DIFFERENTIAL AGONIST-INDUCED DISPLACEMENT OF QUINACRINE AND ETHIDIUM FROM THEIR RESPECTIVE HISTRIONICOTOXIN-SENSITIVE BINDING-SITES ON THE TORPEDO ACETYLCHOLINE-RECEPTOR

Fluorescence spectroscopy was used to begin to localize the agonist in hibitory binding site on the nicotinic acetylcholine receptor (AcChR) from Torpedo californica. High concentrations of three cholinergic ago nists, suberyldicholine (SubCh), acetylcholine (AcCh), and carbamylcho line (CCh), differentially inhibited the binding of two noncompetitive inhibitors (nCIs), quinacrine and ethidium, which bind at distinctly different loci on the desensitized AcChR at zero membrane potential. T he agonist-induced inhibition of quinacrine binding occurred at signif icantly lower (17-fold) concentrations than the inhibition of ethidium binding. Schild plots of SubCh inhibition of ethidium and quinacrine binding showed the competitive nature of the agonist inhibition of the binding of these two NCIs. The quenching constants for short-range qu enching of receptor-bound quinacrine and ethidium fluorescence by spin -labeled acetylcholine were about the same as their inhibition constan ts for agonist-induced displacement of AcChR-bound quinacrine and ethi dium. The results demonstrate that agonists can directly bind to both the quinacrine and the ethidium binding sites, albeit at different ago nist concentrations. Because the agonist-induced displacement of recep tor-bound quinacrine occurs at significantly lower concentrations than the displacement of ethidium, the quinacrine binding site is more lik ely than the ethidium binding site to form part of the agonist inhibit ory binding site.