Molecular Pharmacology, Vol 18, 33-39, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Modulation of Acetylcholinesterase Activity by Peripheral Site Ligands

¹ Department of Chemistry, University of Winnipeg, Winnipeg, Manitoba R3B 2E9, Canada

The steady-state hydrolysis of acetylthiocholine by soluble acetylcholinesterase from Electrophorus electricus (EC 3.1.1.7) under conditions of low ionic strength is inhibited by "peripheral" ligands such as gallamine, propidium, and d-tubocurarine. Curvature in the Lineweaver-Burk plots obtained when either gallamine or propidium is introduced to the enzyme at the same time as substrate is eliminated by 10-min preincubation of the enzyme with ligand and the resulting (inhibited) rates exhibit a moderate enhancement. No preincubation effects are detectable with d-tubocurarine; however the uncompetitive component of inhibition is more pronounced than that observed with gallamine or propidium. Metal ions such as Ca²⁺ and Mg²⁺ are activators at high substrate concentrations; in contrast, Zn²⁺ is a powerful inhibitor. The pre-steady-state carbamoylation of acetylcholinesterase by 7-(dimethylcarbamoyloxy)-N-methylquinolinium iodide (M7C) is subject to substrate activation at M7C concentrations above 5 µM. Gallamine and propidium inhibit this reaction at lower substrate concentrations but have little influence at higher substrate concentrations. The decarbamoylation of the dimethylcarbamoyl enzyme is accelerated by both ligands, as well as by Ca²⁺, Mg²⁺ and M7C itself. d-Tubocurarine similarly has little effect upon the carbamoylation rate at high M7C concentrations; instead the amplitude of the reaction is decreased. This effect approaches a maximum at around 4 mM d-tubocurarine and corresponds to approximately 50% of the original amplitude. Zn²⁺ also decreases the carbamoylation amplitude; in this case the ligand is capable of abolishing all of the carbamoylation amplitude and at an order of magnitude lower concentration than is required for maximal d-tubocurarine effects. The effect of Zn²⁺ is reversed by addition of EDTA. These results are discussed in terms of peripheral ligand-induced conformational changes in acetylcholinesterase.

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