Characterization of six toxins from the venom of the Moroccan scorpion Buthus occitanus mardochei

Mass spectrometry-based top-down and bottom-up approaches for proteomic analysis of the Moroccan Buthus occitanus scorpion venom

Buthus occitanus (B. occitanus) is one of the most dangerous scorpions in the world. Despite the involvement of B. occitanus scorpion in severe cases of envenomation in Morocco, no study has focused yet on the proteomic composition of the Moroccan B. occitanus scorpion venom. Mass spectrometry‐based proteomic techniques are commonly used in the study of scorpion venoms. The implementation of top‐down and bottom‐up approaches for proteomic analyses facilitates screening by allowing a global view of the structural aspects of such complex matrices. Here, we provide a partial overview of the venom of B. occitanus scorpion, in order to explore the diversity of its toxins and hereafter understand their effects. To this end, a combination of top‐down and bottom‐up approaches was applied using nano‐high liquid chromatography coupled to nano‐electrospray tandem mass spectrometry (nano‐LC‐ESI MS/MS). The LC‐MS results showed that B. occitanus venom contains around 200 molecular masses ranging from 1868 to 16 720 Da, the most representative of which are those between 5000 and 8000 Da. Interestingly, combined top‐down and bottom‐up LC‐MS/MS results allowed the identification of several toxins, which were mainly those acting on ion channels, including those targeting sodium (NaScTxs), potassium (KScTxs), chloride (ClScTxs), and calcium channels (CaScTx), as well as antimicrobial peptides (AMPs), amphipathic peptides, myotropic neuropeptides, and hypothetical secreted proteins. This study reveals the molecular diversity of B. occitanus scorpion venom and identifies components that may have useful pharmacological activities.

Isolation and Characterization of Insecticidal Toxins from the Venom of the North African Scorpion, Buthacus leptochelys

Various bioactive peptides have been identified in scorpion venom, but there are many scorpion species whose venom has not been investigated. In this study, we characterized venom components of the North African scorpion, Buthacus leptochelys, by mass spectrometric analysis and evaluated their insect toxicity. This is the first report of chemical and biological characterization of the B. leptochelys venom. LC/MS analysis detected at least 148 components in the venom. We isolated four peptides that show insect toxicity (Bl-1, Bl-2, Bl-3, and Bl-4) through bioassay-guided HPLC fractionation. These toxins were found to be similar to scorpion α- and β-toxins based on their N-terminal sequences. Among them, the complete primary structure of Bl-1 was determined by combination of Edman degradation and MS/MS analysis. Bl-1 is composed of 67 amino acid residues and crosslinked with four disulfide bonds. Since Bl-1 shares high sequence similarity with α-like toxins, it is likely that it acts on Na⁺ channels of both insects and mammals.

Comparative analyses and implications for antivenom serotherapy of four Moroccan scorpion Buthus occitanus venoms: Subspecies tunetanus, paris, malhommei, and mardochei

Temporary passive immunity such as serotherapy against venoms requires the full knowledge of all venom's components. Here, four venoms from Moroccan common yellow scorpions belonging to Buthus occitanus, subspecies tunetanus, paris, malhommei, and mardochei, all collected in four different restricted areas, were analysed in deep. They were fractionated by reversed-phase high-performance liquid chromatography (RP-HPLC) and their molecular masse profile determined by off-line MALDI-TOF mass spectrometry. Characterisation of their main components was achieved by enzyme-linked immunosorbent assay (ELISA) using specific antisera against the major lethal scorpion toxins identified so far, i.e. voltage-gated sodium channels (Na_v) modulators α- and β-toxins, as well as diverse potassium channel pore blocker toxins. For fractions with identical RP-HPLC retention times, we observe that their relative quantities show large differences. Moreover, identical masses present simultaneously in the four venoms are infrequent. ELISAs show that the majority of the RP-HPLC compounds cross-react with the antiserum against the "α-like" toxin Bot I, which has been previously identified in the Algerian Buthus occitanus tunetanus venom. Moreover, minor fractions were recognised by the antiserum against the highly lethal "classical" α-toxin of reference AaH II from the Androctonus australis venom. As such, our results bring new sights for further improving scorpion venom serotherapy in Morocco.

A first exploration of the venom of the Buthus occitanus scorpion found in southern France

Even though Buthus occitanus scorpions are found throughout the Mediterranean region, a lack of distinctive characteristics has hampered their classification into different subspecies. Yet, stings from this particular scorpion family are reported each year to result in pain followed by various toxic symptoms. In order to determine the toxicity origin of the rare French B. occitanus Amoreux scorpion, we collected several specimens and studied their venom composition using a nano ultra high performance liquid chromatography and matrix assisted laser desorption/ionisation time-of-flight mass spectrometry (nano UHPLC/MALDI-TOF-MS) automated workflow combined with an enzyme-linked immunosorbent assay (ELISA) approach. Moreover, we compared this dataset to that obtained from highly lethal Androctonus australis and Androctonus mauretanicus scorpions collected in North Africa. As a result, we found that the B. occitanus Amoreux venom is toxic to mice, an observation that is most likely caused by venom components that inhibit voltage-gated sodium channel inactivation. Moreover, we identified similarities in venom composition between B. occitanus scorpions living in the South of France and other Buthidae collected in Morocco and Algeria. As such, the results of this study should be taken into consideration when treating stings from the B. occitanus species living in the South of France.

Peptides of arachnid venoms with insecticidal activity targeting sodium channels

Arachnids have a venom apparatus and secrete a complex chemical mixture of low molecular mass organic molecules, enzymes and polypeptide neurotoxins designed to paralyze or kill their prey. Most of these toxins are specific for membrane voltage-gated sodium channels, although some may also target calcium or potassium channels and other membrane receptors. Scorpions and spiders have provided the greatest number of the neurotoxins studied so far, for which, a good number of primary and 3D structures have been obtained. Structural features, comprising a folding that determines a similar spatial distribution of charged and hydrophobic side chains of specific amino acids, are strikingly common among the toxins from spider and scorpion venoms. Such similarities are, in turn, the key feature to target and bind these proteins to ionic channels. The search for new insecticidal compounds, as well as the study of their modes of action, constitutes a current approach to rationally design novel insecticides. This goal tends to be more relevant if the resistance to the conventional chemical products is considered. A promising alternative seems to be the biotechnological approach using toxin-expressing recombinant baculovirus. Spider and scorpion toxins having insecticidal activity are reviewed here considering their structures, toxicities and action mechanisms in sodium channels of excitable membranes.

The differential preference of scorpion α-toxins for insect or mammalian sodium channels: Implications for improved insect control

Receptor site-3 on voltage-gated sodium channels is targeted by a variety of structurally distinct toxins from scorpions, sea anemones, and spiders whose typical action is the inhibition of sodium current inactivation. This site interacts allosterically with other topologically distinct receptors that bind alkaloids, lipophilic polyether toxins, pyrethroids, and site-4 scorpion toxins. These features suggest that design of insecticides with specificity for site-3 might be rewarding due to the positive cooperativity with other toxins or insecticidal agents. Yet, despite the central role of scorpion alpha-toxins in envenomation and their vast use in the study of channel functions, molecular details on site-3 are scarce. Scorpion alpha-toxins vary greatly in preference for sodium channels of insects and mammals, and some of them are highly active on insects. This implies that despite its commonality, receptor site-3 varies on insect vs. mammalian channels, and that elucidation of these differences could potentially be exploited for manipulation of toxin preference. This review provides current perspectives on (i) the classification of scorpion alpha-toxins, (ii) their mode of interaction with sodium channels and pharmacological divergence, (iii) molecular details on their bioactive surfaces and differences associated with preference for channel subtypes, as well as (iv) a summary of the present knowledge about elements involved in constituting receptor site-3. These details, combined with the variations in allosteric interactions between site-3 and the other receptor sites on insect and mammalian sodium channels, may be useful in new strategies of insect control and future design of anti-insect selective ligands.

Voltage-gated sodium channel modulation by scorpion alpha-toxins

Voltage-gated Na(+) channels are integral membrane proteins that function as a gateway for a selective permeation of sodium ions across biological membranes. In this way, they are crucial players for the generation of action potentials in excitable cells. Voltage-gated Na(+) channels are encoded by at least nine genes in mammals. The different isoforms have remarkably similar functional properties, but small changes in function and pharmacology are biologically well-defined, as underscored by mutations that cause several diseases and by modulation of a myriad of compounds, respectively. This review will stress on the modulation of voltage-gated Na(+) channels by scorpion alpha-toxins. Nature has designed these two classes of molecules as if they were predestined to each other: an inevitable 'encounter' between a voltage-gated Na(+) channel isoform and an alpha-toxin from scorpion venom indeed results in a dramatically changed Na(+) current phenotype with clear-cut consequences on electrical excitability and sometimes life or death. This fascinating aspect justifies an overview on scorpion venoms, their alpha-toxins and the Na(+) channel targets they are built for, as well as on the molecular determinants that govern the selectivity and affinity of this 'inseparable duo'.

The role of voltage-gated Na+ channels in excitation–contraction coupling of rat heart determined by BmK I, an α-like scorpion neurotoxin

A mechanism underlying the increase in rat heart contractility modulated by BmK I, an alpha-like scorpion neurotoxin, was investigated using whole-cell patch-clamp and fluorescence digital imaging techniques. Results showed that (a) L-type Ca2+ current could not be modified by 500 nM BmK I; (b) The inactivation process of Na+ current was significantly delayed with no change of its amplitude; (c) The overall intracellular Na+ and Ca2+ concentration could be augmented in the presence of BmK I; (p<0.05); (d) The increase of free intracellular Ca2+ concentration induced by BmK I was inhibited completely by 5 mM NiCl2 (p<0.05), an inhibitor of Na+-Ca2+ exchanger; (e) The spontaneous Ca2+ release induced by 10 mM caffeine from sarcoplasmic reticulum could not be modulated by 500 nM BmK I in the absence of external Ca2+. These results indicate that cardiac voltage-gated Na+ channels are also targets of BmK I. Na+ accumulation through Na+ channels can trigger sarcoplasmic reticulum Ca2+ release in rat cardiac myocytes via reverse-mode Na+-Ca2+ exchanger. Furthermore, Ca2+ release from sarcoplasmic reticulum induced by BmK I most likely involves a Ca2+-induced release mechanism.

Exploration of the functional site of a scorpion alpha-like toxin by site-directed mutagenesis

Scorpion alpha-neurotoxins can be classified into distinct subgroups according to their sequence and pharmacological properties. Using toxicity tests, binding studies, and electrophysiological recordings, BmK M1, a toxin from the Asian scorpion Buthus martensi Karsch, was experimentally identified as an alpha-like toxin. Being the first alpha-like toxin available in a recombinant form, BmK M1 was then modified by site-directed mutagenesis for investigation of the molecular basis of its activity. The results suggested a functional site which protrudes from the molecular scaffold as a unique tertiary arrangement, constituted by the five-residue reverse turn 8-12 and the C-terminal segment. The C-terminal basic residues Lys62 and His64 together with Lys8 in the turn, which are critical for the bioactivities, may directly interact with the receptor site on the sodium channel. Residues Asn11 and Arg58, indispensable for the activities, are mainly responsible for stabilizing the distinct conformation of the putative bioactive site. Among others, His10 and His64 seem to be involved in the preference of BmK M1 for phylogenetically distinct target sites. The comparison of BmK M1 with Aah2 (classical alpha-toxin) and Lqh(alpha)IT (alpha-insect toxin) showed that the specific orientation of the C-terminus mediated by the reverse turn might be relevant to the preference of alpha-toxin subgroups for phylogenetically distinct yet closely related receptor sites. The Y5G mutation indicated the "conserved hydrophobic surface" might be structurally important for stabilizing the beta-sheet in the alpha/beta-scaffold. The observations in this work shed light on the nature and roles of the residues possibly involved in the biological activity of a scorpion alpha-like toxin.

Characterization of scorpion alpha-like toxin group using two new toxins from the scorpion Leiurus quinquestriatus hebraeus

Two novel toxins, Lqh6 and Lqh7, isolated from the venom of the scorpion Leiurus quinquestriatus hebraeus, have in their sequence a molecular signature (8Q/KPE10) associated with a recently defined group of alpha-toxins that target Na channels, namely the alpha-like toxins [reviewed in Gordon, D., Savarin, P., Gurevitz, M. & Zinn-Justin, S. (1998) J. Toxicol. Toxin Rev. 17, 131-159]. Lqh6 and Lqh7 are highly toxic to insects and mice, and inhibit the binding of alpha-toxins to cockroach neuronal membranes. Although they kill rodents by intracerebroventricular injection, they do not inhibit the binding of antimammal alpha-toxins (e.g. Lqh2) to rat brain synaptosomes, not even at high concentrations. Furthermore, in voltage-clamp experiments, rat brain Na channels IIA (rNav1.2A) expressed in Xenopus oocytes are not affected by Lqh6 nor by Lqh7 below 3 micro m. In contrast, muscular Na channels (rNav1.4 and hNav1.5) expressed in the same cells respond to nanomolar concentrations of Lqh6 and Lqh7 by slowing of Na current inactivation and a leftward shift of the peak conductance-voltage curve. The structural and pharmacological properties of the new toxins are compared to those of other scorpion alpha-toxins in order to re-examine the hallmarks previously set for the alpha-like toxin group.

The toxin Tx4(6-1) from the spider Phoneutria nigriventer slows down Na(+) current inactivation in insect CNS via binding to receptor site 3

Tx4(6-1) a neurotoxic peptide from the venom of the aggressive South American 'armed' spider Phoneutria nigriventer, has been previously isolated and sequenced. It shows no detectable activity in mice but affects the peripheral nervous system of insects by stimulating glutamate release at the neuromuscular junction. Here we investigate possible interactions of the toxin with voltage-activated sodium channels (Na(v)). We confirm that it is ineffective on mammalian Na(v) channels, and establish that it competes with the alpha-like toxin 125I-Bom IV, for binding on the site 3 of insect Na(v) channel (IC(50) value around 25nM). The physiological consequences of this binding to the insect Na(v) channel are shown by electrophysiology: Tx4(6-1) prolongs evoked axonal action potentials (APs) (<500&mgr;s duration in control). Prolonged 8-10ms or 'plateau' 500-800ms APs accompanied by repetitive firing at 80-150Hz are recorded after 4-8min of toxin action. This modification of evoked activity is due to a slowing down of sodium current inactivation. Effects of Tx4(6-1) on sodium current are compared with those of a typical scorpion alpha-toxin and of some other spider toxins active on insect Na(v) channels. At the end of long voltage pulses, the maintained inward sodium current may represent 50% of the peak current after scorpion alpha-toxin but only about 8-10% after spider toxins. To understand the slight differences in the effects of alpha-scorpion and spider toxins on the insect Na(v) channel, structural studies of toxin-channels interactions would be necessary.

The scorpion α-like toxin Lqh III specifically alters sodium channel inactivation in frog myelinated axons

The effects of 1-100 nM Lqh III, an alpha-like toxin isolated from the scorpion Leiurus quinquestriatus hebraeus, were assessed on the nodal membrane potential and ionic currents of single frog myelinated axons. In current-clamped axons, Lqh III increased the duration of action potentials without markedly affecting the peak amplitude and the resting membrane potential. The toxin was less effective when the resting membrane potential of axons was increasingly more positive. The Lqh III-induced increase in action potential duration was not due to the blockade of K(+) channels, since the toxin had no significant effect upon the K(+) current. In contrast, Lqh III inhibited the inactivation of a fraction of the Na(+) current, leading to a maintained late inward Na(+) current which represented about 45% of the peak Na(+) current, as observed during long-lasting depolarisations and in steady-state Na(+) current inactivation-voltage relationships when the pre-pulse potential was more positive than about -30mV. The activation kinetics of the late Na(+) current were well described by a single exponential whose time constant was 8.53+/-0.78 ms (n=3). Finally, Lqh III slowed the time-course of the remaining peak Na(+) current inactivation by altering initial amplitudes (to time zero of depolarisation) and time constants of its fast and slow phases. No significant additional effect was detected during the action of the toxin. In conclusion, we propose that, in frog myelinated axons, the effects of Lqh III are those typically attributed to classical scorpion alpha-toxins.

Nine novel precursors of Buthus martensii scorpion alpha-toxin homologues

The cDNAs encoding nine novel alpha-toxin homologues were isolated from the venom gland cDNA library of the Chinese scorpion Buthus martensii Karsch (BmK). They are rich in AAAA and TTTT elements at the 5' UTRs. The flanking region of the translation initiation codon ATG is AAAATGAA, which is highly conserved in scorpion Na(+), K(+) and Cl(-) channel toxin genes. These putative scorpion alpha-toxins shared 45.5-98.4% homology with the characterized BmK alpha-toxins, and were completely conserved in the positions of all eight cysteines. This showed, together with higher homology at nucleotide level than that at amino acid level, that these toxins may originate from a common ancestor. The discovery of a series of homologues of scorpion alpha-toxin with a different degree of natural mutation in the primary structure will provide us with a valuable system for studying the structure-function relationship of scorpion toxins.

Structural implications on the interaction of scorpion alpha-like toxins with the sodium channel receptor site inferred from toxin iodination and pH-dependent binding

The alpha-like toxin from the venom of the scorpion Leiurus quinquestriatus hebraeus (Lqh III) binds with high affinity to receptor site 3 on insect sodium channels but does not bind to rat brain synaptosomes. The binding affinity of Lqh III to cockroach neuronal membranes was fivefold higher at pH 6.5 than at pH 7.5. This correlated with an increase in the electropositive charge on the toxin surface resulting from protonation of its four histidines. Radioiodination of Tyr(14) of Lqh III abolished its binding to locust but not cockroach sodium channels, whereas the noniodinated toxin bound equally well to both neuronal preparations. Radioiodination of Tyr(10) or Tyr(21) of the structurally similar alpha-toxin from L. quinquestriatus hebraeus (LqhalphaIT), as well as their substitution by phenylalanine, had only minor effects on binding to cockroach neuronal membranes. However, substitution of Tyr(21), but not Tyr(14), by leucine decreased the binding affinity of LqhalphaIT approximately 87-fold. Thus, Tyr(14) is involved in the bioactivity of Lqh III to locust receptor site 3 and is not crucial for the binding of LqhalphaIT to this site. In turn, the aromatic ring of Tyr(21) takes part in the bioactivity of LqhalphaIT to insects. These results highlight subtle architectural variations between locust and cockroach receptor site 3, in addition to previous results demonstrating the competence of Lqh III to differentiate between insect and mammalian sodium channel subtypes.

Binding characteristics of BmK I, an alpha-like scorpion neurotoxic polypeptide, on cockroach nerve cord synaptosomes

In this study, the binding characteristics of BmK I, an alpha-like neurotoxic polypeptide purified from the venom of the Chinese scorpion Buthus martensi Karsch, were investigated on rat brain and cockroach nerve cord synaptosomes. The results showed that BmK I can bind to a single class of noninteracting binding sites on cockroach nerve cord synaptosomes with medium affinity (Kd = 16.5 +/ - 4.4 nM) and low binding capacity (Bmax = 1.05 +/- 0.23 pmol/mg protein), but lacks specific binding on rat brain synaptosomes. BmK AS, BmK AS-1 (two novel sodium channel-blocking ligands), BmK IT (an excitatory insect-selective toxin) and BmK IT2 (a depressant insect-selective toxin) from the same venom were found to be capable of depressing BmK I binding in cockroach nerve cord synaptosomes, which might be attributed to either allosteric modulation of voltage-gated Na+ channels by these toxic polypeptides or partial overlapping between the receptor binding sites of BmK I and these toxins. This thus supported the notion that alpha-like scorpion neurotoxic polypeptides bind to a distinct receptor site on sodium channels, which might be similar to the binding receptor site of alpha-type insect toxins, and also related to those of BmK AS type and insect-selective scorpion toxins on insect sodium channels.

A scorpion alpha-like toxin that is active on insects and mammals reveals an unexpected specificity and distribution of sodium channel subtypes in rat brain neurons

Several scorpion toxins have been shown to exert their neurotoxic effects by a direct interaction with voltage-dependent sodium channels. Both classical scorpion alpha-toxins such as Lqh II from Leiurus quiquestratus hebraeus and alpha-like toxins as toxin III from the same scorpion (Lqh III) competitively interact for binding on receptor site 3 of insect sodium channels. Conversely, Lqh III, which is highly toxic in mammalian brain, reveals no specific binding to sodium channels of rat brain synaptosomes and displaces the binding of Lqh II only at high concentration. The contrast between the low-affinity interaction and the high toxicity of Lqh III indicates that Lqh III binding sites distinct from those present in synaptosomes must exist in the brain. In agreement, electrophysiological experiments performed on acute rat hippocampal slices revealed that Lqh III strongly affects the inactivation of voltage-gated sodium channels recorded either in current or voltage clamp, whereas Lqh II had weak, or no, effects. In contrast, Lqh III had no effect on cultured embryonic chick central neurons and on sodium channels from rat brain IIA and beta1 subunits reconstituted in Xenopus oocytes, whereas sea anemone toxin ATXII and Lqh II were very active. These data indicate that the alpha-like toxin Lqh III displays a surprising subtype specificity, reveals the presence of a new, distinct sodium channel insensitive to Lqh II, and highlights the differences in distribution of channel expression in the CNS. This toxin may constitute a valuable tool for the investigation of mammalian brain function.

Scorpion toxins specific for Na+-channels

Na+-channel specific scorpion toxins are peptides of 60-76 amino acid residues in length, tightly bound by four disulfide bridges. The complete amino acid sequence of 85 distinct peptides are presently known. For some toxins, the three-dimensional structure has been solved by X-ray diffraction and NMR spectroscopy. A constant structural motif has been found in all of them, consisting of one or two short segments of alpha-helix plus a triple-stranded beta-sheet, connected by variable regions forming loops (turns). Physiological experiments have shown that these toxins are modifiers of the gating mechanism of the Na+-channel function, affecting either the inactivation (alpha-toxins) or the activation (beta-toxins) kinetics of the channels. Many functional variations of these peptides have been demonstrated, which include not only the classical alpha- and beta-types, but also the species specificity of their action. There are peptides that bind or affect the function of Na+-channels from different species (mammals, insects or crustaceans) or are toxic to more than one group of animals. Based on functional and structural features of the known toxins, a classification containing 10 different groups of toxins is proposed in this review. Attempts have been made to correlate the presence of certain amino acid residues or 'active sites' of these peptides with Na+-channel functions. Segments containing positively charged residues in special locations, such as the five-residue turn, the turn between the second and the third beta-strands, the C-terminal residues and a segment of the N-terminal region from residues 2-11, seems to be implicated in the activity of these toxins. However, the uncertainty, and the limited success obtained in the search for the site through which these peptides bind to the channels, are mainly due to the lack of an easy method for expression of cloned genes to produce a well-folded, active peptide. Many scorpion toxin coding genes have been obtained from cDNA libraries and from polymerase chain reactions using fragments of scorpion DNAs, as templates. The presence of an intron at the DNA level, situated in the middle of the signal peptide, has been demonstrated.

Scorpion alpha-like toxins, toxic to both mammals and insects, differentially interact with receptor site 3 on voltage-gated sodium channels in mammals and insects

alpha-Like toxins, a unique group designated among the scorpion alpha-toxin class that inhibit sodium channel inactivation, are highly toxic to mice but do not compete for alpha-toxin binding to receptor site 3 on rat brain sodium channels. We analysed the sequence of a new alpha-like toxin, which was also highly active on insects, and studied its action and binding on both mammalian and insect sodium channels. Action of the alpha-like toxin on isolated cockroach axon is similar to that of an alpha-toxin, and the radioactive toxin binds with a high affinity to insect sodium channels. Other sodium channel neurotoxins interact competitively or allosterically with the insect alpha-like toxin receptor site, similarly to alpha-toxins, suggesting that the alpha-like toxin receptor site is closely related to receptor site 3. Conversely, on rat brain sodium channels, specific binding of 125I-alpha-like toxin could not be detected, although at high concentration it inhibits sodium current inactivation on rat brain sodium channels. The difficulty in measuring binding to rat brain channels may be attributed to low-affinity binding due to the acidic properties of the alpha-like toxins that also impair the interaction with receptor site 3. The results suggest that alpha-like toxins bind to a distinct receptor site on sodium channels that is differentially related to receptor site 3 on mammalian and insect sodium channels.

Makatoxin I, a novel toxin isolated from the venom of the scorpion Buthus martensi Karsch, exhibits nitrergic actions

Buthus martensi Karsch venom exhibits nitrergic action in rat anococcygeus muscle (ACM). We have purified a novel toxin, makatoxin I (MkTx I), which exhibits nitrergic action, to homogeneity from this venom by a combination of gel-filtration, cation-exchange chromatography, and reverse-phase chromatography. Its purity was assessed by capillary electrophoresis and mass spectrometry. Its molecular weight was found to be 7031.71 +/- 2.88 as calculated from electrospray mass spectrographic data. The complete amino acid sequence was elucidated by sequencing of reduced and S-pyridylethylated toxin and a carboxyl-terminal peptide, P55-64, generated by the cleavage of toxin with endoproteinase Lys-C. The complete sequence of MkTx I is GRDAYIADSENCTYTCALNPYCNDLCTKNGAKSGYCQWAGRYGNACWCIDLPDKVPIRISGSCR. This toxin is composed of 64 amino acid residues and contains 8 half-cystine residues. Structurally, MkTx I has high similarity to Bot I and Bot II when compared with toxins from other scorpion species. The effects of MkTx I on nitrergic responses were investigated using the rat isolated ACM mounted in Krebs solution (37 degrees C, 5% CO2 in O2). MkTx I (2 microg/ml) markedly relaxed the carbachol-precontracted ACM; the relaxation was inhibited by the stereoselective inhibitor of nitric oxide synthase, Nomega-nitro-L-arginine methyl ester (50 microM). Thus, MkTx I is the first alpha-toxin that can mediate nitrergic responses in the rat isolated ACM.

A conserved sequence region of scorpion toxins rendered immunogenic induces broadly cross-reactive, neutralizing antibodies

Scorpion toxins constitute a family of proteins with a high degree of sequence diversity but a common mode of action. Neutralization of the toxic effects of scorpion stings by serotherapy is limited due to the various serotypes expressed by these proteins. We explored the possibility of raising antibodies to conserved parts of the toxins which could recognize several members of the family. We established the variability profile of a set of 25 scorpion toxin sequences, then evaluated systematically by peptide-scanning methods the antigenicity of one scorpion toxin. The most conserved regions were generally very poorly antigenic. One exception was the N-terminal region, which is both conserved and antigenic. Antibodies were raised in rabbits against an eight-residue synthetic peptide mimicking the N-terminal region. These peptide antibodies were cross-reactive with several scorpion toxins belonging to different serotypes and neutralized both the pharmacological effects (binding to rat brain synaptosomes) and the biological activity (toxicity in mice) of the parent toxin. The molecular model of the toxin indicates that antibody binding to residues 1-8 probably either masks some residue(s) of the N-terminus critical for the biological activity or overlaps with the epitope previously defined by neutralizing monoclonal antibody. These findings could open the way for new therapeutic strategies for the medical care of envenomations.

A recombinant insect-specific alpha-toxin of Buthus occitanus tunetanus scorpion confers protection against homologous mammal toxins

We have constructed a cDNA library from venom glands of the scorpion Buthus occitanus tunetanus and cloned a DNA sequence that encodes an alpha-toxin. This clone was efficiently expressed in Escherichia coli as a fusion protein with two Ig-binding (Z) domains of protein A from Staphylococcus aureus. After CNBr treatment of the fusion protein and HPLC purification, we obtained approximately 1 mg recombinant apha-toxin/l bacterial culture. The toxin, called Bot XIV, displays no toxicity towards mammals but is active towards insects as shown by its paralytic activity against Blatella germanica cockroach and by electrophysiological studies on Periplaneta americana cockroaches. The Bot XIV protein fused to two Z domains is highly immunogenic in mice and induces production of antisera that specifically recognize and neutralize highly toxic components that had been injected into mice. This fusion protein could be very useful for development of potent protective antisera against scorpion venoms.

Scorpion toxins affecting sodium current inactivation bind to distinct homologous receptor sites on rat brain and insect sodium channels

Sodium channels posses receptor sites for many neurotoxins, of which several groups were shown to inhibit sodium current inactivation. Receptor sites that bind alpha- and alpha-like scorpion toxins are of particular interest since neurotoxin binding at these extracellular regions can affect the inactivation process at intramembranal segments of the channel. We examined, for the first time, the interaction of different scorpion neurotoxins, all affecting sodium current inactivation and toxic to mammals, with alpha-scorpion toxin receptor sites on both mammalian and insect sodium channels. As specific probes for rat and insect sodium channels, we used the radiolabeled alpha-scorpion toxins AaH II and LqhalphaIT, the most active alpha-toxins on mammals and insect, respectively. We demonstrate that the different scorpion toxins may be classified to several groups, according to their in vivo and in vitro activity on mammalian and insect sodium channels. Analysis of competitive binding interaction reveal that each group may occupy a distinct receptor site on sodium channels. The alpha-mammal scorpion toxins and the anti-insect Lqh alphaIT bind to homologous but not identical receptor sites on both rat brain and insect sodium channels. Sea anemone toxin ATX II, previously considered to share receptor site 3 with alpha-scorpion toxins, is suggested to bind to a partially overlapping receptor site with both AaH II and Lqh alphaIT. Competitive binding interactions with other scorpion toxins suggest the presence of a putative additional receptor site on sodium channels, which may bind a unique group of these scorpion toxins (Bom III and IV), active on both mammals and insects. We suggest the presence of a cluster of receptor sites for scorpion toxins that inhibit sodium current inactivation, which is very similar on insect and rat brain sodium channels, in spite of the structural and pharmacological differences between them. The sea anemone toxin ATX II is also suggested to bind within this cluster.

Purification, N-terminal sequence and structural characterization of a toxic protein from the Indian scorpion venom Buthus tamulus

A toxic protein component (Bt-II) was isolated from the venom of the Indian scorpion, Buthus tamulus, by ion-exchange chromatography on Biorex-70 and reverse phase high pressure liquid chromatography (RP-HPLC). This 7800 mol. wt protein was found to have an LD50 value of 2.25 mg/kg body weight in mice by subcutaneous injection. Amino acid composition of the protein revealed the presence of aromatic and basic amino acids; the N-terminal amino acid sequence was also determined. Structural characterization of Bt-II, based on circular dichroic and fluorescent quenching studies, has also been reported.

Analysis by high-performance liquid chromatography of Androctonus mauretanicus mauretanicus (black scorpion) venom

The venom of the black scorpion, Androctonus mauretanicus mauretanicus, was obtained by means of manual stimulation and was analyzed using high-performance liquid chromatography. Starting from 20 mg of venom and using only two chromatographic steps, six toxins were purified to homogeneity. They have been characterized by their amino acid content and compared to those already isolated from a pool of venoms obtained using electric stimulation (Rosso and Rochat, Toxicon 23, 113-125, 1985). The toxins Amm I and Amm II were not found, suggesting either different levels of toxin expression or the existence of Androctonus mauretanicus mauretanicus subspecies. Using rat brain synaptosomes, it was demonstrated that the toxins Amm III, Amm IV and Amm V were alpha-toxins. The toxin Amm VI was neither alpha- or beta-toxin. Unexpectedly, the toxin Amm VII was found to be a beta-toxin, the first one identified in a north African scorpion venom. In addition, some toxins active on mammals exhibited different levels of specificity towards phylogenetically related groups of arthropods.

Isolation and characterization of toxic proteins from the venom of the Venezuelan scorpion Tityus discrepans (Karsch)

Four toxic, electrophoretically homogeneous proteins were isolated by ion-exchange chromatography on CM-cellulose-52 from the venom of the scorpion Tityus discrepans (range North Central Venezuela), named TdIV, TdV, TdVIII and TdIX. Component TdVIII, with 56 amino acid residues and mol. wt 6140 was the most toxic by i.p. injections into mice and had an intracisternal LD50 of 7.9 micrograms protein/kg body weight. Amino acid compositions of components TdIV and TdV were very similar, suggesting that they could be highly homologous proteins, although presumably contaminated one by the other. A fifth component, named TdIII, non-toxic by i.p. injections, was also isolated in homogeneous form. The i.v. and intracisternal LD50 values of the whole T. discrepans venom were 2.5 mg/kg and 16.0 micrograms/kg, respectively.