Analysis of cDNAs encoding the two subunits of crotoxin, a phospholipase A2 neurotoxin from rattlesnake venom: the acidic non enzymatic subunit derives from a phospholipase A2-like precursor

Animal Toxins: A Historical Outlook at the Institut Pasteur of Paris

Humans have faced poisonous animals since the most ancient times. It is recognized that certain animals, like specific plants, produce toxic substances that can be lethal, but that can also have therapeutic or psychoactive effects. The use of the term "venom", which initially designated a poison, remedy, or magic drug, is now confined to animal poisons delivered by biting. Following Louis Pasteur's work on pathogenic microorganisms, it was hypothesized that venoms could be related to bacterial toxins and that the process of pathogenicity attenuation could be applied to venoms for the prevention and treatment of envenomation. Cesaire Phisalix and Gabriel Bertrand from the National Museum of Natural History as well as Albert Calmette from the Institut Pasteur in Paris were pioneers in the development of antivenomous serotherapy. Gaston Ramon refined the process of venom attenuation for the immunization of horses using a formalin treatment method that was successful for diphtheria and tetanus toxins. This paved the way for the production of antivenomous sera at the Institut Pasteur, as well as for research on venom constituents and the characterization of their biological activities. The specific activities of certain venom components, such as those involved in blood coagulation or the regulation of chloride ion channels, raises the possibility of developing novel therapeutic drugs that could serve as anticoagulants or as a treatment for cystic fibrosis, for example. Scientists of the Institut Pasteur of Paris have significantly contributed to the study of snake venoms, a topic that is reported in this review.

Crotalus durissus terrificus crotapotin naturally displays preferred positions for amino acid substitutions

Background

Classically, Crotalus durissus terrificus (Cdt) venom can be described, according to chromatographic criteria, as a simple venom, composed of four major toxins, namely: gyroxin, crotamine, crotoxin and convulxin. Crotoxin is a non-covalent heterodimeric neurotoxin constituted of two subunits: an active phospholipase A₂ and a chaperone protein, termed crotapotin. This molecule is composed of three peptide chains connected by seven disulfide bridges. Naturally occurring variants/isoforms of either crotoxin or crotapotin itself have already been reported.

Methods

The crude Cdt venom was separated by using RP-HPLC and the toxins were identified by mass spectrometry (MS). Crotapotin was purified, reduced and alkylated in order to separate the peptide chains that were further analyzed by mass spectrometry and de novo peptide sequencing.

Results

The RP-HPLC profile of the isolated crotapotin chains already indicated that the α chain would present isoforms, which was corroborated by the MS and tandem mass spectrometry analyses.

Conclusion

It was possible to observe that the Cdt crotapotin displays a preferred amino acid substitution pattern present in the α chain, at positions 31 and 40. Moreover, substitutions could also be observed in β and γ chains (one for each). The combinations of these four different peptides, with the already described chains, would produce ten different crotapotins, which is compatible to our previous observations for the Cdt venom.

Crystallographic characterization of functional sites of crotoxin and ammodytoxin, potent β-neurotoxins from Viperidae venom

This review will focus on a description of the three-dimensional structures of two β-neurotoxins, the monomeric PLA(2) ammodytoxin from Vipera ammodytes ammodytes, and heterodimeric crotoxin from Crotalus durissus terrificus, and a detailed structural analysis of their multiple functional sites. We have recently determined at high resolution the crystal structures of two natural isoforms of ammodytoxin (AtxA and AtxC) (Saul et al., 2010) which exhibit different toxicity profiles and different anticoagulant properties. Comparative structural analysis of these two PLA(2) isoforms, which differ only by two amino acid residues, allowed us to detect local conformational changes and delineate the role of critical residues in the anticoagulant and neurotoxic functions of these PLA(2) (Saul et al., 2010). We have also determined, at 1.35Å resolution, the crystal structure of heterodimeric crotoxin (Faure et al., 2011). The three-dimensional structure of crotoxin revealed details of the binding interface between its acidic (CA) and basic (CB) subunits and allowed us to identify key residues involved in the stability and toxicity of this potent heterodimeric β-neurotoxin (Faure et al., 2011). The precise spatial orientation of the three covalently linked polypeptide chains in the mature CA subunit complexed with CB helps us to understand the role played by critical residues of the CA subunit in the increased toxicity of the crotoxin complex. Since the CA subunit is a natural inhibitor of the catalytic and anticoagulant activities of CB, identification of the CA-CB binding interface describes residues involved in this inhibition. We propose future research directions based on knowledge of the recently reported 3D structures of crotoxin and ammodytoxin.

Structural basis for functional diversity of animal toxins

The diversity of biological functions that are exerted by toxins from snake and scorpion venoms is associated with a limited number of structural frameworks. At present, one predominant basic fold has been observed among scorpion toxins whereas six folds have been found among snake toxins. Most toxin folds have the capacity to accept multiple insertions, deletions and mutations and to exert various recognition functions. We suggest that such folds may serve as guides to engineer new protein functions.

Crotoxin and phospholipases A₂ from Crotalus durissus terrificus showed antiviral activity against dengue and yellow fever viruses

Dengue is the most important arbovirus in the world with an estimated of 50 million dengue infections occurring annually and approximately 2.5 billion people living in dengue endemic countries. Yellow fever is a viral hemorrhagic fever with high mortality that is transmitted by mosquitoes. Effective vaccines against yellow fever have been available for almost 70 years and are responsible for a significant reduction of occurrences of the disease worldwide; however, approximately 200,000 cases of yellow fever still occur annually, principally in Africa. Therefore, it is a public health priority to develop antiviral agents for treatment of these virus infections. Crotalus durissus terrificus snake, a South American rattlesnake, presents venom with several biologically actives molecules. In this study, we evaluated the antiviral activity of crude venom and isolated toxins from Crotalus durissus terrificus and found that phospholipases A₂ showed a high inhibition of Yellow fever and dengue viruses in VERO E6 cells.

Enzymatic toxins from snake venom: structural characterization and mechanism of catalysis

Snake venoms are cocktails of enzymes and non-enzymatic proteins used for both the immobilization and digestion of prey. The most common snake venom enzymes include acetylcholinesterases, l-amino acid oxidases, serine proteinases, metalloproteinases and phospholipases A(2) . Higher catalytic efficiency, thermal stability and resistance to proteolysis make these enzymes attractive models for biochemists, enzymologists and structural biologists. Here, we review the structures of these enzymes and describe their structure-based mechanisms of catalysis and inhibition. Some of the enzymes exist as protein complexes in the venom. Thus we also discuss the functional role of non-enzymatic subunits and the pharmacological effects of such protein complexes. The structures of inhibitor-enzyme complexes provide ideal platforms for the design of potent inhibitors which are useful in the development of prototypes and lead compounds with potential therapeutic applications.

cDNA and deduced primary structure of basic phospholipase A2 with neurotoxic activity from the venom secretion of the Crotalus durissus collilineatus rattlesnake

To illustrate the construction of precursor complementary DNAs, we isolated mRNAs from whole venom samples. After reverse transcription polymerase chain reaction (RT-PCR), we amplified the cDNA coding for a neurotoxic protein, phospholipase A2 D49 (PLA2 D49), from the venom of Crotalus durissus collilineatus (Cdc PLA2). The cDNA encoding Cdc PLA2 from whole venom was sequenced. The deduced amino acid sequence of this cDNA has high overall sequence identity with the group II PLA2 protein family. Cdc PLA2 has 14 cysteine residues capable of forming seven disulfide bonds that characterize this group of PLA2 enzymes. Cdc PLA2 was isolated using conventional Sephadex G75 column chromatography and reverse-phase high performance liquid chromatography (RP-HPLC). The molecular mass was estimated using matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry. We tested the neuromuscular blocking activities on chick biventer cervicis neuromuscular tissue. Phylogenetic analysis of Cdc PLA2 showed the existence of two lines of N6-PLA2, denominated F24 and S24. Apparently, the sequences of the New World's N6-F24-PLA2 are similar to those of the agkistrodotoxin from the Asian genus Gloydius. The sequences of N6-S24-PLA2 are similar to the sequence of trimucrotoxin from the genus Protobothrops, found in the Old World.

Crotalus durissus collilineatus venom gland transcriptome: analysis of gene expression profile

Crotalus durissus rattlesnakes are responsible for the most lethal cases of snakebites in Brazil. Crotalus durissus collilineatus subspecies is related to a great number of accidents in Southeast and Central West regions, but few studies on its venom composition have been carried out to date. In an attempt to describe the transcriptional profile of the C. durissus collilineatus venom gland, we generated a cDNA library and the sequences obtained could be identified by similarity searches on existing databases. Out of 673 expressed sequence tags (ESTs) 489 produced readable sequences comprising 201 singletons and 47 clusters of two or more ESTs. One hundred and fifty reads (60.5%) produced significant hits to known sequences. The results showed a predominance of toxin-coding ESTs instead of transcripts coding for proteins involved in all cellular functions. The most frequent toxin was crotoxin, comprising 88% of toxin-coding sequences. Crotoxin B, a basic phospholipase A(2) (PLA(2)) subunit of crotoxin, was represented in more variable forms comparing to the non-enzymatic subunit (crotoxin A), and most sequences coding this molecule were identified as CB1 isoform from Crotalus durissus terrificus venom. Four percent of toxin-related sequences in this study were identified as growth factors, comprising five sequences for vascular endothelial growth factor (VEGF) and one for nerve growth factor (NGF) that showed 100% of identity with C. durissus terrificus NGF. We also identified two clusters for metalloprotease from PII class comprising 3% of the toxins, and two for serine proteases, including gyroxin (2.5%). The remaining 2.5% of toxin-coding ESTs represent singletons identified as homologue sequences to cardiotoxin, convulxin, angiotensin-converting enzyme inhibitor and C-type natriuretic peptide, Ohanin, crotamin and PLA(2) inhibitor. These results allowed the identification of the most common classes of toxins in C. durissus collilineatus snake venom, also showing some unknown classes for this subspecies and even for C. durissus species, such as cardiotoxins and VEGF.

Systemic and local myotoxicity induced by snake venom group II phospholipases A2: Comparison between crotoxin, crotoxin B and a Lys49 PLA2 homologue

The patterns of myotoxicity induced in mice by crotoxin, crotoxin B and a Lys49 phospholipase A(2) (PLA(2)) homologue were compared. Lys49 PLA(2)-induced local myotoxicity is reflected by creatine kinase (CK) loss in injected gastrocnemius muscle, and by a profile of CK increase in plasma characterized by a rapid increment and drop after intramuscular injection, and by a lack of CK increase in plasma after intravenous injection. In contrast, crotoxin and crotoxin B, which induce local and systemic myotoxicity, provoked a more prolonged increment in plasma CK activity upon intramuscular injection, and induced increments in plasma CK after intravenous injection. The three toxins promoted a similar extent of local myotoxicity, assessed by the loss of CK in injected gastrocnemius. A method for the quantitative assessment of the ability of toxins to induce systemic myotoxicity is proposed, based on the estimation of the ratio between the area under the curve in the plasma CK activity (total myotoxicity) to the loss of CK in injected gastrocnemius (local myotoxicity). The highest ratio corresponded to crotoxin, and the lowest corresponded to Lys49 PLA(2), the former being a systemic myotoxin and the latter a local myotoxin. Neutralization by antivenoms also differed between the toxins: a drastic reduction in plasma CK, with very poor neutralization of local CK loss, was achieved in the case of crotoxin B when antivenom was injected intravenously, whereas no neutralization was achieved in the case of Lys49 PLA(2). When tested in undifferentiated myoblasts in culture, Lys49 PLA(2) induced cytotoxicity, whereas crotoxin and crotoxin B did not, evidencing that the latter are devoid of widespread cytolytic activity. Molecular modeling analysis showed that Lys49 PLA(2) has a conspicuous cationic face, which is likely to interact with diverse membranes. In contrast, crotoxin B, despite its overall basic pI, has a lower density of positively charged residues at this molecular region. It is suggested that Lys49 PLA(2)s homologues interact, through this cationic face, with many different cell types, thus lacking specificity for muscle cells. In contrast, crotoxin B has a more selective interaction with targets in the muscle cell membrane. This selectivity might be the basis for the ability of crotoxin and crotoxin B to induce systemic myotoxicity.

Antibacterial activity of snake, scorpion and bee venoms: a comparison with purified venom phospholipase A2 enzymes

AIMS

Venoms of snakes, scorpions, bees and purified venom phospholipase A(2) (PLA(2)) enzymes were examined to evaluate the antibacterial activity of purified venom enzymes as compared with that of the crude venoms.

METHODS AND RESULTS

Thirty-four crude venoms, nine purified PLA(2)s and two L-amino acid oxidases (LAAO) were studied for antibacterial activity by disc-diffusion assay (100 microg ml(-1)). Several snake venoms (Daboia russelli russelli, Crotalus adamanteus, Naja sumatrana, Pseudechis guttata, Agkistrodon halys, Acanthophis praelongus and Daboia russelli siamensis) showed activity against two to four different pathogenic bacteria. Daboia russelli russelli and Pseudechis australis venoms exhibited the most potent activity against Staphylococcus aureus, while the rest showed only a moderate activity against one or more bacteria. The order of susceptibility of the bacteria against viperidae venoms was -S. aureus > Proteus mirabilis > Proteus vulgaris > Enterobacter aerogenes > Pseudomonas aeruginosa and Escherichia coli. The minimum inhibitory concentrations (MIC) against S. aureus was studied by dilution method (160-1.25 microg ml(-1)). A stronger effect was noted with the viperidae venoms (20 microg ml(-11)) as compared with elapidae venoms (40 microg ml(-1)). The MIC were comparable with those of the standard drugs (chloramphenicol, streptomycin and penicillin).

CONCLUSION

The present findings indicate that viperidae (D. russelli russelli) and elapidae (P. australis) venoms have significant antibacterial effects against gram (+) and gram (-) bacteria, which may be the result of the primary antibacterial components of laao, and in particular, the PLA(2) enzymes. The results would be useful for further purification and characterization of antibacterial agents from snake venoms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The activity of LAAO and PLA(2) enzymes may be associated with the antibacterial activity of snake venoms.

The co-purification of a lectin (BJcuL) with phospholipases A2 from Bothrops jararacussu snake venom by immunoaffinity chromatography with antibodies to crotoxin

Antigens of Bothrops jararacussu snake venom cross-reacting with specific antibodies against crotoxin, an Asp49 PLA(2)-containing heterodimeric complex from Crotalus durissus terrificus snake venom, were purified by two steps of immunoaffinity chromatography. The resulting fraction (Bj-F) was shown to be non-toxic (to mice and rabbits) and immunogenic to rabbits. Antibodies raised against Bj-F were able to protect mice against the lethal effect of both B. jararacussu and Crotalus durissus terrificus snake venoms. Then, the procedure developed showed to be useful for the rapid preparation of an antigen able to elicit neutralizing antibodies against the lethal activities of both venoms. Further fractionation of Bj-F revealed the concomitant presence of two major components: BJcuL, a lectin present in B. jararacussu venom, and BthTX-I, a Lys49 PLA(2) homolog, besides other molecules in minor amounts. Our data are discussed and raise the point that the presence of unrelated molecules may be taken into account when immuno-based methods are considered for purification purposes.

In vitro antimicrobial activity of natural toxins and animal venoms tested against Burkholderia pseudomallei

Background

Burkholderia pseudomallei are the causative agent of melioidosis. Increasing resistance of the disease to antibiotics is a severe problem in treatment regime and has led to intensification of the search for new drugs. Antimicrobial peptides are the most ubiquitous in nature as part of the innate immune system and host defense mechanism.

Methods

Here, we investigated a group of venoms (snakes, scorpions and honey bee venoms) for antimicrobial properties against two strains of Gram-negative bacteria Burkholderia pseudomallei by using disc-diffusion assay for in vitro susceptibility testing. The antibacterial activities of the venoms were compared with that of the isolated L-amino acid oxidase (LAAO) and phospholipase A₂ (PLA₂s) enzymes. MICs were determined using broth dilution method. Bacterial growth was assessed by measurement of optical density at the lowest dilutions (MIC 0.25 mg/ml). The cell viability was measured using tetrazolium salts (XTT) based cytotoxic assay.

Results

The studied venoms showed high antimicrobial activity. The venoms of C. adamanteus, Daboia russelli russelli, A. halys, P. australis, B. candidus and P. guttata were equally as effective as Chloramphenicol and Ceftazidime (30 μg/disc). Among those tested, phospholipase A₂ enzymes (crotoxin B and daboiatoxin) showed the most potent antibacterial activity against Gram-negative (TES) bacteria. Naturally occurring venom peptides and phospholipase A₂ proved to possess highly potent antimicrobial activity against Burkholderia pseudomallei. The XTT-assay results showed that the cell survival decreased with increasing concentrations (0.05–10 mg/mL) of Crotalus adamanteus venom, with no effect on the cell viability evident at 0.5 mg/mL.

Conclusion

This antibacterial profile of snake venoms reported herein will be useful in the search for potential antibacterial agents against drug resistant microorganisms like B. pseudomallei.

Amino acid sequence of a basic aspartate-49-phospholipase A2 from Trimeresurus flavoviridis venom and phylogenetic analysis of Crotalinae venom phospholipases A2

Trimeresurus flavoviridis snakes inhabit the southwestern islands of Japan: Amami-Oshima, Tokunoshima and Okinawa. A phospholipase A2 (PLA2) of basic nature (pI 8.5) was isolated from the venom of Amami-Oshima T. flavoviridis. Its amino acid sequence determined by the ordinary procedures was completely in accord with that predicted from the nucleotide sequence of the cDNA previously cloned from Amami-Oshima T. flavoviridis venom gland, which was named PLA-B'. It consists of 122 amino acid residues and has aspartate at position 49. It induced edema in a mouse footpad assay and caused necrosis in mouse skeletal muscles. PLA-B' is similar in sequence to PLA-B (Tokunoshima) and PL-Y (Okinawa), both basic [Asp49]PLA2s, with a few amino acid substitutions, indicating occurrence of interisland mutation. Although PLA2s of Crotalinae subfamily were phylogenetically classified into four types, PLA2 (acidic or neutral [Asp49]PLA2) type, basic [Asp49]PLA2 type, neurotoxic [Asp49]PLA2 type and [Lys49]PLA2 type, it was ascertained that PLA2s of PLA2 type and [Lys49]PLA2 type are most essential as toxic components for Crotalinae snake venoms and that basic [Asp49]PLA2-type PLA2s are uniquely contained only in the venoms of T. flavoviridis species. Prediction of physiological activities of some PLA2s was made based on their location in the phylogenetic tree. Relationship of divergence of PLA2s via accelerated evolution followed by less rapid mutation and physiological activities was discussed.

Effect of crotapotin on the biological activity of Asp49 and Lys49 phospholipases A(2) from Bothrops snake venoms

Myonecrosis, in addition to edema and other biological manifestations, are conspicuous effects of Bothrops snake venoms, some of them caused by phospholipases A(2) (PLA(2)s). Asp49-PLA(2)s are catalytically active, whereas Lys49-PLA(2)s, although highly toxic, have little or no enzymatic activity upon artificial substrates, due to a substitution of lysine for aspartic acid at position 49. Crotapotin (CA), the acidic counterpart of crotoxin PLA(2) (CB), is a PLA(2)-like protein from Crotalus durissus terrificus snake venom, and is considered a chaperone protein for CB, able to increase its lethality about ten fold, but to inhibit the formation of the rat paw edema induced by carrageenin and by snake venoms. In this study, we demonstrate that CA significantly inhibits the edema induced by BthTX-I (23% inhibition), BthTX-II (27%), PrTX-I (25%), PrTX-III (35%) and MjTX-II (10%) on the mouse paw. CK levels evoked by isolated Asp49 or Lys49-PLA(2)s were reduced by 40% to 54% in the presence of CA and, in all cases, the membrane damaging activity of the toxins was also reduced. Circular dichroism spectra of the PLA(2)s in the presence and absence of CA showed that there was not any detectable secondary structural modification due to association between CA and the myotoxins. However, Fourier Transformed Infrared (FT-IR) analysis indicated that ionic and hydrophobic contacts contributed to stabilize this interaction.

Interisland mutation of a novel phospholipase A2 from Trimeresurus flavoviridis venom and evolution of Crotalinae group II phospholipases A2

Trimeresurus flavoviridis (Crotalinae) snakes inhabit the southwestern islands of Japan: Amami-Oshima, Tokunoshima, and Okinawa. Affinity and conventional chromatographies of Amami-Oshima T. flavoviridis venom led to isolation of a novel phospholipase A2 (PLA2). This protein was highly homologous (91%) in sequence to trimucrotoxin, a neurotoxic PLA2, which had been isolated from T. mucrosquamatus (Taiwan) venom, and exhibited weak neurotoxicity. This protein was named PLA-N. Its LD50 for mice was 1.34 microg/g, which is comparable to that of trimucrotoxin. The cDNA encoding PLA-N was isolated from both the Amami-Oshima and the Tokunoshima T. flavoviridis venom-gland cDNA libraries. Screening of the Okinawa T. flavoviridis venom-gland cDNA library with PLA-N cDNA led to isolation of the cDNA encoding one amino acid-substituted PLA-N homologue, named PLA-N(O), suggesting that interisland mutation occurred and that Okinawa island was separated from a former island prior to dissociation of Amami-Oshima and Tokunoshima islands. Construction of a phylogenetic tree of Crotalinae venom group II PLA2's based on the amino acid sequences revealed that neurotoxic PLA2's including PLA-N and PLA-N(O) form an independent cluster which is distant from other PLA2 groups such as PLA2 type, basic [Asp49]PLA2 type, and [Lys49]PLA2 type. Comparison of the nucleotide sequence of PLA-N cDNA with those of the cDNAs encoding other T. flavoviridis venom PLA2's showed that they have evolved in an accelerated manner. However, when comparison was made within the cDNAs encoding Crotalinae venom neurotoxic PLA2's, their evolutionary rates appear to be reduced to a level between accelerated evolution and neutral evolution. It is likely that ancestral genes of neurotoxic PLA2's evolved in an accelerated manner until they had acquired neurotoxic function and since then they have evolved with less frequent mutation, possibly for functional conservation.

Molecular evolution of myotoxic phospholipases A2 from snake venom

After two decades of study, we draw the conclusion that venom-gland phospholipase A2 (PLA2) isozymes, including PLA2 myotoxins of Crotalinae snakes, have evolved in an accelerated manner to acquire their diverse physiological activities. In this review, we describe how accelerated evolution of venom PLA2 isozymes was discovered. This type of evolution is fundamental for other venom isozyme systems. Accelerated evolution of venom PLA2 isozyme genes is due to rapid change in exons, but not in introns and the flanking regions, being completely opposite to the case of the ordinary isozyme genes. The molecular mechanism by which proper base substitutions had occurred in the particular sites of venom isozyme genes is a puzzle to be solved in future studies. It should be noted that accelerated evolution occurred until the isozymes had acquired their particular function and, since then, they have evolved with less frequent mutation, possibly for functional conservation. We also found that interisland mutations occurred in venom PLA2 isozymes. The relationships between mutation and its driving force are speculative and the real mechanism remains a mystery.

Structure of a cardiotoxic phospholipase A(2) from Ophiophagus hannah with the "pancreatic loop"

The crystal structure of an acidic phospholipase A(2) from Ophiophagus hannah (king cobra) has been determined by molecular replacement at 2.6-A resolution to a crystallographic R factor of 20.5% (R(free)=23.3%) with reasonable stereochemistry. The venom enzyme contains an unusual "pancreatic loop." The conformation of the loop is well defined and different from those in pancreas PLA(2), showing its structural variability. This analysis provides the first structure of a PLA(2)-type cardiotoxin. The sites related to the cardiotoxic and myotoxic activities are explored and the oligomer observed in the crystalline state is described.

Mojave rattlesnakes (Crotalus scutulatus scutulatus) lacking the acidic subunit DNA sequence lack Mojave toxin in their venom

The venom composition of Mojave rattlesnakes (Crotalus scutulatus scutulatus) differs in that some individuals have Mojave toxin and others do not. In order to understand the genetic basis for this difference, genomic DNA samples from Mojave rattlesnakes collected in Arizona, New Mexico, and Texas were analyzed for the presence of DNA sequences that relate to the acidic (Mta) and basic (Mtb) subunits of this toxin. DNA samples were subjected to PCR to amplify nucleotide sequences from second to fourth exons of the acidic and basic subunits. These nucleotide sequences were cloned and sequenced. The nucleotide sequences generated aligned exactly to previously published nucleotide sequences of Mojave toxin. All DNA samples analyzed generated product using the basic subunit primers, and aligned identically to the Mtb nucleotide sequence. However, only 11 out of the 14 samples generated a product with the acidic subunit primers. These 11 sequences aligned identically to the Mta nucleotide sequence. The venom from the three snakes whose DNA did not amplify with the acidic subunit primers were not recognized by antibodies to Mojave toxin. This suggests that snakes with venom lacking Mojave toxin also lack the productive nucleotide sequence for the acidic subunit in their DNA.

Effects of chemical modifications of crotoxin B, the phospholipase A(2) subunit of crotoxin from Crotalus durissus terrificus snake venom, on its enzymatic and pharmacological activities

Crotoxin B, the basic Asp49-PLA(2) subunit from crotoxin, the main component of Crotalus durissus terrificus venom, displays myotoxic, edema-inducing, bactericidal (upon Escherichia coli), liposomal-disrupting and anticoagulant activities. Chemical modifications of His (with 4-bromophenacyl bromide, BPB), Tyr (with 2-nitrobenzenesulphonyl fluoride, NBSF), Trp (with o-nitrophenylsulphenyl chloride, NPSC) and Lys (with acetic anhydride) residues of this protein, in addition to cleavage with cyanogen bromide (CNBr) and inhibition with ethylenediaminetetraacetic acid (EDTA), were carried out in order to study their effects on enzymatic and pharmacological activities. Lethality was reduced after modification of His or Lys residues, as well as after cleavage with CNBr, while enzymatic activity was completely abolished after modification of His or incubation with EDTA. Modification of Lys or Tyr, or cleavage with CNBr, partially reduced enzymatic activity. Anticoagulant activity was modified similarly to enzymatic activity, evidencing the dependency of this pharmacological effect on catalytic activity. Myotoxicity was reduced after modification of His or Lys, as well as after cleavage with CNBr, whereas EDTA reduced this effect to a lesser extent. Bactericidal effect was significantly reduced only after modification of Lys and after cleavage with CNBr. Edema-inducing activity was partially inhibited after treatment with EDTA and strongly reduced after acetylation of Lys residues and cleavage with CNBr, being only partially reduced after His alkylation. On the other hand, liposome disrupting activity was only partially reduced after modification of His and Tyr or after cleavage with CNBr. Modification of Trp residue partially reduced lethality and myotoxicity but did not affect enzymatic or anticoagulant activities. These data indicate that enzymatic activity is relevant for some pharmacological effects induced by crotoxin B (mainly lethal, myotoxic and anticoagulant activities), and also evidence that this subunit of crotoxin displays regions different from the active catalytic site which are involved in some of the toxic and pharmacological effects induced by this phospholipase A(2).

Systemic skeletal muscle necrosis induced by crotoxin

Systemic skeletal muscle necrosis induced by crotoxin, the major component of the venom of Crotalus durissus terrificus, was investigated. Mice received an intramuscular injection of crotoxin (0.35mg/kg body weight) into the right tibialis anterior (TA) muscles, which were evaluated 3h, 24h and 3 days later. Control mice were injected with saline. Right and left TAs, gastrocnemius, soleus and right masseter and longissimus dorsi were removed and frozen. Histological sections were stained with Toluidine Blue or incubated for acidic phosphatase reaction. Three and 24h after the injection, signals of muscle fiber injury were found: (a) in the injected TA muscles; (b) in both right and contralateral soleus and red gastrocnemius; and (c) in the masseter muscles. Contralateral TA, longissimus dorsi and white gastrocnemius muscles were not injured. In conclusion, crotoxin induced a systemic and selective muscle injury in muscles or muscle regions composed by oxidative muscle fibers.

Combining phage display and molecular modeling to map the epitope of a neutralizing antitoxin antibody

Crotoxin is a potent presynaptic neurotoxin from the venom of the rattlesnake Crotalus durissus terrificus. It is composed of the noncovalent and synergistic association of a weakly toxic phospholipase A2, CB, and a nontoxic three-chain subunit, CA, which increases the lethal potency of CB. The A-56.36 mAb is able to dissociate the crotoxin complex by binding to the CA subunit, thereby neutralizing its toxicity. Because A-56.36 and CB show sequence homology and both compete for binding to CA, we postulated that A-56.36 and CB had overlapping binding sites on CA. By screening random phage-displayed libraries with the mAb, phagotopes bearing the (D/S)GY(A/G) or AAXI consensus motifs were selected. They all bound A-56.36 in ELISA and competed with CA for mAb binding, although with different reactivities. When mice were immunized with the selected clones, polyclonal sera reacting with CA were induced. Interestingly, the raised antibodies retained the crotoxin-dissociating effect of A-56.36, suggesting that the selected peptides may be used to produce neutralizing antibodies. By combining these data with the molecular modeling of CA, it appeared that the functional epitope of A-56.36 on CA was conformational, one subregion being discontinuous and corresponding to the first family of peptides, the other subregion being continuous and composed of amino acids of the second family. Phage-displayed peptides corresponding to fragments of the two identified regions on CA reacted with A-56.36 and with CB. Our data support the hypothesis that A-56.36 and CB interact with common regions of CA, and highlight residues which are likely to be critical for CA-CB complex formation.

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.

Molecular mimicry between a monoclonal antibody and one subunit of crotoxin, a heterodimeric phospholipase A2 neurotoxin

Crotoxin is a heterodimeric phospholipase A2 neurotoxin formed by the non-covalent association of an acidic and non-toxic subunit, CA, and a basic and weakly toxic phospholipase A2, CB. The two subunits behave in a synergistic manner. CA enhances the lethal potency of CB by increasing its selectivity of action. The mAb A-56.36, directed against the non-toxic subunit CA, was previously shown to neutralize crotoxin toxicity by dissociating the crotoxin complex. In the present report, a polypeptide sequence similarity was observed between some CDRs of mAb A-56.36 and two regions of CB (pos. 60-80 and 95-110). Phage displayed peptides corresponding to VH2 and VH3 of mAb A-56.36 and to their homologous sequences in CB bind CA to different extents. This observation shows that mAb A-56.36 interacts with a region of CA involved in its interaction with CB, therefore mimicking the binding of CB to CA. A similar approach was used to determine the regions of ammodytoxin A and of agkistrodotoxin, two phospholipase A2 neurotoxins similar to CB, which are involved in the formation of heterocomplexes with CA. The analysis of these data contributes to the determination of stretches of amino acids which could constitute the paratope of mAb A-56.36, as well as the region of association of CB with CA in crotoxin.

Cloning, expression and biochemical characterization of a basic-acidic hybrid phospholipase A2-II from Agkistrodon halys pallas

A cDNA encoding a basic-acidic hybrid phospholipase A2-II from Agkistrodon halys Pallas with an N-terminus highly homologous to that of BPLA2 and a C-terminus sequence almost the same as that of APLA2 was inserted into a bacterial expression vector and effectively expressed in Escherichia coli RR1. The protein was produced as insoluble inclusion bodies. After partial purification by washing, the inclusion bodies with Triton X-100, denaturing and refolding, the renatured recombinant protein was purified by FPLC column superose 12. The purified recombinant enzyme with an isoelectric point of pH 6.8 could cross-react with antiserum prepared against acidic phospholipase A2. The enzymatic activity of the expressed basic-acidic hybrid phospholipase A2-II is close to that of denatured-refolded native basic phospholipase A2, and has the same inhibiting effect on platelet aggregation as denatured-refolded acidic phospholipase A2, but lacks the hemolytic activity of denatured-refolded basic phospholipase A2. To study the structural relationships among basic phospholipase A2, acidic phospholipase A2 and basic-acidic hybrid phospholipase A2-II, molecular modeling of basic-acidic hybrid phospholipase A2-II was done. The roles of various amino acid residues in the enzymatic activity and pharmacological activities of phospholipase A2 are discussed.

Effect of site directed mutagenesis on the activity of recombinant trimucrotoxin, a neurotoxic phospholipase from Trimeresurus mucrosquamatus venom

Trimucrotoxin, the basic phospholipase A2 from Trimeresurus mucrosquamatus venom, is neurotoxic and myotoxic, and structurally similar to crotoxin B subunit. To investigate the amino acid residues responsible for its neurotoxicity, we have mutated its interface-recognition residues including a conserved Asn6 in all the Crotalinae neurotoxic phospholipases. The wild-type and the mutants were expressed in E. coli as fusion-proteins and activated in vitro by factor Xa cleavage after folding. The completion of folding and activation were checked with electrospray ionization mass spectrometry and circular dichroism measurement. Enzymatic activities and neurotoxicities toward the chick tissue of four trimucrotoxin mutants (N6A, N6E, N6R and 6E7T8L) were compared with those of the wild type which was as active as that was isolated from the venom. Mutants N6A and N6E retained more than half of the original enzymatic activity but their neurotoxicities reduced to 33% and 10% that of the wild type, respectively. Mutants N6R and 6E7T8L retained 20-25% of the enzyme activity toward the anionic micellar substrate but were inactive toward the zwitterionic micellar substrate, and their neurotoxicities were less than 3% of that of the wild type. These results demonstrate the importance of residues 6-8 in trimucrotoxin for its neuronal specificity and the specificity toward potential substrates.

Bothrops jararaca snakes produce several bothrojaracin isoforms following an individual pattern

More than one isoform of bothrojaracin (BJC), a potent and specific thrombin inhibitor isolated from Bothrops jararaca venom, has been found in individual venoms collected from adult snakes. Variations in snake venom composition have previously been associated with factors such as age, sex, geographic origin, season of the year and diet. In order to obtain further information concerning individual patterns of expression of BJC isoforms, we have analyzed five individual Bothrops jararaca snake venoms collected at the same time from adult female snakes from the same geographic region. As expected, crude venoms showed a similar migration pattern on SDS-PAGE. BJC was purified using a procedure which includes an affinity chromatography step (PPACK-thrombin Sepharose). A slight variation in the amount of BJC obtained from individual venom samples was noticed. Inhibition of thrombin-induced platelet aggregation as well as migration pattern on SDS-PAGE (under reducing and non-reducing conditions) and isoelectric focusing varied considerably among BJC samples from the five snakes. The amino-terminal sequences (residues 1-34) of individual BJC samples were compared with the sequence deduced from isolated cDNAs encoding alpha and beta chains of BJC. A high degree of homology was detected, although some residues differed from one sample to other. Altogether, data confirmed the heterogeneity found for BJC purified from individual snakes. Thus, the results indicate that: (1) individual specimens of Bothrops jararaca have different patterns of BJC isoform expression; and (2) it seems that genetic factors, at least in part, determine the variability found in BJC production.

A monoclonal antibody directed against the non-toxic subunit of a dimeric phospholipase A2 neurotoxin, crotoxin, neutralizes its toxicity

Crotoxin is the main toxic component of the venom of the South-American rattlesnake Crotalus durissus terrificus. It is a phospholipase A2 neurotoxin constituted by the association of two subunits: an acidic, non-toxic and non-enzymatic subunit (CA) and a basic, weakly toxic phospholipase A2 (CB). A murine monoclonal antibody directed to the non-toxic subunit CA, A-56.36, was shown to fully neutralize the toxicity of crotoxin. When the in vitro pharmacological properties of crotoxin were further tested, A-56.36 was shown to enhance the enzymatic activity on negatively-charged phospholipids and to increase the acetylcholine release triggered by crotoxin on Torpedo synaptosomes. These effects were explained by the fast dissociation of the crotoxin complex in the presence of the monoclonal antibody A-56.36 and the immunocomplexation of CA, with CB being released in solution. CB is less toxic than crotoxin, has a higher enzymatic activity and triggers a higher acetylcholine release than crotoxin, due to its strong enzymatic activity. A single-chain variable fragment antibody was prepared from monoclonal antibody A-56.36. It binds to CA with a similar affinity than the parental immunoglobulin and exhibits similar effects on the in vitro pharmacological properties of crotoxin.

Mitochondrial swelling and oxygen consumption during respiratory state 4 induced by phospholipase A2 isoforms isolated from the South American rattlesnake (Crotalus durissus terrificus) venom

The non-covalent interaction between two molecular entities namely, phospholipase A2 and crotapotin, results in the main toxin, crotoxin, present in the venom of the South American rattlesnake Crotalus durissus terrificus. High performance liquid chromatography has enabled us the isolation of three phospholipase A2 isoforms (F1, F2 and F3), characterized through denaturing and non-denaturing polyacrylamide gel electrophoresis and also through the N-terminal amino acid sequence analysis. The effect of each purified phospholipase A2 isoform on isolated rat liver mitochondria was determined through mitochondrial swelling and O2 consumption during respiratory state 4. F1 showed a dose-dependent stimulation of O2 consumption while F2 and F3 caused stimulation only at low doses and inhibition at high amounts. These effects were completely suppressed by the presence of 0.1% bovine serum albumin or 0.5 mM EGTA in the incubation medium. Taking the mitochondrial swelling as an activity parameter, all of them presented the same behaviour at different intensities, leading to permeabilization of the mitochondrial membrane. In this case, addition of EGTA prevented it whereas bovine serum albumin was ineffective, indicating that the lipid microenvironment was affected. These results suggest that free fatty acids are directly responsible for the observed effects induced by phospholipase A2 isoforms on oxygen consumption experiments. The protection conferred by cyclosporin-A on swelling induced by the isoforms, when present in low concentrations, may suggest that cyclosporin-A binds to a mitochondrial membrane site protecting the membrane against the phospholipase A2 attack.

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.

Characterization and evolution of a gene encoding a Trimeresurus flavoviridis serum protein that inhibits basic phospholipase A2 isozymes in the snake's venom

The proteins that bind phospholipase A2 (PLA2) isozymes of Trimeresurus flavoviridis (habu snake, crotalinae) venom were fractionated from sera on four columns, each conjugated with one of four PLA2 isozymes. Five proteins, termed PLA2 inhibitors (PLI) I-V, were obtained as the binding components. The combinations of the binding components differed depending on the PLA2 isozymes. PLI-IV and PLI-V correspond to PLI-A and PLI-B, respectively, which were known to bind to a major [Asp49]PLA2, PLA2, and contained a segment similar to the carbohydrate-recognition domain of C-type lectins. PLI-I, which is a major component of inhibitory proteins against three basic PLA2 isozymes, PLA-B (a basic [Asp49]PLA2) and basic proteins I and II (both [Lys49]PLA2s), has been isolated, and its partial amino acid sequence has been determined. A cDNA encoding PLI-I was isolated from a T. flavoviridis liver cDNA library and sequenced. PLI-I cDNA encoded 200 amino acid residues, including a signal peptide of 19 amino acid residues. One sugar chain was predicted to occur at position 157. A gene coding for PLI-I was isolated. It is 9.6-kb long and consists of five exons and four introns. Comparison of the exon-intron structure of the PLI-I gene with those of genes encoding urokinase-type-plasminogen-activator receptor (uPAR), Ly-6, CD59 and neurotoxins showed that they have characteristic unit encoding approximately 90 amino acid residues, which is divided over two exons. This strongly suggests that the PLI-I gene belongs to the uPAR, Ly-6, CD59 and neurotoxin gene family. There are two types of structurally different inhibitors against PLA2 isozymes in T. flavoviridis serum with different evolutionary origins.

Pharmacological activity of the interdomain segment between metalloproteinase and disintegrin domains

Metalloproteinases (haemorrhagic or non-haemorrhagic), disintegrins and most probably C-type lectin-related proteins are derived by the proteolysis of a common precursor protein. There is a short interdomain segment between the metalloproteinase and disintegrin domains which will be released into the venom. To determine whether this region of the molecule contributes to the biological role of the precursors or the products derived by the proteolysis of the precursors, we synthesized a peptide based on this short segment and examined its toxicity and biological activity. The synthetic peptide did not show any lethal toxicity, anticoagulant and antiplatelet effects. However, the peptide appeared to lower the blood pressure or normotensive rats upon infusion, but did not affect the blood levels of triglyceride, total cholesterol, high-density lipoproteins or low-density lipoproteins. The peptide, however, failed to exhibit any effect on spontaneously hypertensive rats and hence may not have a potential as an antihypertensive agent. Based on these results, we conclude that this interdomain segment may not contribute significantly to the biological activity of precursor proteins.

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.

Ammodytoxin C gene helps to elucidate the irregular structure of Crotalinae group II phospholipase A2 genes

Ammodytoxin C is a presynaptically neurotoxic phospholipase A2 (PLA2) expressed in the venom glands of Vipera ammodytes (subfamily Viperinae). The gene spans more than 4 kb and consists of five exons and four introns characteristic of group II phospholipase A2 genes. The first exon encodes the 5' untranslated region, the second exon encodes most of the signal peptide, while exons 3-5 encode three parts of the mature protein. Comparison of the Crotalinae and Viperinae PLA2 genes has shown that Crotalinae PLA2 retain the first intron in their mRNAs. The apparent cause of this retention is a deletion of 40 bp in the first exon of PLA2 genes of the subfamily Crotalinae, which prevents splicing of the first intron. Analysis of the secondary structure of the pre-mRNA of the ammodytoxin C gene has shown that the first exon is able to form an intra-exon hairpin which is absent in Crotalinae PLA2 pre-mRNAs. Our results indicate that this intra-exon hairpin structure is essential for the splicing of the retained first intron. Contrary to the predictions of the neutral theory of molecular evolution, the introns of all known snake venom PLA2 genes are conserved up to 90%, that is considerably more than the exons. Consequently it is proposed that highly conserved introns, in multigene families, which evolve under positive Darwinian selection, may have an important role in enabling homologous recombination.

A phospholipase A2-like pseudogene retaining the highly conserved introns of Mojave toxin and other snake venom group II PLA2s, but having different exons

Mojave toxin is a neurotoxic, heterodimeric phospholipase A2 (PLA2) from the venom of the Mojave rattlesnake (Crotalus scutulatus scutulatus) and is characteristic of all rattlesnake presynaptic neurotoxins. Here, we describe a phospholipase A2 pseudogene (psi-Mtx) located 2,000 nucleotides upstream, and on the opposite DNA strand, from a gene for Mojave toxin acidic subunit (Mtx-a). The pseudogene lacks the first exon and a few segments of noncoding DNA found in functional snake venom PLA2 genes, but does have the coding information for a complete PLA2 protein. psi-Mtx retains the unusual gene sequence similarity pattern found in functional viperid PLA2 genes. When compared to genes from C. s. scutulatus and the Hahn snake (Trimeresurus flavoviridus), psi-Mtx shows strong conservation of nocoding regions and variable protein-coding regions. Although the nocoding regions of psi-Mtx are conserved with respect to other viperid PLA2 genes, the three exons code for a unique PLA2-like protein similar in sequence to ammodytoxin b found in the venom of the western sand viper (Vipera ammodytes ammodytes). The structure of these genes suggests a common ancestor for all viperid PLA2 genes. Phylogenetic analysis of psi-Mtx, Mtx-a, Mtx-b, pgPLA 1a, and pgPLA 1b suggest that psi-Mtx diverged from an ancestral sequence before the presumed gene duplication event leading to Mtx-a and Mtx-b. However, analysis of the basis of coding regions alone gives a conflicting result.

Amino acid sequences of a heterodimeric neurotoxin from the venom of the false horned viper (Pseudocerastes fieldi)

The main toxic component of the venom of the false horned viper, Pseudocerastes fieldi, is a heterodimeric neurotoxin composed of a basic subunit, Cb II, and one of two acidic subunits, either Cb I alpha or Cb I beta. The nontoxic acidic subunit increases the toxicity of the basic subunit. Both subunits have phospholipase A2 (PLA2) amino acid sequences. Cb I alpha and Cb I beta themselves are inactive towards phosphatidylcholine and when complexed with Cb II promote a delay in the onset of phospholipase activity of Cb II. Cb I alpha and Cb I beta do hydrolyze the synthetic substrate, 3-octanoyloxy-4-nitrobenzoic acid, but at < 1% the rate of Cb II. Comparisons of the amino acid sequences of Cb II and Cb I alpha with the corresponding acidic and basic subunits of other heterodimeric neurotoxins show high amino acid sequence identity. Some of the amino acids which are different between the acidic and basic subunits are in highly conserved sequences in their respective types of PLA2. This suggests that these amino acid changes in the conserved regions are important for the structure and function of the heterodimeric proteins.

Inhibition of carrageenin-induced rat paw oedema by crotapotin, a polypeptide complexed with phospholipase A2

1. The effect of purified crotapotin, a non-toxic non-enzymatic chaperon protein normally complexed to a phospholipase A2 (PLA2) in South America rattlesnake venom, was studied in the acute inflammatory response induced by carrageenin (1 mg/paw), compound 48/80 (3 micrograms/paw) and 5-hydroxytryptamine (5-HT) (3 micrograms/paw) in the rat hind-paw. The effects of crotapotin on platelet aggregation, mast cell degranulation and eicosanoid release from guinea-pig isolated lung were also investigated. 2. Subplantar co-injection of crotapotin (1 and 10 micrograms/paw) with carrageenin or injection of crotapotin (10 micrograms/paw) into the contralateral paw significantly inhibited the carrageenin-induced oedema. This inhibition was also observed when crotapotin (10-30 micrograms/paw) was administered either intraperitoneally or orally. Subplantar injection of heated crotapotin (15 min at 60 degrees C) failed to inhibit carrageenin-induced oedema. Subplantar injection of crotapotin (10 micrograms/paw) also significantly inhibited the rat paw oedema induced by compound 48/80, but it did not affect 5-HT-induced oedema. 3. In adrenalectomized animals, subplantar injection of crotapotin markedly inhibited the oedema induced by carrageenin. The inhibitory effect of crotapotin was also observed in rats depleted of histamine and 5-HT stores. 4. Crotapotin (30 micrograms/paw) had no effect on either the histamine release induced by compound 48/80 in vitro or on the platelet aggregation induced by both arachidonic acid (1 nM) and platelet activating factor (1 microM) in human platelet-rich plasma. The platelet aggregation and thromboxane B2 (TXB2) release induced by thrombin (100 mu ml-1) in washed human platelets were also not affected by crotapotin. In addition, crotapotin (10 microg/paw) did not affect the release of 6-oxo-prostaglandin Fla, and TXB2 induced by ovalbumin in sensitized guinea-pig isolated lungs.5. Our results indicate that the anti-inflammatory activity of crotapotin is not due to endogenous corticosteroid release or inhibition of cyclo-oxygenase activity. It is possible that crotapotin may interact with extracellular PLA2 generated during the inflammatory process thereby reducing its hydrolytic activity.

Molecular structure and mechanism of action of the crotoxin inhibitor from Crotalus durissus terrificus serum

An antivenom protein has been identified in the blood of the snake Crotalus durissus terrificus and proved to act by specifically neutralizing crotoxin, the main lethal component of rattlesnake venoms. The aim of this study was to purify the crotoxin inhibitor from Crotalus serum (CICS), and to analyze its mechanism of action. CICS has been purified from blood serum of the Crotalus snake by gel filtration on Sephadex G-200, ion-exchange chromatography on DEAE-Sephacel, and FPLC gel filtration on a Superose 12 column. It is an oligomeric glycoprotein of 130 kDa, made by the non-covalent association of 23-25-kDa subunits. Two different subunit peptides were identified by SDS/PAGE, however, their N-terminal sequences are identical. They are characterized by the absence of methionine residues and a high content of acidic, hydrophobic and cysteine residues. The neutralizing effect of purified CICS towards the neurotoxic effects of crotoxin has been demonstrated in vivo by lethality assays. CICS binds to the phospholipase subunit CB of crotoxin, but not to the acidic chaperon subunit CA; it efficiently inhibits the phospholipase activity of crotoxin and its isolated CB subunit and evokes the dissociation of the crotoxin complex. The molecular mechanism of the interaction between CICS and crotoxin seems to be very similar to that of crotoxin with its acceptor. It is, therefore, tempting to suggest that CICS acts physiologically as a false crotoxin acceptor that would retain the toxin in the vascular system, thus preventing its action on the neuromuscular system.

Antipeptide antibodies directed to the C-terminal part of ammodytoxin A react with the PLA2 subunit of crotoxin and neutralize its pharmacological activity

Crotoxin and ammodytoxin A are snake venom neurotoxic phospholipases A2. Polyclonal antibodies against three synthetic peptides selected from the C-terminal part of the primary structure of ammodytoxin A were tested by ELISA for their interaction with crotoxin and its subunits, CA and CB. All three antipeptide antibodies reacted specifically with corresponding parts of ammodytoxin A and CB, either native or reduced. Conversely, polyclonal antibodies produced against ammodytoxin A and CB reacted strongly with all three peptides, suggesting that they constitute at least a part of natural epitopes in both proteins. All antipeptide antibodies reacted also with the corresponding peptides derived from CB by cyanogen bromide cleavage. The biological activity of the immune complexes was tested. No significant change in the enzymatic activity of CB, ammodytoxin A or crotoxin was observed with any of the three antipeptide antibodies. These antibodies were, however, able to protect mice against the lethal potency of CB and to prolong survival time of mice injected with crotoxin. These antipeptide antibodies were assayed in vitro for their protective effect against the action of CB or crotoxin on synaptosomes from Torpedo marmorata electric organ. They partly inhibited the acetylcholine release induced by both proteins. These results indicate that the C-terminal part of CB is likely to be involved in the pharmacological action of crotoxin.

The origin of the diversity of crotoxin isoforms in the venom of Crotalus durissus terrificus

Crotoxin, the main toxin from the venom of the South American rattlesnake Crotalus durissus terrificus, is a beta-neurotoxin which consists of the non-covalent association of two subunits: a phospholipase A2 subunit B (CB), and a non-enzymic subunit A (CA). We have previously purified and characterized several isoforms of each subunit of crotoxin in the venom collected from numerous snakes. Furthermore, three cDNAs encoding two CB isoforms and the precursor, pro-CA, of subunit A have been isolated from a cDNA library prepared from a single venom gland of Crotalus durissus terrificus. The aim of this study is to analyse an individual snake venom from an animal that has been used to construct a cDNA library. Several isoforms of subunit A and two isoforms of subunit B were isolated and compared to purified and characterized subunit isoforms from pooled venom. The result of this study showed that the multiplicity and the diversity of crotoxin isoforms result from post-translational modifications occurring on a precursor and from the expression of different messenger RNAs present in an individual snake. It allowed for the identification of the two CB isoforms encoding cDNAs expressed in the individual venom with two isoforms from pooled venom, CBc and probably CBa2, that belong to two classes of crotoxin complexes which can be distinguished biochemically and pharmacologically.

Genomic sequences encoding the acidic and basic subunits of Mojave toxin: unusually high sequence identity of non-coding regions

Mojave toxin (Mtx) is a heterodimeric, neurotoxic phospholipase A2 (PLA2) found in the venom of the Mojave rattlesnake, Crotalus scutulatus scutulatus, and is characteristic of all rattlesnake presynaptic neurotoxins. This paper describes the isolation and nucleotide (nt) sequence of the genomic clones encoding both the non-neurotoxic, non-enzymatic acidic subunit (Mtx-a) and the toxic, PLA2-active basic subunit (Mtx-b), and compares their structures. Both cloned genes shared virtually identical overall organization, with four exons separated by three introns, which were inserted in the same relative positions of the genes' coding regions. The exon/intron structure was similar to that reported for mammalian PLA2 genes. Most remarkable was the high degree of nt sequence identity between Mtx-a and Mtx-b. While the exons shared about 70% identity, the introns were greater than 90% identical and the 5' and 3' untranslated and flanking regions were greater than 95% identical. These findings support our earlier suggestion [Aird et al., Biochemistry 24 (1985) 7054-7058] that the genes coding for the two subunits arose from a common ancestor. There has clearly been a strong selection on the nt sequence of the non-coding regions during this evolutionary process. This is the first report of genomic sequences of PLA2-like proteins from snakes.

Comparison of crotoxin isoforms reveals that stability of the complex plays a major role in its pharmacological action

Crotoxin from the venom of the South American rattlesnake Crotalus durissus terrificus is a potent neurotoxin consisting of a weakly toxic phospholipase-A2 subunit (CB) and a non-enzymic, non-toxic subunit (CA). Crotoxin complex (CACB) dissociates upon interaction with membranes: CB binds while CA does not. Moreover, CA enhances the toxicity of CB by preventing its non-specific adsorption. Several crotoxin isoforms have been identified. Multiple variants of each subunit give different crotoxin complexes that can be subdivided into two classes: those of high toxicity and low enzymic activity and those of moderate toxicity and a high phospholipase-A2 activity. In this study, we demonstrate that the more-toxic isoforms block neuromuscular transmission of chick biventer cervicis preparations more efficiently than weakly toxic isoforms. The less-toxic crotoxin complexes have the same Km and Vmax as CB alone. In contrast, the more-toxic isoforms are enzymically less active than CB. These differences correlate with the stability of the complexes: less-toxic isoforms are less stable (Kd = 25 nM) and dissociate rapidly (half-life about 1 min), whereas the more-toxic isoforms are more stable (Kd = 4.5 nM) and dissociate more slowly (half-life 10-20 min). The rate of interaction of crotoxin complexes with vesicles of negatively charged phospholipids paralleled the rate of dissociation of the complexes in the absence of vesicles. The differences of pharmacological and biochemical properties of crotoxin isoforms indicate that the stability of crotoxin complexes plays a major role in the synergistic action of crotoxin subunits: a stronger association between the two crotoxin subunits would account for their slower dissociation rate, a weaker enzymic activity, a slower interaction with phosphatidylglycerol vesicles, a faster blockade of neuromuscular transmission and a higher lethal potency.