The chemistry and axonal action of two insect toxins derived from the venom of the scorpion Buthotus judaicus

Two toxic proteins, insect toxins I and II (IT-I and II), selectively paralytic and lethal to insects, were purified from the venom of the Black scorpion Buthotus judaicus. IT-I is approximately 40 times more toxic than the crude venom (according to its fly larvae paralytic activity), is composed of 67 amino acids including six half cystines, and has an estimated molecular weight of 7,532 and a pI value of 8.20. It causes an immediate contraction paralysis of fly larvae and a quick excitatory "knock-down" effect on locusts. IT-II is about 36 times more toxic than the crude venom according to the paralytic potency in fly larvae. It is composed of 69 amino acids including six half cystines and has an estimated molecular weight of 7,894, a unique amino acid composition and a pI value of 8.30. IT-II causes a flaccid paralysis of fly larvae and a slow progressive paralysis and eventually death of locusts. In an isolated axon of a cockroach, IT-I induces repetitive activities, and IT-II, a block of the evoked action potentials. These two opposite effects may be explained by their different effects on sodium permeabilities, as shown by voltage clamp experiments. Both toxins increase the sodium resting permeability (IT-II markedly more) resulting in a progressive depolarization of the axonal membrane. Concerning the activable sodium permeability, both toxins slightly slow the sodium transient inward current turning off. The peak sodium current, however, is increased by IT-I and decreased by IT-II. This essential difference may, at least partially, account for the contrasting symptoms they induce in the whole insect.