences of PS exposure.

Circadian variations in laboratory measurements of coagulation assays after administration of rivaroxaban or warfarin in patients with nonvalvular atrial fibrillation

BACKGROUND

Although rivaroxaban has a relatively shorter half-life and peak and trough plasma concentrations throughout the day than warfarin, rivaroxaban has been found to be non-inferior to warfarin in preventing thromboembolic events in patients with nonvalvular atrial fibrillation (NVAF). We measured circadian variations in laboratory measurements of coagulation assays for chronic treatment with rivaroxaban or warfarin in patients with NVAF.

METHODS

We included 28 consecutive patients with NVAF who were treated with rivaroxaban (n=13) or warfarin (n=15). Blood samples were collected at 6 AM, 11 AM, and 3 PM on the same day and on the next morning at 6 AM. Prothrombin time (PT), international normalized ratio of the PT (PT-INR), activated partial thromboplastin time (APTT), prothrombin fragment 1+2 (F1+2), and protein C level/activity were measured in each patient.

RESULTS

PT and PT-INR were significantly and consistently lower, and the F1+2 and protein C level/activity were significantly and consistently higher throughout the day in rivaroxaban-treated patients than in warfarin-treated patients. Significant increases in PT and PT-INR were observed 3h after oral administration in the patients taking rivaroxaban in the morning, whereas, significant increases in the protein C level/activity were observed 3h after oral administration in the patients taking warfarin in the morning.

CONCLUSIONS

The protein C level/activity was significantly and consistently higher in the rivaroxaban-treated patients than in the warfarin-treated patients throughout the day, which was in contrast to the findings for other coagulation assays. These findings may partly explain the specific persistent anticoagulant effects of rivaroxaban even during the trough phase of the plasma concentration.

Volume and functional outcome of intracerebral hemorrhage according to oral anticoagulant type

OBJECTIVE

To compare intracerebral hemorrhage (ICH) volume and clinical outcome of non-vitamin K oral anticoagulants (NOAC)-associated ICH to warfarin-associated ICH.

METHODS

In this multicenter cross-sectional observational study of patients with anticoagulant-associated ICH, consecutive patients with NOAC-ICH were compared to those with warfarin-ICH selected from a population of 344 patients with anticoagulant-associated ICH. ICH volume was measured by an observer blinded to clinical details. Outcome measures were ICH volume and clinical outcome adjusted for confounding factors.

RESULTS

We compared 11 patients with NOAC-ICH to 52 patients with warfarin-ICH. The median ICH volume was 2.4 mL (interquartile range [IQR] 0.3-5.4 mL) for NOAC-ICH vs 8.9 mL (IQR 4.0-21.3 mL) for warfarin-ICH (p = 0.0028). In univariate linear regression, use of warfarin (difference in cube root volume 1.61; 95% confidence interval [CI] 0.69 to 2.53) and lobar ICH location (compared with nonlobar ICH; difference in cube root volume 1.52; 95% CI 2.20 to 0.85) were associated with larger ICH volumes. In multivariable linear regression adjusting for confounding factors (sex, hypertension, previous ischemic stroke, white matter disease burden, and premorbid modified Rankin Scale score [mRS]), warfarin use remained independently associated with larger ICH (cube root) volumes (coefficient 0.64; 95% CI 0.24 to 1.25; p = 0.042). Ordered logistic regression showed an increased odds of a worse clinical outcome (as measured by discharge mRS) in warfarin-ICH compared with NOAC-ICH: odds ratio 4.46 (95% CI 1.10 to 18.14; p = 0.037).

CONCLUSIONS

In this small prospective observational study, patients with NOAC-associated ICH had smaller ICH volumes and better clinical outcomes compared with warfarin-associated ICH.

Perioperative changes in PIVKA-II

BACKGROUND AND AIMS

Proteins induced by vitamin K absence for factor II (PIVKA-II) is an enzyme-linked immunosorbent assay that monitors uncarboxylated prothrombin and responds to vitamin K deficits prior to changes in the prothrombin test. The aim of this project was to study perioperative PIVKA-II changes during various types of surgery in a prospective observational study.

METHODS

Patients undergoing abdominal or orthopaedic surgery were included. Blood was sampled on the day of surgery (preoperatively) and up to 5 days after surgery. The activated partial thromboplastin time, Quick and Owren prothrombin times were analyzed, together with PIVKA-II.

RESULTS

Thirty-nine patients were included, 27 +male and 12 +female. All but 7 +patients had elevated PIVKA-II levels preoperatively. PIVKA-II levels had already increased significantly (p < 0.017) on day 1 after surgery as compared to presurgery plasma levels. The median PIVKA-II was highest on day 5. Routine tests were mostly normal. No significant difference in PIVKA-II was seen when comparing patients undergoing abdominal versus orthopaedic surgeries. There was no significant correlation between PIVKA-II and routine coagulation tests. Patients with anterior resection, emergency laparotomy and emergency hip fractures had higher postoperative increases, which could be linked to increased gastrointestinal recovery times, paralytic ileus, peritonitis and comorbidities.

CONCLUSIONS

PIVKA-II levels increase during the perioperative period, despite mostly normal routine coagulation tests. Pre- and perioperative vitamin K supplementation in patients with elevated PIVKA-II levels should be studied, and its clinical significance be defined in future studies.

Warfarin Interactions with Medicinal Herbs

Recognition of the adverse effects of medicinal herbs is not routine and the reports on such effects are even less frequent in clinical practice. Potential herb-drug interactions are of a major safety concern, especially for drugs with narrow therapeutic indices like warfarin, which can lead to severe adverse reactions that are sometimes life-threatening. The interactions between warfarin and medicinal herbs described in the literature have been summarized in this paper relying on Medline database (via PubMed) using the key words: warfarin, herbal supplements and interactions. The references on the analyzed literature have been investigated in order to collect the existing data. The case reports with severe adverse effects such as spontaneous postoperative bleeding, formation of hematomas, hematemesis, melena, thrombosis, subarachnoid hemorrhage and/or subdural hematomas after concomitant use of warfarin and the medicinal herbs: Panax ginseng, Hypericum perforatum, Salvia milthiorizza, Gingko biloba, Serenoa repens, Angelica sinensis, Vaccinium species, Allium sativum, Zingiber officinale, Tanacetum parthenium, Lucium barbarum, Matricaria chamomilla, Boswellia serrata and Camellia sinensis have been estimated. Some of the interactions between warfarin and medicinal herbs have been well assessed proving that they are closely-dependent.

The interactions between warfarin and medicinal herbs, not generally reported in previous reviews, are presented in our review. The health professionals who are involved in treating the patients are expected to be fully informed about the interactions between warfarin and medicinal herbs in order to minimize the health risks of the patients.

The linker connecting the two kringles plays a key role in prothrombin activation

The zymogen prothrombin is proteolytically converted by factor Xa to the active protease thrombin in a reaction that is accelerated >3,000-fold by cofactor Va. This physiologically important effect is paradigmatic of analogous cofactor-dependent reactions in the coagulation and complement cascades, but its structural determinants remain poorly understood. Prothrombin has three linkers connecting the N-terminal Gla domain to kringle-1 (Lnk1), the two kringles (Lnk2), and kringle-2 to the C-terminal protease domain (Lnk3). Recent developments indicate that the linkers, and particularly Lnk2, confer on the zymogen significant flexibility in solution and enable prothrombin to sample alternative conformations. The role of this flexibility in the context of prothrombin activation was tested with several deletions. Removal of Lnk2 in almost its entirety (ProTΔ146-167) drastically reduces the enhancement of thrombin generation by cofactor Va from >3,000-fold to 60-fold because of a significant increase in the rate of activation in the absence of cofactor. Deletion of Lnk2 mimics the action of cofactor Va and offers insights into how prothrombin is activated at the molecular level. The crystal structure of ProTΔ146-167 reveals a contorted architecture where the domains are not vertically stacked, kringle-1 comes within 9 Å of the protease domain, and the Gla-domain primed for membrane binding comes in contact with kringle-2. These findings broaden our molecular understanding of a key reaction of the blood coagulation cascade where cofactor Va enhances activation of prothrombin by factor Xa by compressing Lnk2 and morphing prothrombin into a conformation similar to the structure of ProTΔ146-167.

Membrane binding by prothrombin mediates its constrained presentation to prothrombinase for cleavage

Long-standing dogma proposes a profound contribution of membrane binding by prothrombin in determining the rate at which it is converted to thrombin by prothrombinase. We have examined the action of prothrombinase on full-length prothrombin variants lacking γ-carboxyglutamate modifications (desGla) with impaired membrane binding. We show an unexpectedly modest decrease in the rate of thrombin formation for desGla prothrombin but with a major effect on the pathway for substrate cleavage. Using desGla prothrombin variants in which the individual cleavage sites have been singly rendered uncleavable, we find that loss of membrane binding and other Gla-dependent functions in the substrate leads to a decrease in the rate of cleavage at Arg(320) and a surprising increase in the rate of cleavage at Arg(271). These compensating effects arise from a loss in the membrane component of exosite-dependent tethering of substrate to prothrombinase and a relaxation in the constrained presentation of the individual cleavage sites for active site docking and catalysis. Loss of constraint is evident as a switch in the pathway for prothrombin cleavage and the intermediate produced but without the expected profound decrease in rate. Extension of these findings to the action of prothrombinase assembled on platelets and endothelial cells on fully carboxylated prothrombin reveals new mechanistic insights into function on physiological membranes. Cell-dependent enzyme function is probably governed by a differential ability to support prothrombin binding and the variable accumulation of intermediates from the two possible pathways of prothrombin activation.

Kinetic regulation of the binding of prothrombin to phospholipid membranes

A wide range of equilibrium and kinetic constants exist for the interaction of prothrombin and other coagulation factors with various model membranes from a variety of techniques. We have investigated the interaction of prothrombin with pure dioleoylphosphatidylcholine (DOPC) membranes and dioleoylphosphatidlyserine (DOPS)-containing membranes (DOPC:DOPS, 3:1) using surface plasmon resonance (SPR, with four different model membrane presentations) in addition to isotheral titration calorimetry (ITC, with suspensions of phospholipid vesicles) and ELISA methods. Using ITC, we found a simple low-affinity interaction with DOPC:DOPS membranes with a K(D) = 5.1 μM. However, ELISA methods using phospholipid bound to microtitre plates indicated a complex interaction with both DOPC:DOPS and DOPC membranes with K(D) values of 20 and 58 nM, respectively. An explanation for these discrepant results was developed from SPR studies. Using SPR with low levels of immobilised DOPC:DOPS, a high-affinity interaction with a K(D) of 18 nM was obtained. However, as phospholipid and prothrombin concentrations were increased, two distinct interactions could be discerned: (i) a kinetically slow, high-affinity interaction with K(D) in the 10(-8) M range and (ii) a kinetically rapid, low-affinity interaction with K(D) in the 10(-6 )M range. This low affinity, rapidly equilibrating, interaction dominated in the presence of DOPS. Detailed SPR studies supported a heterogeneous binding model in agreement with ELISA data. The binding of prothrombin with phospholipid membranes is complex and the techniques used to measure binding will report K D values reflecting the mixture of complexes detected. Existing data suggest that the weaker rapid interaction between prothrombin and membranes is the most important in vivo when considering the activation of prothrombin at the cell surface.

The transition of prothrombin to thrombin

The proteolytic conversion of prothrombin to thrombin catalyzed by prothrombinase is one of the more extensively studied reactions of blood coagulation. Sophisticated biophysical and biochemical insights into the players of this reaction were developed in the early days of the field. Yet, many basic enzymological questions remained unanswered. I summarize new developments that uncover mechanisms by which high substrate specificity is achieved, and the impact of these strategies on enzymic function. Two principles emerge that deviate from conventional wisdom that has otherwise dominated thinking in the field. (i) Enzymic specificity is dominated by the contribution of exosite binding interactions between substrate and enzyme rather than by specific recognition of sequences flanking the scissile bond. Coupled with the regulation of substrate conformation as a result of the zymogen to proteinase transition, novel mechanistic insights result for numerous aspects of enzyme function. (ii) The transition of zymogen to proteinase following cleavage is not absolute and instead, thrombin can reversibly interconvert between zymogen-like and proteinase-like forms depending on the complement of ligands bound to it. This establishes new paradigms for considering proteinase allostery and how enzyme function may be modulated by ligand binding. These insights into the action of prothrombinase on prothrombin have wide-ranging implications for the understanding of function in blood coagulation.

Review on natural coumarin lead compounds for their pharmacological activity

Coumarin (2H-1-benzopyran-2-one) is a plant-derived natural product known for its pharmacological properties such as anti-inflammatory, anticoagulant, antibacterial, antifungal, antiviral, anticancer, antihypertensive, antitubercular, anticonvulsant, antiadipogenic, antihyperglycemic, antioxidant, and neuroprotective properties. Dietary exposure to benzopyrones is significant as these compounds are found in vegetables, fruits, seeds, nuts, coffee, tea, and wine. In view of the established low toxicity, relative cheapness, presence in the diet, and occurrence in various herbal remedies of coumarins, it appears prudent to evaluate their properties and applications further.

Management of special conditions in patients on vitamin K antagonists

Physicians are occasionally faced with difficult situations in the management of vitamin K antagonists (VKA) due to the lack of sound data available in controlled studies on certain conditions. In this review we would like to address some special but frequent conditions that can be encountered in daily clinical practice. These include the use of VKA in hemodialysis, thromboembolism in patients with liver cirrhosis and the thromboembolic risk in patients who bleed in the course of treatment with VKA. Moreover, two other conditions were examined: what the best way of expressing prothrombin time would be in patients with liver disease and how to behave when a patient treated with VKA shows a subtherapeutic INR. These topics were discussed by a panel of experts during a workshop recently held in Milan by the Italian Federation of Centres for the Diagnosis of Thrombosis and the Surveillance of Antithrombotic Therapies (FCSA). The main aim of the workshop was to provide helpful and practical advice to physicians in the daily management of VKA.

Stroke Prevention in Atrial Fibrillation: Where are We Now?

Atrial fibrillation is the commonest arrhythmia worldwide and is a growing problem. AF is responsible for 25% of all strokes, and these patients suffer greater mortality and disability. Warfarin has traditionally been the only successful therapy for stroke prevention, but its limitations have resulted in underutilisation. Major progress has been made in AF research, leading to improved management strategies. Better risk stratification permits identification of truly low-risk patients who do not require anticoagulation and we are able to simplify ourevaluation of a patient's bleeding risk.The advent of novel anticoagulants means warfarin is no longer the only choice for stroke prophylaxis. These drugs circumvent many of warfarin's inconveniences, but onlylong-term study and use will conclusively demonstrate how they compare to warfarin. The landscape of stroke prevention in AF has changed with effective alternatives to warfarin available for the first time in 60 years-but each new option brings new considerations.

Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

The objective of this article is to summarize the published literature concerning the pharmacokinetics and pharmacodynamics of oral anticoagulant drugs that are currently available for clinical use and other aspects related to their management.

METHODS

We carried out a standard review of published articles focusing on the laboratory and clinical characteristics of the vitamin K antagonists; the direct thrombin inhibitor, dabigatran etexilate; and the direct factor Xa inhibitor, rivaroxaban

RESULTS

The antithrombotic effect of each oral anticoagulant drug, the interactions, and the monitoring of anticoagulation intensity are described in detail and discussed without providing specific recommendations. Moreover, we describe and discuss the clinical applications and optimal dosages of oral anticoagulant therapies, practical issues related to their initiation and monitoring, adverse events such as bleeding and other potential side effects, and available strategies for reversal.

CONCLUSIONS

There is a large amount of evidence on laboratory and clinical characteristics of vitamin K antagonists. A growing body of evidence is becoming available on the first new oral anticoagulant drugs available for clinical use, dabigatran and rivaroxaban.

Fate of membrane-bound reactants and products during the activation of human prothrombin by prothrombinase

Membrane binding by prothrombin, mediated by its N-terminal fragment 1 (F1) domain, plays an essential role in its proteolytic activation by prothrombinase. Thrombin is produced in two cleavage reactions. One at Arg(320) yields the proteinase meizothrombin that retains membrane binding properties. The second, at Arg(271), yields thrombin and severs covalent linkage with the N-terminal fragment 1.2 (F12) region. Covalent linkage with the membrane binding domain is also lost when prethrombin 2 (P2) and F12 are produced following initial cleavage at Arg(271). We show that at the physiological concentration of prothrombin, thrombin formation results in rapid release of the proteinase into solution. Product release from the surface can be explained by the weak interaction between the proteinase and F12 domains. In contrast, the zymogen intermediate P2, formed following cleavage at Arg(271), accumulates on the surface because of a approximately 20-fold higher affinity for F12. By kinetic studies, we show that this enhanced binding adequately explains the ability of unexpectedly low concentrations of F12 to greatly enhance the conversion of P2 to thrombin. Thus, the utilization of all three possible substrate species by prothrombinase is regulated by their ability to bind membranes regardless of whether covalent linkage to the F12 region is maintained. The product, thrombin, interacts with sufficiently poor affinity with F12 so that it is rapidly released from its site of production to participate in its numerous hemostatic functions.

Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This article concerning the pharmacokinetics and pharmacodynamics of vitamin K antagonists (VKAs) is part of the American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). It describes the antithrombotic effect of the VKAs, the monitoring of anticoagulation intensity, and the clinical applications of VKA therapy and provides specific management recommendations. Grade 1 recommendations are strong and indicate that the benefits do or do not outweigh the risks, burdens, and costs. Grade 2 recommendations suggest that the individual patient's values may lead to different choices. (For a full understanding of the grading, see the "Grades of Recommendation" chapter by Guyatt et al, CHEST 2008; 133:123S-131S.) Among the key recommendations in this article are the following: for dosing of VKAs, we recommend the initiation of oral anticoagulation therapy, with doses between 5 mg and 10 mg for the first 1 or 2 days for most individuals, with subsequent dosing based on the international normalized ratio (INR) response (Grade 1B); we suggest against pharmacogenetic-based dosing until randomized data indicate that it is beneficial (Grade 2C); and in elderly and other patient subgroups who are debilitated or malnourished, we recommend a starting dose of < or = 5 mg (Grade 1C). The article also includes several specific recommendations for the management of patients with nontherapeutic INRs, with INRs above the therapeutic range, and with bleeding whether the INR is therapeutic or elevated. For the use of vitamin K to reverse a mildly elevated INR, we recommend oral rather than subcutaneous administration (Grade 1A). For patients with life-threatening bleeding or intracranial hemorrhage, we recommend the use of prothrombin complex concentrates or recombinant factor VIIa to immediately reverse the INR (Grade 1C). For most patients who have a lupus inhibitor, we recommend a therapeutic target INR of 2.5 (range, 2.0 to 3.0) [Grade 1A]. We recommend that physicians who manage oral anticoagulation therapy do so in a systematic and coordinated fashion, incorporating patient education, systematic INR testing, tracking, follow-up, and good patient communication of results and dose adjustments [Grade 1B]. In patients who are suitably selected and trained, patient self-testing or patient self-management of dosing are effective alternative treatment models that result in improved quality of anticoagulation management, with greater time in the therapeutic range and fewer adverse events. Patient self-monitoring or self-management, however, is a choice made by patients and physicians that depends on many factors. We suggest that such therapeutic management be implemented where suitable (Grade 2B).

Characterization of an ideal amphipathic peptide as a procoagulant agent

On the basis of previous evidence that amphipathic helical peptides accelerate Factor IXa activation of Factor X [Blostein, Rigby, Furie, Furie and Gilbert (2000) Biochemistry 39, 12000–12006], the present study was designed to assess the procoagulant activity of an IAP (ideal amphipathic peptide) of Lys7Leu15 composition. The results show that IAP accelerates Factor X activation by Factor IXa in a concentration-dependent manner and accelerates thrombin generation by Factor Xa with a comparable peptide- and substrate-concentration-dependence. A scrambled helical peptide with the same amino acid composition as IAP, but with its amphipathicity abolished, eliminated most of the aforementioned effects. The Gla (γ-carboxyglutamic acid)-rich domain of Factor X is required for IAP activity, suggesting that this peptide behaves as a phospholipid membrane. This hypothesis was confirmed, using fluorescence spectroscopy, by demonstrating direct binding between IAP and the Gla-rich domain of Factor X. In addition, the catalytic efficiencies of the tenase and prothrombinase enzymatic complexes, containing cofactors Factor VIIIa and Factor Va respectively, are enhanced by IAP. Finally, we show that IAP delays clot lysis in vitro. In summary, these observations demonstrate that IAP not only enhances essential procoagulant reactions required for fibrin generation, but also inhibits fibrinolysis, suggesting a potential role for IAP as a haemostatic agent.

The pharmacology and management of the vitamin K antagonists: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy

This article concerning the pharmacokinetics and pharmacodynamics of vitamin K antagonists (VKAs) is part of the Seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. The article describes the antithrombotic effect of VKAs, the monitoring of anticoagulation intensity, the clinical applications of VKA therapy, and the optimal therapeutic range of VKAs, and provides specific management recommendations. Grade 1 recommendations are strong, and indicate that the benefits do, or do not, outweigh the risks, burdens, and costs. Grade 2 suggests that individual patient's values may lead to different choices (for a full understanding of the grading see Guyatt et al, CHEST 2004; 126:179S-187S). Among the key recommendations in this article are the following: for dosing of VKAs, we suggest the initiation of oral anticoagulation therapy with doses between 5 and 10 mg for the first 1 or 2 days for most individuals, with subsequent dosing based on the international normalized ratio (INR) response (Grade 2B). In the elderly and in other patient subgroups with an elevated bleeding risk, we suggest a starting dose at < or = 5 mg (Grade 2C). We recommend basing subsequent doses after the initial two or three doses on the results of INR monitoring (Grade 1C). The article also includes several specific recommendations for the management of patients with INRs above the therapeutic range and for patients requiring invasive procedures. For example, in patients with mild to moderately elevated INRs without major bleeding, we suggest that when vitamin K is to be given it be administered orally rather than subcutaneously (Grade 1A). For the management of patients with a low risk of thromboembolism, we suggest stopping warfarin therapy approximately 4 days before they undergo surgery (Grade 2C). For patients with a high risk of thromboembolism, we suggest stopping warfarin therapy approximately 4 days before surgery, to allow the INR to return to normal, and beginning therapy with full-dose unfractionated heparin or full-dose low-molecular-weight heparin as the INR falls (Grade 2C). In patients undergoing dental procedures, we suggest the use of tranexamic acid mouthwash (Grade 2B) or epsilon amino caproic acid mouthwash without interrupting anticoagulant therapy (Grade 2B) if there is a concern for local bleeding. For most patients who have a lupus inhibitor, we suggest a therapeutic target INR of 2.5 (range, 2.0 to 3.0) [Grade 2B]. In patients with recurrent thromboembolic events with a therapeutic INR or other additional risk factors, we suggest a target INR of 3.0 (range, 2.5 to 3.5) [Grade 2C]. As models of anticoagulation monitoring and management, we recommend that clinicians incorporate patient education, systematic INR testing, tracking, and follow-up, and good communication with patients concerning results and dosing decisions (Grade 1C+).

Properties of recombinant human thromboplastin that determine the International Sensitivity Index (ISI)

Prothrombin Time (PT) clotting tests are widely used to monitor oral anticoagulation therapy and to screen for clotting factor deficiencies. The active ingredient in PT reagents (thromboplastins) is tissue factor, the integral membrane protein that triggers the clotting cascade through the extrinsic pathway. Several years ago, a system for calibrating and using thromboplastin reagents, known as the International Sensitivity Index (ISI) and the International Normalized Ratio (INR), was developed to standardize monitoring of oral anticoagulant therapy. The ISI/INR method, while revolutionizing the monitoring of coumarin therapy, has been criticized for a number of perceived shortcomings. We have undertaken a series of studies aimed at achieving a detailed understanding of which parameters influence the ISI values of thromboplastin reagents, with an ultimate goal of creating 'designer thromboplastins' whose sensitivities to the various clotting factors can be individually tailored. In this study, we demonstrate that ISI values of thromboplastin reagents based on relipidated, recombinant human tissue factor can be controlled by a combination of changes in the phospholipid content (in particular, the levels of phosphatidylserine and phosphatidylethanolamine) and ionic strength. The sensitivity of a given thromboplastin reagent can be increased (i.e. its ISI value decreased) by decreasing the content of phosphatidylserine and/or increasing the ionic strength. The molar ratio of phospholipid to tissue factor, on the other hand, had essentially no impact on ISI value.

Binding of the Factor IX γ-Carboxyglutamic Acid Domain to the Vitamin K-dependent γ-Glutamyl Carboxylase Active Site Induces an Allosteric Effect That May Ensure Processive Carboxylation and Regulate the Release of Carboxylated Product

Propeptides of the vitamin K-dependent proteins bind to an exosite on gamma-glutamyl carboxylase; while they are bound, multiple glutamic acids in the gamma-carboxyglutamic acid (Gla) domain are carboxylated. The role of the propeptides has been studied extensively; however, the role of the Gla domain in substrate binding is less well understood. We used kinetic and fluorescence techniques to investigate the interactions of the carboxylase with a substrate containing the propeptide and Gla domain of factor IX (FIXproGla41). In addition, we characterized the effect of the Gla domain and carboxylation on propeptide and substrate binding. For the propeptide of factor IX (proFIX18), FIXproGla41, and carboxylated FIXproGla41, the Kd values were 50, 2.5, and 19.7 nM and the koff values were 273 x 10(-5), 9 x 10(-5), and 37 x 10(-5) s(-1), respectively. The koff of proFIX18 is reduced 3-fold by FLEEL and 9-fold by the Gla domain (residues 1-46) of FIX. The pre-steady state rate constants for carboxylation of FIXproGla41 was 0.02 s(-1) in enzyme excess and 0.016 s(-1) in substrate excess. The steady state rate in substrate excess is 4.5 x 10(-4) s(-1). These results demonstrate the following. 1) The pre-steady state carboxylation rate constant of FIXproGla41 is significantly slower than that of FLEEL. 2) The Gla domain plays an allosteric role in substrate-enzyme interactions. 3) Carboxylation reduces the allosteric effect. 4) The similarity between the steady state carboxylation rate constant and product dissociation rate constant suggests that product release is rate-limiting. 5) The increased dissociation rate after carboxylation contributes to the release of product.

A common mutation, Arg457-->Gln, links prothrombin deficiencies in the Puerto Rican population

Five unrelated families with Puerto Rican ancestry were identified as having at least one member with bleeding due to a prothrombin deficiency. Genetic prothrombin deficiencies are extremely rare, but at the University of Puerto Rico Hemophilia Center, prothrombin deficiency is the third most common congenital coagulation factor deficiency. Because Puerto Rico is relatively isolated, there was a reasonable expectation of a founder effect. Prothrombin genes from probands and their parents were directly sequenced from PCR amplified exons using forward and reverse primers. Four novel prothrombin mutations were identified. The first, a G-->A substitution at DNA position 10150 predicting an Arg457-->Gln (R457Q) replacement, is common to all five families. In two of the families, the proband children are homozygous for R457Q. In the other three families, the probands are compound heterozygotes for R457Q and one of the other three mutations, which include another point mutation (gamma16Q), a deletion and a splice junction mutation. The two point mutations have been designated Puerto Rico I and Puerto Rico II. The crystal structure of alpha-thrombin predicts that the R457Q mutation removes a salt bridge that links the A- and B-chains of thrombin. The primary effect of this defect appears to be destabilization of the circulating prothrombin, creating a moderate hypoprothrombinemia. However, prothrombin antigen/activity ratios indicate a dysprothrombinemia as well, most likely due to the inability of R457Q prothrombin to activate fully to thrombin.

Very low dose warfarin as prophylaxis against ultrasound detected deep vein thrombosis following primary hip replacement

One mg daily warfarin was compared to variable dose warfarin (PT 1.3-1.5 times the normal PT), as prophylaxis against deep vein thrombosis (DVT) following unilateral hip replacement for degenerative joint disease (DJD). Ninety-eight patients entered onto study after having had negative color Doppler ultrasounds of the legs. Patients receiving 1 mg began therapy 7 days preoperatively and continued daily until discharge. Patients receiving the variable dose took 5 mg the night preoperatively, and thereafter daily based upon the daily PT. Seventy-eight patients completed the study protocol. No patient completing the protocol had DVT or pulmonary embolus (PE). Based upon intent to treat for all registered patients, one from each group had DVT after withdrawal from study. For patients receiving 1 mg warfarin daily, PTs extended none or slightly. Therefore, 1 mg warfarin can be used to prevent postoperative DVT following elective hip surgery.

A thrombin receptor function for platelet glycoprotein Ib-IX unmasked by cleavage of glycoprotein V

Glycoprotein (GP) V is a major substrate cleaved by the protease thrombin during thrombin-induced platelet activation. Previous analysis of platelets from GP V-null mice suggested a role for GP V as a negative modulator of platelet activation by thrombin. We now report the mechanism by which thrombin activates GP V -/- platelets. We show that proteolytically inactive forms of thrombin induce robust stimulatory responses in GP V null mouse platelets, via the platelet GP Ib--IX--V complex. Because proteolytically inactive thrombin can activate wild-type mouse and human platelets after treatment with thrombin to cleave GP V, this mechanism is involved in thrombin-induced platelet aggregation. Platelet activation through GP Ib-IX depends on ADP secretion, and specific inhibitors demonstrate that the recently cloned P2Y(12) ADP receptor (G(i)-coupled ADP receptor) is involved in this pathway, and that the P2Y(1) receptor (G(q)-coupled ADP receptor) may play a less significant role. Thrombosis was generated in GP V null mice only in response to catalytically inactive thrombin, whereas thrombosis occurred in both genotypes (wild type and GP V null) in response to active thrombin. These data support a thrombin receptor function for the platelet membrane GP Ib--IX--V complex, and describe a novel thrombin signaling mechanism involving an initiating proteolytic event followed by stimulation of the GP Ib--IX via thrombin acting as a ligand, resulting in platelet activation.