Abstract

The structure and catalytic activity of acetylcholinesterase (Acetylcholine acetyl-hydrolase, EC 3.1.1.7) (AChE) from Torpedo marmorata depend upon the ionic strength, Γ/2, of the environment. The sedimentation coefficient of the enzyme is 14 S at Γ/2 = 0.3, but is polydisperse in the range of 10-80 S at Γ/2 = 0.003. The optimal velocity of the reaction catalyzed by AChE increases with ionic strength, while under the same conditions the affinity for the substrate and for several reversible competitive inhibitors decreases. The relative decrease of affinity as a consequence of increased ionic strength is higher for inhibitor molecules containing two quaternary ammonium ions than for compounds containing a single quaternary ammonium group. Among the monoquaternary inhibitors, this decrease is greater for phenyltrimethylammonium than for its 3-hydroxy analog.

In solutions of low salt concentration (Γ/2 = 0.003) significant affinity of the enzyme for two pachycurares, flaxedil and d-tubocurarine, can be demonstrated. Both compounds produce partial inhibition of AChE activity, antagonize its inhibition by reversible competitive inhibitors including some leptocurares, and enhance the inhibition by 3-hydroxyphenyltrimethylammonium. By measuring the degree of protection of AChE against thermal inactivation, the dissociation constant for the flaxedil-enzyme complex can be estimated to be about 3 x 10^-7 M.

These observations and their possible physiological significance are interpreted in terms of conformational alterations of the AChE molecule in response to the binding of the pharmacologic agents.