Early processing of prothrombin and factor X by the vitamin K-dependent carboxylase

Production of a self-activating CBM-factor X fusion protein in a stable transformed Sf9 insect cell line using high cell density perfusion culture

Factor Xa is a serine protease, whose high selectivity can be used to cleave protein tags from recombinant proteins. A fusion protein comprised of a self-activating form of factor X linked to a cellulose-binding module, saCBMFX, was produced in a stable transformed Sf9 insect cell line. The activity of the insect cell produced saCBMFX was higher than the equivalent mammalian cell produced material. A 1.5 l batch fermentation reached a maximum cell concentration of 1.6 x 10(7) cells ml(-1) and a final saCBMFX concentration of 4 mg l(-1). The production of saCBMFX by this cell line was also analyzed in a 1.5 l perfusion system using an ultrasonic filter as a cell-retention device for flow rates up to 3.5 l day(-1). The cell-retention efficiency of an air backflush mode of acoustic filter operation was greater than 95% and eliminated the need to pump the relatively shear sensitive insect cells. In the perfusion system over 4 x 10(7) Sf9 cells ml(-1) were obtained with a viability greater than 80%. With a doubling of viable cell concentration from 1.5 to 3 x 10(7) cells ml(-1) the saCBMFX production rate was doubled to 6 mg l(-1) day(-1). The saCBMFX volumetric productivity of the perfusion system was higher than the batch fermentations (0.6 mg l(-1) day(-1)) by an order of magnitude.

Truncated gamma-glutamyl carboxylase in rambouillet sheep

A flock of Rambouillet sheep was examined because of increased lamb mortality due to ineffective hemostasis at parturition. Decreased activities of coagulation factors II, VII, IX, and X, and severely reduced hepatic gamma-glutamyl carboxylase activity with adequate vitamin K 2,3 epoxide reductase activity was determined.(1,)(21) Parenteral vitamin K(1) supplementation did not improve vitamin K-dependent coagulation factor activities in 3 affected lambs. Affected lamb gamma-glutamyl carboxylase deoxyribonucleic acid was sequenced, and 4 single nucleotide polymorphisms (SNPs 2-5) of the gamma-glutamyl carboxylase gene were identified. Single nucleotide polymorphism-4 results in an arginine to stop codon (UGA) substitution, which prematurely terminates the peptide at residue 686 (R686Stop). This genotype (GATT/GATT) has a strong association with the coagulopathy observed in clinically affected lambs, P < 0.001. The frequency of SNP-3 in exon 11 (R486H) within the MARC 1.1 database is high in the US sheep population overall. Gamma-glutamyl carboxylase activity in hepatic microsomes from a SNP-3 homozygous lamb lacking the SNP-4 mutation (GACC/GACC) was similar to control sheep homozygous for arginine at 486 and also lacking SNP-4 (TGCC/TGCC), indicating that the R486H does not measurably impact gamma-glutamyl carboxylase activity. The remaining two SNPs (2 and 5) are located within non-coding intron sequences. These 4 SNPs allowed for determining the genotype associated with the observed fatal coagulopathy. Screening for the premature truncation (SNP-4) based on the presence of a Bbv I restriction site in clinically normal lambs but not in the homozygous affected lambs allows for detection of the heterozygous state (GATT/GACC), because carrier animals are clinically normal.

Enhanced gamma-carboxylation of recombinant factor X using a chimeric construct containing the prothrombin propeptide

Factor Xa is the serine protease component of prothrombinase, the enzymatic complex responsible for thrombin generation. Production of recombinant factor X/Xa has proven to be difficult because of inefficient gamma-carboxylation, a critical post-translational modification. The affinities of the vitamin K-dependent propeptides for the gamma-carboxylase vary over 2 logs, with the propeptide of factor X having the highest affinity followed by the propeptides of factor VII, protein S, factor IX, protein C, and prothrombin [Stanley, T. B. (1999) J. Biol. Chem. 274, 16940-16944]. On the basis of this observation, it was hypothesized that exchanging the propeptide of factor X with one that binds the gamma-carboxylase with a reduced affinity would enhance gamma-carboxylation by allowing greater substrate turnover. A chimeric cDNA consisting of the human prothrombin signal sequence and propeptide followed by mature human factor X was generated and stably transfected into HEK 293 cells, and modified factor X was purified from conditioned medium. The results indicate that on average 85% of the total factor X produced with the prothrombin propeptide was fully gamma-carboxylated, representing a substantial improvement over a system that employs the native factor X propeptide, with which on average only 32% of the protein is fully gamma-carboxylated. These results indicate that the affinity of the gamma-carboxylase for the propeptide greatly influences the extent of gamma-carboxylation. It was also observed that regardless of which propeptide sequence is directing gamma-carboxylation (factor X or prothrombin), two pools of factor X are secreted; one is uncarboxylated and a second is fully gamma-carboxylated, supporting the notion that the gamma-carboxylase is a processive enzyme.

Factor X fusion proteins: improved production and use in the release in vitro of biologically active hirudin from an inactive alpha-factor-hirudin fusion protein

Many recombinant proteins are synthesized as fusion proteins containing affinity tags to aid in the downstream processing. After purification, the affinity tag is often removed by using a site-specific protease such as factor Xa (FXa). However, the use of FXa is limited by its expense and availability from plasma. To develop a recombinant source of FXa, we have expressed two novel forms of FXa using baby hamster kidney (BHK) cells as host and the expression vector pNUT. The chimeric protein FIIFX consisted of the prepropeptide and the Gla domain of prothrombin linked to the activation peptide and protease region of FXa, together with a cellulose-binding domain (CBD(Cex)) as an affinity tag. A second variant consisted of the transferrin signal peptide linked to the second epidermal growth factor-like domain and the catalytic domain of FX and a polyhistidine tag. Both FX variants were secreted into the medium, their affinity tags were functional, and following activation, both retained FXa-specific proteolytic activity. However, the yield of the FIIFX-CBD(Cex) fusion protein was 10-fold higher than that of FX-CBD(Cex) and other forms of recombinant FX reported to date. The FXa derivatives were used to cleave two different fusion proteins, including a biologically inactive alpha-factor-hirudin fusion protein secreted by Saccharomyces cerevisiae. After cleavage, the released hirudin demonstrated biological activity in a thrombin inhibition assay, suggesting that this method may be applicable to the production of toxic or unstable proteins. The availability of novel FX derivatives linked to different affinity tags allows the development of a versatile system for processing fusion proteins in vitro.

Co-purification of microsomal epoxide hydrolase with the warfarin-sensitive vitamin K1 oxide reductase of the vitamin K cycle

Vitamin K1 oxide reductase activity has been partially purified from rat liver microsomes. A three-step procedure produced a preparation in which warfarin-sensitive vitamin K1 oxide reductase activity was 118-fold enriched over the activity in intact rat liver microsomes. A major component of the multi-protein mixture was identified as a 50 kDa protein that strongly cross-reacts with antiserum prepared against homogeneous rat liver microsomal epoxide hydrolase. The reductase preparation also had a high level or epoxide hydrolase activity against two xenobiotic epoxide substrates. The K(m) values for hydrolysis by the reductase preparation were similar to those for homogeneous microsomal epoxide hydrolase itself, and the specific hydrolase activities of the reductase preparation were 25-35% of the specific activities measured for the homogeneous hydrolase preparation. Antibodies prepared against homogeneous microsomal epoxide hydrolase inhibited up to 80% of reductase activity of the reductase preparation. Homogeneous microsomal epoxide hydrolase had no vitamin K1 oxide reductase activity. This evidence suggests that microsomal epoxide hydrolase, or a protein that is very similar to it, is a major functional component of a multi-protein complex that is responsible for vitamin K1 oxide reduction in rat liver microsomes.

Assembly of the warfarin-sensitive vitamin K 2,3-epoxide reductase enzyme complex in the endoplasmic reticulum membrane

gamma-Carboxylation of vitamin K-dependent proteins requires a functional vitamin K cycle to produce the active vitamin K cofactor for the gamma-carboxylase which posttranslationally modifies precursors of these proteins to contain gamma-carboxyglutamic acid residues. The warfarin-sensitive enzyme vitamin K epoxide reductase (VKOR) of the cycle reduces vitamin K 2,3-epoxide to the active vitamin K hydroquinone cofactor. Because of the importance of warfarin as an anticoagulant in prophylactic medicine and as a poison in rodent pest control, numerous attempts have been made to understand the molecular mechanism underlying warfarin-sensitive vitamin K 2,3-epoxide reduction. In search for protein components that could be involved in this reaction we designed an in vitro gamma-carboxylation test system where the warfarin-sensitive VKOR produces the cofactor for the gamma-carboxylase. Dissection of this system by chromatographic techniques has identified a member(s) of the glutathione S-transferase gene family as one component of the VKOR enzyme complex in the endoplasmic reticulum membrane. The affinity-purified glutathione S-transferase(s) was sensitive to warfarin but lost its warfarin sensitivity and glutathione S-transferase activity upon association with lipids in the presence of Mn2+ or Ca2+. In the gamma-carboxylation test system, loss of warfarin-sensitive glutathione S-transferase activity coincided with formation of the VKOR enzyme complex. It is proposed that formation of VKOR in the endoplasmic reticulum membrane resembles formation of the lipoxygenase enzyme complex where the glutathione S-transferase-related FLAP protein binds cytosolic lipoxygenase to form a membrane enzyme complex.

Processing and expression of rat and human clotting factor-X-encoding cDNAs

The cDNA encoding clotting factor X, which participates in the middle stage of the blood coagulation cascade was cloned from a rat liver cDNA library. Sequencing of the rat factor-X-encoding cDNA revealed that this vitamin-K-dependent protein has a dibasic Arg-Arg sequence at the propeptide cleavage site, as occurs in other vitamin-K-dependent proteins. Although the human and rat deduced amino acid sequences are remarkably similar (76% identical), they do significantly differ in that human factor-X contains a unique Thr-Arg sequence at the propeptide cleavage site [Fung et al., Proc. Natl. Acad. Sci. USA 82 (1985) 3591-3595], where a dibasic sequence would normally be expected. This specific site is the recognition motif for the endoprotease, furin, which is located in the Golgi apparatus. Both rat and human cDNAs expressed in Cos-1 cells resulted in secretion of a mixture of single- and two-chain forms of factor X. The two-chain forms were devoid of the propeptide and were produced at similar rates by the transfected cells. The efficient processing of human factor X, when compared to rat factor X, may indicate that an additional protease(s), which recognizes the Thr-Arg motif, may be involved in proteolytic processing of the human enzyme.

Evidence for competition between vitamin K-dependent clotting factors for intracellular processing by the vitamin K-dependent gamma-carboxylase

This study demonstrates an apparent competition between newly synthesized precursors of prothrombin and factor X for binding to and processing by the gamma-carboxylase in the ER membrane of hepatocytes. The precursor of factor X appears to exhibit a strong affinity for the carboxylase than the prothrombin precursor. This conclusion is supported by the findings that 1) in normal hepatocytes, the factor X precursor prevents increased prothrombin precursor binding to the ER membrane, 2) increased prothrombin binding to the ER membrane was measured in H4-II-E-C3 Reuber H-35 cells where factor X synthesis is suppressed. The variations in the concentrations of the prothrombin and the factor X precursors that were as associated with the ER membrane correlated with the available prothrombin and factor X substrate pools for the gamma-carboxylase.

Intracellular proteolytic processing of the two-chain vitamin K-dependent coagulation factor X

The vitamin K-dependent clotting factors require posttranslational proteolytic processing before they are secreted by the liver into blood as mature zymogens. For most of these proteins, the sequences around the cleavage sites show a common motif (Arg-X-Lys/Arg-Arg) which define the substrate specificity for the endoprotease furin/PACE of the Golgi apparatus. In this paper, we present data which suggest that an additional Ca(++)-dependent endoprotease(s) is located in the endoplasmic reticulum, and may participate in processing of the two-chain vitamin K-dependent coagulation factor X. The single-chain 70 kDa factor X precursor in microsomes from rat liver was labeled by 14C-gamma-carboxylation and its conversion to a two-chain form followed in an incubation system with microsomal membrane fragments. A Ca(++)-dependent endoprotease(s) converted the factor X precursor to a two-chain form with a light-chain of 21 kDa. The endoprotease(s) showed little reactivity towards release of the factor X propeptide. The data provide new information about the endoprotease system in liver which participates in clotting factor proteolytic processing.

Cloning, sequence and overexpression of NADH peroxidase from Streptococcus faecalis 10C1. Structural relationship with the flavoprotein disulfide reductases

DNA fragments encoding streptococcal NADH peroxidase (NPXase) have been amplified, cloned and sequenced from the genome of Streptococcus (Enterococcus) faecalis 10C1 (ATCC 11700). The NPXase gene (npr) comprises 1341 base-pairs and is preceded by a typical ribosome binding site. Upstream from the structural gene, putative -10 and -35 promoter regions have been identified, as has a possible factor-independent terminator that occurs in 3'-flanking sequences. The deduced relative molecular mass (Mr = 49,551), amino acid composition and isoelectric point of NPXase are in good agreement with previous values obtained with the purified enzyme. In addition, three sequenced peptides totaling approximately 20% of the protein were located in the npr gene product. From the sequencing data the deduced NPXase sequence shares low but significant homology with the flavoprotein disulfide reductase class of enzymes ranging from 21% for glutathione reductase (GRase) to 28% for thioredoxin reductase. Alignment of NPXase to Escherichia coli GRase allowed the identification of three previously reported fingerprints for the FAD, NADP+ and central domains of GRase, in the peroxidase sequence. In addition, Cys42 of NPXase, which is present as an unusual stabilized cysteine-sulfenic acid in the oxidized enzyme, aligns favorably with the charge-transfer cysteine in E. coli GRase, and both residues closely follow FAD-binding folds found near their respective amino termini. Such sequence characteristics can also be seen in mercuric reductase, lipoamide dehydrogenase and trypanothione reductase, suggesting that all these enzymes may have originally diverged from a common ancestor. Sequences that are on average 50% identical with three previously reported peptides of the related streptococcal NADH oxidase were also identified in the NPXase primary structure, suggesting a strong similarity between these flavoenzymes. Using the E. coli phage T7 expression system the npr gene has now been overexpressed in an E. coli genetic background. The resultant overexpressing clone produced a recombinant NPXase that was catalytically active and immunoreactive to NPXase antisera.

The effects of warfarin on HepG2 cells suggest that prothrombin and factor X interact differently with the vitamin K-dependent carboxylase in the secretory pathway

HepG2 cells have been shown to respond to warfarin by 1) enhanced vitamin K-dependent carboxylase activity; 2) enhanced intracellular concentration of the factor X clotting factor precursor and 3) enhanced vitamin K-dependent 14C-labelling of a 74 kDa microsomal protein which has been identified as the factor X precursor. There was no difference in any of these measured parameters whether the cells had been treated for 4 or 24 hours with warfarin. In contrast to the intracellular factor X concentration, the intracellular prothrombin precursor concentration was not affected by the drug which suggests there is a difference in the mechanism of processing of these two clotting factors by HepG2 cells. The data are consistent with the view that warfarin maintains its effect on the vitamin K-dependent carboxylation system in HepG2 cells for 24 hours and support the hypothesis that clotting factor X and prothrombin precursors interact differently with the vitamin K-dependent carboxylase.

Vitamin K-dependent carboxylation in articular chondrocytes

A nonhepatic vitamin K-dependent protein, matrix Gla protein, has recently been identified in cartilage where it may play an important role in the control of mineralization or matrix development. We have investigated the vitamin K cycle in chondrocytes isolated from bovine and rabbit articular cartilage and examined these cells for their ability to synthesize vitamin K-dependent proteins. Chondrocytes were found to have an active vitamin K-dependent carboxylation system. Preincubation of the cells with warfarin resulted in a significant increase in the measured carboxylase activity. Both vitamin K epoxide reductase and DT-diaphorase (EC 1.6.99.2) activity were present indicating that chondrocytes are capable of producing reduced vitamin K1H2, the cofactor for the vitamin K-dependent carboxylase. Specific 14C-labeling of microsomal vitamin K-dependent protein precursors demonstrated synthesis of several vitamin K-dependent proteins by chondrocytes. 35S-labeling of chondrocyte proteins provided evidence that matrix Gla protein is synthesized by these cells.