title: Covalent labeling of the acetylcholine receptor from Torpedo electric tissue with the channel blocker [3H]triphenylmethylphosphonium by ultraviolet irradiation.
abstract: The lipophilic cation [3H]triphenylmethylphosphonium, frequently used as a voltage sensor in membrane systems, binds reversibly to a site different from the acetylcholine binding site. This is concluded from the different pH dependences of the binding of these two ligands. Furthermore [3H]triphenylmethylphosphonium, previously identified as a channel blocker, can be covalently incorporated into acetylcholine receptor-rich membranes from Torpedo electric tissue by UV irradiation of the receptor-ligand complex. In the absence of effector, predominantly the alpha-polypeptide chains (Mr 40000) of the receptor protein are labeled by the radioactive ligand. The agonist carbamoylcholine strongly stimulates the labeling, but it directs the label predominantly to the delta- and beta-polypeptide chains. The antagonist D-tubocurarine and the virtually irreversible competitive antagonist alpha-bungarotoxin have qualitatively the same effect as the agonist carbamoylcholine. Significant differences were obtained with receptor-rich membranes prepared from Torpedo marmorata and Torpedo californica: No agonist- or antagonist-stimulated reaction was observed with the latter. The results are interpreted as an indication of a rearrangement of the receptor's quaternary structure caused by cholinergic effector binding preceding discrimination between agonists and antagonists.
mesh_terms: D000109:Acetylcholine / Q000378:metabolism; D000345:Affinity Labels / Q000378:metabolism; D000818:Animals; D002038:Bungarotoxins / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D004557:Electric Organ / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007473:Ion Channels / Q000378:metabolism; D046911:Macromolecular Substances; D009861:Onium Compounds; D011950:Receptors, Cholinergic / Q000378:metabolism*; D014101:Torpedo; D014320:Triphenylmethyl Compounds / Q000378:metabolism* / Q000528:radiation effects; D014403:Tubocurarine / Q000494:pharmacology; D014466:Ultraviolet Rays
chemical_list: D000345:Affinity Labels; D002038:Bungarotoxins; D007473:Ion Channels; D046911:Macromolecular Substances; D009861:Onium Compounds; D011950:Receptors, Cholinergic; D014320:Triphenylmethyl Compounds; C011561:triphenylmethylphosphonium; D002217:Carbachol; D000109:Acetylcholine; D014403:Tubocurarine