Agonist and Antagonist Binding of Muscarinic Acetylcholine Receptors Purified from Porcine Brain: Interconversion of High‐ and Low‐Affinity Sites by Sulfhydryl Reagents

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Abstract
The affinity for muscarinic ligands of a preparation of muscarinic acetylcholine receptors purified from porcine brain was examined by means of competitive binding of [3H]quinuclidinylbenzylate and unlabeled ligands, followed by computer‐assisted nonlinear regression analysis. The displacements by antagonists fitted a single‐site model. In contrast, the displacements by agonists did not fit the single‐site model and could be explained by assuming two populations of binding sites. The proportion of the sites with high affinity for muscarinic agonists (H‐sites) ranged from 25 to 35% of the total number of sites. GTP had no effect on the displacements by agonists, a finding indicating that H‐sites did not result from interaction between receptors and GTP‐binding proteins. In the presence of dithiothreitol, the affinity for muscarinic ligands decreased. The largest effects were observed on the affinity for pirenzepine and that of H‐sites for carbachol. Preincubation of the preparation with 5,5′‐dithiobis(2‐nitrobenzoic acid) resulted in an increase in the proportion of H‐sites to 75% of the total number of binding sites. The results of sucrose density gradient centrifugation of the preparation indicated apparent heterogeneity as to molecular size of the receptors, but this heterogeneity did not correlate with that of the affinity for agonists. In addition, the receptors were detected as a single band on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis of the preparation, regardless of the presence or absence of disulfide‐reducing reagents. These results suggest that the redox state of thiol groups in the receptor molecules is relevant to their affinities for ligands. In particular, molecules carrying (an) intact, probably intramolecular, disulfide bond(s) show high affinity for agonists, whereas those carrying the reduced state of the relevant thiol group(s) show low affinity for agonists.

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