Selective labeling of the .delta. subunit of the acetylcholine receptor by a covalent local anesthetic

Abstract

A radioactive photoaffinity derivative of the potent local anesthetic trimethisoquin, 5-azido[3H]trimethisoquin, was used to label the acetylcholine receptor from Torpedo marmorata electric organ. The product labeled the 66,000-dalton (.delta.) subunit of the receptor with the selectivity expected for an affinity label of the site for noncompetitive blockers. The labeling was enhanced by cholinergic agonists and inhibited by other noncompetitive blockers. The 40,000-dalton (.alpha.) subunit of the receptor was labeled in a manner consistent with the attachment of 5-azido[3H]trimethisoquin to an acetylcholine binding site as the incorporation of radioactivity into the .alpha. chain was inhibited by cholinergic agonists and antagonists, such as carbamylcholine, d-tubocurarine and .alpha.-bungarotoxin. The reversible binding of [3H]phencyclidine, a potent noncompetitive blocker, to acetylcholine receptor rich membranes resembled qualitatively and quantitatively the 5-azido[3H]trimethisoquin labeling of the .delta. subunit and was inhibited by the prior covalent labeling of the membranes with nonradioactive 5-azidotrimethisoquin. 5-Azido[3H]-trimethisoquin labels at least a portion of the binding site for noncompetitive blockers at the level of the .delta. subunit. The functional significance of this site and the use of 5-azidotrimethisoquin in the study of acetylcholine receptor structure and function are discussed.