Tityustoxin

Tityustoxin is a toxin found in the venom of scorpions from the subfamily of Tityinae. By binding to voltage-dependent sodium ion channels and potassium channels, they cause sialorrhea, lacrimation and rhinorrhea.

Sources

Scorpions are distributed among six families. Only those of the Buthidae family are dangerous to humans. Within this family, the Tityinae subfamily cause the most scorpion poisoning in humans. (Correa, Sampaio et al. 1997). These species are found in Brazil (Tityus serrulatus, T. bahiensis and T. stigmurus) and in Northern and Southern Venezuela (T. discrepans) (Novello, Arantes et al. 1999). The venom of Tityus Serrulatus is the most potent of these (Correa, Sampaio et al. 1997).

Chemistry

The crude venom of T. serrulatus contains different toxins. Some of the strongest derivates are the neurotoxins tityustoxin and toxin Ts-g (Clemente, Rossoni et al. 1999). Two types of toxins are considered to be responsible for the main toxic effect: toxin gamma (an β-type toxin) and tityustoxin (TsTX, a α-type toxin), both with a specific affinity to the sodium channel. (Mesquita, Moraes-Santos et al. 2003) Other types of venom are: TsTX-kα, a 37 amino acid (Werkman, Gustafson et al. 1993), toxin gamma (TsTX-γ and TsTX-I) with 61 amino acid residues is the major neurotoxin of this venom. TsTX-Kβ has a longer chain. (Novello, Arantes et al. 1999) K+ channels blocking peptides are single chain polypeptides of 30-40 amino acids with three sulfide bridges (Legros, Oughuideni et al. 1996). The toxin with four disulfide bonds is from TsTX-IV. This contains 41 amino acid residues. (Novello, Arantes et al. 1999)

Target

-Na⁺ Channel affecting peptides: TsTX-γ and TsTX-I (Novello, Arantes et al. 1999). -Ca²⁺ activated K⁺ Channel affecting peptides: TsTX-Kβ and TsTX-IV. The latter has got a high affinity. TsTx-IV blocks Ca²⁺ activated K⁺ channels of high conductance. (Novello, Arantes et al. 1999)

Mode of action

Two types of toxin are interesting: α-Scorpion toxins bind at site 3 of Na1 channels, causing a slowing of their inactivation. β-Scorpion toxins bind at site 4, shifting the activation of Na1 currents (INa) toward more negative potentials (Conceicao, Lebrun et al. 1998). Tityustoxin causes cell depolarization, activating Na⁺ channels and increasing the Na⁺ uptake that can affect Ca²⁺ uptake and can increase acetylcholine release from cerebral cortical slices (Casali, Gomez et al. 1995).

Na⁺ channels: The α-toxins bind to the subunit 3 of the sodium channel, slowing the inactivation and increasing peak current without changing time to peak (Mesquita, Moraes-Santos et al. 2003). This causes cell depolarization that opens calcium channels allowing the influx of Ca²⁺, triggering ACh release (Casali, Gomez et al. 1995). Both the steady-state activation and inactivation curves are shifted to more negative potentials. (Mesquita, Moraes-Santos et al. 2003)

K⁺ channels; TsTX-I, Ts1 or toxin gamma is a β-type toxin that binds to receptor site 4 and shift the voltage dependence of the sodium channel activation to more negative potentials (Pessini, Takao et al. 2001) TsTX-Ka selectively blocks voltage-gated noninactivating (possibly delayed rectifier) K⁺ channels in synaptosomes (Rogowski, Krueger et al. 1994).

Toxicity

The venom of Tityus serrulatus is the most potent of the toxins from the species (Clemente, Rossoni et al. 1999). Tityustoxin-1, TsTX-I is the most toxic protein among the neurotoxins in this venom, with an intravenous and intracisternal LD₅₀ (mouse) of 76 ± 9 and 1.1 ± 0.3 µg/kg, respectively. The identification of TsTX-I as a potent component of T. serrulatus venom characterized it as the major and main neurotoxin from this venom (Correa, Sampaio et al. 1997). Poisoning effects in man evoked by T. serrulatus venom are sialorrhea, lacrimation and rhinorrhea. (Clemente, Rossoni et al. 1999) and acute pancreatitis (Correa, Sampaio et al. 1997). Catecholamines by the adrenal glands and postganglionic nerve terminals and Ach by ganglions and postganglionic nerve terminals are released when the poison strikes. Also other neurotransmitters are released by the whole venom and isolated toxins (Correa, Sampaio et al. 1997). In rats, the Tityustoxin caused dramatic effects on the circulatory and respiratory systems, consisting of hypotension, tachypnea, hyperpnea, ataxic and gasping breathing. Following these initial effects, 5 or 10 µg of TsTX induced hypertension and hyperpnea. The largest dose produced apnea and death about 70 min later (Lima and Freire-Maia 1977).

Treatment

The lung edema induced by TsTX is blocked by 170 mg/kg of phenobarbital. (Mesquita, Moraes-Santos et al. 2002) Rabbit anti-TsNTxP antibodies displayed cross-reactivity with the scorpion toxins and showed in vitro neutralizing capacity. Thus, this protein emerges as a strong candidate for the production of antiserum to be used in the treatment of scorpion stings. The nontoxic recombinant protein can induce a level of circulating antibodies sufficient to neutralize the toxic effects of Tityus toxins and is a good candidate for use in the production of a new generation of neutralizing polyclonal antibodies for clinical use (Guatimosim, Kalapothakis et al. 2000).

References

Casali TA, Gomez RS, Moraes-Santos T, Gomez MV (June 1995). "Differential effects of calcium channel antagonists on tityustoxin and ouabain-induced release of [3H]acetylcholine from brain cortical slices". Neuropharmacology. 34 (6): 599–603. doi:10.1016/0028-3908(95)00019-3. PMID 7566495. 

Clemente GT, Rossoni RB, Safe JM, Freire-Maia L (February 1999). "Effects of crude venom, tityustoxin and toxin Ts-gamma from Tityus serrulatus scorpion on secretion and structure of the rat submandibular gland". Arch. Oral Biol. 44 (2): 103–10. doi:10.1016/S0003-9969(98)00109-5. PMID 10206328. 

Conceicao IM, Lebrun I, Cano-Abad M, et al. (June 1998). "Synergism between toxin-gamma from Brazilian scorpion Tityus serrulatus and veratridine in chromaffin cells". Am. J. Physiol. 274 (6 Pt 1): C1745–54. PMID 9611141. 

Corrêa MM, Sampaio SV, Lopes RA, et al. (July 1997). "Biochemical and histopathological alterations induced in rats by Tityus serrulatus scorpion venom and its major neurotoxin tityustoxin-I". Toxicon. 35 (7): 1053–67. doi:10.1016/S0041-0101(96)00219-X. PMID 9248004. 

Guatimosim SC, Kalapothakis E, Diniz CR, Chávez-Olórtegui C (January 2000). "Induction of neutralizing antibodies against Tityus serrulatus toxins by immunization with a recombinant nontoxic protein". Toxicon. 38 (1): 113–21. doi:10.1016/S0041-0101(99)00138-5. PMID 10669016. 

Legros C, Oughuideni R, Darbon H, Rochat H, Bougis PE, Martin-Eauclaire MF (July 1996). "Characterization of a new peptide from Tityus serrulatus scorpion venom which is a ligand of the apamin-binding site". FEBS Lett. 390 (1): 81–4. doi:10.1016/0014-5793(96)00616-3. PMID 8706835. 

Lima EG, Freire-Maia L (1977). "Cardiovascular and respiratory effects induced by intracerebroventricular injection of scorpion toxin (tityustoxin) in the rat". Toxicon. 15 (3): 225–34. doi:10.1016/0041-0101(77)90048-4. PMID 867438. 

Matteson DR, Blaustein MP (September 1997). "Scorpion toxin block of the early K+ current (IKf) in rat dorsal root ganglion neurones". J. Physiol. (Lond.). 503 (Pt 2): 285–301. doi:10.1111/j.1469-7793.1997.285bh.x. PMC 1159863. PMID 9306273. 

Mesquita MB, Moraes-Santos T, Moraes MF (October 2002). "Phenobarbital blocks the lung edema induced by centrally injected tityustoxin in adult Wistar rats". Neurosci. Lett. 332 (2): 119–22. doi:10.1016/S0304-3940(02)00932-1. PMID 12384225. 

Mesquita MB, Moraes-Santos T, Moraes MF (February 2003). "Centrally injected tityustoxin produces the systemic manifestations observed in severe scorpion poisoning". Toxicol. Appl. Pharmacol. 187 (1): 58–66. doi:10.1016/S0041-008X(02)00036-4. PMID 12628585. 

Novello JC, Arantes EC, Varanda WA, Oliveira B, Giglio JR, Marangoni S (April 1999). "TsTX-IV, a short chain four-disulfide-bridged neurotoxin from Tityus serrulatus venom which acts on Ca²⁺-activated K⁺ channels". Toxicon. 37 (4): 651–60. doi:10.1016/S0041-0101(98)00206-2. PMID 10082164. 

Pessini AC, Takao TT, Cavalheiro EC, et al. (October 2001). "A hyaluronidase from Tityus serrulatus scorpion venom: isolation, characterization and inhibition by flavonoids". Toxicon. 39 (10): 1495–504. doi:10.1016/S0041-0101(01)00122-2. PMID 11478957. 

Rogowski RS, Krueger BK, Collins JH, Blaustein MP (February 1994). "Tityustoxin K alpha blocks voltage-gated noninactivating K+ channels and unblocks inactivating K+ channels blocked by alpha-dendrotoxin in synaptosomes". Proc. Natl. Acad. Sci. U.S.A. 91 (4): 1475–9. doi:10.1073/pnas.91.4.1475. PMC 43182. PMID 7509073. 

Salgado AH, Prado MA, Moraes-Santos T, Romano-Silva MA, Gomez MV (June 1997). "Tityustoxin-induced release of ATP from rat brain cortical synaptosomes". Neurosci. Lett. 229 (2): 113–6. doi:10.1016/S0304-3940(97)00436-9. PMID 9223604. 

Werkman TR, Gustafson TA, Rogowski RS, Blaustein MP, Rogawski MA (August 1993). "Tityustoxin-K alpha, a structurally novel and highly potent K⁺ channel peptide toxin, interacts with the alpha-dendrotoxin binding site on the cloned Kv1.2 K⁺ channel". Mol. Pharmacol. 44 (2): 430–6. PMID 8355670.