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Activation and desensitization of Torpedo acetylcholine receptor: Evidence for separate binding sites

Dunn, Susan M. J. and Raftery, Michael A. (1982) Activation and desensitization of Torpedo acetylcholine receptor: Evidence for separate binding sites. Proceedings of the National Academy of Sciences of the United States of America, 79 (22). pp. 6757-6761. ISSN 0027-8424.

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The acetylcholine receptor from Torpedo californica was labeled by reaction with the fluorescent probe 4-[N-(iodoacetoxy)ethyl-N-methyl]amino-7-nitrobenz-2-oxa-1, 3-diazole without apparent effect on its in vitro ligand binding and functional properties. Addition of acetylcholine or carbamoylcholine to the labeled-receptor preparations enhanced the fluorescence of the bound probe, and this effect was specific for agonists and inhibited by prior incubation with excess -bungarotoxin. Equilibrium fluorescence titrations gave apparent dissociation constants of 0.86± 0.14 mM for carbamoylcholine and 79± 11µ M for acetylcholine, in good agreement with the dissociation constants measured for the permeability response of the receptor. Stopped-flow experiments showed that the fluorescence change was a single exponential process whose rate increased with ligand concentration, reaching a saturating value for carbamoylcholine of approximately 400 s-1. The equilibrium binding of carbamoylcholine was not significantly affected by prior incubation of the receptor with d-tubocurarine or histrionicotoxin and the dissociation constant was only slightly increased in the presence of lidocaine. These inhibitory ligands do not, therefore, compete directly with agonists for this low-affinity binding site, suggesting that their mode of action may be indirect.

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Additional Information:© 1982 by the National Academy of Sciences. Communicated by Harry Gray, March 29, 1982. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Subject Keywords:carbamoylcholine; low affinity; multiple sites; parallel mechanisms
Issue or Number:22
Record Number:CaltechAUTHORS:DUNpnas82
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7601
Deposited By: Tony Diaz
Deposited On:12 Mar 2007
Last Modified:02 Oct 2019 23:43

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