Photochromic Activators of the Acetylcholine Receptor

Two photochromic activators of the electrogenic membrane of the electroplax of Electrophorus electricus are described. Trans-3,3'-bisalpha-(trimethylammonium)methylazobenzene dibromide (Bis-Q), one of the most potent ever reported, is active at concentrations of less than 10(-7) M. Its cis isomer, which is obtained from the trans by exposure to light of 330 nm, is practically devoid of activity. Photoregulation of the potential of the membrane takes place in the presence of Bis-Q, presumably because of the conversion of the active trans isomer to the inactive cis isomer in the single-cell electroplax system. The second activator, 3-(alpha-bromomethyl)-3'-alpha-(trimethylammonium)methylazobenzene bromide (QBr) can be covalently attached to the electroplax membrane after reduction of the membrane with dithiothreitol. Activation of the membrane is induced by the covalently linked reagent. Its cis isomer, obtained from the trans by exposure to light of 330 nm, is, like cis-Bis-Q, of very low activity. Both isomers of Bis-Q are equally active as inhibitors of acetylcholinesterase, 50% inhibition occurring at a concentration of 10(-5) M. The possibility of using trans-Bis-Q and trans-QBr to characterize and isolate the receptor protein is discussed.