8.B.1 The Long (4C-C) Scorpion Toxin (L-ST) Superfamily
The NaC- L-ST superfamily contains a large number of scorpion-derived peptide toxins. These are tabulated below with descriptions. They include the well-characterized scorpion α- and β-toxins that act on tetrodotoxin-inhibitable, voltage-gated Na+ channels (TC #1.A.1.10). While the α-toxins (e.g., from Buthinea venom) prolong the Na+-inactivation phase of the activated action potential-causing channels, thereby blocking neuronal transmission, the β-toxins (e.g., from Centrurinae sculpturatus venom) affect the Na+-activation phase. These toxins are derived from a variety of scorpions. They can affect both insect and mammalian Na+ channel activities (Tian et al., 2007).
In many cases the details of their toxic actions are known. β-scorpion toxin, for example, targets neurotoxin receptor site 4 in Na+ channels and induces a negative shift in the voltage dependence of activation through a voltage sensor-trapping mechanism (Cestèle et al., 2006). A single organism may produce many of these toxins, some closely related, others more distantly related. This superfamily includes hundreds of sequenced members as revealed by PSI-BLAST searches with six iterations, some of which are reported to be non-toxic. They are usually characterized by four disulfide bridges, but some have three or five.