Cloning and expression in Escherichia coli of a synthetic DNA for hirudin, the blood coagulation inhibitor in the leech

From ancient leech to direct thrombin inhibitors and beyond: New from old

Medicinal leeches have been used in health care since before written history, with widely varying popularity over the centuries. Nowadays, medicinal leech therapy is mainly used in plastic and reconstructive microsurgery, with new interesting potential therapeutic applications in many other diseases. The leech's best-known salivary product, hirudin - one of the most powerful natural anticoagulants - was the only remedy to prevent blood clotting until the discovery of heparin. Starting from hirudin, pharmacological research succeeded in developing new anticoagulants, which represent a cornerstone of prevention and treatment of thromboembolic disease. While we are perhaps on the threshold of a new era of anticoagulation, with the development of FXI and XII inhibitors and direct reversible covalent thrombin inhibitors, which promise to achieve effective anticoagulation without bleeding risk. This review retraces the intriguing journey of these drugs in cardiovascular disease, highlighting the fil rouge that links the ancient leech to the current and oncoming antithrombotic therapy. We think that knowledge of the past is key to understanding and appreciating the present and to seize future opportunities.

Cell-free synthesis of the hirudin variant 1 of the blood-sucking leech Hirudo medicinalis

Synthesis and purification of peptide drugs for medical applications is a challenging task. The leech-derived factor hirudin is in clinical use as an alternative to heparin in anticoagulatory therapies. So far, recombinant hirudin is mainly produced in bacterial or yeast expression systems. We describe the successful development and application of an alternative protocol for the synthesis of active hirudin based on a cell-free protein synthesis approach. Three different cell lysates were compared, and the effects of two different signal peptide sequences on the synthesis of mature hirudin were determined. The combination of K562 cell lysates and the endogenous wild-type signal peptide sequence was most effective. Cell-free synthesized hirudin showed a considerably higher anti-thrombin activity compared to recombinant hirudin produced in bacterial cells.

A factor XIa-activatable hirudin-albumin fusion protein reduces thrombosis in mice without promoting blood loss

Background

Hirudin is a potent thrombin inhibitor but its antithrombotic properties are offset by bleeding side-effects. Because hirudin’s N-terminus must engage thrombin’s active site for effective inhibition, fusing a cleavable peptide at this site may improve hirudin’s risk/benefit ratio as a therapeutic agent. Previously we engineered a plasmin cleavage site (C) between human serum albumin (HSA) and hirudin variant 3 (HV3) in fusion protein HSACHV3. Because coagulation factor XI (FXI) is more involved in thrombosis than hemostasis, we hypothesized that making HV3 activity FXIa-dependent would also improve HV3’s potential therapeutic profile. We combined albumin fusion for half-life extension of hirudin with positioning of an FXIa cleavage site N-terminal to HV3, and assessed in vitro and in vivo properties of this novel protein.

Results

FXIa cleavage site EPR was employed. Fusion protein EPR-HV3HSA but not HSAEPR-HV3 was activated by FXIa in vitro. FVIIa, FXa, FXIIa, or plasmin failed to activate EPR-HV3HSA. FXIa-cleavable EPR-HV3HSA reduced the time to occlusion of ferric chloride-treated murine arteries and reduced fibrin deposition in murine endotoxemia; noncleavable mycHV3HSA was without effect. EPR-HV3HSA elicited less blood loss than constitutively active HV3HSA in murine liver laceration or tail transection but extended bleeding time to the same extent. EPR-HV3HSA was partially activated in citrated human or murine plasma to a greater extent than HSACHV3.

Conclusions

Releasing the N-terminal block to HV3 activity using FXIa was an effective way to limit hirudin’s bleeding side-effects, but plasma instability of the exposed EPR blocking peptide rendered it less useful than previously described plasmin-activatable HSACHV3.

Locally activity-released bifunctional fusion protein enhances antithrombosis and alleviates bleeding risk

Despite the fact that lytic therapy of thromboembolic disorder has been achieved, reocclusion of the damaged vessels and bleeding complication frequently reduce the therapeutic effect. In order to prevent the vessel reocclusion and enhance the therapeutic effect, combining the anticoagulant with the thrombolytic was assumed. Herein, we propose that restraining but locally releasing anticoagulant activity in the vicinity of thrombus is a way to alleviate the bleeding risk. A bifunctional fusion protein, termed as SFH (Staphylokinase (SAK) linked by FXa recognition peptide at N-terminus of Hirudin (HV)), was designed. SFH retained thrombolytic activity but no anticoagulant activity in thrombus-free blood due to the extension of the N-terminus of HV. However, it could locally liberate intact HV and exhibit anticoagulant activity when FXa or fresh thrombus was present. At equimolar dose, both improved antithrombotic and thrombolytic effects of SFH were observed in kappa-carrageenin inducing mouse-tail thrombosis model and rat inferior vena cava thrombosis model, respectively. Moreover, we observed significantly lower bleeding risk in mice and rats treated with SFH than with the mixture of SAK and HV with monitoring TT (P < 0.01), aPTT (P < 0.05) and PT (P < 0.05), and bleeding time (P < 0.05). In conclusion, SFH is a promising bifunctional therapeutic candidate with lower bleeding risk.

Finding of an isoleucine derivative of a recombinant protein for pharmaceutical use

Protein modification generally occurs by addition to the amino acid side-chains of protein at the post-translational stage, for example, by enzymatic or chemical reactions after polypeptide synthesis. Recently, the recombinant hirudin analog CX-397, a potent thrombin inhibitor, was found to contain methylated Ile residues when it was overproduced by Escherichia coli in the absence of amino acids in the culture medium. The Ile derivatives, deduced to be beta-methylnorleucine [betaMeNle; (2S, 3S)-2-amino-3-methylhexanoic acid] by systematic chromatographic analysis, do not appear to be normal post-translational modifications of the protein because Ile has no functional group in its side-chain. We, therefore, propose that betaMeNle is biosynthesized by E. coli, activated by E. coli isoleucyl-tRNA synthetase (IleRS), then incorporated into the overproduced recombinant hirudin analog. The biosynthesis of betaMeNle in E. coli is thought to occur as follows: alpha-ketovalerate is synthesized from alpha-ketobutyrate by three Leu biosynthetic enzymes, alpha-isopropylmalate synthase (IPMS) (EC 4.1.3.12), alpha-isopropylmalate isomerase (ISOM) (EC 4.2.1.33) and beta-isopropylmalate dehydrogenase (IPMD) (EC 1.1.1.85), which have broad substrate specificities. alpha-Ketovalerate is then converted to alpha-keto-beta-methylcaproate by three Ile and Val biosynthetic enzymes, acetohydroxy acid synthase (AS) (EC 4.1.3.18), acetohydroxy acid isomeroreductase (IR) (EC 1.1.1.86) and dihydroxy acid dehydratase (DH) (EC 4.2.1.9). Finally, this is converted to betaMeNle by branched-chain amino acid transaminase (EC 2.6.1.42), one of the Ile and Val biosynthetic enzymes.

Modular design of a novel chimeric protein with combined thrombin inhibitory activity and plasminogen-activating potential

In order to design plasminogen activators with improved thrombolytic properties we sought to construct the bifunctional protein HLS-2 which combines both a plasminogen-activating and an anticoagulative activity. The chimeric protein comprises four elements: a derivative of thrombin inhibitor hirudin, a 6-amino acid spacer, the sequence of plasminogen-activator staphylokinase (Sak), and a 13-amino acid expression tag at the C-terminus. The gene of the fusion protein was obtained by SOE-PCR, cloned into pCANTAB5E, and expressed in E. coli BL21. HLS-2 was purified from periplasmatic extracts and characterized by Western blotting. Plasminogen-activation of HLS-2 and of Sak in equimolar mixtures with plasminogen showed near equivalence as measured by plasmin-mediated cleavage of chromogenic substrate S-2403. For catalytic amounts of plasminogen-activator, however, HLS-2 was less effective by a factor of 1.7. HLS-2 also inhibited both the amidolytic and the fibrinolytic activities of thrombin. Similar concentrations of either commercial HV1 (42 pmol/L) or HLS-2 (250 pmol/L) were required to halve the initial rate of thrombin reaction with fluorogenic substrate Tos-Gly-Pro-Arg-AMC, suggesting the retention of high-affinity inhibition of thrombin by the fusion protein sufficiently strong to substitute anticoagulative comedication during fibrinolytic treatment. The results provide a rationale for further testing the efficacy of HLS-2 for the lysis of platelet-rich arterial blood clots and for the prevention of reocclusion after thrombolysis.

A new reporter gene system suited for cell-free protein synthesis and high-throughput screening in small reaction volumes

The properties of M-hirudin as a new reporter gene system were examined using rabbit reticulocyte lysate for cell-free protein expression. In contrast to the luciferase gene, in vitro translation of M-hirudin is highly robust against changes in concentrations of K+ (and Rb+). In addition, M-hirudin can be detected very sensitively using a reasonably priced fluorimetric thrombin assay. To show that the new reporter gene system is well suited for (u)HTS-applications, cell-free synthesis as well as the fluorimetric assay of M-hirudin were carried out in nanotiter and microtiter plates, respectively.

Anticoagulants and inhibitors of platelet aggregation derived from leeches

Increased life expectancy is associated with aging populations in the developed countries, and we can expect an increased incidence of cardiovascular and inflammatory diseases and cancers. A priority for medical research is to reduce such morbidity. Leeches have been demonstrated to be a useful source of drugs to treat cardiovascular diseases, as they have evolved highly specific mechanisms to feed on their hosts by blocking blood coagulation. Powerful molecules acting at different points in the coagulation cascade or in the inhibition of platelet aggregation have been purified from these animals. Moreover, clinical trials confirm their potential to treat cardiovascular diseases.

Display of functional thrombin inhibitor hirudin on the surface of phage M13

A synthetic gene for hirudin was ligated into phagemid pCANTAB5E. This construct allows production of either soluble hirudin or phage having hirudin displayed on the surface. Similarly, hirudin variants with extensions either at their N- or C-terminus were generated. The genes were expressed in their soluble form in a non-suppressor strain of E. coli. Periplasmatic fractions were evaluated in standard thrombin inhibition assays. Extending hirudin by a single Gln residue at the N-terminus reduces the activity by two orders of magnitude. This suggests that either the terminal amine group makes an important interaction or that steric constraints do not allow additional amino acids here. Only C-terminal extensions maintain most of the thrombin inhibitor activity of r-hirudin. The r-hirudin gene was also expressed on the tips of filamentous phage as a fusion protein with protein III (pIII). The hirudin-pIII fusion protein was detected with anti-hirudin antibody and with anti-E-tag antibody by Western blot analysis. Recombinant phages were shown to bind to immobilized thrombin in a dose-dependent manner. Upon addition of soluble thrombin, recombinant hirudin phages could be eluted specifically. Finally, purified phages carrying displayed r-hirudin were shown to inhibit thrombin in a standard amidolytic assay for thrombin inhibitor activity. These results demonstrate that hirudin can be C-terminally extended without diminishing the antithrombic activity. Beyond that, active hirudin can be displayed on the surface of M13 phage. As a conclusion, applied molecular evolution, i.e. the selection of hirudin-based thrombin inhibitor variants with tailored properties from (partially) randomized peptide pools should now be possible.

Local adenoviral-mediated expression of recombinant hirudin reduces neointima formation after arterial injury

Catalytically active thrombin, acting locally, is thought to mediate neointima formation after arterial injury. We constructed an adenovirus vector, AdHV-1.2, containing a complementary DNA for the thrombin inhibitor hirudin. AdHV-1.2 directed the synthesis and secretion of biologically active hirudin from vascular cells in vitro. In vivo gene transfer of hirudin into smooth muscle cells of injured rat carotid arteries resulted in peak secretion of at least 34+/-23 pg hirudin per vessel per 24 hours, and resulted in a significant (P<0.05) 35% reduction in neointima formation. Systemic partial thromboplastin times were not affected by local hirudin expression. These results support the hypothesis that local thrombin activity contributes to neointima formation after arterial injury and suggest that local delivery of a highly specific antithrombin may constitute an effective intervention for arterial proliferative disease.

Production of biologically active hirudin in plant seeds using oleosin partitioning

A plant oleosin was used as a 'carrier' for the production of the leech anticoagulant protein, hirudin (variant 2). The oleosin-hirudin fusion protein was expressed and accumulated in seeds. Seed-specific expression of the oleosin-hirudin fusion mRNA was directed via an Arabidopsis oleosin promoter. The fusion protein was correctly targeted to the oil body membrane and separated from the majority of other seed proteins by flotation centrifugation. Recombinant hirudin was localized to the surface of oil bodies as determined by immunofluorescent techniques. The oleosin-hirudin fusion protein accumulated to ca. 1% of the total seed protein. Hirudin was released from the surface of the oil bodies using endoprotease treatment. Recombinant hirudin was partially purified through anion exchange chromatography and reverse-phase chromatography. Hirudin activity, measured in anti-thrombin units (ATU), was observed in seed oil body extracts, but only after the proteolytic release of hirudin from its oleosin 'carrier'. About 0.55 ATU per milligram of oil body protein was detected in cleaved oil body preparations. This activity demonstrated linear dose dependence. The oleosin fusion protein system provides a unique route for the large-scale production of recombinant proteins in plants, as well as an efficient process for purification of the desired polypeptide.

Experiments on the possible role of leeches as vectors of animal and human pathogens: a light and electron microscopy study

The presence and survival of pathogens inside the gut of leeches were studied by means of light and electron microscopy. In African leeches from Cameroon, blood was serologically positive for human immunodeficiency virus (HIV) and hepatitis B; blood of Hirudo medicinalis bought in German pharmacies contained up to 11 different species of bacteria. In experiments done at low (3 degrees C) and high (22 degrees, 32 degrees C) temperatures, it was shown that ingested red and white blood cells survive for long periods. The time was prolonged to at least 6 months in cases in which the leeches were stored at 3 degrees C. The same effect occurred with pathogens. Bacteriophages (viruses of bacteria) and bacteria persisted in large numbers for at least 6 months in the gut of experimentally infected leeches. Protozoan parasites such as Toxoplasma gondii, Trypanosoma brucei brucei, or Plasmodium berghei were even capable of reproducing inside the gut of the leech. In the case of Plasmodium parasites, this proceeded at low (3 degrees C) and high (22 degrees C) temperatures until all erythrocytes were used up. These parasites survived as long as the erythrocytes and lymphocytes were of good shape, i.e., around 5-6 weeks p.i. Single stages survived longer, especially at low temperatures. However, electron microscopy studies gave no hint of penetration of such pathogens into the unicellular salivary glands, which would initiate a direct transmission. Such transmission, however, is possible--many fish leeches directly transmit several blood parasites--when the leeches are squeezed during skin attachment or when they are manipulated by dropping salt solution on their backs while they are sucking. Consequently, the leech is a potential vector of many pathogens, especially in regions with an endemic spread of human and/or animal pathogens.

Production of recombinant hirudin by high cell density fed-batch cultivations of a Saccharomyces cerevisiae strain: physiological considerations during the bioprocess design

The conditions for the high cell density fed-batch culture of a Saccharomyces cerevisiae strain producing recombinant hirudin (rHV2-Lys47) have been established. A Leu+ derivative of S. cerevisiae c13ABYS86 was used as the host strain transformed with an expression plasmid containing the gene encoding rHV2-Lys47 and driven by the MF alpha 1 promoter. In order to develop the fed-batch culture protocol, the recombinant strains' physiology was first of all investigated in chemostat culture. The maximum respiratory capacity of the recombinant strain was observed to be between dilution rates of 0.2 and 0.26 h-1, which is typical for laboratory strains as compared to values published for baker's yeasts. Furthermore, maximum biomass yield and product secretion were observed at a dilution rate of approx. 0.15 h-1. The plasmid segregational stability of the recombinant strain showed that the expression plasmid was stable, irrespective of the dilution rates used, for more than 80 generations of growth between dilution rates of 0.043 h-1 and 0.3 h-1. The chemostat data was used to define a fed-batch process. The fed-batch results demonstrated a biomass production of 60 g l-1 CDW and a high production level of recombinant hirudin of 500 mg l-1. Stability of the expression of the gene coding for rHV2-Lys47 was maintained during all the studied fed-batch conditions. The plasmid copy number in the fed-batch remained constant at approx. 43 at a specific growth rate of 0.12 h-1, whereas it increased by 60-95% at a lower dilution rate (mu = 0.06 h-1). Although a variation of the plasmid copy number could be expected, it was postulated from the experimental data that the observed amplification could have been influenced by an environmental effect due to an accumulation of medium components in the supernatant. The results presented here illustrate the importance of a well-balanced medium when considering the production of a recombinant protein in a high cell density cultivation process with high production levels.

Studies on the pharmacokinetics and pharmacodynamics of recombinant hirudin (rHV2-Lys 47) after intravenous and subcutaneous administration in dogs

In recent years, the pharmacological and biochemical characterization of hirudins has taken a major upswing due to the availability of this natural polypeptide in recombinant form. Despite this, the current knowledge on the pharmacokinetics and pharmacodynamics of recombinant hirudin (rH) appears to be incomplete. The present study was designed to investigate the relationship between plasma concentrations of rH with corresponding antithrombin responses after intravenous (i.v.) and subcutaneous (s.c.) administration in dogs. Four male, Mongrel dogs were each injected with an i.v. (bolus) dose (1 mg/kg) of one specific variant of rH, i.e. rH with a lysine residue in position 47 (rHV2-Lys 47). The dogs were injected with a s.c. dose (1 mg/kg) of rHV2-Lys 47 after one week. After each dose, blood was collected at different time intervals, plasma separated and stored at -70 degrees C. Plasma concentrations of rHV2-Lys 47 were determined using an enzyme-linked immunosorbent assay (ELISA) method and pharmacokinetic parameters were determined using standard non-compartmental methods. The ex vivo antithrombin activity of the drug was measured using activated partial thromboplastin time (APTT), calcium-thrombin time (Ca++TT) and a chromogenic anti-IIa assay. The results from this study indicate that the pharmacokinetic behavior of rHV2-Lys 47 is strongly influenced by the route of administration. In all three functional assays used, a significant correlation was obtained after i.v. administration between plasma concentrations and corresponding responses over the time period of the study when compared to s.c. administration. The results are indicative of a probable structural and functional modification of this rH variant after s.c. administration which may be responsible for the altered pharmacokinetics and pharmacodynamics after s.c. dosing.

Alteration of pharmacokinetics and pharmacodynamics of recombinant hirudin (rHV2-Lys 47) after repeated intravenous administration in dogs

Native hirudin is a heterogenous polypeptide obtained from the medicinal leech, Hirudo medicinalis. Recent advances in molecular biological techniques have led to the availability of large amounts of hirudin in the recombinant form. Recombinant hirudins (rH) are currently being investigated for potential use in the prophylaxis and treatment of deep venous thrombosis (DVT), in disseminated intravascular coagulation (DIC) and during cardiovascular bypass surgery. In this study, one specific variant of rH with a lysine residue in position 47 (rHV2-Lys 47) was administered in dogs in a multiple dose regimen of 2 mg/kg (i.v. bolus) for three weeks with a dosing interval of one week. After each dose, blood samples were collected at regular time intervals, plasma separated and stored at -4 degrees C. Concentrations of rHV2-Lys 47 in each sample were determined using an enzyme-linked immunosorbent assay (ELISA). Ex vivo antithrombin responses measured included activated partial thromboplastin time (APTT), calcium-thrombin time (Ca++TT-10 NIH units/ml) and a chromogenic anti-IIa assay. It was the purpose of this study to detect any sensitization or desensitization of antithrombin responses when rHV2-Lys 47 is used in a repeated fashion such as would be expected in the prophylaxis of DVT. The results indicated that there was no attenuation in the responses; however, there was a sensitization of response as measured by the Ca++TT (10 NIH units/ml). These findings could have major implications in the clinical use of rH where this drug is expected to be used in a multiple dose regimen.

Hirudins: antithrombin anticoagulants

OBJECTIVE

To review the chemistry, pharmacology, available clinical data, and adverse effects of the hirudin anticoagulants.

DATA SOURCES

A MEDLINE search and a review of recent scientific abstracts was conducted to identify pertinent literature.

STUDY SELECTION

Focus was placed on studies conducted in humans. Because hirudin is still an investigational agent, however, relevant animal data, particularly pharmacokinetic studies and studies of preclinical efficacy, were also selected.

DATA EXTRACTION

Data from both human and animal studies were evaluated; emphasis was placed on human trials.

DATA SYNTHESIS

Hirudin has demonstrated potent anticoagulant effects. Although hirudin could have a significant impact on the therapeutic management of patients requiring anticoagulant therapy, only a limited number of human studies have been published to date. Trials comparing hirudin and heparin in specific patient populations are still ongoing.

CONCLUSIONS

Although still in clinical trials, hirudin is a unique agent that may represent a breakthrough in anticoagulant therapy. The specific role that this agent will play in the management of patients has yet to be determined.

Analysis of recombinant proteins by isoelectric focusing in immobilized pH gradients

Isoelectric focusing in immobilized pH gradients (IEF-IPG) was used to analyze three different recombinant proteins. Recombinant leech hirudin (65 amino acids, three disulfide bonds) expressed in Saccharomyces cerevisiae as a secreted protein and purified by anion-exchange and reversed-phase chromatography proved to be homogeneous with regard to its isoelectric point (pI). In addition, the theoretical pI, calculated on the basis of the primary structure, corresponded precisely to the measured pI of 4.30. IEF-IPG was further employed to follow the stability of recombinant hirudin at pH 9, indicating that deamidation occurred under these conditions. A variant of recombinant human alpha 1-antitrypsin (AAT) (389 amino acids, one cysteine residue) expressed in Escherichia coli and purified by anion-exchange, metal chelate and hydrophobic-interaction chromatography appeared to be homogeneous by polyacrylamide gel electrophoresis under reducing and denaturing conditions as well as by various high performance liquid chromatography methods. However, some heterogeneity was detected by IEF-IPG between pH 5-6. The measured pI values of 5.43-5.58 were slightly lower than the calculated pI based on the primary structure (5.72). This indicated deamidations of Asn or Gln residues. A recombinant Schistosoma mansoni parasite antigen, p28 (210 amino acids, one cysteine residue) obtained after intracellular expression in Saccharomyces cerevisiae and affinity purification on glutathione agarose was analyzed by IEF-IPG in a pH 7.3-8.3 gradient. It appeared to be heterogeneous with regard to its pI, with the major component having a pI of 7.81 compared to the calculated value of 7.17.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization, stability and refolding of recombinant hirudin

A recombinant variant of hirudin, the blood-clotting inhibitor of the leech Hirudo medicinalis, has been characterized employing spectroscopic and hydrodynamic techniques. Conditions have been defined for efficient reconstitution of the native, disulfide-bonded inhibitor from completely unfolded, reduced polypeptide chains. The spectral properties of the native inhibitor are consistent with previous results on the solution structure of hirudin. Extremely low circular dichroism in the far ultraviolet ([theta]Mr,220 nm = -8 +/- 1 x 10(2) deg.cm2.dmol-1) indicates a very low content of regular secondary structure. Although both tyrosine residues of the recombinant inhibitor titrate around pH 10.6, typical for solvent-exposed tyrosines, fluorescence emission and near-ultraviolet circular dichroism suggest that at least one of the tyrosines is partially shielded from solvent quench, and immobilized in an asymmetric environment. Reversible thermal unfolding of hirudin around 65 degrees C is indicated by the disappearance of its dichroic absorption in the near ultraviolet and by a fourfold increase in ellipticity at 225 nm. The transition can be approximated by a two-state model with a transition enthalpy of delta Hvan't Hoff = 159 kJ/mol and a transition entropy of 464 J.mol-1.K-1. Reduced hirudin at room temperature is largely unfolded and inactive as an inhibitor of thrombin assayed with a low-molecular-mass substrate. Refolding and reoxidation are observed at alkaline pH in the presence of a mixture of glutathione and glutathione disulfide. Spectroscopy, thrombin inhibition, and reversed-phase HPLC indicate reconstitution yields close to 100% and that the reconstituted inhibitor is identical to the native starting material.

Secretion of biologically active leech hirudin from baculovirus-infected insect cells

The thrombin inhibitor, hirudin, from the leech Hirudo Medicinalis, is the most powerful natural anticoagulant known. It has been characterized as a polypeptide of 65 amino acids which exhibits its anticoagulant properties by binding tightly and specifically to alpha-thrombin. The potency and specificity of hirudin have generated interest on its possible use in the treatment or prophylaxis of various thrombotic diseases. We have used the baculovirus expression system to efficiently produce active hirudins in insect cells. The Autographa californica nuclear polyhedrosis virus has proved useful as a helper-independent viral expression vector for high-level production of recombinant proteins in cultured insect cells. Hirudin variants (HV1 and HV2) were produced in infected insect cells as secreted proteins by joining their coding sequences to the leader peptide sequence of the vescicular stomatitis virus G protein. The recombinant products were biologically active and, interestingly, N-terminal sequencing of HV1 revealed that the heterologous leader peptide is correctly removed.

Selective inhibition by a synthetic hirudin peptide of fibrin-dependent thrombosis in baboons

To determine the importance of the thrombin substrate recognition exosite for fibrinogen binding in the formation of both arterial and venous thrombi, we evaluated the antithrombotic effects of the tyrosine-sulfated dodecapeptide from residues 53-64 of hirudin (H peptide) in a nonhuman primate model. This peptide was studied because it inhibits thrombin cleavages of fibrinogen by simple competition without blocking enzyme catalytic-site function. When an exteriorized arteriovenous access shunt model was used in baboons (Papio anubis), thrombus formation was induced by placing a thrombogenic device made of (i) a segment of tubing coated covalently with type I collagen, which generated platelet-rich thrombi under arterial flow conditions, and (ii) two subsequent annular regions of flow expansion that produced fibrin-rich thrombi typically associated with venous valves and veins. Thrombus formation was quantified by measurements of 111In-labeled platelet and 125I-labeled fibrinogen deposition in both arterial-flow and venous-flow portions of the device. Continuous infusion of H peptide (0.5, 15, and 75 mg/kg) proximal to the device for 40 min interrupted, in a dose-response fashion, formation of fibrin-rich thrombus in the regions of disturbed flow and generation of fibrinopeptide A. In contrast, H peptide did not inhibit the capacity of platelets to deposit on the collagen surface (P greater than 0.2 at all doses) or to form hemostatic plugs (as assessed by measurements of bleeding time; P greater than 0.1 at all doses). These findings suggest that, by competitive inhibition of fibrinogen binding to thrombin, fibrin-rich venous-type thrombus formation may be selectively prevented. This strategy may be therapeutically attractive for preserving normal platelet function when conventional anticoagulant therapy is contraindicated.

Effect of recombinant hirudin, a specific inhibitor of thrombin, on endotoxin-induced intravascular coagulation and acute lung injury in pigs

We hypothesized that thrombin activation may play a prominent role in endotoxin-induced secondary organ failure, such as acute lung injury. To test this hypothesis, we administered a thrombin-specific inhibitor, recombinant hirudin, in endotoxemic pigs. The pigs were anesthetized, mechanically ventilated, and prepared with Swan-Ganz and extravascular lung water (EVLW) catheters. A total of 18 randomly selected animals received a pretreatment of 1,000 U/kg of hirudin, followed by a continuous infusion over 6 h of 500 U/kg/h given simultaneously with the infusion of 10 micrograms/kg/h of Salmonella abortus equi endotoxin. Another 18 animals received a continuous infusion over 6 h of endotoxin but did not receive hirudin. All animals were fluid resuscitated with 17 ml/kg/h of saline for the duration of the experiment. Data are expressed as the mean (95% confidence interval). Hirudin reduced the endotoxin-induced consumption of plasma fibrinogen from -110 (-138 to -82) mg/100 ml to -39 (-67 to -12) mg/100 ml (p = 0.0001) and endotoxin-induced increases in the soluble fibrin in plasma from 434 (369 to 499) ng/ml to 236 (171 to 300) ng/ml (p = 0.0002). These data suggest an effective inhibition of the endotoxin-generated thrombin by hirudin. Furthermore, hirudin significantly reduced endotoxin-induced increases in pulmonary vascular resistance from 32 (27 to 37) kdyn x s x cm-5 x kg to 20 (15 to 25) kdyn x s x cm-5 x kg (p = 0.0015) and increases in EVLW from 15.4 (13.2 to 17.6) ml/kg to 12.2 (10.0 to 14.4) ml/kg (p = 0.0299).(ABSTRACT TRUNCATED AT 250 WORDS)

Use of fragments of hirudin to investigate thrombin-hirudin interaction

Site-directed mutagenesis was used to create hirudin in which Asn52 was replaced by methionine. Cyanogen bromide cleavage at this unique methionine resulted in two fragments. These fragments have been used to study the kinetic mechanism of the inhibition of thrombin by hirudin and to identify areas of the two molecules which interact with each other. The binding of the C-terminal fragment (residues 53-65) to thrombin resulted in a decrease in the Michaelis constant for the substrate D-phenylalanylpipecolylarginyl-p-nitroanilide (DPhe-Pip-Arg-NH-Ph). The N-terminal fragment (residues 1-52) was a competitive inhibitor of thrombin. There was a small amount of cooperativity in the binding of the two fragments. Whereas hirudin and its C-terminal fragment protected alpha-thrombin against cleavage by trypsin, the N-terminal fragment did not. Hirudin and the N-terminal fragment completely prevented the cleavage of alpha-thrombin by pancreatic elastase while the C-terminal fragment afforded a lesser degree of protection. The results of these experiments with trypsin and elastase are discussed in terms of interaction areas on thrombin and hirudin.

Contribution of the N-terminal region of hirudin to its interaction with thrombin

Hirudin is a 65-residue polypeptide that specifically inhibits thrombin by forming a tight, noncovalent complex with the enzyme. The role of the two amino-terminal valine residues and the N-terminal alpha-amino group of hirudin in the formation of the complex has been investigated by site-directed mutagenesis and chemical modification. Replacement of the two N-terminal valyl residues of recombinant hirudin by polar amino acids resulted in an increase in the inhibition constant (KI). In contrast, replacement of these residues by hydrophobic amino acids had little effect on the value for KI. These results demonstrated that the hydrophobic nature of the N-terminal residues of hirudin was important for its interaction with thrombin. Addition of a single amino acid to the N-terminus of hirudin resulted in a marked increase in the value of KI. A similar effect was observed when the positive charge of the alpha-amino group was removed by acetylation. In contrast, amidination of this group, which preserves the positive charge, resulted in a less pronounced increase in the value of KI. Thus, it appears that a positive charge immediately adjacent to the N-terminal hydrophobic residue is required for optimal binding to thrombin.

Isolation of recombinant hirudin by preparative high-performance liquid chromatography

The purification of recombinant hirudin variant 2-Lys47 (rHV2-Lys47), produced by a genetically engineered yeast strain, is described. rHV2-Lys47 expressed and secreted into the culture medium was the starting material for the purification process of hirudin from the culture broth after cell harvesting by centrifugation. Initial purification of the product by preparative reversed-phase high-performance liquid chromatography (HPLC) using step-gradient elution, followed by precipitation of rHV2-Lys47 in the presence of acetone, removed most of the contaminants from the culture medium. The pure product was obtained by successive preparative anion-exchange and reversed-phase HPLC on silica based stationary phases. Characterization of the final product by analytical HPLC, isoelectric focusing gel electrophoresis, quantitative amino acid composition and sequence analysis did not reveal any contaminants. Liquid secondary ion mass spectrometry was used to confirm its primary structure. The isolated product was tested in an inhibition assay of human alpha-thrombin and proved to be fully active.

Interaction of hirudin with thrombin: identification of a minimal binding domain of hirudin that inhibits clotting activity

Hirudin, isolated from the European leech Hirudo medicinalis, is a potent inhibitor of thrombin, forming an almost irreversible thrombin-hirudin complex. Previously, we have shown that the carboxyl terminus of hirudin (residues 45-65) inhibits clotting activity and without binding to the catalytic site of thrombin. In the present study, a series of peptides corresponding to this carboxyl-terminal region of hirudin have been synthesized, and their anticoagulant activity and binding properties to thrombin were examined. Binding was assessed by their ability to displace 125I-hirudin 45-65 from Sepharose-immobilized thrombin and by isolation of peptide-thrombin complexes. We show that the carboxyl-terminal 10 amino acid residues 56-65 (Phe-Glu-Glu-Ile-Pro-Glu-Glu-Tyr-Leu-Gln) are minimally required for binding to thrombin and inhibition of clotting. Phe-56 was critical for maintaining anticoagulant activity as demonstrated by the loss of activity when Phe-56 was substituted with D-Phe, Glu, or Leu. In addition, we found that the binding of the carboxyl-terminal peptide of hirudin with thrombin was associated with a significant conformational change of thrombin as judged by circular dichroism. This conformational change might be responsible for the loss of clotting activity of thrombin.

The design of synthetic genes

Computer programs are described that aid in the design of synthetic genes coding for proteins that are targets of a research program in site directed mutagenesis. These programs "reverse-translate" protein sequences into general nucleic acid sequences (those where codons have not yet been selected), map restriction sites into general DNA sequences, identify points in the synthetic gene where unique restriction sites can be introduced, and assist in the design of genes coding for hybrids and evolutionary intermediates between homologous proteins. Application of these programs therefore facilitates the use of modular mutagenesis to create variants of proteins, and the implementation of evolutionary guidance as a strategy for selecting mutants.

Regen: program for designing gene assembly

One of the main problems in constructing synthetic genes is the incorrect hybridisation between the oligonucleotides. The problem is resolved if the sequence uniquely defines the position of the oligonucleotide in the assembled gene. This can be accomplished through the wise partition of dsDNA sequence in the fragments. We describe a program for use in designing such gene assembly. For a given DNA sequence and the approximate location of oligonucleotide boundary it generates all sets of protruding ends that share the smallest homology.

Effect of heparin on the interaction between thrombin and hirudin

The effect of heparin on the interaction between thrombin and hirudin has been examined by kinetic methods. Three forms of heparin fractionated on the basis of their affinity for antithrombin III and unfractionated heparin were found to act as noncompetitive inhibitors of the formation of the thrombin-hirudin complex. A three--four fold increase in the dissociation constant of the complex was observed at saturating heparin concentrations. This increase in the dissociation constant was due to a twofold decrease in the rate of association of thrombin and hirudin together with a similar increase in the rate of dissociation of the complex. Implications for the location of the heparin binding site on thrombin and the possible therapeutic use of the hirudin are discussed.