Biphasic effect on platelet aggregation by phospholipase a purified from Vipera russellii snake venom

The role of platelets in hemostasis and the effects of snake venom toxins on platelet function

The human body has a set of physiological processes, known as hemostasis, which keeps the blood fluid and free of clots in normal vessels; in the case of vascular injury, this process induces the local formation of a hemostatic plug, preventing hemorrhage. The hemostatic system in humans presents complex physiological interactions that involve platelets, plasma proteins, endothelial and subendothelial structures. Disequilibrium in the regulatory mechanisms that control the growth and the size of the thrombus is one of the factors that favors the development of diseases related to vascular disorders such as myocardial infarction and stroke, which are among the leading causes of death in the western world. Interfering with platelet function is a strategy for the treatment of thrombotic diseases. Antiplatelet drugs are used mainly in cases related to arterial thrombosis and interfere in the formation of the platelet plug by different mechanisms. Aspirin (acetylsalicylic acid) is the oldest and most widely used antithrombotic drug. Although highly effective in most cases, aspirin has limitations compared to other drugs used in the treatment of homeostatic disorders. For this reason, research related to molecules that interfere with platelet aggregation are of great relevance. In this regard, snake venoms are known to contain a number of molecules that interfere with hemostasis, including platelet function. The mechanisms by which snake venom components inhibit or activate platelet aggregation are varied and can be used as tools for the diagnosis and the treatment of several hemostatic disorders. The aim of this review is to present the role of platelets in hemostasis and the mechanisms by which snake venom toxins interfere with platelet function.

Proteomic Analyses of Agkistrodon contortrix contortrix Venom Using 2D Electrophoresis and MS Techniques

Snake venom is a complex mixture of proteins and peptides which in the Viperidae is mainly hemotoxic. The diversity of these components causes the venom to be an extremely interesting object of study. Discovered components can be used in search for new pharmaceuticals used primarily in the treatment of diseases of the cardiovascular system. In order to determine the protein composition of the southern copperhead venom, we have used high resolution two dimensional electrophoresis and MALDI ToF/ToF MS-based identification. We have identified 10 groups of proteins present in the venom, of which phospholipase A₂ and metalloprotease and serine proteases constitute the largest groups. For the first time presence of 5′-nucleotidase in venom was found in this group of snakes. Three peptides present in the venom were also identified. Two of them as bradykinin-potentiating agents and one as an inhibitor.

Haemostatically active proteins in snake venoms

Snake venom proteins that affect the haemostatic system can cause (a) lowering of blood coagulability, (b) damage to blood vessels, resulting in bleeding, (c) secondary effects of bleeding, e.g. hypovolaemic shock and organ damage, and (d) thrombosis. These proteins may, or may not, be enzymes. We review the data on the most relevant haemostatically active proteinases, phospholipases A₂, L-amino acid oxidases and 5'-nucleotidases from snake venoms. We also survey the non-enzymatic effectors of haemostasis from snake venoms--disintegrins, C-type lectins and three-finger toxins. Medical applications have already been found for some of these snake venom proteins. We describe those that have already been approved as drugs to treat haemostatic disorders or are being used to diagnose such health problems. No clinical applications, however, currently exist for the majority of snake venom proteins acting on haemostasis. We conclude with the most promising potential uses in this respect.

Characterization of a human coagulation factor Xa-binding site on Viperidae snake venom phospholipases A2 by affinity binding studies and molecular bioinformatics

Background

The snake venom group IIA secreted phospholipases A₂ (SVPLA₂), present in the Viperidae snake family exhibit a wide range of toxic and pharmacological effects. They exert their different functions by catalyzing the hydrolysis of phospholipids (PL) at the membrane/water interface and by highly specific direct binding to: (i) presynaptic membrane-bound or intracellular receptors; (ii) natural PLA₂-inhibitors from snake serum; and (iii) coagulation factors present in human blood.

Results

Using surface plasmon resonance (SPR) protein-protein interaction measurements and an in vitro biological test of inhibition of prothrombinase activity, we identify a number of Viperidae venom SVPLA₂s that inhibit blood coagulation through direct binding to human blood coagulation factor Xa (FXa) via a non-catalytic, PL-independent mechanism. We classify the SVPLA₂s in four groups, depending on the strength of their binding.

Molecular electrostatic potentials calculated at the surface of 3D homology-modeling models show a correlation with inhibition of prothrombinase activity. In addition, molecular docking simulations between SVPLA₂ and FXa guided by the experimental data identify the potential FXa binding site on the SVPLA₂s. This site is composed of the following regions: helices A and B, the Ca²⁺ loop, the helix C-β-wing loop, and the C-terminal fragment. Some of the SVPLA₂ binding site residues belong also to the interfacial binding site (IBS). The interface in FXa involves both, the light and heavy chains.

Conclusion

We have experimentally identified several strong FXa-binding SVPLA₂s that disrupt the function of the coagulation cascade by interacting with FXa by the non-catalytic PL-independent mechanism. By theoretical methods we mapped the interaction sites on both, the SVPLA₂s and FXa. Our findings may lead to the design of novel, non-competitive FXa inhibitors.

Snake venoms and hemostasis

Snake venoms are complex mixtures of biologically active proteins and peptides. Many of them affect hemostasis by activating or inhibiting coagulant factors or platelets, or by disrupting endothelium. Based on sequence, these snake venom components have been classified into various families, such as serine proteases, metalloproteinases, C-type lectins, disintegrins and phospholipases. The various members of a particular family act selectively on different blood coagulation factors, blood cells or tissues. For almost every factor involved in coagulation or fibrinolysis there is a venom protein that can activate or inactivate it. Venom proteins affect platelet function by binding or degrading vWF or platelet receptors, activating protease-activated receptors or modulating ADP release and thromboxane A2 formation. Some venom enzymes cleave key basement membrane components and directly affect capillary blood vessels to cause hemorrhaging. L-Amino acid oxidases activate platelets via H2O2 production.

Russell's viper snakebite in Taiwan: differences from other Asian countries

Formosan Russell's viper (Daboia russelli siamensis) is the sixth most frequent cause of snakebite in Taiwan. Its venom has been thought to have both neurotoxic and hematoxic properties. This viper's snakebite is rare and thus scarcely subjected to systemic studies. In this paper, we retrospectively analyzed and described 18 cases of viper snakebite from 1987 to 1999. Like that of the Russell's viper snakebite in other South East Asian areas, varied degrees of acute renal failure, incoagulable blood with bleeding diathesis and hemolysis were the major symptoms found in the systemic envenoming patients. Systemic thrombosis seems to be the distinguishing feature in Formosan Russell's viper snakebite. Neither symptoms nor signs of neuromuscular junction blocking effects were observed, which is another difference from symptoms observed after bites of some other Russell's viper subspecies, suggesting a significant geographic variation. These findings confirmed the clinical importance of Russell's viper snakebite in Taiwan.

Role of catalytic function in the antiplatelet activity of phospholipase A2 cobra (Naja naja naja) venom

Three acidic phospholipases A2 from Indian cobra (Naja naja naja) venom inhibited platelet aggregation in platelet rich plasma induced separately by ADP, collagen and epinephrine with different potencies. The order of inhibition was epinephrine > collagen > ADP. They did not inhibit platelet aggregation induced by arachidonic acid (10 microM). The inhibition was dependent on concentration of the protein and the time of incubation of the phospholipases A2 with platelet rich plasma. Parabromophenacyl bromide modified PLA2 enzymes lost their enzymatic activity as well as platelet aggregation inhibition activity suggesting the involvement of catalytic function in platelet aggregation inhibitory activity.

Hemostatic disturbances observed in patients with snakebite in south China

To investigate the hematological disorders after snakebite, we measured the maximum platelet aggregation rate (MAR), antithrombin III (AT-III) activity, alpha(2)-plasmin inhibitor (alpha(2)-PI) activity, concentration of fibrinogen (Fg) and fibrin degradation products (FDP) in 25 samples from 17 patients with snakebite in south China. The results obtained in the patients before application of antivenom and patients with Ophiophagus hannah (Oh.) bite were as follows: (1) the mean MAR values were significantly decreased in the case of the snakebites from Vipera russellii (Vr.) and Trimeresurus mucrosquamatus (Tm.); (2) the mean activities of AT-III were decreased in all patients in the present study; 3) the mean activities of alpha(2)-PI were significantly decreased in patients bitten by Deinagkistrodon acutus (Da.), Agkistrodon halys (Ah.), Vr., Trimeresurus stejnegeri (Ts.), Tm. and Naja naja atra (Nn.); (4) the mean concentrations of Fg were markedly decreased in patients bitten by Da., Ah., Vr., Ts. and Tm.; and (5) the mean levels of FDP were significantly increased in cases of Da., Vr. and Ts. bite, but not in Ah., Tm., Nn. and Oh. bite. The results of the present study indicate that disorders of platelet aggregation and the coagulation-fibrinolysis system are liable to occur in patients with snakebite from Da., Ah., Vr., Ts., Tm. and Nn. Furthermore, it appeared that disseminated intravascular coagulation (DIC) was evoked in some patients. Specific antivenom was found to be useful for improving the hemostatic disturbances after snakebite from Ah. and Nn.

Phospholipase A(2) with platelet aggregation inhibitor activity from Austrelaps superbus venom: protein purification and cDNA cloning

Four phospholipase A(2) (PLA(2)) enzymes (Superbins a, b, c, and d) with varying platelet aggregation inhibitor activities have been purified from Austrelaps superbus by a combination of gel filtration, ion-exchange, and reversed-phase high-pressure liquid chromatography. Purity and homogeneity of the superbins have been confirmed by high-performance capillary zone electrophoresis and mass spectrometry. The electron spray ionization mass spectrometry data showed that their molecular masses range from 13,140 to 13,236 Da. Each of the proteins has been found to be basic and exhibit varying degrees of PLA(2) activity. They also displayed different platelet aggregation inhibitory activities. Superbin a was found to possess the most potent inhibitory activity with an IC(50) of 9.0 nM, whereas Superbin d was found to be least effective with an IC(50) of 3.0 microM. Superbins b and c were moderately effective with IC(50) values of 0.05 and 0.5 microM, respectively. The amino-terminal sequencing confirmed the identity of these superbins. cDNA cloning resulted in the identification of 17 more PLA(2) isoforms in A. superbus venom. It has also provided complete information on the precursor PLA(2). The precursor PLA(2) contained a 27-amino-acid signal peptide and 117- to 125-amino-acid PLA(2) (molecular mass ranging from 13,000 to 14,000 Da). Two of these PLA(2) enzymes resembled more closely (87%) Superbin a in structure. Two unique PLA(2) enzymes containing an extra pancreatic loop also have been identified among the isoforms.

Sea snake Hydrophis cyanocinctus venom. I. Purification, characterization and N-terminal sequence of two phospholipases A2

Two phospholipases A2 (PLA2, H1 and H2) from sea snake Hydrophis cyanocinctus venom were purified to homogeneity in a single step using reversed-phase high performance liquid chromatography on a Nucleosil 7C18 column. The molecular weights of H1 and H2, as estimated by MALDI MS, were 13588.1 and 13247.2 Da, respectively. The N-terminal 60 amino acid residues were determined by direct automated Edman degradation analysis. Since both PLA2s show close sequence homologies to those of PLA2s from other Elapid snakes (60-84%) they have been tentatively classified as belonging to group-IA and Asp-49 phospholipases A2. Despite the sequence variation (18%) between H1 and H2, their general structural organization is very similar as shown by their clearly related CD spectra. Furthermore, both enzymes are quite thermostable (60-65 degrees C) as determined by temperature variable CD spectra, indicating that the enzymes contain compact folded structure, mainly based on the core structure of disulfide bridges. However, the major PLA2 (H1) shows higher toxicity to albino rats (LD50 i.p. 0.04 mg/kg) and purification resulted in 18-fold increase in toxicity over the crude or whole venom (LD50 i.p. 0.80 mg/kg). H1 also shows edema-inducing and indirect haemolytic but no haemorrhagic activity. Unlike the toxic PLA2-H1, enzyme H2 was not toxic to albino rats but showed edema-inducing and indirect haemolytic activities.

Pharmacological effects of the leaf-nosed viper snake (Eristocophis macmahoni) venom and its HPLC fractions

Crude venom from Eristocophis macmahoni was demonstrated to exert a potent inhibition of human blood platelet aggregation mediated by adenosine diphosphate (ADP), platelet activating factor (PAF) and arachidonic acid (AA). The venom caused lysis of the platelets, however, the red blood cells were not lysed by the venom. Substantial oedema was produced upon injection of the venom into the rat hind paw. Contrarily, the intraperitoneal injection of the venom to the rats caused an inhibition of the carrageenin-induced rat paw oedema. However, an 100% lethality within 24 h was observed with a dose of 40 mg/kg body weight. The venom was fractionated by reverse phase high pressure liquid chromatography (HPLC) and the fractions were analyzed for their effect on ADP-induced platelet aggregation. The fraction eluted at 15.5 min (20% acetonitrile concentration) exhibited an inhibitory effect of several-fold greater potency than that of the crude venom. Fractions eluted at 18.5 min (25.4% acetonitrile concentration) and onward showed a proaggregatory but insignificant effect. It is suggested that although the venom contains pro aggregatory components, inhibition of platelet aggregation seems to be its predominant activity.

Snake venom hemorrhagins

Viperine and crotaline snake venoms contain one or more hemorrhagic principles called hemorrhagins. These are zinc-containing metalloproteases characterized by the presence of a protease domain, with additional domains in some of them. They act essentially by degrading the component proteins of basement membrane underlying capillary endothelial cells. The toxins also act on these cells causing lysis or drifting apart, resulting in hemorrhage per rhexis or per diapedesis. Some of these toxins have been found to exert additional effects such as fibrinogenolysis and platelet aggregation that facilitate hemorrhage. The structural and functional features of this class of toxins have been discussed in this review in an attempt to get a better understanding of their toxicity. This can be of immense therapeutic value in the management of snake venom poisoning, as hemorrhagins are among the major lethal factors in snake venom.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.

Purification and preliminary characterisation of praelongin phospholipases, antiplatelet agents from the snake venom of Acanthophis praelongus

Three praelongin phospholipases were chromatographically purified from the snake venom of Acanthophis praelongus. The purity and homogeneity of the praelongins were assessed by RP-HPLC, HPCE and mass spectrometry. The purified enzymes, praelongins 2bIII, 2cII and 2cIV were found to have phospholipase A2 activities with specific activities of 31.4 +/- 0.4, 326.1 +/- 10.2 and 362.5 +/- 12.0 U/mg, respectively. Mass spectrometry studies showed the molecular mass of praelongin 2bIII to be 12,782.9 +/- 2.6 and praelongins 2cII and 2cIV to have very similar molecular mass values, 12,971.4 +/- 4.5 and 12,971.9 +/- 3.6, respectively. However, platelet aggregation studies showed the praelongins to display different IC50 values, 180 microM for praelongin 2cII and 55 microM for praelongin 2cIV; praelongin 2bIII was found to be a more potent antiplatelet agent, having an IC50 of 0.65 microM. Praelongins 2bIII, 2cIV and 2cII were found to have pI values of 10.3 +/- 0.3, 9.6 +/- 0.6 and 9.4 +/- 0.6 as determined by HPCE. The antiplatelet potencies do not correspond to their in vitro phospholipase catalytic potencies, but appear to be related to the enzyme isoelectric points.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

Isolation and purification of superbins I and II from Austrelaps superbus (copperhead) snake venom and their anticoagulant and antiplatelet effects

Two proteins with anticoagulant and antiplatelet activities were purified from Austrelaps superbus (copperhead) venom by gel filtration, ion-exchange and reverse-phase chromatographic methods. These purified proteins were designated superbins I and II. Superbin I was homogeneous, as indicated by electrospray ionization-mass spectrometry, with a mol. wt of 13,252.3 +/- 1.6, whereas superbin II contained two closely related proteins of mol. wts 13,235.5 +/- 1.1 and 13,212.9 +/- 1.2. Both superbins showed phospholipase A2 activity and exhibited weak anticoagulant effects when tested by one-step prothrombin time clotting assays. The 'dissection approach' was used to identify the coagulation complex(es) inhibited by these enzymes in the extrinsic coagulation cascade. The results indicate that both the enzymes inhibit the extrinsic tenase complex, but not the prothrombinase complex, similarly to other weakly anticoagulant phospholipases. Superbins I and II also inhibited aggregation of human platelets induced by collagen.

A platelet aggregation inhibitor phospholipase A2 from Russell's viper (Vipera russelli) venom: isolation and characterization

Vipera russelli venom is reported to contain multiple forms of phospholipase A2 (PLA2). Three PLA2s (VRV-PL-V, VRV-PL-VI and VRV-PL-VIIIa) have been purified and characterized in this laboratory. A PLA2(VRV-PL-IIIb) inhibiting platelet aggregation was purified from the same venom in two steps involving CM-Sephadex C-25 column chromatography followed by gel filtration on Sephadex G-50. VRV-PL-IIIb is a basic (pI 7.3-7.7) isoenzyme, with a mol. wt between 14,000 and 15,000. It induced neurotoxic symptoms in experimental mice with an i.p. LD50 of 5.2 mg/kg body weight. VRV-PL-IIIb inhibits ADP-induced platelet aggregation in a dose-dependent manner. It induced oedema in the foot pads of mice and is devoid of anticoagulant, myotoxic and direct haemolytic activities. Antibodies to VRV-PL-IIIb showed a single precipitin line against the antigen as well as whole venom.

Molecular cloning and characterization of a neurotoxic phospholipase A2 from the venom of Taiwan habu (Trimeresurus mucrosquamatus)

Using gel-filtration chromatography and reverse-phase (RP) HPLC we have purified a presynaptic neurotoxin (designated as trimucrotoxin) from the crude venom of Taiwan habu (Trimeresurus mucrosquamatus). Its complete primary structure was solved by an automated N-terminal sequencing and cDNA sequencing method. The enzyme inhibited the twitch of the chick biventer cervicis muscle at 0.1-1 micrograms/ml and showed lethality in mice (LD50 = 1.2 micrograms/g, when given intravenously). Trimucrotoxin exists mainly as a homodimer of 14 kDa subunits as shown by a gel-filtration experiment, and dissociates into monomers during SDS/PAGE in the absence of Ca2+. However, most of trimucrotoxin migrated as slowly as a trimer during nondenaturing SDS/PAGE in the presence of Ca2+ or Sr2+. Its amino acid sequence identity to crotoxin B and agkistrodotoxin is about 75%, and its cDNA sequence is 82% identical to that of crotoxin B. Rabbit antiserum against trimucrotoxin also cross-reacted with the other crotalid neurotoxic phospholipases A2. Furthermore, the purified acidic subunit of crotoxin potentiated the neurotoxicity of trimucrotoxin. A comparison of the sequences of these crotalid neurotoxins revealed some common features of the possible neurotoxic sites, including residues 6, 11, 76-81 and 119-125.

Platelet secretory phospholipase A2 fails to induce rabbit platelet activation and to release arachidonic acid in contrast with venom phospholipases A2

The ability of platelet secretory phospholipase A2 (sPLA2) to induce platelet activation was investigated. sPLA2 (group II) contained in an activated platelet supernatant, as well as high concentrations of purified recombinant platelet sPLA2, failed to induce platelet activation. Furthermore, sPLA2 did not modify platelet activation induced by various agonists. The possible relationship between the failure of this enzyme to induce platelet activation and its origin (mammalian) or its structural group (group II) was then investigated, using pancreatic PLA2s (group I) and venom PLA2s from groups I, II and III. All venom PLA2s induced platelet activation that was accompanied by the liberation of arachidonic acid and was abolished by aspirin. In contrast, as observed for platelet sPLA2, enzymes from hog or bovine pancreas were unable to induce platelet activation even when used at high concentrations. Interestingly, PLA2 able to induce platelet activation efficiently hydrolyse phosphatidylcholine, while those inactive on platelets did not. Taken together, these results suggest that the catalytic activity of added PLA2 is necessary but not sufficient to induce platelet activation. Moreover, the ability of PLA2 to induce platelet activation is not related to its structural group (I, II, III) but rather to its origin (venom vs. mammalian) and capacity to hydrolyse phosphatidylcholine, the major phospholipid of the outer leaflet of the plasma membrane.

Effect on human platelet aggregation of phospholipase A2 purified from Heloderma horridum (beaded lizard) venom

By means of gel filtration, ionic exchange chromatography and DEAE-column HPLC, an acidic phospholipase A2 (PLA2) was purified from beaded lizard (Heloderma horridum) venom. The purified PLA is a single-chain polypeptide, consisting of about 163 amino acid residues with a molecular mass of 19,000 Da as calculated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino acid analysis. HHV-PLA showed a rather specific inhibitory effect on platelet aggregation induced by U46619 and epinephrine in human platelet-rich plasma in a dose- and time-dependent manner, whereas it had little effect on collagen- and ADP-induced aggregation. ATP-release reaction induced by various agonists were dose- and time-dependently inhibited by HHV-PLA, even though platelet aggregation was apparently not affected in human washed platelets. When HHV-PLA was chemically modified with p-bromophenacyl bromide, both of its enzymatic activity and antiplatelet activity were lost. Furthermore, exogenous lysophosphatidylcholine and HHV-PLA treated phosphatidylcholine inhibited platelet aggregation induced by U46619 in human washed platelets. In conclusion, PLA enzyme from H. horridum venom inhibits exclusively U46619- or thromboxane-induced platelet aggregation of human platelet-rich plasma probably by virtue of their PLA enzymatic activity on plasma phospholipids, converting phospholipids (e.g., phosphatidylcholine) into lysophospholipids, which in turn interfere with the coupling of TXA2 receptor and its signalling transduction system.

Human recombinant phospholipase A2 inhibits platelet aggregation in vitro and in vivo in rat and guinea pig

Platelets contain a phospholipase A2 in their granules which can be released in response to various activating stimuli in vitro as well as in vivo. Human recombinant phospholipase A2 (1-10 micrograms/ml) had no direct effect on platelet aggregation in vitro using rabbit platelet rich plasma. In contrast human recombinant phospholipase A2 (1-20 micrograms/ml) was able to inhibit aggregation of washed rabbit platelet in vitro induced by collagen (0.250-2.0 micrograms/ml). When rabbit platelet rich plasma was recalcified with CaCl2 1 M in the presence of the thrombin inhibitor hirulog (10 micrograms/ml), human recombinant phospholipase A2 (10-40 micrograms/ml) was able to inhibit platelet aggregation. The anti-aggregatory effect was removed by incubation of platelet rich plasma with a monoclonal anti-human recombinant phospholipase A2 antibody. Human recombinant phospholipase A2 (1-10 micrograms) inhibited 111In-labelled platelet accumulation within the thoracic region of rats and guinea pigs induced by i.v. administration of submaximal doses of collagen or adenosine diphosphate. Phospholipase A2 (1-20 micrograms/ml) from Naja mocambique mocambique snake venom had no direct effect on platelet aggregation in vitro. However, Naja phospholipase A2 administered i.v. to rats or guinea pigs was able to induce a dose related accumulation of 111In-labelled platelet within the thoracic region. The inhibitory effect of exogenously added human recombinant phospholipase A2 on platelet aggregation in vivo suggests a possible pathophysiological role for the extracellular form of phospholipase A2 but this property is not a feature of all phospholipase A2 preparations.

Purification and characterisation of a snake venom phospholipase A2: a potent inhibitor of platelet aggregation

An inhibitor of human platelet aggregation was identified from the venom of an Australian Copperhead snake, Austrelaps superba, as a novel phospholipase A2. The inhibitor was purified to homogeneity by chromatography on Q-Sepharose, S-Sepharose and C8 reverse phase HPLC. The purified phospholipase A2 has a molecular weight of 15 kDa as assessed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). N-terminal sequence analysis of the platelet inhibitor revealed 70-80% sequence identity to other previously described secretory phospholipase A2. Phospholipase activity of the purified protein was confirmed by the ability of the enzyme to hydrolyse lecithin. Pretreatment of the purified protein with the specific phospholipase A2 inhibitor p-bromophenacyl bromide, resulted in abrogation of both its enzyme and platelet inhibitory activity. The phospholipase A2 inhibited platelet aggregation and serotonin release, induced by a variety of platelet agonists, in a time and dose dependent manner.

Characterization and molecular cloning of neurotoxic phospholipases A2 from Taiwan viper (Vipera russelli formosensis)

Two phospholipases A2 (PLA2s), designated as RV-4 and RV-7 were purified from venom of the Taiwan Russell's viper (Vipera russelli formosensis) by gel-filtration and reverse-phase HPLC. Their primary structures were solved by both protein sequencing and cDNA cloning and sequencing. The cDNA synthesized was amplified by the polymerase-chain reaction using a pair of synthetic oligonucleotide primers corresponding to the N- and the C-terminal flanking regions of the enzymes. The deduced amino acid sequences of RV-4 and RV-7 were 92% identical to those of the vipoxin and vipoxin inhibitor, respectively, from the Bulgarian Vipera a. ammodytes. RV-4 itself was neurotoxic, whereas RV-7 had much lower enzymatic activity and was not toxic. The low enzymatic activity of RV-7 may be attributed to five acidic residues at positions 7, 17, 59, 114 and 119, which presumably impair its binding to aggregated lipid substrates. Based on the sequence comparison among all the known group II PLA2s, residues 6, 12, 76-81, and 119-125 were identified as important for the neurotoxicity. RV-4 and RV-7 exist in the crude venom as heterodimers, which were again formed by mixing together the HPLC-purified RV-4 and RV-7. Moreover, RV-7 inhibited the enzymatic activity of RV-4 in vitro but potentiated its lethal potency and neurotoxicity. It is suggested that RV-7 may facilitate the specific binding of RV-4 to its presynaptic binding sites, probably by preventing its non-specific adsorption.

Characterization of snake venom components acting on blood coagulation and platelet function

Snake venoms can affect blood coagulation and platelet function in various ways. The physicochemical properties and the mechanisms of actions of the snake venom components affecting blood coagulation and platelet function are discussed.

Phospholipase A2 is associated with albumin in patients with acute pancreatitis

Evidence for the association of serum phospholipase A2 (PLA2) (EC 3.1.1.4) and serum proteins was examined. The effect of this on the PLA2 results from patients diagnosed as having acute pancreatitis was investigated. Two distinct zones of PLA2 activity were found on agarose electrophoresis of purified human PLA2 in the presence of albumin, and in the sera from the acute pancreatitis patients. One of the zones was coincident with albumin. To investigate this finding, a comparison of the PLA2 activity in sera and protein-free ultrafiltrates prepared from the sera of the same patients, showed that PLA2 was not completely ultrafiltered as would be expected from its molecular weight. The PLA2 method used employed a radiolabelled E. coli membrane-phospholipid substrate. It has been previously shown that there is an association between PLA2 and albumin and there is good evidence that albumin interferes with certain methods used for the measurement of PLA2. The recovery of PLA2 activity from the ultrafiltrates of patients' serum was highly variable and it may be that serum proteins, in particular albumin, provide a protective 'buffer' against small increases in PLA2 activity. Inhibitor proteins such as lipocortin which were originally postulated as binding to the enzyme, have been subsequently shown to be substrate-binding agents. However, direct protein effects may be a factor in the inconsistent PLA2 results reported in studies of patients with acute pancreatic disease.

Comparison of the enzymatic and edema-producing activities of two venom phospholipase A2 enzymes

The edema-producing activity of NNAVPLA2, an acidic phospholipase A2 (PLA2) enzyme from Naja naja atra venom (NNAV), was less potent than that of TMVPLA2 II, a basic PLA2 from Trimeresurus mucrosquamatus venom (TMV). These edema-forming effects were greatly suppressed by pretreatment of rats with diphenhydramine/methysergide or compound 48/80, which reduced the tissue content of histamine and serotonin. Heparin abolished and suppressed the paw edema caused by protamine and TMVPLA2 II, respectively, but had no effect on the NNAVPLA2-induced response. In isolated rat peritoneal mast cells, both PLA2 concentration dependently induced the release of histamine and beta-glucuronidase. Again, TMVPLA2 II was more potent than NNAVPLA2. This degranulation effect of mast cells caused by TMVPLA2 II and protamine was inhibited by heparin, while that caused by NNAVPLA2 was unaffected. The edema-forming and mast cell degranulation effects were greatly decreased in both PBPB-modified NNAVPLA2 and PBPB-modified TMVPLA2 II, in which the catalytic activity of the enzymes was completely lost. PBPB-modified TMVPLA2 II-induced paw edema was also suppressed by heparin. Furthermore, this edematous response was totally reversed in rat pretreated with aspirin in combination with diphenhydramine and methysergide. These results suggest that the edema-forming effect of PLA2 is probably dependent on the presence of catalytic, positive charge and pharmacological sites on its molecule.

Bothrops jararaca snake venom: effects on platelet aggregation

Crude venom from Bothrops jararaca has procoagulant, platelet aggregating and phospholipase A2 (PLA2) activities. By chromatographic fractionation of the venom on Sephacryl S-200 it was possible to separate these three activities and to show that distinct protein components are involved. The procoagulant activity appears to involve the synergistic action of several components and was not further studied in the present work. The aggregating activity results from the action of two components: 1) a serino-proteinase PMSF-inhibitable similar to thrombocytin and 2) a PMSF-resistant, calcium- and plasma-dependent factor distinct from other previously described aggregating principles. Fractions possessing PLA2 activity were also able to inhibit platelet aggregation induced by collagen and accelerated the slow reversal of aggregation induced by ADP. Both PLA2 activity and inhibition of collagen-induced platelet aggregation displayed by these fractions were abolished by reaction with p-bromophenacyl bromide and 2-mercapto-ethanol. These results indicate that in B. jararaca venom the PLA2 activity and the factor inhibiting platelet aggregation may be related to the same protein molecule.

Effects of snake venom proteins on blood platelets

Snake venoms are complex mixtures which contain pharmacologically active polypeptides and proteins. Several snake venom constituents interfere in platelet aggregation, an important cellular process in thrombosis and hemostasis. These components range in size from small molecular weight polypeptides to high molecular weight proteins. Some of the proteins are enzymes, such as phospholipase A2, proteinases, nucleotidases, or L-amino acid oxidase, while others do not exhibit enzymatic activity. These components may initiate and/or inhibit platelet aggregation. Some venom factors induce platelet agglutination. This review deals with the physical characteristics of these venom factors, the mechanisms of their platelet effects, structure-function relationships, and their physiological significance.

Triwaglerin: a potent platelet aggregation inducer purified from Trimeresurus wagleri snake venom

Trimeresurus wagleri venom is the most potent inducer of platelet aggregation among the seven Trimeresurus snake venoms tested. By means of CM-Sephadex C-50 column chromatography, T. wagleri venom was separated into 19 fractions. Fraction XVI possessed the strongest aggregating activity and was further purified by Sephadex G-75 and on heparin-agarose columns, and finally Triwaglerin, with a molecular weight of 68000, was obtained. Its aggregating and ATP-releasing activity was dose-dependent and 10-times more potent than the crude venom. Triwaglerin was devoid of any of the enzymatic activities possessed by the crude venom. Triwaglerin-induced aggregation was not affected by indomethacin, creatine phosphate/creatine phosphokinase (CP/CPK), platelet-activating factor (PAF) antagonists, verapamil or heparin, but was inhibited completely by mepacrine, imipramine and forskolin and markedly by tetracaine and sodium nitroprusside. Thromboxane B2 formation caused by Triwaglerin was suppressed by mepacrine, imipramine and indomethacin. R59022 and TMB-8 caused a synergistic inhibitory effect against Triwaglerin-induced aggregation. These data suggest that Triwaglerin activates platelets in a unique action which is independent of formation of thromboxane A2 and PAF, or release of ADP.

A model to explain the pharmacological effects of snake venom phospholipases A2

Snake venom phospholipase A2 enzymes induce a wide variety of pathological symptoms in animals, despite sharing a common catalytic activity and similar structural features with nontoxic mammalian pancreatic enzymes. A hypothetical model is described to explain how specific pharmacological effects, such as presynaptic neurotoxicity, cardiotoxicity, myotoxicity, anticoagulant and platelet effects are exhibited by venom PLA2 enzymes. The model is an effort to elucidate many controversial and contradictory observations which have previously been difficult to interpret. The essential feature of the model is the targeting of venom PLA2 enzymes to the specific tissue or cell due to their affinity towards specific proteins, rather than lipid domains. After the initial binding, PLA2 enzymes induce various pharmacological effects by mechanisms which are either dependent or independent of their enzymatic activity. The model and its predicted target proteins thus provide a new focus for toxin research.

Isolation of an acidic phospholipase A2 from the venom of Agkistrodon acutus (five pace snake) and its effect on platelet aggregation

A phospholipase A2 from the venom of the snake Agkistrodon acutus was purified by immunoaffinity chromatography and fast protein liquid chromatography (FPLC) as a single band by PAGE and SDS-PAGE. The estimated mol.wt was 16,400 by SDS-PAGE and 16,900 by gel filtration and the isoelectric point was 4.9. The ten N-terminal amino acid residues are homologous to those of the acidic phospholipases A2 from other crotalid venoms. The purified enzyme showed a potent inhibitory effect on platelet aggregation induced by ADP, collagen and sodium arachidonate in human platelet-rich plasma. The platelet aggregation by these inducers was completely suppressed when the concentration of the venom phospholipase A2 was 10-100 micrograms/ml. However, at 20 micrograms/ml, platelet aggregation could be elicited in washed human platelet suspension. Aspirin (28 micrograms/ml), an inhibitor of cyclooxygenase, inhibited the aggregating effect of the phospholipase A2. It is proposed that the stimulatory mechanism of the phospholipase A2 might be due to the liberation of arachidonic acid from phospholipids in the membrane of platelets and the formation of thromboxane A2.

Mechanism of platelet effects of cardiotoxins from Naja nigricollis crawshawii (spitting cobra) snake venom

Four cardiotoxins isolated from Naja nigricollis crawshawii venom show inhibition of platelet aggregation when tested on whole blood aggregation in an electronic aggregometer. A similar inhibitory effect is observed by adding hemolyzed erythrocytes to whole blood before initiation of aggregation with collagen. Qualitatively, ADP-induced aggregation in whole blood appears to be different from collagen-induced aggregation in that the change in impedance is smaller than that induced by collagen. Thus, addition of ADP apparently "inhibits" collagen-induced aggregation as measured by the electronic aggregometer. Inclusion of apyrase in the aggregation cuvet stimulates the rate of aggregation initiated by collagen. The cardiotoxins lyse blood cells and release their cellular contents including ADP, AMP and other inhibitory substances, which reduce the impedance changes associated with collagen-induced aggregation. The cardiotoxins also lyse platelets coated onto the electrodes and reduce the impedance after aggregation is completed. Thus the lytic effects of these polypeptides cause an apparent inhibition of platelet aggregation in whole blood by both release of inhibitory components and removal of platelets from the electrodes. The lytic ability of these cardiotoxins can also explain the apparent "potentiation" and "aggregation" observed by previous workers using turbidometric aggregometers. Under these conditions, the cardiotoxins from N. nigricollis appear to both potentiate ADP-initiated aggregation and initiate aggregation themselves, but lysis is responsible, as shown by the release of cytoplasmic lactate dehydrogenase. Neurotoxin II from N. naja oxiana venom, although structurally homologous with cardiotoxins, does not lyse cells, nor did it show any effects on platelets.

Haemostatic disturbances in patients bitten by Russell's viper (Vipera russelli siamensis) in Burma

Patients who are severely envenomed by Russell's viper develop DIC which is frequently associated with spontaneous bleeding and incoagulable blood. These haemostatic disturbances may be responsible for death or organ/tissue damage both through haemorrhage and microvascular occlusion by fibrin thrombi. The most striking laboratory features of the coagulopathy developing after Russell's viper bite in the 42 patients studied were depletion of fibrinogen (mean 0.09 g/l, range 0-0.6), factor V (6.5 u/dl, range 0-17), factor X (35 u/dl, range 1-85), factor XIIIa (57 u/dl, range 15-82), plasminogen (61 u/dl, range 10-92), antiplasmin (36 u/dl, range 14-62). Protein C (49 u/dl, range 15-100) and platelets (104 x 10(9)/l, range 25-197). Intense fibrinolytic activity was detected in all cases with marked elevation of FDPs (1614 micrograms/ml, range 350-3000), a large proportion of which were cross-linked (1058 micrograms/ml, range 38-3000). The monospecific Burmese antivenom appeared to be very effective in neutralizing the venom procoagulants and in restoring blood coagulability. Moreover, the unexpectedly normal level of AT III provides a theoretical basis for the use of heparin to enhance the inactivation of those serine proteases present before antivenom administration.

Venom from southern copperhead snake (Agkistrodon contortrix contortrix). II. A unique phospholipase A2 that induces platelet aggregation

A platelet aggregation factor was purified from the venom of southern copperhead snake (Agkistrodon contortrix contortrix) by DEAE-cellulose ion-exchange chromatography, precipitation with ammonium sulfate, affinity chromatography using bovine serum albumin as ligand, and gel filtration on Cellulofine GCL-2000. It had molecular weights of 11,000 and 14,000, as determined by gel filtration chromatography and sodium dodecyl sulfate--polyacrylamide gel electrophoresis (SDS-PAGE), respectively. It consists of a single polypeptide, and was identified as a phospholipase A2. It was quite resistant to heat and various denaturing reagents including urea and SDS. It lost both phospholipase A2 activity and platelet aggregating activity upon modification of histidine residue(s) with p-bromophenacyl bromide. Its specificity towards the beta-position of phospholipid in esterolytic reaction was confirmed by gas-liquid chromatography using a pure synthetic phosphatidylcholine. Platelet aggregation by this phospholipase A2 was completely inhibited by prostacyclin, but was little inhibited by aspirin which indicates almost no direct participation of released arachidonic acid in the aggregation mechanism.

Effect of cobra venom phospholipase A2 on platelet aggregation in comparison with those produced by arachidonic acid and lysophophatidylcholine

Cobra venom phospholipase A2 induced a biphasic effect on washed rabbit platelets. The first phase was a reversible aggregation which was dependent on stirring and extracellular calcium. The aggregation and thromboxane B2 formation were inhibited by indomethacin, mepacrine, tetracaine and imipramine, while PGE1 and sodium nitroprusside inhibited only the aggregation, but not the thromboxane B2 formation. The second phase was an inhibitory effect on platelet aggregation induced by arachidonic acid, PAF, ADP or collagen but not that by thrombin or ionophore A23187. The longer the incubation time of cobra venom phospholipase A2 with platelets, the more the inhibitory effect. The aggregating and anti-aggregating effects could be overcome by bovine serum albumin. Lysophosphatidylcholine (Lyso-PC) and arachidonic acid showed synergistic inhibition in platelet aggregation. Lyso-PC decreased thromboxane B2 formation in platelets formed by collagen. The inhibitory effect of Lyso-PC on platelet aggregation was more marked at lower calcium concentrations. It is concluded that the aggregating effect of exogenous addition of venom phospholipase A2 is due to thromboxane formation and the antiplatelet effect is similar to those produced by arachidonic acid and lysophosphatidylcholine.

Purification and characterization of a platelet aggregation inducer from Calloselasma rhodostoma (Malayan pit viper) snake venom

A potent platelet aggregation inducer was purified from Calloselasma rhodostoma snake venom by Sephadex G-75, CM-Sephadex C-50 and Sephacryl S-300 column chromatography. It was homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with a molecular weight estimated to be 28,160 +/- 1280. It was devoid of phospholipase A2, fibrino(geno)lytic and thrombin-like activities. The venom inducer elicited platelet aggregation and the serotonin release reaction in rabbit platelet-rich plasma and platelet suspension. Exogenous calcium was required for its platelet activation. Creatine phosphate/creatine phosphokinase and indomethacin did not inhibit the venom inducer-induced aggregation and release reaction. Mepacrine and verapamil preferentially inhibited aggregation, while PGE1 completely blocked both aggregation and release reaction. It is concluded that the venom inducer activates platelets through the activation of endogenous phospholipase A2 or C, leading to intracellular calcium mobilization, but independent of the ADP release reaction or thromboxane A2 formation.

Mechanism of action of the platelet function inhibitor from Vipera russelli siamensis snake venom

Human platelet aggregation induced by ADP, adrenaline, collagen or thrombin was inhibited by the venom inhibitor. Heating reduced both its phospholipase A2 enzymatic and anti-aggregatory activities, although not in parallel. The inhibitor caused significant dose-related inhibitory effects on the clot retraction of rabbit platelet-rich plasma caused by thrombin, while platelet malondialdehyde formation stimulated by thrombin was not affected. Furthermore, the venom inhibitor increased basal cyclic AMP levels in platelets, while cyclic GMP content was slightly lowered, but not in a dose-dependent manner. In addition, microscopic study revealed that the cytoskeleton was disordered after treatment of platelets with the venom inhibitor. The platelets lost their discoid form, while the ultrastructural changes of platelet aggregation induced by ADP were blocked. It is concluded that increasing platelet cyclic AMP and the disorder of the cytoskeleton may be the mechanism of action of the venom inhibitor on platelet function.

A platelet function inhibitor purified from Vipera russelli siamensis (Smith) snake venom

By means of CM-Sephadex C-50 column chromatography and gel filtration on Sephadex G-75, a potent platelet function inhibitor was purified from Vipera russelli siamensis venom. It appeared as a single protein band on polyacrylamide gel electrophoresis in the presence or absence of SDS, and consists of 123 amino acid residues. Its NH2-terminal residue is serine. It showed the following characteristics: molecular weight, 13,800; isoelectric point, 10.4; LD50, 0.5 +/- 0.12 mg/kg (i.v.). The platelet inhibitor exhibited phospholipase A2 activity with a specific activity of 35 mumoles/min/mg. From 2 g of the venom, 70 mg of the purified inhibitor was obtained. Inhibition of human platelet aggregation induced by ADP or adrenaline was dose-dependent, with ID50 of 1.14 micrograms/ml or 0.37 microgram/ml, respectively. The platelet aggregation induced by thrombin or collagen was also inhibited and the inhibitory activity on platelet aggregation was heat stable (at 100 degrees C, 20 min) in an acidic medium (pH 5.8), while its phospholipase A2 activity was relatively heat labile under the same condition. The release of 3H-serotonin in platelets stimulated by ADP was also inhibited and this was positively correlated with inhibition of platelet aggregation induced by ADP (r = 0.998, P less than 0.002).