155Eu As a probe of cholinergic ligand interactions with acetylcholine receptor proteins isolated from Drosophila melanogaster and Torpedo Californica

Abstract

155Eu 3+ exchange experiments were performed on acetylcholine receptor proteins isolated from the electroplax tissue of Torpedo californica and the cerebral ganglion of Drosophila melanogaster utilizing a dual-chambered flow dialysis nuclear counting apparatus designed and constructed in our laboratory. The apparatus continuously monitors 155Eu 3+ γ ray emission from the protein compartments of two flow dialysis cells facilitating the measurement of exchange half-lives. Receptor protein is dialyzed against buffer in the first cell, while in the second cell the receptor is perturbed by the presence of nicotinic ligand in the dialysate. Nicotinic ligands induce 155Eu 3+ displacement from the receptor proteins of both species in a manner directly related to the structure of the ligand. Nicotine and nikethamide molecules with protonated pyridyl nitrogen atoms induce 155Eu 3+ exchange significantly, while the deprotonated forms of the molecules do not effect exchange. Acetylcholine, tetraethylammonium ion and carbamylcholine all possess a quaternary nitrogen. Acetylcholine displaces bound 155Eu 3+ from the acetylcholine receptor proteins more readily than the tetraethylammonium ion does, which in turn induces exchange better than carbamylcholine does.