The binding of an insect-selective neurotoxin and saxitoxin to insect neuronal membranes

Abstract

(1) The radioiodinated insect-selective toxin derived from the venom of the scorpion Androctonus australis 125I-labeled AaIT ( 125I-AaIT) was used in a series of equilibrium-saturation binding assays to synaptosomal plasma membrane vesicles derived from locust brains, locust ventral nerve cords, cricket central nervous system, fly larvae central nervous system and fly heads. AaIT binds to single classes of non-interacting binding sites of high affinities K d = 1.2–3 nM) and low capacities (0.5–2.0 pmol/mg of membrane protein), thus indicating the existence of an evident homogeneity in the AaIT-binding constants among various insect neuronal membranes. (2) Assays on the binding of tritiated saxitoxin to the locust central nervous system synaptosomal membrane vesicles indicate that it (a) binds to a single class of non-interacting binding sites with K d = 0.14 nm and a capacity of 1.4 pmol/mg of membrane protein closely resembling the capacity of AaIT (1.2–2 pmol/mg protein), and (b) is competitively displaced with a high affinity ( K d = 0.35 nM) by tetrodotoxin. (3) The estimated binding capacity of AaIT to a crude preparation of fly heads (0.12–0.14 pmol/mg protein) is in accordance with previous data [31,32] concerning the binding of saxitoxin. The close resemblance in the binding capacity of saxitoxin and insect toxin to insect neuronal membranes supports the suggestion, previously given by electrophysiological studies, that the insect toxin's binding sites are related to the insect neuronal sodium channels.