GGCX variants leading to biallelic deficiency to γ-carboxylate GRP cause skin laxity in VKCFD1 patients.

γ-Glutamyl carboxylase (GGCX) catalyzes the γ-carboxylation of 15 different vitamin K dependent (VKD) proteins. Pathogenic variants in GGCX cause a rare hereditary bleeding disorder called Vitamin K dependent coagulation factor deficiency type 1 (VKCFD1). In addition to bleedings, some VKCFD1 patients develop skin laxity and skeletal dysmorphologies. However, the pathophysiological mechanisms underlying these non-hemorrhagic phenotypes remain elusive. Therefore, we have analyzed 20 pathogenic GGCX variants on their ability to γ-carboxylate six non-hemostatic VKD proteins in an in vitro assay, where GGCX variants were expressed in GGCX-/- cells and levels of γ-carboxylated co-expressed VKD proteins were detected by a functional ELISA. We observed that GGCX variants causing markedly reduced γ-carboxylation of Gla rich protein (GRP) in vitro were reported in patients with skin laxity. Reduced levels of γ-carboxylated Matrix gla protein (MGP) are not exclusive for causing skeletal dysmorphologies in VKCFD1 patients. In silico docking of vitamin K hydroquinone on a GGCX model revealed a binding site, which was validated by in vitro assays. GGCX variants affecting this site result in disability to γ-carboxylate VKD proteins and hence are involved in the most severe phenotypes. This genotype-phenotype analysis will help to understand the development of non-hemorrhagic phenotypes and hence improve treatment in VKCFD1 patients.

Recurrent Intracranial Bleed in 3 Siblings: Short of a Shot of Vitamin K!

We present a family who suffered recurrent sibling losses due to vitamin K deficiency bleed. The index child was asymptomatic at presentation, had normal clinical examination, and was investigated for coagulation disorders in view of previous 3 sibling losses as a result of intracranial hemorrhage. His investigations showed deranged coagulogram and clotting factors' assay. The baby was given vitamin K1 1 mg intramuscularly following which his coagulogram and clotting factors' assay returned to normal. The genetic analysis did not identify any inherited cause of bleeding tendency. The significant family history, exclusive breastfeeding, no diarrhea, failure to thrive or drug use, no prophylaxis with vitamin K at birth, recovery of clotting factors on vitamin K administration, and a corroborative molecular analysis confirmed diagnosis of vitamin K deficiency in the index child. This case gives a strong reminder not to miss birth dose of vitamin K in any neonate.

The Arg98Trp mutation in human VKORC1 causing VKCFD2 disrupts a di-arginine-based ER retention motif.

Vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) is an enzyme localized to the endoplasmic reticulum (ER) membrane. VKORC1 catalyzes the reduction of vitamin K 2,3-epoxide to vitamin K and to vitamin K hydroquinone, the latter required by the enzyme γ-carboxylase for γ-carboxylation of all vitamin K-dependent (VKD) proteins. Until now, only 1 human VKORC1 mutation, p.Arg98Trp, is known to cause combined deficiency of VKD clotting factors type 2 (VKCFD2), a disease phenotype reported in 3 unrelated families. VKCFD2 patients suffer from spontaneous bleeding episodes because of decreased levels of γ-carboxylated VKD clotting factors. Daily supraphysiological vitamin K supplementation restores clotting for VKCFD2 patients and results in high serum levels of vitamin K 2,3-epoxide, suggesting that supplemented vitamin K is reduced in vivo. Although the p.Arg98Trp mutation results in reduced vitamin K 2,3-epoxide reductase activity, the molecular mechanism underlying this pathophysiology is unknown. Using a combination of in silico analysis and confocal microscopy, we demonstrate for the first time that VKORC1:p.Arg98Trp disrupts a di-arginine ER retention motif resulting in 20% ER colocalization only. As a consequence, VKORC1 exits the ER membrane by cellular quality control systems and results in the observed VKCFD2 phenotype.

Human VKORC1 mutations cause variable degrees of 4-hydroxycoumarin resistance and affect putative warfarin binding interfaces.

Since the discovery of warfarin-sensitive vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1), 26 human VKORC1 (hVKORC1) missense mutations have been associated with oral anticoagulant resistance (OACR). Assessment of warfarin resistance using the "classical" dithiothreitol-driven vitamin K 2,3-epoxide reductase (VKOR) assay has not reflected clinical resistance phenotypes for most mutations. Here, we present half maximal inhibitory concentrations (IC50) results for 21 further hVKORC1 mutations obtained using a recently validated cell-based assay (J Thromb Haemost 11(5):872). In contrast to results from the dithiothreitol-driven VKOR assay, all mutations exhibited basal VKOR activity and warfarin IC50 values that correspond well to patient OACR phenotypes. Thus, the present assay is useful for functional investigations of VKORC1 and oral anticoagulant inhibition of the vitamin K cycle. Additionally, we modeled hVKORC1 on the previously solved structure of a homologous bacterial enzyme and performed in silico docking of warfarin on this model. We identified one binding site delineated by 3 putative binding interfaces. These interfaces comprise linear sequences of the endoplasmic reticulum-lumenal loop (Ser52-Phe55) and the first (Leu22-Lys30) and fourth (Phe131-Thr137) transmembrane helices. All known OACR-associated hVKORC1 mutations are located in or around these putative interfaces, supporting our model.

Tris(3-hydroxypropyl)phosphine is superior to dithiothreitol for in vitro assessment of vitamin K 2,3-epoxide reductase activity.

Use of the reductant dithiothreitol (DTT) as a substrate for measuring vitamin K 2,3-epoxide reductase (VKOR) activity in vitro has been reported to be problematic because it enables side reactions involving the vitamin K1 2,3-epoxide (K1>O) substrate. Here we characterize specific problems when using DTT and show that tris(3-hydroxypropyl)phosphine (THPP) is a reliable alternative to DTT for in vitro assessment of VKOR enzymatic activity. In addition, the pH buffering compound imidazole was found to be problematic in enhancing DTT-dependent non-enzymatic side reactions. Using THPP and phosphate-based pH buffering, we measured apparent Michaelis-Menten constants of 1.20 µM for K1>O and 260 µM for the active neutral form of THPP. The Km value for K1>O is in agreement with the value that we previously obtained using DTT (1.24 µM). Using THPP, we successfully eliminated non-enzymatic production of 3-hydroxyvitamin K1 and its previously reported base-catalyzed conversion to K1, both of which were shown to occur when DTT and imidazole are used as the reductant and pH buffer, respectively, in the in vitro VKOR assay. Accordingly, substitution of THPP for DTT in the in vitro VKOR assay will ensure more accurate enzymatic measurements and assessment of warfarin and other 4-hydroxycoumarin inhibition constants.

Determination of the warfarin inhibition constant Ki for vitamin K 2,3-epoxide reductase complex subunit-1 (VKORC1) using an in vitro DTT-driven assay.

BACKGROUND: Warfarin directly inhibits vitamin K 2,3-epoxide reductase (VKOR) enzymes. Since the early 1970s, warfarin inhibition of vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1), an essential enzyme for proper function of blood coagulation in higher vertebrates, has been studied using an in vitro dithiothreitol (DTT) driven enzymatic assay. However, various studies based on this assay have reported warfarin dose-response data, usually summarized as half-maximal inhibitory concentration (IC50), that vary over orders of magnitude and reflect the broad range of conditions used to obtain VKOR assay data. METHODS: We standardized the implementation of the DTT-driven VKOR activity assay to measure enzymatic Michaelis constants (Km) and warfarin IC50 for human VKORC1. A data transformation is defined, based on the previously confirmed bi bi ping-pong mechanism for VKORC1, that relates assay condition-dependent IC50 to condition-independent Ki. RESULTS: Determination of the warfarin Ki specifically depends on measuring both substrate concentrations, both Michaelis constants for the VKORC1 enzyme, and pH in the assay. CONCLUSION: The Ki is not equal to the IC50 value directly measured using the DTT-driven VKOR assay. GENERAL SIGNIFICANCE: In contrast to warfarin IC50 values determined in previous studies, warfarin inhibition expressed as Ki can now be compared between studies, even when the specific DTT-driven VKOR assay conditions differ. This implies that warfarin inhibition reported for wild-type and variant VKORC1 enzymes from previous reports should be reassessed and new determinations of Ki are required to accurately report and compare in vitro warfarin inhibition results.

Genetic variants of the vitamin K dependent coagulation system and intraventricular hemorrhage in preterm infants.

BACKGROUND: Pathogenesis of intraventricular hemorrhage (IVH) in premature infants is multifactorial. Little is known about the impact of genetic variants in the vitamin K-dependent coagulation system on the development of IVH. METHODS: Polymorphisms in the genes encoding vitamin K epoxide reductase complex 1 (VKORC1 -1639G>A) and coagulation factor 7 (F7 -323Ins10) were examined prospectively in 90 preterm infants <32 weeks gestational age with respect to coagulation profile and IVH risk. RESULTS: F7-323Ins10 was associated with lower factor VII levels, but not with individual IVH risk. In VKORC1-wildtype infants, logistic regression analysis revealed a higher IVH risk compared to carriers of the -1639A allele. Levels of the vitamin K-dependent coagulation parameters assessed in the first hour after birth did not differ between VKORC1-wildtype infants and those carrying -1639A alleles. CONCLUSIONS: Our data support the assumption that genetic variants in the vitamin K-dependent coagulation system influence the coagulation profile and the IVH risk in preterm infants. Further studies focussing on short-term changes in vitamin K-kinetics and the coagulation profile during the first days of life are required to further understand a possible link between development of IVH and genetic variants affecting the vitamin K-metabolism.

VKORC1-dependent pharmacokinetics of intravenous and oral phylloquinone (vitamin K1) mixed micelles formulation.

OBJECTIVE: The pharmacokinetics of phylloquinone (vitamin K1) were evaluated in healthy human adult volunteers (15 male and 15 female) following oral and intravenous administration of a mixed micelles formulation (Konakion MM 2 mg) in an open label study design. The subjects were allocated to one of three genotype-specific groups (n = 10 in each group) in terms of VKORC1 promoter polymorphism c.-1639 G > A to explore the relationship between genotype and pharmacokinetic parameters. METHODS: Blood samples were collected for up to 24 h after administration. Phylloquinone serum levels were determined by reversed phase HPLC with fluorometric detection after post-column zinc reduction. Pharmacokinetic evaluation was performed using non-compartmental analysis. RESULTS: Pharmacokinetic analysis of serum phylloquinone concentration versus time profiles revealed significant differences in the main pharmacokinetic parameters between groups. Upon oral administration, VKORC1 AG carriers showed 41 % higher mean bioavailability (p = 0.01) compared with homozygous AA individuals. Furthermore, AG subjects exhibited 30 % (p = 0.042) and 36 % (p = 0.021) higher mean AUC compared with GG and AA respectively. Terminal half-life was 32 % and 27 % longer for AG carriers in comparison to GG (p = 0.004) and AA (p = 0.015) genotypes respectively. CONCLUSION: Pharmacokinetic differences indicated significant inter-individual variance of vitamin K fate in the human body. The influence of the VKORC1 promoter polymorphism c.-1639 G > A on the pharmacokinetic properties of phylloquinone could be demonstrated in humans. To gain deeper insight in other potential genetic determinants of systemic vitamin K exposure, further correlation of the phenotype-genotype relationship of different players in vitamin K turnover has to be gained.

Human vitamin K 2,3-epoxide reductase complex subunit 1-like 1 (VKORC1L1) mediates vitamin K-dependent intracellular antioxidant function.

Human vitamin K 2,3-epoxide reductase complex subunit 1-like 1 (VKORC1L1), expressed in HEK 293T cells and localized exclusively to membranes of the endoplasmic reticulum, was found to support both vitamin K 2,3-epoxide reductase (VKOR) and vitamin K reductase enzymatic activities. Michaelis-Menten kinetic parameters for dithiothreitol-driven VKOR activity were: K(m) (µM) = 4.15 (vitamin K(1) epoxide) and 11.24 (vitamin K(2) epoxide); V(max) (nmol·mg(-1)·hr(-1)) = 2.57 (vitamin K(1) epoxide) and 13.46 (vitamin K(2) epoxide). Oxidative stress induced by H(2)O(2) applied to cultured cells up-regulated VKORC1L1 expression and VKOR activity. Cell viability under conditions of no induced oxidative stress was increased by the presence of vitamins K(1) and K(2) but not ubinquinone-10 and was specifically dependent on VKORC1L1 expression. Intracellular reactive oxygen species levels in cells treated with 2,3-dimethoxy-1,4-naphthoquinone were mitigated in a VKORC1L1 expression-dependent manner. Intracellular oxidative damage to membrane intrinsic proteins was inversely dependent on VKORC1L1 expression and the presence of vitamin K(1). Taken together, our results suggest that VKORC1L1 is responsible for driving vitamin K-mediated intracellular antioxidation pathways critical to cell survival.

Prediction of phenprocoumon maintenance dose and phenprocoumon plasma concentration by genetic and non-genetic parameters.

PURPOSE: The anticoagulation response to vitamin K antagonists is characterised by high inter-individual variability. The impact of single nucleotide polymorphisms (SNPs) in several genes of enzymes involved in the vitamin K cycle on phenprocoumon dose variability and phenprocoumon plasma concentrations is still under investigation. METHODS: We assessed the influence of VKORC1 c.-1639G>A, CYP2C9*2, CYP2C9*3, CYP4F2 c.1297G>A, CALU c.*4A>G, EPHX1 c.337T>C, GGCX c.214+597G>A, F7 c.-402G>A, F7 c.-401G>T, PROC c.-228C>T and PROC c.-215G>A along with clinical and demographic parameters on steady-state phenprocoumon therapy in 75 patients. A prediction model was developed for total phenprocoumon plasma concentrations and daily phenprocoumon doses required for therapeutic anticoagulation. RESULTS: The VKORC1 c.-1639 genotype was the main predictor of the phenprocoumon daily dose (adjusted R(2) = 37.6%) and the total phenprocoumon concentration (adjusted R(2) = 38.3%). CYP2C9 affected the phenprocoumon concentration, but not the dose requirements. SNPs in the other genes of the vitamin K cycle, concomitant medication, nicotine use and alcohol consumption did not predict phenprocoumon concentrations and phenprocoumon dose requirements in a multiple linear regression model. Phenprocoumon concentrations were predicted by VKORC1 c.-1639, CYP2C9 genotype, age and BMI. The final prediction model for the daily phenprocoumon dose requirements comprised VKORC1 c.-1639 genotype, age and height accounting for 48.6% of the inter-individual variability. CONCLUSIONS: A rough prediction of phenprocoumon maintenance doses can be achieved by a limited set of parameters (VKORC1, age, height). The investigated SNPs in CYP4F2, CALU, EPHX1, GGCX, F7, and PROC did not improve the predictive value of a pharmacogenetic-based dosing equation for phenprocoumon.

GGCX mutations show different responses to vitamin K thereby determining the severity of the hemorrhagic phenotype in VKCFD1 patients.

BACKGROUND: Vitamin K dependent coagulation factor deficiency type 1 (VKCFD1) is a rare hereditary bleeding disorder caused by mutations in γ-glutamyl carboxylase (GGCX). VKCFD1 patients are treated life-long with high doses of vitamin K in order to correct the bleeding phenotype. However, normalization of clotting factor activities cannot be achieved for all VKCFD1 patients. OBJECTIVE: The current study aims to investigate the responsiveness to vitamin K for all reported GGCX mutations with respect to clotting factors in order to optimize treatment. METHODS: This study developed an assay using genetically engineered GGCX-/- cells, in which GGCX mutations were analyzed with respect to their ability to γ-carboxylate vitamin K dependent pro-coagulatory and anti-coagulatory clotting factors by ELISA. Additionally, factor VII activity was measured in order to proof protein functionality. For specific GGCX mutations immunofluorescent staining was performed to assess the intracellular localization of clotting factors with respect to GGCX wild-type and mutations. RESULTS: All GGCX mutations were categorized into responder and low responder mutations, thereby determining the efficiency of vitamin K supplementation. Most VKCFD1 patients have at least one vitamin K responsive GGCX allele that is able to γ-carboxylate clotting factors. In few patients, the hemorrhagic phenotype cannot be reversed by vitamin K administration because GGCX mutations on both alleles affect either structural or catalytically important sites thereby resulting in residual ability to γ-carboxylate clotting factors. CONCLUSION: With these new functional data we can predict the hemorrhagic outcome of each VKCFD1 genotype, thus recommending treatments with either vitamin K or prothrombin complex concentrate.

VKORC1 deficiency in mice causes early postnatal lethality due to severe bleeding.

Vitamin K hydroquinone is oxidised to the epoxide form (K>O) during vitamin K-dependent posttranslational gamma-glutamyl carboxylation resulting in biological active so called vitamin K-dependent proteins. In turn, K>O is reduced by the enzyme VKORC1 (vitamin K epoxide reductase complex component 1) to complete the vitamin K cycle. To investigate the biological role of VKORC1 in vivo, we generated VKORC1 knockout mice. Homozygous VKORC1-deficient mice developed normally until birth. Within 2-20 days after birth, the knockout mice died due to extensive, predominantly intracerebral haemorrhage. Bleeding resulted from a severe deficiency of gamma-carboxylated clotting factors. This lethal phenotype could be rescued by oral administration of vitamin K. Additionally, morphometric analysis of the limbs in VKORC1-deficient animals revealed reduced length of bone calcification relative to wild-type control mice. The observed phenotype of VKORC1 knockout mice excludes the existence of other enzymes with VKOR activity that can substitute to supply vitamin K hydroquinone required for maturation of blood clotting factors. Thus, our study underscores the essential role of VKORC1 in vitamin K-dependent gamma-glutamyl carboxylation.

Vitamin K-dependent coagulation factors deficiency.

All vitamin K-dependent coagulation factors require normal function of gamma-glutamyl carboxylase and vitamin K epoxide reductase enzyme complex (VKORC1). Heritable dysfunction of gamma-glutamyl carboxylase or of the VKORC1 complex results in the secretion of poorly carboxylated vitamin K-dependent proteins that play a role in coagulation. The following review will summarize the clinical manifestations of vitamin K-dependent coagulation factors deficiency I and II and will provide a detailed explanation about the gene and protein structure, the function of the protein, and an analysis of the previously reported mutations. Laboratory assays used for diagnosis will be discussed, and treatment for various clinical settings will be recommended.

The first case of combined coagulation factor V and coagulation factor VIII deficiency in Poland due to a novel p.Tyr135Asn missense mutation in the MCFD2 gene.

Congenital combined coagulation factor V and coagulation factor VIII deficiency (F5F8D) is a rare bleeding disorder due to mutations in the LMAN1 or MCFD2 genes. Here we report the first Polish family with F5F8 deficiency due to a mutation in the MCFD2 gene. The proposita suffered from mild bleeding including epistaxis, menorrhagia, bleeding after dental extraction, and bruising after minor traumas. The F5F8 deficiency was diagnosed due to an excessive postpartum bleeding at the age of 31. Analysis of further family members revealed a second affected individual. Sequencing of the MCFD2 gene and its flanking regions in both patients demonstrated a novel homozygous missense mutation within the second elongation factor hand domain resulting in a substitution of tyrosine by asparagine at amino acid position 135 (p.Tyr135Asn). This variant represents the third missense mutation found in the MCFD2 gene and most likely disrupts the MCFD2-LMAN1 interaction, thus leading to the disease phenotype.

VKORC1 and VKORC1L1 have distinctly different oral anticoagulant dose-response characteristics and binding sites.

Vitamin K reduction is catalyzed by 2 enzymes in vitro: the vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) and its isozyme VKORC1-like1 (VKORC1L1). In vivo, VKORC1 reduces vitamin K to sustain γ-carboxylation of vitamin K-dependent proteins, including coagulation factors. Inhibition of VKORC1 by oral anticoagulants (OACs) is clinically used in therapy and in prevention of thrombosis. However, OACs also inhibit VKORC1L1, which was previously shown to play a role in intracellular redox homeostasis in vitro. Here, we report data for the first time on specific inhibition of both VKOR enzymes for various OACs and rodenticides examined in a cell-based assay. Effects on endogenous VKORC1 and VKORC1L1 were independently investigated in genetically engineered HEK 293T cells that were knocked out for the respective genes by CRISPR/Cas9 technology. In general, dose-responses for 4-hydroxycoumarins and 1,3-indandiones were enzyme-dependent, with lower susceptibility for VKORC1L1 compared with VKORC1. In contrast, rodenticides exhibited nearly identical dose-responses for both enzymes. To explain the distinct inhibition pattern, we performed in silico modeling suggesting different warfarin binding sites for VKORC1 and VKORC1L1. We identified arginine residues at positions 38, 42, and 68 in the endoplasmatic reticulum luminal loop of VKORC1L1 responsible for charge-stabilized warfarin binding, resulting in a binding pocket that is diametrically opposite to that of VKORC1. In conclusion, our findings provide insight into structural and molecular drug binding on VKORC1, and especially on VKORC1L1.

Current pharmacogenetic developments in oral anticoagulation therapy: the influence of variant VKORC1 and CYP2C9 alleles.

For decades coumarins have been the most commonly prescribed drugs for therapy and prophylaxis of thromboembolic conditions. Despite the limitation of their narrow therapeutic dosage window, the broad variation of intra- and inter-individual drug requirement, and the relatively high incidence of bleeding complications, prescriptions for coumarins are increasing due to the aging populations in industrialised countries. The identification of the molecular target of coumarins, VKORC1, has greatly improved the understanding of coumarin treatment and illuminated new perspectives for a safer and more individualized oral anticoagulation therapy. Mutations and SNPs within the translated and non-translated regions of the VKORC1 gene have been shown to cause coumarin resistance and sensitivity, respectively. Besides the known CYP2C9 variants that affect coumarin metabolism, the haplotype VKORC1*2 representing a frequent SNP within the VKORC1 promoter has been identified as a major determinant of coumarin sensitivity, reducing VKORC1 enzyme activity to 50% of wild type. Homozygous carriers of the VKORC1*2 allele are strongly predisposed to coumarin sensitivity. Using individualized dose adaptation, a significant reduction of bleeding complications can be expected, especially in the initial drug saturation phase. Furthermore, concomitant application of low dose vitamin K may significantly reduce intra-individual coumarin dose variation and, thus, may stabilize oral anticoagulation therapy. The use of new pharmacogenetics-based dosing schemes and the concomitant application of low-dose vitamin K with coumarins will decidedly influence the current practice of oral anticoagulation and greatly improve coumarin drug safety.

Functional promoter polymorphism in the VKORC1 gene is no major genetic determinant for coronary heart disease in Northern Germans.

Recently, the C-allele of polymorphism rs2359612 (VKORC1: c.283+837C>T) in the VKORC1 gene has been reported to represent a major risk factor for coronary heart disease (CHD), stroke, and aortic dissection in Chinese patients. VKOR activity itself is the rate-limiting step in gamma-carboxylation of vitamin K-dependent coagulation factors (factors II, VII, IX, X, protein C, S, and Z) and proteins of calcium metabolism (matrix Gla protein and osteocalcin). Gamma-carboxylation is essential for the biological activity of these proteins that have been previously hypothesised to play a role in the pathogenesis of atherosclerosis. It was the objective of this study to analyse the VKORC1 genotype frequency in patients with CHD and controls from Northern Germany and to investigate the association of VKORC1 and CHD risk in patients with an European background. CHD patients (n = 901) and healthy controls (n = 521) were part of the PopGen biobank. Case and control samples were matched for ethnic and geographic origin, age and gender. After typing German CHD patients and control individuals, no evidence for a statistically significant association was detected between VKORC1 genotype and CHD phenotype. Also stratification for gender and myocardial infarction yielded no significant results. In conclusion, the discrepant association findings in Chinese and German populations may be explained by ethnic differences in genetic and perhaps environmental predisposition, modifying the polygenic CHD phenotype by interacting with VKORC1 variants and thus conferring disease susceptibility in some populations, but not in others.

Warfarin and vitamin K compete for binding to Phe55 in human VKOR.

Vitamin K epoxide reductase (VKOR) catalyzes the reduction of vitamin K quinone and vitamin K 2,3-epoxide, a process essential to sustain γ-carboxylation of vitamin K-dependent proteins. VKOR is also a therapeutic target of warfarin, a treatment for thrombotic disorders. However, the structural and functional basis of vitamin K reduction and the antagonism of warfarin inhibition remain elusive. Here, we identified putative binding sites of both K vitamers and warfarin on human VKOR. The predicted warfarin-binding site was verified by shifted dose-response curves of specified mutated residues. We used CRISPR-Cas9-engineered HEK 293T cells to assess the vitamin K quinone and vitamin K 2,3-epoxide reductase activities of VKOR variants to characterize the vitamin K naphthoquinone head- and isoprenoid side chain-binding regions. Our results challenge the prevailing concept of noncompetitive warfarin inhibition because K vitamers and warfarin share binding sites on VKOR that include Phe55, a key residue binding either the substrate or inhibitor.

Vitamin K epoxide reductase complex subunit 1 (VKORC1): the key protein of the vitamin K cycle.

Vitamin K epoxide, a by-product of the carboxylation of blood coagulation factors, is reduced to vitamin K by an enzymatic system possessing vitamin K epoxide reductase (VKOR) activity. This system is the target of coumarin-derived drugs widely used in thrombosis therapy and prophylaxis. Recently, the key protein of the VKOR system has been identified. The human VKORC1 gene maps to chromosome 16 and consists of 3 exons encoding a 163-amino acid integral ER membrane protein with three or four predicted transmembrane alpha- helices. Expression of human VKORC1 in Spodoptera frugiperda (Sf9) cells and in Pichia pastoris results in enhanced VKOR activity over low endogenous constitutive levels. Sequence based search methods reveal that human VKORC1 belongs to a large family of homologous genes found in vertebrates, insects, plants, protists, archea, and bacteria. All orthologs share five completely conserved amino acids, including two cysteines found in a tetrapeptide motif presumably required for redox function. The recent discovery of the VKORC1 gene has initiated renewed interest in understanding VKOR activity. Analysis of VKORC1 protein structure and function will be crucial in understanding the VKOR catalytic mechanism, how anticoagulant drugs modulate VKOR activity, and the role of VKORC1 in downstream physiological and pathological pathways.

Detection of heterozygous large deletions in the antithrombin gene using multiplex polymerase chain reaction and denatured high performance liquid chromatography.

The present study reports a method for the easy, rapid and cost effective detection of heterozygous large deletions. As a model gene all exons of the antithrombin gene were amplified in a one tube multiplex polymerase chain reaction (PCR) and the products separated according to their size by reverse-phase ion-pair high performance liquid chromatography. A significant reduction in the height of a peak in the probandOs sample compared to in the control indicates the presence of a large deletion of the corresponding allele. Using this approach we identified heterozygous deletions in four patients: the deletions affected exons 1 and 2, exon 7 and the whole antithrombin gene.