Purification, structure and activity of three insect toxins from Buthus occitanus tunetanus venom

One contractive and two depressant toxins active on insect were purified by high-performance liquid chromatography from the venom of Buthus occitanus tunetanus (Bet). The two depressant toxins, BotIT4 and BotIT5, differ only at position 6 (Arg for Lys) and are equally toxic to insects (LD(50) to Blatella germanica = 110 ng/100 mg body weight). They show a strong antigenic cross-reaction with a depressive toxin from Leiurus quinquestriatus quinquestriatus (LqqIT2). The two toxins are able to inhibit with high affinity (K-0.5 between 2 and 3 nM) the specific binding of the radioiodinated excitatory insect toxin (I-125-AaHIT) on its receptor site on Periplaneta americana synaptosomal membranes. These toxins depolarize the cockroach axon, irreversibly block the action potential, and slow down and very progressively block the transmembrane transient Naf current. The contracturant toxin BotIT1 is highly toxic to B. germanica (LD(50) = 60 ng/100 mg body weight) and barely toxic to mice (LD(50) = 1 mu g/20 g body weight) when injected intracerebroventricularly. It does not compete with I-125-AaHIT for its receptor site on P. americana synaptosomal membranes. On cockroach axon, BotIT1 develops plateau potentials and slows down the inactivation mechanism of the Na+ channels. Thus, BotIT1 belongs to the group of alpha insect-selective toxins and shows a strong sequence identity (> 90%) with Lqh alpha IT and LqqIII, two insect alpha-toxins previously purified from the venom of L. g. hebraeus and L. q. quinquestriatus, respectively. (C) 1997 Elsevier Science Ltd.