MULTIPLE BINDING-SITES INVOLVED IN THE EFFECT OF CHOLINE ESTERS ON DECARBAMOYLATION OF MONOMETHYLCARBAMOYL-ACETYLCHOLINESTERASE OR DIMETHYLCARBAMOYL-ACETYLCHOLINESTERASE

Multiple binding sites for inhibitory choline esters in spontaneous de carbamoylation of dimethylcarbamoyl-acetylcholinesterase (AChE) were s uggested from a wide range of IC50 values, in contrast with a limited range of AC(50) values (concentration giving 50% of maximal activation ) at a peripheral activatory site. Association of choline esters conta ining a long acyl chain (C-7-C-12) with the hydrophobic zone in the ac tive site could be deduced from a linear relationship between the size of the acyl group and the inhibitory potency in either spontaneous de carbamoylation or acetylthiocholine hydrolysis. Direct support for lau rylcholine binding to the active site might come from the competitive inhibition (K-i 33 mu M) of choline-catalysed decarbamoylation by laur ylcholine. Moreover, its inhibitory action was greater for monomethylc arbamoyl-AChE than for dimethylcarbamoyl-AChE, where there is a greate r steric hindrance at the active centre. In further support, the inhib ition of pentanoylthiocholine-induced decarbamoylation by laurylcholin e was suggested to be due to laurylcholine binding to a central site r ather than a peripheral site, similar to the inhibition of spontaneous decarbamoylation by laurylcholine. Supportive data for acetylcholine binding to the active site are provided by the results that acetylchol ine is a competitive inhibitor (K-i 7.6 mM) of choline-catalysed decar bamoylation, and its inhibitory action was greater for monomethylcarba moyl-AChE than for dimethylcarbamoyl-AChE. Meanwhile, choline esters w ith an acyl group of an intermediate size (C-4-C-6), more subject to s teric exclusion at the active centre, and less associable with the hyd rophobic zone, appear to bind preferentially to a peripheral activity site. Thus the multiple effects of choline esters may be governed by h ydrophobicity and/or a steric effect exerted by the acyl moiety at the binding sites.