Binding Sites for Cholinergic Ligands in a Particulate Fraction of Electrophorus Electroplax

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Abstract
The binding of four radioactive cholinergic ligands to a particulate fraction of Electrophorus electricus electroplax was investigated by equilibrium dialysis and found to be reversible. A single site (concentration, 0.02-0.03 nmol/g electroplax) that bound muscarone and nicotine was found to be on a phospholipoprotein; it is suggested that this is the acetylcholine receptor. Four binding sites for decamethonium were found; the highest-affinity site (0.03 nmol/g) may be the same one that binds muscarone and nicotine, both because of the chemical nature of the binding macromolecule and because decamethonium competes with muscarone and nicotine in binding. Dimethylcurare bound to three sites. The lower-affinity sites appeared to be on different macromolecules.When binding of the four cholinergic ligands (at 0.01 muM) to 28 arbitrarily selected proteins was determined, it was concluded that muscarone would be the most specific for the acetylcholine receptor, in that it bound to none of the 28 proteins tested; nicotine bound to one, dimethylcurare to two, and decamethonium to ten of the proteins. Nicotine or muscarone appear to be the ligands of choice in studying binding to the acetylcholine receptor; decamethonium appears to have inadequate specificity.