Rates and equilibria for a photoisomerizable antagonist at the acetylcholine receptor of Electrophorus electroplaques

Voltage-jump and light-flash experiments have been performed on isolated Electrophorus electroplaques exposed simultaneously to nicotinic agonists and to the photoisomerizable compound 2,2'-bis- [alpha-(trimethylammonium)methyl]-azobenzene (2BQ). Dose-response curves are shifted to the right in a nearly parallel fashion by 2BQ, which suggests competitive antagonism; dose-ratio analyses show apparent dissociation constants of 0.3 and 1 microM for the cis and trans isomers, respectively. Flash-induced trans----cis concentration jumps produce the expected decrease in agonist-induced conductance; the time constant is several tens of milliseconds. From the concentration dependence of these rates, we conclude that the association and dissociation rate constants for the cis-2BQ-receptor binding are approximately 10(8) M-1 s-1 and 60 s-1 at 20 degrees C; the Q10 is 3. Flash-induced cis----trans photoisomerizations produce molecular rearrangements of the ligand-receptor complex, but the resulting relaxations probably reflect the kinetics of buffered diffusion rather than of the interaction between trans-2BQ and the receptor. Antagonists seem to bind about an order of magnitude more slowly than agonists at nicotinic receptors.