Several properties of the enzyme acetylcholinesterase (AChE) isolated in vitro are compared with those of the membrane receptor(s) of acetylcholine expressed by the in vivo electrical response of the electroplax membrane. AChE strongly binds in vitro effectors of the electroplax: agonists e.g., decamethonium or antagonists, e.g., d-tubocurarine and flaxedil. It also reacts covalently with an affinity labeling reagent of the acetylcholine receptor site(s) in vivo (TDF). Two classes of sites on AChE molecule account for the binding of these quaternary nitrogen containing compounds: (1) the anionic site of the active center and (2) noncatalytic "peripheral anionic centers" located outside the active center. A disulfide bond breaking agent, dithiothreitol (DTT) alters in a parallel manner the reaction of AChE and the excitable membrane of the electroplax to TDF. The irreversibility of TDF action is lost in both cases, after exposure to DTT. Both AChE and the acetylcholine receptor thus contain disulfide bonds—they are closely related but not necessarily identical proteins.

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