Thrombin functions as an inflammatory mediator through activation of its receptor.

A rat model of inflammation was used to investigate the biological effects of thrombin. The thrombin-specific inhibitor Hirulog markedly attentuated the carrageenin-induced edema of the paw of the rat. Injection of thrombin into the paw also produced edema. The effect of thrombin was due to activation of its receptor; a thrombin receptor activating peptide (TRAP) reproduced the effects of thrombin in causing edema. TRAP also increased vascular permeability as demonstrated by extravasation of Evans blue and 125I-labeled serum albumin. The release of bioactive amines played an important role in mediating the TRAP-induced edema; the serotonin/histamine antagonist cryproheptadine and the histamine H2 receptor antagonist cimetidine reduced significantly the edema caused by TRAP. Treatment of rats with the mast cell degranulator 48/80 to deplete these cells of their stores of histamine and serotonin abolished completely the ability of TRAP to produce edema. Histochemical examination confirmed that TRAP treatment led to mast cell degranulation. Thus, it has been possible to demonstrate that thrombin acts as an inflammatory mediator in vivo by activating its receptor, which in turn leads to release of vasoactive amines from mast cells.

Relation between binding and the action of phospholipases A2 on Escherichia coli exposed to the bactericidal/permeability-increasing protein of neutrophils.

Exposure of Escherichia coli to the bactericidal/permeability-increasing protein (BPI) of neutrophils renders the bacterial phospholipids susceptible to hydrolysis by only a few of numerous phospholipases A2 tested. To explore further the determinants of hydrolysis we measured the binding of 125I-labeled phospholipase A2 to E. coli in the presence and absence of BPI. Phospholipases A2 from Aqkistrodon piscivorus piscivorus venom and pig pancreas neither degraded nor bound to BPI-treated E. coli. In contrast, the phospholipases A2 from Aqkistrodon halys blomhoffii and Aqkistrodon halys palas venoms actively hydrolyzed the phospholipids of BPI-treated E. coli: they also bound to E. coli in the presence but not in the absence of BPI. Carbamylation of lysines of the A.h. blomhoffii phospholipase A2 progressively reduced binding in parallel with reduced phospholipid hydrolysis. Both binding and hydrolysis increased with increasing BPI dose. However, maximal binding occurred at 25% of the BPI dose that produced optimal hydrolysis. Thus, binding may be necessary but is not sufficient for maximal BPI-mediated phospholipid hydrolysis. Comparison of the NH2-terminal amino sequences of the active and inactive phospholipase A2 suggests that this portion of the phospholipase A2 molecule plays a role in BPI-independent binding and hydrolysis.

Structure of the hirugen and hirulog 1 complexes of alpha-thrombin.

The isomorphous structures of the hirugen (N-acetylhirudin 53'-64' with sulfato-Tyr63') and hirulog 1 (D-Phe-Pro-Arg-Pro-(Gly)4 desulfato-Tyr63'-hirugen) complexes of human alpha-thrombin have been determined and refined at 2.2 A resolution to crystallographic R-factors of 0.167 and 0.163, respectively. The binding of hirugen to thrombin is similar to that of the binding of the C-terminal dodecapeptide of hirudin, including that of the terminal 3(10) helical turn. The sulfato Tyr63', which, as a result of sulfation, increases the binding affinity by an order of magnitude, is involved in an extended hydrogen bonding network utilizing all three sulfato oxygen atoms. The hirugen-thrombin complex is the first thrombin structure determined to have an unobstructed active site; this site is practically identical in positioning of catalytic residues and in its hydrogen bonding pattern with that of other serine proteinases. Hirulog 1, which is a poor thrombin substrate, is cleaved at the Arg3'-Pro4' bond in the crystal structure. The Arg3' of hirulog 1 occupies the specificity site, the D-Phe-Pro-Arg tripeptide is positioned like that of D-Phe-Pro-Arg chloromethylketone in the active site and the Pro4'(Gly)4 spacer to hirugen is disordered in the structure, as is the 3(10) turn of hirugen. The latter must be related to the simultaneous absence both of sulfation and of the last residue of hirudin (Gln65'). In addition, the autolysis loop of thrombin (Lys145-Gly150) is disordered in both structures. Changes in circular dichroism upon hirugen binding are therefore most likely the result of the flexibility associated with this loop.

Structure-function relationships of hirulog peptide interactions with thrombin.

Using hirudin as a model, a novel class of bivalent thrombin inhibitors has been designed and characterized (Maraganore et al. (1990) Biochemistry 29, 7095-7101). These peptides, designated 'hirulogs', interact with both thrombin's catalytic center and its anion-binding exosite for fibrinogen recognition. In order to investigate structure-activity relationships in hirulog peptides, a number of peptide and peptidomimetic derivatives with alterations in catalytic-site binding and anion-binding exosite binding moieties were prepared. Replacements or modifications in the catalytic site and exosite binding moieties were achieved with the consequences of maintaining or improving antithrombin activity. In addition to showing improved affinity for thrombin, some derivatives with Ki's in the sub-nanomolar range showed increased anticoagulant activities. These findings highlight the versatility of hirulog peptides in their bivalent interactions with thrombin.

Anticoagulant effects of hirulog, a novel thrombin inhibitor, in patients with coronary artery disease.

Selective thrombin inhibitors are a new class of antithrombotic drugs that, unlike heparin, can effectively inhibit clot-bound thrombin and escape neutralization by activated platelets. Hirulog is a 20 amino acid hirudin-based synthetic peptide that has shown promise in experimental models of thrombosis. Little information is available about the effects of hirulog in patients with coronary artery disease. Forty-five patients undergoing cardiac catheterization, who were taking aspirin, were randomized to receive either (1) hirulog, 0.05 mg/kg intravenous bolus followed by 0.2 mg/kg/hour intravenous infusion until the end of the catheterization; (2) hirulog, 0.15 mg/kg intravenous bolus followed by 0.6 mg/kg/hour intravenous infusion; or (3) heparin; 5,000 U intravenous bolus. Serial activated partial thromboplastin time (APTT), prothrombin time, activated clotting time and fibrinopeptide A were measured. Hirulog produced a dose-dependent prolongation of all coagulation parameters; the 0.6 mg/kg/hour dose prolonged the APTT to 218 +/- 50% of baseline after 2 minutes and 248 +/- 50% of baseline after 15 minutes. The half-life of the effect on APTT was 40 minutes. The hirulog blood level correlated well with the APTT, prothrombin time and activated clotting time (r = 0.77, 0.73, and 0.82 respectively, all p < 0.001). Both doses of hirulog potently suppressed the generation of fibrinopeptide A (p < 0.05). There were no major hemorrhagic, thrombotic or allergic complications in patients treated with hirulog or heparin. Thus, hirulog, a direct thrombin inhibitor, provides a predictable level of anticoagulation and appears to have a potent yet well-tolerated anticoagulant profile in patients with coronary artery disease.

Comparison of enzymatic and pharmacological activities of lysine-49 and aspartate-49 phospholipases A2 from Agkistrodon piscivorus piscivorus snake venom.

The basic Lys-49 phospholipase A2 (PLA2) from Agkistrodon piscivorus piscivorus venom is homologous to the basic Asp-49 PLA2 from the same venom as well as other snake venom PLA2 enzymes. It differs, however, in several respects, most important being replacement of the previously invariant Asp-49 at the calcium binding site by Lys, resulting in a reversed order of addition of calcium and phospholipid, phospholipid binding first. Although the preferences for phospholipid substrates of the two enzymes are identical, the apparent Vmax of the Lys-49 PLA2 was only 1.4 to 3% that of the Asp-49 enzyme. Similarly, the Lys-49 PLA2, compared to the Asp-49 PLA2 had less than 3% of the intraventricular lethal potency and 4% of the anticoagulant activity. The intravenous lethal potency of the Lys-49 enzyme was 20% that of the Asp-49 PLA2 and both had little direct hemolytic activity. In contrast, both enzymes were approximately equipotent on the phrenic nerve-diaphragm preparation and on the isolated ventricle strip of the heart. On the cardiac and neuromuscular preparations, the effects of the Asp-49 PLA2 were accompanied by hydrolysis of phosphatidylcholine and phosphatidylethanolamine, whereas no phospholipid hydrolysis was observed with the Lys-49 PLA2. Evaluation of the present results, along with earlier findings using Asp-49 PLA2 enzymes from Naja nigricollis, Hemachatus haemachatus and Naja naja atra venoms, allows us to conclude that: The A. p. piscivorus Asp-49 PLA2 enzyme resembles the Asp-49 enzymes from N. n. atra and H. haemachatus. In contrast, the A. p. piscivorus Lys-49 PLA2 has much lower enzymatic and anticoagulant activities than the Asp-49 enzymes, but equal cardiotoxic and junctional effects. In contrast to some previous suggestions, basic PLA2 enzymes are not necessarily more toxic than neutral or acidic enzymes. Pharmacological effects upon the heart and phrenic nerve-diaphragm preparation correlate neither with in vitro measurements of PLA2 activity nor with actual levels of phospholipid hydrolysis in the heart or diaphragm. This suggests that PLA2 enzymes exert effects independent of phospholipid hydrolysis.

A rapid method for the purification of monomeric and/or dimeric phospholipases A2 in crotalid snake venoms.

We have developed a simple two-step procedure for the separation of monomeric (14,000 mol. wt) and dimeric (28,000 mol. wt) phospholipases A2 from the venoms of Crotalidae family snakes. All venom phospholipases A2 studied thus far exist as monomers under acidic conditions and are chromatographed as such on a column of G-50 Sephadex (superfine) equilibrated in 5% acetic acid. Separation of dimeric phospholipases A2 from any monomeric enzyme(s) in pools of enzyme thus obtained is achieved by chromatography on a second column of G-50 Sephadex (superfine) identical to the first but developed in 1% ammonium bicarbonate. This method has been applied to an investigation of the prevalence of monomeric and/or dimeric enzymes in venoms of the Crotalidae family. The distribution of monomeric and dimeric phospholipases correlates well with phylogeny. In the more primitive Crotalidae genera, such as Trimeresurus and Agkistrodon, monomeric phospholipases A2 are predominant. In the more highly evolved Crotalus genus, only dimeric enzymes are found.

Characterization of the structure and function of three phospholipases A2 from the venom of Agkistrodon halys pallas.

Three monomeric phospholipases A2 with isoelectric points 4.5, 6.9 and 9.3 were purified from the venom of Agkistrodon halys pallas. The complete amino acid sequence of the acidic enzyme and partial amino acid sequences of the neutral and basic phospholipases were determined in order to relate differences in enzymatic reactivities, pharmacologic activities and cytotoxicities to aspects of structure. Studies reported here and elsewhere demonstrate that the three phospholipases A2 exhibit pronounced differences relative to function. The acidic enzyme maintains the highest reactivity toward hydrolysis of monolayers at the air-water interface and may share a feature in common with the acidic enzyme from A. h. blomhoffii, namely the inhibition of platelet aggregation. The neutral phospholipase A2 designated agkistrotoxin, is characterized by potent activity as a pre-synaptic neurotoxin. Agkistrotoxin is the first single polypeptide chain, neurotoxic phospholipase A2 to be documented with a Group II disulfide pattern and, in several respects, may be considered functionally and structurally analogous to notexin from the Australian tiger snake venom. Finally, the basic membranes in the presence of a bactericidal-permeability-increasing protein from neutrophil sources.

Hirulog effect in rat endotoxin shock.

Hirulog is a thrombin catalytic site inhibitor which exhibits specificity for the anionic binding exosite of alpha thrombin. Here, we have evaluated the effect of Hirulog (1, 5 and 10 mg/kg, 30 min pretreatment) in a rat model of endotoxemia. Intravenous injection of lipopolysaccharide from E. coli (25 mg/kg; serotype 0127:B8) caused decreases in blood pressure which were significantly reduced (about 60%) in animals pretreated with Hirulog. Rat survival to endotoxin was significantly increased in Hirulog pretreated group (5 and 10 mg/kg) up to 24 hours. Hirulog at the dose of 10 mg/kg inhibited both endotoxin-induced leukopenia at 30 and 60 minute points and thrombocytopenia at 30 minute point but not at 90 and 120 minute points. Fibrinogen levels were significantly reduced after 2 hours following endotoxin administration. Pretreatment with Hirulog (5-10 mg/kg i.v.) 30 min prior to administration of endotoxin prevented changes in fibrinogen plasma levels. These results demonstrate that Hirulog-induced inhibition of thrombin is effective in reducing toxic and lethal effects of endotoxin.

Hirulog: a direct thrombin inhibitor for management of acute coronary syndromes.

Hirulog therapy has been studied extensively in numerous settings including prevention of DVT, treatment of unstable angina, treatment of acute myocardial infarction during thrombolysis, and prevention of acute complications of PTCA. Being one of the first direct thrombin inhibitors in clinical development, it has had to 'test the waters', so to speak, of the relationship between pathophysiology and clinical trial design: what are the correct indications, patient entry criteria, endopoints, frequency and duration of dosing, and so on? Our findings validate a role for thrombin in treating arterial thromboembolism and demonstrate clinical activity and tolerability of Hirulog.

Thrombin structure and function: why thrombin is the primary target for antithrombotics.

Thrombin has both beneficial and harmful effects. In order of importance, at very low concentrations, alpha-thrombin firstly amplifies its own generation through the activation of factors V and VIII, which are the primary targets of antithrombotic agents. It secondly functions at the cellular level where, at low concentrations it activates platelets, and at higher concentrations, induces endothelial cell changes (e.g., shape changes, albumin transport release of plasminogen activators and other substances). It thirdly converts fibrinogen into clottable fibrin and becomes actively incorporated into the forming thrombus. In addition, it activates protein C, which in turn degrades factors V and VIII (and/or their activated forms) and causes the shutdown of thrombin generation. When compared to other serine proteinases of the blood coagulation and fibrinolytic systems, alpha-thrombin is unique in that it loses most of its proenzyme activation fragment and has developed multisite short-ranged bridge-binding interactions, which appear to explain thrombin specificity. To understand thrombin is to understand haemostasis.

The arterial thrombotic process and emerging drugs for its control.

Unabated, the arterial thrombotic process continues to be a major challenge in the management of acute coronary artery disease. Pharmacologic and mechanical revascularization therapies, which have proliferated over the last decade, remain impeded by arterial thrombosis and its clinical sequelae. New antithrombotic drugs aimed at specific points in the arterial thrombotic process offer the potential for substantial improvements in the management of coronary artery disease. The use of these agents as a mainstay of patient care is becoming a reality as controlled clinical studies test their safety and potential benefits.

The role of lysyl residues of phospholipases A2 in the formation of the catalytic complex.

The aspartyl residue at position 49 in phospholipases A2 (PLA) has been viewed as a component of the catalytic apparatus because of its involvement in binding the essential cofactor, calcium. We recently discovered a new class of PLA's in which, among other changes in highly invariant residues, Asp-49 is replaced by a lysine (Maraganore et al. (1984) J. Biol. Chem. 259, 13839). These Lys-49 PLA's are also calcium-dependent, but, in contrast to the Asp-49 enzymes, they bind phospholipid strongly in the absence of calcium. Lys-49 PLA's are, therefore, ideal for studying structural and mechanistic aspects of these enzymes. Attempts to modify Lys-49 with the amino group-specific reagent, trinitrobenzenesulfonic acid (TNBS) led to the inactivation of the PLA, but reaction occurred not as expected at position 49, but at Lys-53. These findings lead us to propose a model, applicable to PLA's in general, in which cationic side chains at position 53 in these enzymes participate in phospholipid binding on the path to formation of the catalytic complex. This model serves to explain a number of unresolved observations in the current literature relating to enzyme-substrate interactions in the PLA's.

The lysine-49 phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus. Relation of structure and function to other phospholipases A2.

A new class of phospholipases A2 that have a lysine at position 49 differ from the more conventional Asp-49 enzymes with respect to the sequential binding of the essential cofactor, calcium, and the substrate, phospholipid, in the formation of the catalytic complex (Maraganore, J.M., Merutka, G., Cho, W., Welches, W., Kézdy, F.J., and Heinrikson, R.L. (1984) J. Biol. Chem. 259, 13839-13843). We report here the complete amino acid sequence of the Lys-49 enzyme from Agkistrodon piscivorus piscivorus. The sequence was determined by automated Edman degradation of the intact, S-carboxymethylcysteinyl protein and of peptides derived therefrom by cleavage with cyanogen bromide, chymotrypsin, trypsin, and endoproteinase Lys-C. Despite several changes at amino acid residues previously considered to be invariant, the Lys-49 enzymes are homologous to the Asp-49 phospholipases. Homology is especially apparent in the following: 1) the pattern of 14 half-cystine residues, 2) conservation of hydrophobic residues which have been shown to encircle the active site, and 3) conservation of Asp-99 and His-48 which have been implicated in the catalytic reaction itself. These observations together with kinetic and binding data imply that the Lys-49 phospholipases have a catalytic mechanism and a three-dimensional architecture similar to those of the Asp-49 enzymes. Modeling of the Lys-49 enzyme based upon the structure of bovine pancreatic phospholipase reveals that the epsilon-amino group of Lys-49 can fit easily in the calcium-binding site and, moreover, that this orientation of a cationic side chain at position 49 could account for the characteristic and novel feature of the Lys-49 phospholipases, i.e. that they are able to form complexes with phospholipid in the absence of calcium.

Inhibition of coagulation and thrombin-induced platelet activities by a synthetic dodecapeptide modeled on the carboxy-terminus of hirudin.

A synthetic, tyrosine-sulfated, dodecapeptide (BG8865) modeled on residues 53-64 of hirudin was found to elevate the activated partial thromboplastin time (APTT), prothrombin time (PT), and thrombin time (TT) of human plasma in a dose-dependent manner. The most sensitive assay was the TT, which was prolonged 2 and 3 times control values at 2.2 and 4.1 micrograms/mL hirudin peptide, respectively. The sulfated dodecapeptide exhibited no dependency on antithrombin III as monitored by the APTT in the presence of sheep anti-human antithrombin III antibodies, and its activity was not neutralized by platelet releasates or platelet factor 4. In studies of thrombin-induced platelet activation, the hirudin peptide was found to block aggregation, serotonin release and thromboxane A2 generation. At thrombin concentrations of 0.25 U/mL, the IC50 (concentration resulting in 50% inhibition) for inhibition of platelet aggregation was 0.72 micrograms/mL peptide. Inhibition of TXA2 generation and serotonin release correlated closely with inhibition of aggregation. Using platelets from patients with clinically documented heparin-induced thrombocytopenia anticoagulant doses of heparin were found to induce platelet aggregation and thromboxane A2 generation. In sharp contrast, anticoagulant-equivalent doses of hirudin peptide had no effect on patient platelets, as evidenced by a lack of platelet aggregation and thromboxane A2 generation. These data provide compelling in vitro evidence that the hirudin peptide has several potential advantages over heparin, namely effective inhibition of thrombin-induced platelet activities, co-factor independence, insensitivity to endogenous heparin-neutralizing factors, and an apparent lack of direct or immune-mediated platelet stimulating properties.

Affinity labeling of lysine-149 in the anion-binding exosite of human alpha-thrombin with an N alpha-(dinitrofluorobenzyl)hirudin C-terminal peptide.

In order to define structural regions in thrombin that interact with hirudin, the N alpha-dinitrofluorobenzyl analogue of an undecapeptide was synthesized corresponding to residues 54-64 of hirudin [GDFEEIPEEY(O35SO3)L (DNFB-[35S]Hir54-64)]. DNFB-[35S]Hir54-64 was reacted at a 10-fold molar excess with human alpha-thrombin in phosphate-buffered saline at pH 7.4 and 23 degrees C for 18 h. Autoradiographs of the product in reducing SDS-polyacrylamide gels revealed a single 35S-labeled band of Mr approximately 32,500. The labeled product was coincident with a band on Coomassie Blue stained gels migrating slightly above an unlabeled thrombin band at Mr approximately 31,000. Incorporation of the 35S affinity reagent peptide was found markedly reduced when reaction with thrombin was performed in the presence of 5- and 20-fold molar excesses of unlabeled hirudin peptide, showing that a specific site was involved in complex formation. The human alpha-thrombin-DNFB-Hir54-64 complex was reduced, S-carboxymethylated, and treated with pepsin. Peptic fragments were separated by reverse-phase HPLC revealing two major peaks containing absorbance at 310 nm. Automated Edman degradation of the peptide fragments allowed identification of Lys-149 of human thrombin as the major site of DNFB-Hir54-64 derivatization. These data suggest that the anionic C-terminal tail of hirudin interacts with an anion-binding exosite in human thrombin removed 18-20 A from the catalytic apparatus.