A phospholipase A2 with anticoagulant activity. II. Inhibition of the phospholiped activity in coagulation

Daboialipase, a phospholipase A2 from Vipera russelli russelli venom posesses anti-platelet, anti-thrombin and anti-cancer properties

Snake venoms are known to contain toxins capable of interfering with normal physiological processes of victims. Specificity of toxins from snake venoms give scope to identify new molecules with therapeutic action and/or help to understand different cellular mechanisms. Russell's viper venom (RVV) is a mixture of many bioactive molecules with enzymatic and non-enzymatic proteins. The present article describes Daboialipase (DLP), an enzymatic phospholipase A2 with molecular mass of 14.3 kDa isolated from RVV. DLP was obtained after cation exchange chromatography followed by size-exclusion high performance liquid chromatography (SE-HPLC). The isolated DLP presented strong inhibition of adenosine di-phosphate (ADP) and collagen induced platelet aggregation. It also showed anti-thrombin properties by significantly extending thrombin time in human blood samples. Trypan blue and resazurin cell viability assays confirmed time-dependent cytotoxic and cytostatic activities of DLP on MCF7 breast cancer cells, in vitro. DLP caused morphological changes and nuclear damage in MCF7 cells. However, DLP did not cause cytotoxic effects on non-cancer HaCaT cells. Peptide sequences of DLP obtained by O-HRLCMS analysis showed similarity with a previously reported PLA2 (Uniprot ID: PA2B_DABRR/PDB ID: 1VIP_A). An active Asp at 49th position, calcium ion binding site and anticoagulant activity sites were identified in 1 VIP_A. These findings are expected to contribute to designing new anti-platelet, anticoagulant and anti-cancer molecules.

Biochemistry and toxicology of proteins and peptides purified from the venom of Vipera berus berus

The isolation and characterization of individual snake venom components is important for a deeper understanding of the pathophysiology of envenomation and for improving the therapeutic procedures of patients. It also opens possibilities for the discovery of novel toxins that might be useful as tools for understanding cellular and molecular processes. The variable venom composition, toxicological and immunological properties of the common vipers (Vipera berus berus) have been reviewed. The combination of venom gland transcriptomics, bottom-up and top-down proteomics enabled comparison of common viper venom proteomes from multiple individuals. V. b. berus venom contains proteins and peptides belonging to 10–15 toxin families: snake venom metalloproteinase, phospholipases A₂ (PLA₂), snake venom serine proteinase, aspartic protease, L-amino acid oxidase (LAAO), hyaluronidase, 5′-nucleotidase, glutaminyl-peptide cyclotransferase, disintegrin, C-type lectin (snaclec), nerve growth factor, Kunitz type serine protease inhibitor, snake venom vascular endothelial growth factor, cysteine-rich secretory protein, bradykinin potentiating peptide, natriuretic peptides. PLA₂ and LAAO from V. b. berus venom produce more pronounced cytotoxic effects in cancer cells than normal cells, via induction of apoptosis, cell cycle arrest and suppression of proliferation. Proteomic data of V. b. berus venoms from different parts of Russia and Slovakian Republic have been compared with analogous data for Vipera nikolskii venom. Proteomic studies demonstrated quantitative differences in the composition of V. b. berus venom from different geographical regions. Differences in the venom composition of V. berus were mainly driven by the age, sex, habitat and diet of the snakes. The venom variability of V. berus results in a loss of antivenom efficacy against snakebites. The effectiveness of antibodies is discussed. This review presents an overview with a special focus on different toxins that have been isolated and characterized from the venoms of V. b. berus. Their main biochemical properties and toxic actions are described.

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Vipera berus berus venom composition is variable among different populations.

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Venom contains about 15 protein/peptide families.

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It disturbs blood coagulation inducing pro- or anticoagulant effects.

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Venom contains different types of blood factor X activators.

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PLA₂ and L-amino acid oxidase produce cytotoxic effects in cancer cells.

Pharmacological Characterisation of Pseudocerastes and Eristicophis Viper Venoms Reveal Anticancer (Melanoma) Properties and a Potentially Novel Mode of Fibrinogenolysis

Venoms are a rich source of potential lead compounds for drug discovery, and descriptive studies of venom form the first phase of the biodiscovery process. In this study, we investigated the pharmacological potential of crude Pseudocerastes and Eristicophis snake venoms in haematological disorders and cancer treatment. We assessed their antithrombotic potential using fibrinogen thromboelastography, fibrinogen gels with and without protease inhibitors, and colourimetric fibrinolysis assays. These assays indicated that the anticoagulant properties of the venoms are likely induced by the hydrolysis of phospholipids and by selective fibrinogenolysis. Furthermore, while most fibrinogenolysis occurred by the direct activity of snake venom metalloproteases and serine proteases, modest evidence indicated that fibrinogenolytic activity may also be mediated by selective venom phospholipases and an inhibitory venom-derived serine protease. We also found that the Pseudocerastes venoms significantly reduced the viability of human melanoma (MM96L) cells by more than 80%, while it had almost no effect on the healthy neonatal foreskin fibroblasts (NFF) as determined by viability assays. The bioactive properties of these venoms suggest that they contain a number of toxins suitable for downstream pharmacological development as candidates for antithrombotic or anticancer agents.

Variability in venom composition of European viper subspecies limits the cross-effectiveness of antivenoms

Medically relevant cases of snakebite in Europe are predominately caused by European vipers of the genus Vipera. Systemic envenoming by European vipers can cause severe pathology in humans and different clinical manifestations are associated with different members of this genus. The most representative vipers in Europe are V. aspis and V. berus and neurological symptoms have been reported in humans envenomed by the former but not by the latter species. In this study we determined the toxicological profile of V. aspis and V. berus venoms in vivo in mice and we tested the effectiveness of two antivenoms, commonly used as antidotes, in counteracting the specific activities of the two venoms. We found that V. aspis, but not V. berus, is neurotoxic and that this effect is due to the degeneration of peripheral nerve terminals at the NMJ and is not neutralized by the two tested antisera. Differently, V. berus causes a haemorrhagic effect, which is efficiently contrasted by the same antivenoms. These results indicate that the effectiveness of different antisera is strongly influenced by the variable composition of the venoms and reinforce the arguments supporting the use polyvalent antivenoms.

Exactin: A specific inhibitor of Factor X activation by extrinsic tenase complex from the venom of Hemachatus haemachatus

Unwanted clots lead to heart attack and stroke that result in a large number of deaths. Currently available anticoagulants have some drawbacks including their non-specific actions. Therefore novel anticoagulants that target specific steps in the coagulation pathway are being sought. Here we describe the identification and characterization of a novel anticoagulant protein from the venom of Hemachatus haemachatus (African Ringhals cobra) that specifically inhibits factor X (FX) activation by the extrinsic tenase complex (ETC) and thus named as exactin. Exactin belongs to the three-finger toxin (3FTx) family, with high sequence identity to neurotoxins and low identity to the well-characterized 3FTx anticoagulants-hemextin and naniproin. It is a mixed-type inhibitor of ETC with the kinetic constants, Ki’ and Ki determined as 30.62 ± 7.73 nM and 153.75 ± 17.96 nM, respectively. Exactin does not bind to the active site of factor VIIa and factor Xa based on its weak inhibition (IC₅₀ ≫ 300 μM) to the amidolytic activities of these proteases. Exactin shows exquisite macromolecular specificity to FX activation as compared to factor IX activation by ETC. Exactin thus displays a distinct mechanism when compared to other anticoagulants targeting ETC, with its selective preference to ETC-FX [ES] complex.

Daboxin P, a Major Phospholipase A2 Enzyme from the Indian Daboia russelii russelii Venom Targets Factor X and Factor Xa for Its Anticoagulant Activity

In the present study a major protein has been purified from the venom of Indian Daboia russelii russelii using gel filtration, ion exchange and Rp-HPLC techniques. The purified protein, named daboxin P accounts for ~24% of the total protein of the crude venom and has a molecular mass of 13.597 kDa. It exhibits strong anticoagulant and phospholipase A2 activity but is devoid of any cytotoxic effect on the tested normal or cancerous cell lines. Its primary structure was deduced by N-terminal sequencing and chemical cleavage using Edman degradation and tandem mass spectrometry. It is composed of 121 amino acids with 14 cysteine residues and catalytically active His48 -Asp49 pair. The secondary structure of daboxin P constitutes 42.73% of α-helix and 12.36% of β-sheet. It is found to be stable at acidic (pH 3.0) and neutral pH (pH 7.0) and has a Tm value of 71.59 ± 0.46°C. Daboxin P exhibits anticoagulant effect under in-vitro and in-vivo conditions. It does not inhibit the catalytic activity of the serine proteases but inhibits the activation of factor X to factor Xa by the tenase complexes both in the presence and absence of phospholipids. It also inhibits the tenase complexes when active site residue (His48) was alkylated suggesting its non-enzymatic mode of anticoagulant activity. Moreover, it also inhibits prothrombinase complex when pre-incubated with factor Xa prior to factor Va addition. Fluorescence emission spectroscopy and affinity chromatography suggest the probable interaction of daboxin P with factor X and factor Xa. Molecular docking analysis reveals the interaction of the Ca+2 binding loop; helix C; anticoagulant region and C-terminal region of daboxin P with the heavy chain of factor Xa. This is the first report of a phospholipase A2 enzyme from Indian viper venom which targets both factor X and factor Xa for its anticoagulant activity.

Comparative analysis of the venom proteome of four important Malaysian snake species

BACKGROUND

Naja kaouthia, Ophiophagus hannah, Bungarus fasciatus and Calloselasma rhodostoma are four venomous snakes indigenous to Malaysia. In the present study, their proteomic profile by two-dimensional gel electrophoresis (2-DE) have been separated and compared.

RESULTS

The 2-DE of venoms of the four species snake demonstrated complexity and obvious interspecies differences in proteome profiles. A total of 63 proteins were identified in the four species: C. rhodostoma - 26, N. kaouthia - 16, O. hannah - 15 and B. fasciatus - 6.

CONCLUSIONS

Despite the identifications of major proteins in the four snake species, a large number of protein spots from the 2-DE were unidentified even though the spots displayed high-quality MALDI-TOF-MS spectra. Those identified included phospholipase A2 proteins in all four venoms, long neurotoxins in both cobra species and the C. rhodostoma venom found with the most varied types of peptidases, i.e. metalloproteinase kistomin, halystase and L-amino acid oxidase.

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Haemostatic abnormalities and clinical findings in Vipera palaestinae-envenomed dogs

The venomous viper Vipera palaestinae (Vp) is responsible for most envenomations in humans and animals in Israel. Its venom contains proteases, haemorrhagins, L-amino acid oxidase and phospholipase A2 but its effects on haemostasis have yet to be characterised. This prospective study aimed to characterise haemostatic abnormalities in Vp-envenomed dogs from presentation to discharge or death, and their association with mortality. Samples from 39 Vp-envenomed dogs were collected periodically and examined for haematology, prothrombin time (PT), activated partial thromboplastin time (aPTT), antithrombin activity (ATA), fibrinogen level and D-dimer concentration. All dogs presented with severe local signs and most (79%) had systemic signs. Six dogs (15%) died. Haemostatic abnormalities were present in 37/39 dogs. Increased D-dimer concentrations were detected in 28/31 dogs. Disseminated intravascular coagulation was diagnosed in 10 dogs and in all non-survivors. Platelet and leucocyte counts at presentation, maximum PT and aPTT, and minimum ATA during hospitalisation were significantly different between survivors and non-survivors and were good predictors of the outcome. The results show that hypercoagulability, consumption and derangement of haemostasis are common in Vp-envenomed dogs and are associated with mortality. Haemostasis should be closely monitored in such dogs.

Expression of human recombinant antibody fragments capable of partially inhibiting the phospholypase activity of Crotalus durissus terrificus venom

Crotoxin is the main toxic component of the South American rattlesnake Crotalus durissus terrificus venom. It is composed of two different subunits: CA, crotapotin, and CB (basic subunit of cortoxin isolated from C. d. terrificus), a weakly toxic phospholipase A(2) with high enzymatic activity. The phospholipases A(2) are abundant in snake venoms and are responsible for disruption of cell membrane integrity via hydrolysis of its phospholipids. However, in addition to their normal digestive action, a wide range of pharmacological activities, such as neurotoxic, myotoxic, oedema-inducing, hypotensive, platelet-aggregating, cardiotoxic, and anticoagulant effects have been attributed to venom phospholipases A(2). In this study, we used a non-immune human single-chain fragment variable library, Griffin.1 (Medical Research Council, Cambridge, UK) for selection of recombinant antibodies against antigens present in C. d. terrificus venom and identification of specific antibodies able to inhibit the phospholipase activity. Two clones were identified as capable of inhibiting partially this activity in vitro. These clones were able to reduce in vivo the myotoxic and oedema-inducing activity of CB and the lethality of C. d. terrificus venom and crotoxin, but had no effect on the in vitro anticoagulant activity of CB. These results demonstrate the potential of using recombinant single-chain fragment variable libraries in the production of antivenoms.

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.

Expression of recombinant human antibody fragments capable of inhibiting the phospholipase and myotoxic activities of Bothrops jararacussu venom

Phospholipases A(2) are components of Bothrops venoms responsible for disruption of cell membrane integrity via hydrolysis of its phospholipids. This study used a large nonimmune human scFv library named Griffin.1 (MRC, Cambridge, UK) for selection of recombinant antibodies against antigens present in Bothrops jararacussu venom and identification of specific antibodies able to inhibit phospholipase activity. Four clones were identified as capable of inhibiting this activity in vitro. These clones were able to reduce in vivo the myotoxic activity of BthTX-I and BthTX-II PLA(2), but had no effect on the in vitro anticoagulant activity of BthTX-II. This work shows the potential of using recombinant scFv libraries in the search for antibodies that neutralize relevant venom components.

Anticoagulant proteins from snake venoms: structure, function and mechanism

Over the last several decades, research on snake venom toxins has provided not only new tools to decipher molecular details of various physiological processes, but also inspiration to design and develop a number of therapeutic agents. Blood circulation, particularly thrombosis and haemostasis, is one of the major targets of several snake venom proteins. Among them, anticoagulant proteins have contributed to our understanding of molecular mechanisms of blood coagulation and have provided potential new leads for the development of drugs to treat or to prevent unwanted clot formation. Some of these anticoagulants exhibit various enzymatic activities whereas others do not. They interfere in normal blood coagulation by different mechanisms. Although significant progress has been made in understanding the structure-function relationships and the mechanisms of some of these anticoagulants, there are still a number of questions to be answered as more new anticoagulants are being discovered. Such studies contribute to our fight against unwanted clot formation, which leads to death and debilitation in cardiac arrest and stroke in patients with cardiovascular and cerebrovascular diseases, arteriosclerosis and hypertension. This review describes the details of the structure, mechanism and structure-function relationships of anticoagulant proteins from snake venoms.

The C-terminal and β-wing regions of ammodytoxin A, a neurotoxic phospholipase A2 from Vipera ammodytes ammodytes, are critical for binding to factor Xa and for anticoagulant effect

Ammodytoxin A (AtxA) from the venom of Vipera ammodytes ammodytes belongs to group IIA secreted phospholipase A2 (sPLA2), for which the major pathologic activity is presynaptic neurotoxicity. We show here that this toxin also affects hemostasis because it exhibits strong anticoagulant activity. AtxA binds directly to human coagulation factor Xa (FXa) with Kdapp of 32 nM, thus inhibiting the activity of the prothrombinase complex with an IC50 of 20 nM. To map the FXa-interaction site on AtxA, various mutants of AtxA produced by site-directed mutagenesis and expressed in Escherichia coli were tested in the study. In surface plasmon resonance (SPR) measurements, with FXa covalently attached to the sensor chip, we show that the FXa-binding site on AtxA includes several basic amino acid residues at the C-terminal and beta-wing regions of the molecule. Applying an in vitro biological test for inhibition of prothrombinase activity, we further demonstrate that the same residues are also very important for the anticoagulant activity of AtxA. We conclude that the anticoagulant site of AtxA is located in the C-terminal and beta-wing regions of this phospholipase A2. Synthetic peptides comprising residues of the deduced anticoagulant site of AtxA provide a basis to synthesize novel anticoagulant drugs.

Structure–function relationships and mechanism of anticoagulant phospholipase A2 enzymes from snake venoms

Phospholipase A(2) (PLA(2)) enzymes from snake venom are toxic and induce a wide spectrum of pharmacological effects, despite similarity in primary, secondary and tertiary structures and common catalytic properties. Thus, the structure-function relationships and the mechanism of this group of small proteins are subtle, complex and intriguing challenges. This review, taking the PLA(2) enzymes from spitting cobra (Naja nigricollis) venom as examples, describes the mechanism of anticoagulant effects. The strongly anticoagulant CM-IV inhibits both the extrinsic tenase and prothrombinase complexes, whereas the weakly anticoagulant PLA(2) enzymes (CM-I and CM-II) inhibit only the extrinsic tenase complex. CM-IV binds to factor Xa and interferes in its interaction with factor Va and the formation of prothrombinase complex. In contrast, CM-I and CM-II do not affect the formation of prothrombinase complex. In addition, CM-IV inhibits the extrinsic tenase complex by a combination of enzymatic and nonenzymatic mechanisms, while CM-I and CM-II inhibit by only enzymatic mechanism. These functional differences explain the disparity in the anticoagulant potency of N. nigricollis PLA(2) enzymes. Similarly, human secretory enzyme binds to factor Xa and inhibits the prothrombinase complex. We predicted the anticoagulant region of PLA(2) enzymes using a systematic and direct comparison of amino acid sequences. This region between 54 and 77 residues is basic in the strongly anticoagulant PLA(2) enzymes and neutral or negatively charged in weakly and non-anticoagulant enzymes. The prediction is validated independently by us and others using both site directed mutagenesis and synthetic peptides. Thus, strongly anticoagulant CM-IV binds to factor Xa (its target protein) through the specific anticoagulant site on its surface. In contrast, weakly anticoagulant enzymes, which lack the anticoagulant region fail to bind specifically to the target protein, factor Xa in the coagulation cascade. Thus, these studies strongly support the target model which suggests that protein-protein interaction rather than protein-phospholipid interaction determines the pharmacological specificity of PLA(2) enzymes.

Snake venom proteins acting on hemostasis

The venoms of Viperidae and Crotalidae snakes are a rich source of proteins with activity against various factors involved in coagulation and fibrinolysis. These proteins are very specific for their molecular targets, resistant to physiological inhibitors and stable in vitro and in vivo. They have therefore proved to be useful for diagnostic tests. Based on sequence similarities, these snake venom proteins have been classified into various families, such as serine proteinases, metalloproteinases, C-type lectins, disintegrins and phospholipases A(2). The various members of a given family, although structurally similar, act selectively on different blood coagulation factors. This opens up the possibility of characterizing the structural elements involved in target molecule recognition. Thus, snake venom proteins provide excellent models for studies of structure-function relationships.

Three-dimensional structure of a presynaptic neurotoxic phospholipase A2 from Daboia russelli pulchella at 2.4 A resolution

The phospholipase A(2 )from Daboia russelli pulchella (DPLA(2)) is the only known member of subclass II of group IIA. The three-dimensional structure of this presynaptic neurotoxic DPLA(2) enzyme has been determined at 2.4 A resolution. The structure was determined by the molecular replacement method using the model Crotalus atrox, and refined using X-PLOR to a final R-factor of 18.8 % for all data in the resolution range 20.0 A-2.4 A. The final refined model comprises 1888 atoms from two crystallographically independent protein molecules and 160 water oxygen atoms. The overall folding of DPLA(2), with three long helices and two short antiparallel beta-strands is grossly similar to those observed for other PLA(2)s. In the present structure, the calcium binding site is empty but the conformation of the calcium binding loop is similar to those observed in the calcium bound states. Two spatially adjacent regions of residues 55-61 (a typical beta-turn I) and 83-94 (a well defined loop) are remarkably different in conformation, electrostatic characteristics and inter-segmental interactions from those found in non-neurotoxic PLA(2)s. Yet another striking structural feature in DPLA(2 )pertains to the stretch of residues 53-77, which has a series of positively charged residues protruding outwardly. The above segment is presumed to be involved in the anticoagulant activity. A unique hydrophobic patch including residues Leu17, Ala18, Ile19, Pro20, Phe106 and Leu110 is found on the surface together with an equally emphatic region of -OH groups containing residues such as Ser21, Tyr22, Ser23, Ser24, Tyr25 and Tyr28. The interactions between two molecules of DPLA(2) in the asymmetric unit are remarkably different from those observed in the standard dimers and trimers of PLA(2)s, leaving the enzyme's active site fully exposed for enzyme-substrate reactions, it makes this structure one of the most favourable examples for structure-based drug design through soaking experiments.

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.

Structural elements of Trimeresurus flavoviridis serum inhibitors for recognition of its venom phospholipase A2 isozymes

Five inhibitors (PLI-I-V) against Trimeresurus flavoviridis (Tf, habu snake, Crotalinae) venom phospholipase A2 (PLA2) isozymes have been isolated from its serum. PLI-I, which is composed of two repeated three-finger motifs, and PLI-IV and PLI-V, which contain a sequence similar to the carbohydrate recognition domain (CRD) of C-type lectins, were expressed in the forms fused with glutathione S-transferase (GST). The resulting GST-PLIs showed ability to bind to three Tf venom PLA2 isozymes. The binding study with the truncated forms indicated that one of two three-finger motifs of PLI-I was able to bind to PLA2 isozymes. The N-terminal 37-amino acid fragment and the CRD-like domain of PLI-IV and PLI-V were bound to PLA2 isozymes. On the other hand, their C-terminal 12-amino acid segment also associated with PLA2 isozymes. When either of two units of a hydrophobic tripeptide in this sequence was replaced by trialanine, the binding was completely abolished, indicating that the C-terminal hydrophobic cores of PLI-IV and PLI-V were critically responsible for the binding to venom PLA2 isozymes.

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.

Accelerated evolution of snake venom phospholipase A2 isozymes for acquisition of diverse physiological functions

The nucleotide sequences of two cDNAs and four genes encoding Trimeresurus gramineus venom gland phospholipase A2 (PLA2) isozymes were determined and compared internally and externally with those encoding Trimeresurus flavoviridis venom gland PLA2 isozymes. It was revealed that the protein-coding regions are much more diversified than the 5' and 3' untranslated regions (UTRs) and the introns except for the signal peptide domain. The numbers of nucleotide substitutions per site (KN) for the UTRs and the introns were approximately one-quarter of the numbers of nucleotide substitutions per synonymous site (KS) for the protein-coding regions and were at the same level as the KN value of T. gramineus and T. flavoviridis TATA box-binding protein (TBP) genes, indicating that the protein-coding regions of PLA2 isozyme genes are unusually variable and that the UTRs including the introns of venom gland PLA2 isozyme genes have evolved at similar rate to those of non-venomous genes. The numbers of nucleotide substitutions per non-synonymous site (KA) values were close to or larger than the KS values for the protein-coding regions in venom gland PLA2 isozyme genes, indicating that the protein-coding regions of snake venom gland PLA2 isozyme genes have evolved via accelerated evolution. Furthermore, the evolutionary trees derived from the combined sequences of the 5' and 3' UTRs and the signal peptide domain of cDNAs were in accord with the consequences from taxonomy. In contrast, the evolutionary trees from the mature protein-coding region sequences of cDNAs and from the amino acid sequences showed random patterns. Estimations of nucleotide divergence of genes and the phylogenetic analysis reveal that snake venom group IJ PLA2 isozyme genes have been evolving under adaptive pressure to acquire new physiological activities.

The role of enzymatic activity in inhibition of the extrinsic tenase complex by phospholipase A2 isoenzymes from Naja nigricollis venom

Three phospholipase A2 isoenzymes from Naja nigricollis venom inhibit the extrinsic tenase complex. We examined the role of their enzymatic activity in this inhibition by studying the effects of native and His-modified enzymes. Only CM-IV of the His-modified, catalytically inactive proteins showed significant inhibition of the activity of the complex. This indicates that strongly anticoagulant CM-IV inhibits the complex by both enzymatic and nonenzymatic mechanisms, whereas the weakly anticoagulant isoenzymes, CM-I and CM-II, inhibit primarily by catalytic degradation of phospholipids. This indicates a functional difference in the mode of inhibition between strongly and weakly anticoagulant phospholipase A2 enzymes.

Cobras

Human encounters with cobras in Africa and Asia are common with substantial mortality and primarily respiratory paralysis in untreated cases. First aid includes maintaining calm, placing a lymphatic retardant on the bitten extremity, and transportation to a medical facility. Medical management is antivenin. Physiologic monitoring and the ability to manage the compromised airway are important.

Biological properties of the venom of the Papuan black snake (Pseudechis papuanus): presence of a phospholipase A2 platelet inhibitor

The whole venom of Pseudechis papuanus, in addition to its anticoagulant activity, powerfully inhibited platelet aggregation induced by ADP, adrenaline, collagen, ristocetin and thrombin. High levels of phospholipase A2 (PLA2) activity were detected. A mild procoagulant activity was also observed. Following exposure of platelets to P. papuanus venom, platelet factor 3 (procoagulant platelet phospholipid) showed decreased cofactor activity in factor X activation by Russell's viper, venom suggesting that the venom PLA2 plays a major role in the inhibition of the coagulation mechanism. In vivo rodent assays confirmed the inhibitory effect on platelets and the haemorrhagic and neurotoxic activities. It is possible that PLA2 is responsible for anticoagulation and that this, combined with the effect on platelet aggregation, a mild procoagulant and a moderately potent haemorrhagin, is responsible for the haemorrhagic diathesis observed in systemically envenomed patients. Polyvalent (Australia-Papua New Guinea) Commonwealth Serum Laboratories antivenom, currently used for clinical treatment of snakebite in Papua New Guinea, proved highly effective against P. papuanus venom in rodent and in vitro assays, despite the absence of this particular venom from the immunising mixture.

Human recombinant non pancreatic secreted platelet phospholipase A2 has anticoagulant activity in vitro on human plasma

Extracellular phospholipase A2 is secreted from the platelets upon activation by a stimulus such as thrombin. The secreted enzyme has been recently cloned and the recombinant protein produced. Snake venom PLA2 effect on platelet and coagulation has been extensively studied (for review see 3,4) and it has been proposed that the anticoagulant phospholipases may inhibit coagulation by competing with clotting proteins for the lipid surface. Structure function relationship for PLA2s with anticoagulant activity indicates that the activity is conferred by positively charged aminoacids between residues 54 and 77. The corresponding segment of human recombinant secreted platelet PLA2 (r-hnps-PLA2) possesses five positively charged aminoacids in this region being at the identical positions to those of PLA2s with known anticoagulant activities. Here using human and rat plasma we have demonstrated for the first time that the recombinant human extracellular secreted platelet PLA2 increases activated partial thromboplastin time but not prothrombin time.

Isolation, partial characterization and complete amino acid sequence of the toxic phospholipase A2 from the venom of the common viper, Vipera berus berus

A basic, toxic phospholipase A2 was purified from the venom of Vipera berus berus (Vbb) by a single purification step, using hydrophobic chromatography. The primary structure of isolated protein was established from peptides generated by Gly-specific papaya proteinase IV, beta-trypsin, CNBr and mild acid hydrolysis. The enzyme consists of a single chain of 122 amino acid residues with 14 Cys in positions characteristic for the phospholipase A2 subgroup IIA. As far as we know, this is the first complete Vipera berus phospholipase A2 amino acid sequence reported.

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 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.

Characterization of the anticoagulants from Taiwan cobra (Naja naja atra) snake venom

Taiwan cobra (Naja naja atra) snake venom was separated into 19 fractions by means of CM-Sephadex C-50 column chromatography. Anticoagulant Fractions V-VII were refractionated by gel filtration on Sephadex G-50 and the purified component possessed phospholipase A2 activity and an inhibitory effect on collagen-induced platelet aggregation. The anticoagulant action could be antagonized by phospholipid or platelet factor 3. Anticoagulant Fraction XVII was also further refractionated by gel filtration on Sephadex G-50 and the purified component was shown to be cardiotoxin. It was a weak anticoagulant, caused direct hemolysis and potentiated collagen-induced platelet aggregation. Thromboelastographic studies showed that the anticoagulant action of cobra venom is due to the synergistic effects of phospholipase A2 and cardiotoxin.

Some examples of the clinical importance of the basic and heparin-induced phospholipase A in human plasma

Phospholipase A activity was determined in human plasma with biological 32P-labelled phosphatidylethanolamine from rat liver following cardiac operations with the aid of the heart-lung-machine. The basic activity before heparin and the initiation of bypass was 72 nmol . ml-1 . h-1 plasma; thereby large differences in the basic activity were detected between cyanotic and acyanotic heart diseases. A sharp increase of phospholipase activity was observed after heparin (a smaller one in intensive care patients with low-dose heparin), and a further increase up to the end of perfusion. Normal values are reached after giving protamine at the end of the operation. An important role for plasma phospholipase in the regulation of physiological and pathophysiological processes may be supposed from these results.

Characterization of a fibrinogenase from northern copperhead (Agkistrodon contortrix mokasen) venom

One of the fractions obtained by the carboxymethylcellulose ion-exchange chromatography of northern copperhead (Agkistrodon contortrix mokasen) venom prevented the thrombin-induced clotting of fibrinogen by proteolytically degrading the fibrinogen. The active component has been further purified to apparent electrophoretic homogeneity by molecular sieve chromatography. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis indicated a molecular weight of 22 900 +/- 600 for the purified enzyme. In addition to its fibrinogenase activity, it catalyzed the hydrolysis of hide power azure and had an intraperitoneal LD50 value in mice of less than 5.1 microgram/g body weight. The enzyme rapidly destroyed fibrinogen's ability to form clots. Electrophoresis of fibrinogen which had been incubated only a few minutes with the fibrinogenase revealed the rapid disappearance of the alpha-chain and the appearance of lower molecular weight fragments. The neutral pH optimum and ethylenediamine-tetraacetic acid (EDTA) and dithiothreitol sensitivity indicated that this enzyme belonged to the class metalloproteinases. Atomic absorption studies have revealed one zinc atom per molecule of protein. The apoenzyme's activity was restored by incubation with ZnCl2.

Correlation of enzymatic activity and anticoagulant properties of phospholipase A2

Some highly purified phospholipases A from the venom of viperidae, crotalidae and elapidae were found to hve anticoagulant properties. All phospholipases which exhibited anticoagulant properties are characterized by a high isoelectric point, but not all strongly basic phospholipases are anticoagulant. Anticoagulant phospholipases hydrolyse highly packed monomolecular films of phospholipids without any lag time while non-anticoagulant phospholipases present considerable induction times indicative of a low penetrating power. When the ester linkages in the procoagulant lipids were replaced by the non-hydrolysable ether bonds, the mixture retained its clotting ability even in the presence of phospholipases, thus suggesting that anticoagulant phospholipases prevent clot formation by hydrolysis of phospholipids. This was confirmed by chemical modification of phospholipases, viz. alkylation of the active-centre histidine with 1-bromo-octan-2-one. This modification yielded proteins which had lost their anticoagulant properties but which retained a high affinity for phospholipids.