Platelet aggregation inhibitors from Agkistrodon acutus snake venom

Beyond the 'big four': Venom profiling of the medically important yet neglected Indian snakes reveals disturbing antivenom deficiencies

Background

Snakebite in India causes the highest annual rates of death (46,000) and disability (140,000) than any other country. Antivenom is the mainstay treatment of snakebite, whose manufacturing protocols, in essence, have remained unchanged for over a century. In India, a polyvalent antivenom is produced for the treatment of envenomations from the so called ‘big four’ snakes: the spectacled cobra (Naja naja), common krait (Bungarus caeruleus), Russell’s viper (Daboia russelii), and saw-scaled viper (Echis carinatus). In addition to the ‘big four’, India is abode to many other species of venomous snakes that have the potential to inflict severe clinical or, even, lethal envenomations in their human bite victims. Unfortunately, specific antivenoms are not produced against these species and, instead, the ‘big four’ antivenom is routinely used for the treatment.

Methods

We characterized the venom compositions, biochemical and pharmacological activities and toxicity profiles (mouse model) of the major neglected yet medically important Indian snakes (E. c. sochureki, B. sindanus, B. fasciatus, and two populations of N. kaouthia) and their closest ‘big four’ congeners. By performing WHO recommended in vitro and in vivo preclinical assays, we evaluated the efficiencies of the commercially marketed Indian antivenoms in recognizing venoms and neutralizing envenomations by these neglected species.

Findings

As a consequence of dissimilar ecologies and diet, the medically important snakes investigated exhibited dramatic inter- and intraspecific differences in their venom profiles. Currently marketed antivenoms were found to exhibit poor dose efficacy and venom recognition potential against the ‘neglected many’. Premium Serums antivenom failed to neutralise bites from many of the neglected species and one of the ‘big four’ snakes (North Indian population of B. caeruleus).

Conclusions

This study unravels disturbing deficiencies in dose efficacy and neutralisation capabilities of the currently marketed Indian antivenoms, and emphasises the pressing need to develop region-specific snakebite therapy for the ‘neglected many’.

Snakebite is a ‘neglected tropical disease’ that majorly affects the rural populations in developing countries. India bears the brunt of snakebites with over 46,000 deaths and 140,000 disabilities, annually. A significant number of these bites are attributed to the widely distributed ‘big four’ snakes, namely spectacled cobra (Naja naja), common krait (Bungarus caeruleus), Russell’s viper (Daboia russelii), and saw-scaled viper (Echis carinatus). The commercial antivenoms marketed in India are only manufactured against these four species, while neglecting many other medically relevant snakes with restricted geographic distribution. Snakebite pathology is dependent on the venom composition of the population/species, which can, in turn, vary intra- and inter-specifically. Though this variation severely limits the cross-population/species antivenom efficacy, envenomations by the neglected snakes in India are treated with the ‘big four’ antivenom. Therefore, to unravel the underlying venom variability, we investigated venom proteomic, biochemical/pharmacological and toxicity profiles of the major neglected Indian snakes and their ‘big four’ relatives. To assess the effectiveness of the ‘big four’ antivenom in treating bites from these neglected snakes, we performed preclinical experiments, which revealed alarming inadequacies of the commercial antivenoms. Our findings accentuate the compelling necessity for the innovation of highly efficacious next-generation snakebite therapy in India.

Clinical and laboratory features distinguishing between Deinagkistrodon acutus and Daboia siamensis envenomation

Background

There are 6 species of venomous snakes in Taiwan. Two of them, Deinagkistrodon acutus (D. acutus) and Daboia siamensis (D. siamensis), can cause significant coagulopathy. However, a significant proportion of patients with snakebites cannot identify the correct snake species after envenomation, which hampers the application of antivenom. Hence, the differential diagnosis between the two snakebites by clinical presentations is important. This study aims to compare their clinical and laboratory features for the purpose of differential diagnosis between the two snakebites.

Methods

We retrospectively reviewed the medical records of patients who arrived at the emergency department due to D. acutus or D. siamensis envenomation, between 2003 and 2016, in one medical center in eastern Taiwan. Since these snakebites are rare, we also included 3 cases reported from another hospital in central Taiwan.

Results

In total, 15 patients bitten by D. acutus and 12 patients by D. siamensis were analyzed. Hemorrhagic bulla formation and the need for surgical intervention only presented for D. acutus envenomation cases (Both 53.3% vs. 0.0%, P = 0.003). As to laboratory features, lower platelet counts (20.0 ×  10³/μL [interquartile range, 14–66 × 10³/μL] vs. 149.0 × 10³/μL [102.3–274.3 × 10³/μL], P = 0.001), lower D-dimer level (1423.4 μg/L [713.4–4212.3 μg/L] vs. 12,500.0 μg/L [2351.4–200,000 μg/L], P = 0.008), higher proportion of patients with moderate-to-severe thrombocytopenia (platelet count < 100 × 10³/μL) (80% vs. 16.7%, odds ratio (OR) = 20.0, 95% CI, 2.77–144.31; P = 0.002), and lower proportion of patients with extremely high D-dimer (> 5000 ng/mL) (16.7% vs. 66.7%, adjusted OR = 0.1 (95% CI, 0.01–0.69; P = 0.036) were found among cases of D. acutus envenomation compared to D. siamensis envenomation. The combination of hemorrhagic bulla, thrombocytopenia, and a lack of extremely high D-dimer had good discriminatory power (area under the curve (AUC) = 0.965; 95% CI, 0.904–1.00) for distinguishing D. acutus from D. siamensis envenomation.

Conclusions

The presentation of moderate to severe thrombocytopenia (platelet count < 100 × 10³/μL) and hemorrhagic bulla formation may indicate D. acutus envenomation. However, the envenomed patient with extremely high D-dimer levels may indicate a D. siamensis envenomation. These findings may help diagnose and select the right antivenom in patients with unknown snakebites who present significant coagulopathy.

Proteomic Characterization of Two Medically Important Malaysian Snake Venoms, Calloselasma rhodostoma (Malayan Pit Viper) and Ophiophagus hannah (King Cobra)

Calloselasma rhodostoma (CR) and Ophiophagus hannah (OH) are two medically important snakes found in Malaysia. While some studies have described the biological properties of these venoms, feeding and environmental conditions also influence the concentration and distribution of snake venom toxins, resulting in variations in venom composition. Therefore, a combined proteomic approach using shotgun and gel filtration chromatography, analyzed by tandem mass spectrometry, was used to examine the composition of venoms from these Malaysian snakes. The analysis revealed 114 proteins (15 toxin families) and 176 proteins (20 toxin families) in Malaysian Calloselasma rhodostoma and Ophiophagus hannah species, respectively. Flavin monoamine oxidase, phospholipase A₂, phosphodiesterase, snake venom metalloproteinase, and serine protease toxin families were identified in both venoms. Aminopeptidase, glutaminyl-peptide cyclotransferase along with ankyrin repeats were identified for the first time in CR venom, and insulin, c-type lectins/snaclecs, hepatocyte growth factor, and macrophage colony-stimulating factor together with tumor necrosis factor were identified in OH venom for the first time. Our combined proteomic approach has identified a comprehensive arsenal of toxins in CR and OH venoms. These data may be utilized for improved antivenom production, understanding pathological effects of envenoming, and the discovery of biologically active peptides with medical and/or biotechnological value.

Deinagkistrodon acutus envenomation: a report of three cases

Background

Deinagkistrodon acutus envenomation is associated with severe hematological and wound complications but is rarely described.

Case presentation

Herein, we report three cases of victims bitten by D. acutus and indicate that rapid-onset severe coagulopathy and thrombocytopenia are distinct features of D. acutus snakebite, which are not observed in other crotaline snakebites (i.e., Trimeresurus stejnegeri and Protobothrops mucrosquamatus) in Taiwan. The toxic effects could occur as early as 2 to 3 h following D. acutus envenomation and persist if the administration of specific antivenom is delayed or even not commenced. Based on our findings, 2 to 4 vials of specific antivenom as the first dose should be administered to victims and repeated at 6 to 8 h intervals if coagulopathy or thrombocytopenia persists. Fresh frozen plasma or platelet replacement is probably safe as an adjunct therapy for D. acutus bite in the presence of venom-induced consumptive coagulopathy.

Conclusion

Severe coagulopathy and thrombocytopenia could occur as early as 2 to 3 h after D. acutus envenomation. The current recommendation for antivenom is 2 to 4 vials as the first dose and repeated every 6– to 8 h if coagulopathy or thrombocytopenia persists. These cases studied may be helpful to first-line medical personnel in the early diagnosis and management of D. acutus envenomation among other crotaline snakebites in Taiwan.

Purification and characterization of a platelet aggregation inhibitor and anticoagulant Cc 5_NTase, CD 73-like, from Cerastes cerastes venom

The present study is the first attempt to report the characterization of a nucleotidase from Cerastes cerastes venom. A 70 kDa 5'-nucleotidase (Cc-5'NTase) was purified to homogeneity. The amino acid sequence of Cc-5'NTase displayed high homology with many nucleotidases. Its activity was optimal at pH 7 with a specific hydrolytic activity toward mono-, di-, and triphosphate adenylated nucleotides. Cc-5'NTase preferentially hydrolyzed ADP and obeyed Michaelis-Menten kinetics. Among the metals and inhibitors tested, Ni²⁺ and Mg²⁺ completely potentiated enzyme activity, whereas EGTA, PMSF, iodoacetamide, vanillic acid, vanillyl mandelic acid, and 1,10-phenanthroline partially abolished its activity. Cc-5'NTase was not lethal for mice at 5 mg/kg and exhibited in vivo anticoagulant effect. It also dose-dependently inhibited adenosine diphosphate-induced platelet aggregation by converting adenosine diphosphate to adenosine and prohibited arachidonic acid-induced aggregation but was not effective on fibrinogen-induced aggregation. Cc-5'NTase could be a good tool as pharmacological molecule in thrombosis diagnostic and/or therapy.

5'-Nucleotidase from Vipera lebetina venom

5'-Nucleotidase (5'-NT) is widely represented in animal tissues (CD73) as well as in almost all snake venoms. In the present study, a 5'-NT isoform has been isolated from Vipera lebetina venom. The homodimeric isoform consists of monomers with molecular masses of 60 kDa. The enzyme is thermolabile and has pH optimum at 7.5. The 5'-NT activity is inhibited by metal ions Fe(3+), Cu(2+) and Zn(2+), enhanced by Mn(2+) while Mg(2+) and Ca(2+) have no remarkable effect. In addition to 120-kDa protein there are higher molecular forms of 5'-NT present in the V. lebetina venom. The cloning and sequencing of the 5'-NT coding cDNA resulted in 5'-truncated construct. MALDI-TOF and Orbitrap mass-spectrometry of the tryptic peptides confirmed the translated N-terminally truncated protein sequence concordance to the 5'-NT isolated from the venom. The isolated protein strongly inhibited ADP- or collagen-induced platelet aggregation.

The pharmacological role of phosphatases (acid and alkaline phosphomonoesterases) in snake venoms related to release of purines - a multitoxin

Snake venom components, acting in concert in the prey, cause their immobilization and initiate digestion. To achieve this, several hydrolytic enzymes of snake venom have evolved to interfere in various physiological processes, which are well defined. However, hydrolytic enzymes such as phosphatases (acid and alkaline phosphomonoesterases) are less studied and their pharmacological role in venoms is not clearly defined. Also, they show overlapping substrate specificities and have other common biochemical properties causing uncertainty about their identity in venoms. The near-ubiquitous distribution of these enzymes in venoms, suggests a significant role for these enzymes in envenomation. It appears that these enzymes may play a central role in liberating purines (mainly adenosine) - a multitoxin and through the action of purines help in prey immobilization. However, apart from this, these enzymes could also possess other pharmacological activities as venom enzymes have been evolved to interfere in diverse physiological processes. This has not been verified by pharmacological studies using purified enzymes. Further research is needed to biologically characterize these enzymes in snake venoms, such that their role in venom is clearly established.

Anticoagulant and antithrombotic activity of a new peptide pENW (pGlu-Asn-Trp)

Objectives

The aim was to test a newly discovered oligopeptide, pENW (pGlu-Asn-Trp), for its anticoagulant and antithrombotic activity in vivo, and try to investigate its underlying mechanisms.

Methods

We measured coagulation time by the glass slide method and bleeding time by cutting of mice tails. The thrombosis models employed here included an arterio-venous shunt model and inferior vena ligation model. An ELISA (enzyme-linked immunosorbent assay) was used to analyse t-PA/PAI (tissue-type plasminogen activator/plasminogen activator inhibitor) in the blood drawn from the rats with thrombosis. The ultrastructural changes of the endothelium in the vessels developing thrombosis were observed under a transmission electron microscope.

Key findings

We found that pENW-treated mice exhibited a prolonged coagulation time in a dose-dependent manner, but not an extended haemorrhage time. On the other hand, pENW significantly inhibited thrombus formation in both arterio-venous shunt models and inferior vena ligation models. Plasma t-PA/PAI was significantly higher as measured by ELISA. Transmission electron microscope photos of pENW-treated groups also displayed a better condition than model controls, with less erythrocytes in the vascular lumens. In addition, pENW concentration-dependently inhibited aggregation of platelets induced by ADP (adenosine 5′-diphosphate sodium salt) in rabbit platelet-rich plasma.

Conclusions

These findings support the suggestion that pENW possesses antithrombotic activity and could be a promising drug in the prevention and treatment of unwanted clot formation.

NPP-BJ, a nucleotide pyrophosphatase/phosphodiesterase from Bothrops jararaca snake venom, inhibits platelet aggregation

Enzymes of the pyrophosphatase/phosphodiesterase family have multiple roles in extracellular nucleotide metabolism and in the regulation of nucleotide-based intercellular signaling. Snake venoms contain enzymes that hydrolyze nucleic acids and nucleotides, but their function is poorly understood. Here we describe for the first time the isolation and functional characterization of a soluble phosphodiesterase from Bothrops jararaca venom, which shows amino acid sequence similarity to mammalian nucleotide pyrophosphatase/phosphodiesterase 3 (NPP3), and inhibits ADP-induced platelet aggregation. The enzyme, named NPP-BJ, showed an apparent molecular mass of 228 kDa by size exclusion chromatography. NPP-BJ exhibited nuclease activity as well as pyrophosphatase and phosphatase activities, preferentially hydrolyzing nucleoside 5'-triphosphates over nucleoside 5'-diphosphates, but was not active upon nucleoside 5'-monophosphates. Depending on the substrate used, dithiothreitol and EDTA differently inhibited the catalytic activity of NPP-BJ. Platelet aggregation induced by ADP was also abrogated by NPP-BJ, whereas thrombin-induced platelet aggregation was only slightly attenuated. However, polyclonal antibodies raised against NPP-BJ could not abolish the lethal activity of B. jararaca venom. Altogether, these results show that NPP-BJ has a minor contribution to the lethal activity of this venom, but interferes with mechanisms of ADP-induced platelet aggregation.

Inhibition of adrenaline and adenosine diphosphate induced platelet aggregation by Lansberg's hognose pit viper (Porthidium lansbergii hutmanni) venom

The haemostatic components of venom from the genus Porthidium has been poorly studied, although it is known that severe manifestations occur when humans are envenomed, which include invasive oedema and disseminated ecchymosis. The effects of venom on blood platelets are commonly studied and are normally carried out with platelet-rich plasma (PRP). A series of crude venom dilutions was used to determine the effects of adenosine diphosphate (2 microM) and adrenaline (11 microM) induced platelet aggregation. Venom of Porthidium lansbergii hutmanni was fractioned by anionic exchange chromatography, and the fractions were also used to determine the 50% inhibition of adenosine diphosphate (ADP) and adrenaline-induced platelet aggregating dose (AD50). Crude venom has more effect in inhibiting adrenaline-induced aggregation (AD50 = 0.0043 microg) followed by the adenosine diphosphate (AD50 = 17 microg). Peaks I and II obtained by chromatography also inhibited adrenaline-induced platelet aggregation with an AD50 of 3.2 and 0.013 microg, respectively, and both peaks inhibited ADP-induced platelet aggregation with an AD50 of 10 microg. The main purpose of this work was to characterise the in vitro effects caused by P. lansbergii hutmanni crude venom and its fractions on the platelet aggregation mediated by adrenaline and ADP agonists.

Inhibition of collagen, and thrombin-induced platelet aggregation by Lansberg’s hognose pit viper (Porthidium lansbergii hutmanni) venom

The Porthidium genus is represented by the P. lansbergii rozei and P. lansbergii hutmanni (Plh) subspecies in Venezuela. The venom components of these have been little studied, probably due to the low incidence of reported accidents, although acute and serious local effects such as invasive edema and disseminated ecchymosis are present during human envenonation. The aim of this work was to characterize the in vitro effects of crude P. l. hutmanni venom, and its fractions, on platelet aggregation triggered by two physiologic agonists: thrombin and collagen. The effects of thrombin and collagen were observed on a platelet-rich plasma (PRP) solution (3 x 10(5) platelets/microL) using serial dilutions of P. l. hutmanni venom (0.625-40 microg). The crude venom was fractionated by anionic exchange chromatography and two peaks obtained. Crude venom and both fractions were highly inhibitory on platelet aggregation mediated by the two agonists. The anti-aggregating dose (AD(50)) for both agonists was determined. PRP collagen-triggered aggregation was most inhibited by the crude venom (AD(50) = 0.67 microg) when compared with PRP thrombin-triggered aggregation (AD(50) = 4.92 microg). Collagen-induced aggregation was more intensely inhibited by venom than thrombin-induced aggregation. In conclusion, to specify the inhibition mechanisms involved for each of the active components in the venom from these subspecies, we must characterize and purify the inhibitors of aggregation from P. l. hutmanni venom, with the purpose of suggesting new pharmacological substances to be incorporated into the therapeutic arsenal to treat hemostatic pathologies related to high levels of platelet aggregation.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Prey specificity, comparative lethality and compositional differences of coral snake venoms

Toxicities of crude venoms from 49 coral snake (Micrurus sp.) populations, representing 15 nominal taxa, were examined in both laboratory mice and in native prey animals and compared with data gathered from two non-micrurine elapids and a crotalid, which served as outgroups. These venoms were further compared on the basis of 23 enzymatic activities. Both toxicities and enzymatic activities were analyzed with respect to natural prey preferences, as determined from stomach content analyses and literature reports. Venoms of nearly all Micrurus for which prey preferences are known, are more toxic to natural prey than to non-prey species. Except for amphisbaenians, prey are more susceptible to venoms of Micrurus that feed upon them, than to venoms of those that eat other organisms. All venoms were more toxic i.v.>i.p.>i.m. Route-specific differences in toxicity are generally greatest for preferred prey species. Cluster analyses of venom enzymatic activities resulted in five clusters, with the fish-eating M. surinamensis more distant from other Micrurus than even the crotalid, Bothrops moojeni. Ophiophagous and amphisbaenian-eating Micrurus formed two close subclusters, one allied to the outgroup species Naja naja and the other to the fossorial, ophiophagous Bungarus multicinctus. Prey preference is shown to be the most important determinant of venom composition in Micrurus.

Antithrombotic and thrombolytic activities of Agkisacutacin, a snake venom proteinase, in experimental models

The antithrombotic and thrombolytic activities of Agkisacutacin (Agk), a component isolated from Agkistrodon acutus, were determined in vitro and in vivo. The models employed included Chandler's model, arterio-venous shunt model and pulmonary embolus model. The effects of Agkisacutacin on coagulation, plasma fibrinogen and platelet aggregation induced by collagen, adenosine diphosphate (ADP) and thrombin were also investigated. The results showed that Agkisacutacin can significantly inhibit thrombus formation in Chandler's and arterio-venous shunt models, and accelerate thrombolysis of pulmonary emboli in rats. The data suggested that Agkisacutacin possessed antithrombotic and thrombolytic activities. Agkisacutacin was also partial characterized.

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.

Molecular cloning and deduced primary structures of acidic and basic phospholipases A2 from the venom of Deinagkistrodon acutus

This paper presents the nucleotide sequences of the cDNAs encoding three acidic phospholipases A2 and one basic phospholipase A2 from Deinagkistrodon acutus venom. The deduced primary structure of the basic enzyme is closest to that of the basic neurotoxic enzyme from Trimeresurus mucrosquamatus venom, while the acidic phospholipases from D. acutus have highest sequence similarity to that from Agkistrodon halys pallas. The phylogeny of this monotypic species is discussed.

Triflavin, an Arg-Gly-Asp-containing peptide, prevents platelet plug formation in in vivo experiments

Triflavin, an Arg-Gly-Asp-containing peptide from Trimeresurus flavoviridis snake venom (M(r) of 7500 Da) inhibits platelet aggregation through the blockade of fibrinogen binding to activated platelets. The present study demonstrated that the intravenous injection of triflavin (0.1 and 0.25 mg/kg) significantly prolonged the bleeding time about 1.8- to 2.4-fold as compared with control (normal saline) of severed mesenteric arteries in rats, whereas the injection of Gly-Arg-Gly-Asp-Ser (GRGDS) (2-8 mg/kg) failed to increase the bleeding time in this model. Continuous infusion of triflavin (0.08 mg/kg/min) significantly increased the bleeding time about 2.6-fold, and the bleeding time returned to normal within 20 min after the cessation of triflavin infusion. Triflavin (10-20 mg/kg) significantly prolonged the occlusion time of platelet plug formation induced by irradiation of mesenteric venules of fluorescein sodium-pretreated mice. In contrast, trigramin (10-20 mg/kg) and GRGDS (500 and 1000 mg/kg) showed no significant effect. These results suggest that triflavin has an effective antiplatelet effect in vivo and this peptide may be a useful therapeutic agent for arterial thrombosis.

Characterization of a potent platelet aggregation inducer from Cerastes cerastes (Egyptian sand viper) venom

A potent, proteinaceous inducer of platelet aggregation designated as IVa, has been purified to homogeneity from Cerastes cerastes venom by molecular sieve and ion exchange chromatography. It is composed of 2 subunits with total M(r) of 62,000 as shown by native gel chromatography and chemical cross-linking with disuccinimidyl suberate. It is not clear at the present time whether both subunits are identical gene products, however, both have identical N-terminal sequences for the first 15 amino acids. The protein has a pI above 9.6. IVa (0.1 micrograms/ml) could aggregate platelets up to 80% and was inhibited by p-APMSF, leupeptin, iodoacetamide, protein kinase C inhibitor, phosphatase inhibitor, ATP and PGE1, while it was insensitive to acetylsalicylic acid, ADP scavenger system, protein kinase A inhibitor and hirudin. Protein IVa is a serine proteinase with thrombin-like activity as it hydrolysed thrombin chromogenic substrate CBS 34.47, its aggregatory activity was partially inhibited by monoclonal antibodies against GPIb and the thrombin receptor, as was the thrombin, and its ability to induce intracellular Ca2+ release was blocked by pretreating platelets with thrombin. Unlike thrombin, the IVa protein showed very weak coagulant activity as indicated by plasma recalcification time and fibrinogen clotting time although it could hydrolyse fibrinogen alpha-chains.

An antiplatelet peptide, gabonin, from Bitis gabonica snake venom

Interaction of fibrinogen with its receptors (glycoprotein IIb/IIIa complex) on platelet membranes leads to platelet aggregation. By means of gel filtration, CM-Sephadex C-50, and reverse-phase HPLC, an antiplatelet peptide, gabonin, was purified from the venom of Bitis gabonica. The purified protein migrates as a 21,100-Da polypeptide on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions and as a 11,000-Da peptide in the presence of beta-mercaptoethanol, indicating that gabonin is a disulfide-linked dimer. It is a polypeptide consisting of about 84 amino acid residues, rich in Asp, Pro, and half-cystine. Gabonin dose-dependently inhibited human platelet aggregation stimulated by ADP, collagen, U46619, or thrombin in preparations of platelet-rich plasma and platelet suspension (IC50 = 340-1600 nM). It also blocked platelet aggregation of whole blood. However, it apparently did not affect the initial shape change and only slightly reduced ATP release caused by aggregation agonists. Gabonin did not inhibit the rise of cytosolic calcium in Quin-2-loaded platelets stimulated by thrombin. In addition, gabonin dose-dependently inhibited fibrinogen-induced aggregation of elastase-treated platelets. In conclusion, gabonin inhibits platelet aggregation mainly through the blockade of fibrinogen binding toward fibrinogen receptors of the activated platelets.

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.

A potent antiplatelet peptide, triflavin, from Trimeresurus flavoviridis snake venom

The interaction of fibrinogen with its receptors on platelet surfaces leads to platelet aggregation. A snake-venom peptide, trigramin, has previously been demonstrated to inhibit platelet aggregation by acting as a fibrinogen-receptor antagonist. By means of gel filtration, ionic-exchange chromatography and reverse-phase h.p.l.c., a potent platelet-aggregation inhibitor, triflavin, has now been purified from the venom of Trimeresurus flavoviridis. The purified triflavin is a single-chain polypeptide, consisting of about 71 amino acid residues with a molecular mass of 7600 Da, and its N-terminal sequence is Gly-Glu-Glu-Cys-Asp. Triflavin dose-dependently inhibited human platelet aggregation stimulated by ADP, adrenaline, collagen, thrombin or prostaglandin endoperoxide analogue U46619 in preparations of platelet-rich plasma, platelet suspension and whole blood. Its IC50 ranged from 38 to 84 nM, depending on the aggregation inducer used and the platelet preparation. However, triflavin apparently did not affect the platelet shape change and ATP-release reactions caused by these agonists. Triflavin inhibited fibrinogen-induced aggregation of human elastase-treated platelets in a dose-dependent manner, indicating that it directly interferes with the binding of fibrinogen to its receptors on platelet membranes exposed by elastase treatment. Additionally, triflavin dose-dependently blocked 125I-labelled fibrinogen binding to ADP-activated platelets. In conclusion, triflavin inhibits platelet aggregation through the blockade of fibrinogen binding to fibrinogen receptors on platelet membranes.

Purification and characterization of an antiplatelet peptide, arietin, from Bitis arietans venom

By means of Fractogel TSK-50, CM-Sephadex C-50 column chromatography, gel filtrations on Sephadex G-75 and Sephacryl S-200 columns and reverse-phase HPLC, an antiplatelet peptide, arietin, was purified from venom of Bitis arietans. Arietin was shown to be an Arg-Gly-Asp-containing peptide with a NH2-terminus, Ser-Pro-Pro-Val-Cys-Gly-Asn-Lys- (Mr 8500). Arietin dose-dependently inhibited aggregation of human platelet suspension stimulated by ADP, thrombin, collagen and U46619 with IC50, 1.3-2.7.10(-7) M, while it had no effect on the initial shape changes and only slightly affected ATP release of platelets caused by thrombin and collagen. Arietin also blocked platelet aggregation in platelet-rich plasma and whole blood, and inhibited thrombin-induced clot retraction of platelet-rich plasma. Furthermore, arietin (6.5.10(-8) M) completely blocked the fibrinogen-induced aggregation of elastase-treated platelets, indicating that arietin interferes with the fibrinogen binding to fibrinogen receptors on platelet membranes. In conclusion, arietin, an Arg-Gly-Asp-containing peptide, inhibits platelet aggregation probably through the blockade of fibrinogen binding to the activated platelets.

Effects of snake venoms on hemostasis

Proteins found in venoms, especially of the Viperidae snake family, exert, often with a narrow specificity, activating, inactivating, or other converting effects on different components of the hemostatic and fibrinolytic systems, respectively. Some purified snake venom proteins have become valuable tools in basic research and in diagnostic procedures in hemostaseology. "Procoagulant" as well as "anticoagulant" venom components have been identified in in vitro test systems. "Procoagulant" snake venom components may cause in vivo, upon massive application as in the case of snake-bite of small prey animals, intravascular coagulation leading to circulatory arrest and rapid death. Smaller doses of procoagulant venom components applied to large organisms as in the case of snake-bite accidents in humans, may cause a consumption coagulopathy with localized or generalized bleeding. Highly purified, specific fibrinogen coagulant venom proteinases are used in human medicine to produce therapeutic defibrinogenation. These practically nontoxic venom enzymes may act synergistically with other components aggravating their toxic effects.

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.

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.