Purification, Characterization, and Primary Structure of Four Depressant Insect-Selective Neurotoxin Analogs from Scorpion (Buthus sindicus) Venom

Four depressant insect-selective neurotoxin analogs (termed Bs-dprIT1 to 4) from the venom of the scorpion Buthus sindicus were purified to homogeneity in a single step using reverse-phase HPLC. The molecular masses of the purified toxins were 6820.9, 6892.4, 6714.7, and 6657.1 Da, respectively, as determined by mass spectrometry. These long-chain neurotoxins were potent against insects with half lethal dose values of 67, 81, 103, and 78 ng/100 mg larva and 138, 160, 163, and 142 ng/100 mg cockroach, respectively, but were not lethal to mice even at the highest applied dose of 10 μg/20 g mouse. When injected into blowfly larvae (Sarcophaga falculata), Bs-dprIT1 to 4 induced classical manifestations of depressant toxins, i.e., a slow depressant flaccid paralysis. The primary structures of Bs-dprIT 1 to 4 revealed high sequence homology (60–75%) with other depressant insect toxins isolated from scorpion venoms. Despite the high sequence conservation, Bs-dprIT1 to 4 showed some remarkable features such as (i) the presence of methionine (Met6 in Bs-dprIT1 and Met24 in Bs-dprIT2 to 4) and histidine (His53 and His57 in Bs-dprIT1) residues, i.e., amino acid residues that are uncommon to this type of toxin; (ii) the substitution of two highly conserved tryptophan residues (Trp43 → Ala and Trp53 → His) in the sequence of Bs-dprIT1; and (iii) the occurrence of more positively charged amino acid residues at the C-terminal end than in other depressant insect toxins. Multiple sequence alignment, sequence analysis, sequence-based structure prediction, and 3D homology modeling studies revealed a protein fold and secondary structural elements similar to those of other scorpion toxins affecting sodium channel activation. The electrostatic potential calculated on the surface of the predicted 3D model of Bs-dprIT1 revealed a significant positive patch in the region of the toxin that is supposed to bind to the sodium channel.