Molecular Pharmacology, Vol 11, 352-360, Copyright © 1975 by the American Society for Pharmacology and Experimental Therapeutics

Acetylcholinesterase: Specificity of the Peripheral Anionic Site for Cholinergic Ligands

¹ Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1W5

The activation of erythrocyte acetylcholinesterase by various ligands, including CaCl₂, was almost completely and irreversibly blocked by a water-soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC), at a peripheral anionic site. CaCl₂ either enhanced, antagonized, or had little effect on the inhibition of the unmodified enzyme by cholinergic ligands, depending on the affinity of these ligands for the peripheral anionic site. These effects of CaCl₂ were abolished after modification of the peripheral site by EDAC. Whereas both cholinergic agonists and antagonists had affinity for the peripheral site, antagonists (gallamine, pentamethonium, hexamethonium, tetraethylammonium, and atropine) accelerated acetylcholine hydrolysis, while agonists [tetramethylammonium, nicotine, and EDAC (reversible component)] had little effect. Acetylcholine and butyrylcholine inhibited substrate hydrolysis when bound to the peripheral site, but this inhibition accounted for only part of the total substrate inhibition observed, since substrate inhibition of the EDAC-modified enzyme, though delayed, still occurred at high substrate concentrations. Hydrophobic molecules (chlorpromazine and tetracaine) and ligands with structural complementarity to the active site (edrophonium) had little or no affinity for the peripheral anionic site. The peripheral anionic site may be located in a more polar environment than that of the catalytic anionic site. Binding of cholinergic ligands to the peripheral anionic site of acetylcholinesterase appeared to elicit responses parallel to those of cholinoreceptors.