An insect selective neurotoxic
polypeptide from
venom of the scorpion Androctonus australis (
AaIT, M(r) 8,000) was shown to cross the midgut of the flesh fly Sarcophaga falculata, using assays of oral toxicity, column chromatography, and microscopic autoradiography of the native and radioiodinated toxin.
AaIT induced
paralysis of flies within 1-2 h after
oral administration, with a lethal dose (LD50) of 10 micrograms/100 mg of
body weight. Oral toxicity was about 0.14% of toxicity by injection. Hemolymph collection 70-85 min after feeding flies with [125I]
AaIT showed that 5% of ingested radioactivity appeared in hemolymph. Most of this represented degradation products, but included about 0.3% of the chromatographically intact toxin. In contrast, hemolymph of identically treated lepidopterous larvae (Manduca, Helioverpa [= Heliothis]) contained degradation products but no intact toxin. [125I]
AaIT was shown to cross the midgut of Sarcophaga through a morphologically distinct segment of the midgut previously shown to be permeable to a cytotoxic, positively charged
polypeptide of similar molecular weight. These results suggest that Sarcophaga midgut contains a morphologically and functionally distinct segment that transports small
peptides, and that employment of neurotoxic
polypeptides for insect control may be feasible. Activity might be greatly improved through modification and metabolic stabilization of active
peptides.