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Localization of phencyclidine binding sites on alpha and beta subunits of the nicotinic acetylcholine receptor from Torpedo ocellata electric organ using azido phencyclidine

A photolabile derivative of phencycliding (PCP), azido phencyclidine (AZ-PCP), was synthesized and used to localize PCP binding sites on the acetylcholine receptor from Torpedo ocellata electric organ. In the dark, the binding of micromolar concentrations of [3H]AZ-PCP to a receptor-enriched membrane preparation fits a single dissociation constant (Kd = 2.65 microM) and is very similar to the binding of [3H]PCP. The agonist carbamylcholine increases the association rate (and the affinity) of these ligands to the receptor, but it does not alter the total number of available binding sites. Following UV irradiation and gel electrophoresis, [3H]AZ-PCP was found to label specifically the alpha and beta subunits of the receptor. The labeling of the alpha subunit band was heavier, and it was inhibited by tetracaine and PCP but not by alpha-bungarotoxin (alpha-Bgt). The addition of carbamylcholine enhanced the labeling of the beta subunit; this effect was diminished by alpha-Bgt. The labeling of the beta subunit was also inhibited by tetracaine and PCP. The effect of carbamylcholine, which binds to the alpha subunit, could be the result of an induced conformational change, which is propagated to the beta subunit and increases its labeling by [3H]AZ-PCP. A simple model which accommodates the binding and photoaffinity labeling data is described. According to the model, the high affinity PCP binding site is located between the alpha and beta receptor subunits, and the drug thus becomes attached simultaneously to both. Hypothetical overlapping recognition sites for PCP on these receptor subunits would allow binding (and labeling) with increased affinity in the presence of carbamylcholine with no increase in the number of available sites.