Tityustoxin

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).