SCORPIONS

An interesting pattern develops between the size of the claws and the toxicity of the scorpion. Typically, the smaller and more slender the claws the more toxic the scorpion. However, all scorpions should be treated with caution.

Androctonus

This genus is very toxic, with symptoms of envenomation including malignant hyperthermia, myocarditis and pulmonary edema and with many lethal stings on record [1-3]. Two main types of toxic compounds are present, those specific for insects and those for mammals, differences in the specificity between these related proteins, discriminating between mammalian and insect voltage-dependent sodium channels of excitable tissues, is likely are due to the position of the C-terminal peptide in relation to a hydrophobic surface common to all examined scorpion toxins [4].

The yellow scorpion Androctonus australis hector, contains many alpha toxins acting upon sodium channels of mammals, Aah, and insects, AaHIT. A structural motif of the surface of these compounds is an aromatic cluster, surrounded by long hydrophobic side-chain residues with loops protruding out of it [4]. Structure/function studies have established the molecular mechanisms by which scorpion alpha-toxins act, results indicating that the aromatic residues belonging to the conserved hydrophobic surface, C-terminal and the loop region (residues 37-44) are all involved along with contributions by charged residues in the N- terminal and C-terminal [5]. multipoint interaction with the proteins of the sodium channel is indicated by all important residues being clustered on one face of the toxin [5]. Thus, the interaction of a given scorpion toxin with its receptor might well be governed by the presence of this solvent-exposed hydrophobic surface, whereas adjacent areas modulate the specificity of the interaction [4].

AaH toxin I, along with AaHII, is the lethal component of australis venom [6]. AaH II, a 64 amino acid neurotoxin, is very similar to toxin I but more toxic, being the most toxic of all scorpion alpha-toxins [7, 8]. The toxic region appears to be composed of residues belonging to the conserved hydrophobic surface and to the C-terminal and N-terminal sequences. [9] The antigenic regions have been identified as sequences: 1-8, 4-12, 27-35, 39-45, 52-58 and 55-61), regions appeared to corresponding to either beta-turn or extended parts of the molecule [7]. AaH IV, on the other hand, is a weakly toxic 64 residues protein that is sequentially similar to AaH I and III. However, AaH IV does not react strongly with anti-AaH I or III antibodies, giving weight to the theroy amino acids regions of AaH I and III not found in AaH IV are the portions involved in the toxic binding of [10].

Australis also contains four identified proteins (AaHITI, AaHIT2, AaHIT3, and AaHIT4) that are, with one exception (AaHIT4) specific for insects and may have potential for use as insecticidal agents. AaHITI toxin gene linked to a sendai virus has been used in viral infections of mosquitos, with the mosquitoes successfully assimilating and expressing the genes resulting in their death from the toxin [11]. AaHIT2 and AaHIT3 are insect specific toxins similar to AaHTI [12]. AaHIT4 is a 65 amino acid toxin that, like the preceeding AaHITs, binds to the sodium channel of insects. However, it also has a toxic effect on mammals in addition to modifying the binding to the mammal sodium channel by alpha-type and beta-type anti-mammal scorpion toxins. It has been proposed that this toxin could be the first member of a new class of toxins to have ancestral structural features and a wide toxic range [13].

Androctonus crassicauda is of mecidal importance in the middle east, with stings being common and fatalities on record. Typcial effects of the sting are pain and tenderness at the injection site, with severe neurological effects and hypertension [14]. With proper medical, including the use of antivenom, recovery is uneventful [15].

No specific proteins have been isolated and reported for this specie. Androctonus mauretanicus mauretanicus (Black scorpion) is a dangerous specie with effects similar to that of Androctonus australis hector but not as pronounced. Kaliotoxin, found in very high amounts in A.m.mauretanicus (Black scorpion), is a blocker of high conductance Ca(2+)-activated K+ channels, with a significant degree of homology with Ca(2+)-activated or voltage- gated K+ channels blockers: 44% homology with charybdotoxin (CTX), 52% with noxiustoxin (NTX), and 44% with iberiotoxin (IbTX) [16]. However, unlike charbdotoxin, the kaliotoxin-induced block is not voltage dependent, signifying a definative distinction in the method of blockace of KCa channels. Early studies speculated that the amino acid sequence that interacts with the channel is likely to be (26-33) since this is the region of difference between these two proteins and this was proven to be true through studies of the secondary structure. [16].Extensive studies have been done of kaliotoxin isolated from this specie, comparing it to charbdotoxin from Leirus quinquestriatus hebraeus. The secondary structure of these two proteins is homologous, but alpha-helix of kaliotoxin is shorter and bent [17]. In addition, the longer length of the extended structure just preceeding the helix results in a different tertiary structure in this region with direct implications upon the bioactivity kaliotoxin. This is due to changes the degree of accessability of amino acid 27, a conserved lysine that has been shown to be intimately involved in the channel blocking process structure which is. [17].

Other proteins of interest from mauretanicus are the neurotoxic proteins PO1 and PO5. PO5, 31 amino acids with three disulfide bridges, is similar PO1 but two amino acids longer and much more active [18]. PO5 is highly neurotoxic and competes with apamin, from bee venom, in binding to the same spot on calcium- activated potassium channels [18-20] . In addition, PO5 is similar to leiurotoxin I (scyllatoxin) in structure and activity [21].

Buthus (Indian and Middle Eastern) genus contains some very toxic species with B. tamulus being of particular toxicity.

Buthus martensi Karsch venom contains BmKIT, a 70 amino acid insect specific protein that has 75% sequence homology with AaHIT and also may have similar potential as a bioinsecticide [22]. Like most other scorpion toxins of this sort this protein is highly basic and thus positively charged. Modification by flurochrome results in a complete loss of insect toxicity, proving that the positive charges on the molecular surface are crucial in the bioactivity of these proteins [23]. Since several scorpion venoms can cause peripheral sympathetic nerve stimulation with enhanced adrenergic responses, the effect of Buthus martensi Karsch venom (MKV) on adrenergic responses was investigated using the rat isolated anococcygeus muscle (Acm). Results of this study of the adrenergic agonist action of MKV in the rat isolated anococcygeus muscle indicates that it is mediated by some prejunctional mechanism(s) of action, stimulating the release of the neurotransmitter noradrenaline. [24]

Bom III is the most significant of the toxins contained by Buthus occitanus mardochei venom. This 66 amino acid neurotoxic protein contains an amino acid sequence that is 56% immunologically related to B.o.tunetanus toxin I and II, markedly differing from toxins in the same structural and immunological group. In addition, Bom III was found to be unable to compete in binding to the sodium channel with both toxin II of Androctonus australis Hector and with toxin II of Centruroides s.suffusus (alpha and beta toxins, respectively) [25].

Buthus occitanus tunetanus contains a potentially very useful venom protein in the production of antivenom. Bot XIV is an insectidal toxin does not possess toxicological bioactivity towards mammals yet is highly antigenic in mice with the resulting antibodies having significant effectiveness in neutralising other, more toxic proteins [26].

Buthus tamulus (Indian red scorpion) is a specie best left in isolation, this will kill you [27]. The pathology of envenomation, particularily in children, is insiduous with pronounced heart damage from venom proteins acting directly upon the myocardium. Initial symptoms in children are peripheral failure and hypotension with myocarditis and other serious complications showing up in follow-up [28]. Other cardiovascular symptoms are hypertension with accompanying bradycardia and tachycardia. Pulmonary edema with either hypo or hypertension also often seen [29] The exact mechanism of action oftamulus venom is unclear with the main effects likely to be due to toxins that affect the opening of Na+ channels in nerves and muscles causing an increase in the release of neurotransmitters in the peripheral nervous system, resulting in cardiovascular abnormalities and respiratory paralysis. [30] Increased peripheral sympathetic activity with consequent enhancement of adrenergic responses stimulates the release of neurotransmitter [31]. In lab animals venom was shown to cause an increase in osmotic fragility of red cells in addition to initial hypertension followed by hypotension, suggesting a venom triggered autonomic storm and that the released catecholamines were responsible for acute myocarditis, changes in the blood pressure and increased osmotic fragility of red cells. [32].

The main venom protein isolated from tamulus is iberiotoxin, a37 amino acid neurotoxic peptide containing three disulfide bridges forming three loops, that potently block large conductance kCa channels [33, 34]. This results in a concentration dependent contraction of muscles and it has been shown expirementally that no single loop is a mediator of this action activity [33, 35]. In addition, iberiotoxin has been shown to have a 68% homology with charybdotoxin from the venom of Leirus quinquestriatus hebraeus [36].

Centruroides

The clinical effects of these scorpions from the Americas Centruroides genus is neurological, with stings in young children being life threatening, resulting in extreme neuromuscular activity, tachycardia, and respiratory distress [37]. Of this genus C. sculpturatus is likely the most toxic.

Centruroides infamatus infamatus contains a beta toxin of 66 amino acids in length that show high degrees of similarity with toxins from other Centruroides species especially with C.l.limpidus and noxius [38]. Experiments conducted with chick dorsal root ganglion cells showed that toxin 1 is a Na+ channel effector, causing a decrease in the peak Na+ permeability, similar to decreases observed for typical beta- scorpion toxins. [39]. The sodium channel is blocked in such a way that as to not affect inactivation.[40].

C. limpidus venom contains a beta toxin similar to that in infamatus which also has 66 amino acid residues and is stabilized by four disulfide bridges (Cys12-Cys65, Cys16-Cys41, Cys25-Cys46, and Cys29- Cys48). [41]. The mode of action of this toxin is the same as its counterpart in infamatus, blocking the sodium channel [40]. In addition, this neurotoxin shares 91% of the amino acid sequence of for toxin 1 from C. limpidus tecomanus and 89% of toxin 4 from C. noxius venom, both of the beta-scorpion toxin class [42]. Another toxin from C. l. limpidus, toxin 1, shown to be composed of 38 amino acid residues, containing six half-cystines, causes a partially reversible K+ current reduction that is not voltage-dependent. Comparison of the primary structure of C. l. limpidus toxin 1 with other known toxins shows 74% identity with margatoxin, 64% with NTX, 51% with kaliotoxin, 39% with iberiotoxin, 37% with charybdotoxin and Lq2, and 29% with leirutoxin 1 with the only variation among amino acids in all these toxins being the six cysteines [43].

Margatoxin of C. margaritatus is a highly potent K+ channel blocker selecting for only one subtype of potassium channel. This particular channel is directly involved in activating lymphocytes blocks lymphocyte activation and the production if interleukin-2 by human T-lymphocytes [44-46]. There exists the definate potential of use as immunosuppresent in treatment of autoimmune disease or in preventing the refection of organ transplants [44, 47]

C. noxius contains primarily beta toxins such as beta-scorpion toxin 2 which blocks voltage-gated K+ channels binding to a site different than that of other beta toxins [48, 49]. Noxiustoxin from the Mexican scorpion Centruroides noxius Hoffmann is a 39 amino acid beta toxin that also blocks neuronal K+ channels. The toxic active site of Noxiustoxin is thought to be located in or near the N-terminal amino acid portion of the molecule. [50]. A significant activity of noxiustoxin is displacement of bound dendrotoxin, a potent toxin from the mambas, from synaptosomal membranes from rat brain indicating obvious shared pharmacological properties. [51]. Comparison with charybdotoxin, potent inhibitor of the high conductance Ca(2+)-activated K+ channel from Leirus quinquestriatus hebraeus, reveals a 48% amino acid sequence similarity with noxiustoxin [52]. Noxiustoxin also shows significant similarity with leiurutoxin III (scyllatoxin), also from L.q.hebraeus [53].

Envenomation by the scorpion C. sculpturatus can be life threatening, with the the venom causing prolonged and excessive firing of neuronal axons resulting in a wide variety of signs and symptoms [54]. Fasciculation, local pain, hypertension,restlessness, paresthesiae and roving eye movements are the most common symptoms of envenomation [55-58]. The two main toxins identified from sculpturatus are CsE-M1 and CsE-V, both of significance for very different reasons. CsE-M1 is a 65 amino acid beta-toxin that is the most lethal toxin identified in sculpturatus venom and shows little similarity with other toxins isolated in sculpturatus but has a 92% sequential similarity with the most potent beta toxin found in C. suffusus [59]. CsE-V is and alpha toxin targeting sodium channels, producing spontaneous rythmic muscular contractions and is significant in that it structurally and functionally represents a transition between New World and Old World scorpion alpha toxins [60].

The primary venom component of C. suffusus is CssII, a beta-type antimammal neurotoxin [12]. As has been shown in other toxins, noteably kaliotoxin from Androctonus mauretanicus mauretanicus and charbdotoxin from Leirus quinquestriatus hebraeus, the lysine found at position Amino acid 27, is a crucial in binding to the sodium ion channel [61]. In addition, none of these three toxins, as opposed to tetrodotoxin, affect contraction of the muscle confirming a separate class of neurotoxins specific for a different receptor on the Na+ channel [62].

Euscorpius Despite being ubiquitous in Italy, this genus does not pose a medical threat, excluding anaphalaxis. The venom is primarily one of localised pain and edema [63].

Heterometrus

After Buthus, is the genus responsible for most of the serious srings in India. The lethal toxic antigenic protein isolated from the medically important H. bengalensis is toxin Hb which produces irreversible nerve blockage [64]. The venom of the less toxic H. Longimanus acts directly upon the postjunctional alpha-adrenocepotors in the anococcygeus muscles thus mimicing the agonist actions of noradrenaline. In depth studies confirmed the presense of noradrenaline in significant quantities within this venom accounting for the postjunctional alpha-agonist actions of the venom [65]. The venom of H. fulvipes has hemotoxic effects in addition to inhibiting the activity levels of acetycholine and acetylcholine esterase [66, 67].

Leirus

Leirus quinquestriatus is a highly lethal member of this genus producing pronounced cadiovascular and respiratory effects. Often seen effects are cardiogenic shock, severe systemic hypertension, central nervous system manifestations, including lethargy, confusion and agitation, and markedly reduced level of consciousness with hemodynamic studies revealing cardiogenic pulmonary edema [68]. Either the CNS or the cardiovascular effects could occur first in the early phases of the scorpion envenoming syndrome; the CNS manifestations, however, always preceded the terminal hypotension and cardiac arrest with electrocardiographic studies showing that L. quinquestriatus venom causes myocardial ischaemia and either inferior or anterior wall infarction [69].

Five main venom proteins have been isolated from the venom of L. quinquestriatus: agitoxin I and II, charybdotoxin, cholorotoxin, and scyllatoxin (leiurotoxin I).

Agitoxin is a 38 amino acid toxin that binds to the external pore entryway of the shaker K+ channel as well as the mammalian homologues of shaker [70]. Agitoxin is related to previously described K+ channel neurotoxins, but forms an ew subclass of scorpion derived K+ channel inhibitors [70]. Agitoxin II significantly differs in specificity of interaction from other calcium channel blockers with surface exposed Arg24, Lys27, and Arg31 of the beta-sheet shown to functionally important for blocking the Shaker K+ channel. [71].

Charybdotoxin is a 37 residue neurotoxin that causes concentration dependent contraction through the blocking of conductance kCa channels in addition to blocking the same receptor on voltage-dependent K+ channels as the mamba protein dendrotoxin and the bee mast cell degranulating peptide [72] This protein is of particular significance having been shown to be a good natural scaffold for protein engineering [73].

Chlorotoxin is a small insect toxin that blocks chloride channels that has a similar structure as charybdotoxin and other members of the short scorpion toxin family, consisting of a small three- stranded antiparallel beta-sheet packed against an alpha-helix, thereby adopting the same fold [74, 75]

Scyllatoxin (leiurotoxin I) meanwhile has 87 % homology structural homology and quite similar bioactivity as PO5 from Androctonus mauretanicus mauretanicus binding to high conductance calcium-activated potassium channels as apamin, from the honey bee Apis mellifera. However, current data concerning bioactivities and tertiary structures of these toxins suggest a different toxin-receptor interaction of scyllatoxin as compared to apamin and P05 [76].

Palamneus

Mild effects, venom contains histamine which results in localised irritation, edema, and itching but no systemic effects have been reported for this specie [77]

Pandinus

Pandinus imperator is a very mild specie with effects similar to that of Palamneus. Pandinus imperator scorpion venom blocks voltage-gated potassium channels [78] Pandinus venom contains toxins that bind tightly to voltage-dependent potassium channels [79] Because of its high affinity for voltage-gated potassium channels and its irreversibility, Pandinus venom may be useful in the isolation, mapping, and characterization of voltage-gated potassium channels. [80].

Two main toxins have been identified from the venom, imperatoxin A and Pil. Imperatoxin A is peptide that selectively activates skeletal-type ryanodine receptor [81]. PITxa may prove a useful tool to identify regulatory domains critical for channel gating and to dissect the contribution of skeletal-type Ca2+ release channels/ryanodine receptors to intracellular Ca2+ waveforms generated by stimulation of different ryodine receptor isoforms. [82]. Pil, composed of only 35 amino acids but cross-linked twith 4 disulfide, selectively blocks Shaker B K+ -channels [80]. This toxin belongs to a new structural class of K+-channel blocking peptide and is 40, 43 and 46% identical to noxiustoxin, margatoxin and toxin 1 of Centruroides limpidus respectively. However, it is less similar (26 to 37% identity) to toxins from scorpions of the geni Leiurus, Androctonus and Buthus. [83]

Parabuthus

Parabuthus granulatus is a toxic specie to beware of [84]. Initial effects are localised burning pain but systemic effects showing up within four hours of envenomation. The general pathology is general paraesthesia, hyperaesthesia, muscle pain and cramps with other noteable effects such as dysphagia, dysarthria and sialorrhoea with varying degrees of loss of pharyngeal reflexes [85]. In addition, the prognosis was worsened by the appearance respiratory distress in a high percentage of the cases, with this being complicated further in children by an extreme form of restlessness characterised by excessive neuromuscular activity. [85]. No specific proteins have been isolated from this genus.

Tityus

A genus containing one of the more lethal scorpions know, T. serrulatus.

Tityus bahiensis is a more common but less toxic specie than T. serrulatus [86]. Two significant proteins from this specie are gamma-b and toxin IV-5b. Gamma-b is the more abundant peptide and is 95% identical with that of toxin gamma from the venom of Tityus serrulatus. [87] Toxin IV-5b is 80% identical to the corresponding segment of T. serrulatus toxin IV-5 and was found to markedly slow inactivation of Na channel in dorsal root ganglion neurons [88].

Tityus serrulatus envenomations, like all poisoning, is much more severe in children and these are unfortunately the typical victim of the severe central nervous system dysfunction produced [89]. Death often is the result of heart failurefrom diffuse areas of myocardiocytolysis in addition to pulmonary edema [86, 90, 91]. Many potent toxins have been isolated from the venom of serrulatus, most of which show a far higher degree of homology with Old World scorpion venoms than with venoms from other scorpions from the Americas [92].

Ts-gamma produces very complex cardiological effects, characterized by an initial reduction of both rate and contractile force, followed by increase of force and reduction of rate and finally by reduction of both rate and force due to the release of acetylcholine from vagal endings [93]. This neurotoxin apparantly produces these effects on cell currents primarily by retarding activation gating of cardiac sodium channels [94].

Tityustoxin variations cause release of acetylcholine through the stimulation of calcium uptake and are responsible for the acute pulmonary edema sign in clinical settings [95, 96]. Tityustoxin V is a 64 amino acid alpha-toxin that effects K+ channels. By extending the depolarized period this neurotoxin indirectly affects beta-cell voltage-dependent K+ channels resulting in a increased K+ permeability. [97]. Ts-VII binds to both insect and mammalian nervous systems, has a much more flexible structure than the anti-insect toxin AaHIT2 (from Androctonus australis hector ) or the anti-mammal toxin CssII (from C. sculpturatus sculpturatus ) which is what allows it to compete with both of these proteins in binding to sodium channels of insects and also mammals [12, 98]. Tityustoxin-K is a selective blocker of voltage-gated, non-inactivating potassium K+ currents [99. The binding site is the same as that of charybdotoxin from Leirus quinquestriatus and the snake vneom alpha-dendrotoxin jsut as it is for [Werkman, 1993 #339].

Tityus stigmurus has yielded one main neurotoxin, gamma-st which is 95% identical with that of toxin gamma from the venom of Tityus serrulatus [87].