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[Acetylcholinesterase. II. Experimental aspects of interaction with reversible effectors under conditions of high
Abstract:
Interaction of usual effectors with acetylcholinesterase (EC 3.1.1.7) from bovine erythrocytes was examined under conditions of high ionic strength (gamma/2 greater than or equal to 0,1). Detailed kinetic investigation of the hydrolysis of acetylcholine by acetylcholinesterase in the presence of modifiers shows that the effects produced by numerous quaternary nitrogen compounds on the enzyme can be explained on the basis of binding of the effectors to the anionic subsite of the active center. The various kinetic behaviors, that are observed, are dependent on the relative values of the deacetylation rate constant ak of the complex acetylated enzyme-modifier and of the rate constant k-2 defined by : (see article) with respect to the value of the deacetylation rate constant K of the acetylated enzyme. If a identical to [1--(k/k-2)]-1, it is shown that interaction of the enzyme with tetraethylammonium, pentamethonium, hexamethonium and gallamine ions is characterized by : a greater than a and k-2 greater than k therefore, these modifiers accelerate deacetylation. On the other hand, inhibition of acetylcholinestase by methylpyridinium, d-tubocurarine, tetra-n-propylammonium, tetra-n-butylammonium, decamethonium and succinylbischoline is consistent with one of the conditions : a less than a and k-2 greater than or equal to k or a greater than a and k-2 less than k and inhibition by tetramethylammonium, phenyltrimethylammonium, 3-hydroxyphenyl-triethylammonium, N-methylacridinium and bis (3-aminopyridinium)-1,10-decane ions agrees with one of the two previous conditions or with : (see article) consequently, the effect of these ligands on the deacetylation step is undetermined. However, the effects of choline chloride, thiazinamium methyl sulfate and thioridazine hydrochloride are not entirely consistent with this mechanism but support the existence of a functional peripheral anionic site which is distinct from the anionic subsite of the active center.