Biochemical and pharmacological characterization of a depressant insect toxin from the venom of the scorpion Buthacus arenicola

A depressant toxin active on insects, Buthacus arenicola IT2, was isolated from the venom of the North African scorpion B. arenicola and its structural and pharmacological properties were investigated. B. arenicola IT2 is a single polypeptide of 61 amino acid residues, including 8 half-cystines but no methionine and histidine, with a molecular mass of 6835 Da. Its amino acid sequence is 79-95% identical to other depressant toxins from scorpions. When injected into the cockroach Blatella germanica, B. arenicola IT2 induced a slow depressant flaccid paralysis with a LD(50) of 175 ng. B. arenicola IT2 has two non-interacting binding sites in cockroach neuronal membranes: one of high affinity (K-d1 = 0.11 +/- 0.04 nM) and low capacity (B-max1 = 2.2 +/- 0.6 pmol/mg), and one of low affinity (K-d2 = 24 +/- 7 nM) and high capacity (B-max2 = 226 +/- 92 pmol/mg). Its binding to these two sites was completely inhibited by Leiurus quinquestriatus quinquestriatus IT2, a depressant toxin from L. quinquestriatus quinquestriatus. Reciprocal-binding experiments between B. arenicola IT2 and the excitatory insect-toxin A. australis Hector IT revealed competition between the two toxins for the high-affinity sites of B. arenicola IT2. B. arenicola IT2 has a higher affinity than L. quinquestriatus hebraeus IT2, a depressant toxin from L. quinquestriatus hebraeus. Thus, B. arenicola IT2 represents an interesting tool to study the receptor site for depressant toxins on insect sodium channels.