Relationships Between The Structure Of Organophosphorus Compounds And Their Activity As Acetylcholinesterase Inhibitors*

Bull. Org. mond. Sante Bull. Wid Hith Org.

1971, 44, 31-42

Relationships Between the Structure of Organophosphorus Compounds and their Activity as Acetylcholinesterase Inhibitors*
T. R. FUKUTO 1

This paper discusses the relation between chemical structure and inactivation of the enzyme acetylcholinesterase (AChE) by organophosphorus esters, in terms of reactivity and steric effects. Thediscussion is centered in organophosphorus esters of the type (R) (R')P(O)X, where X is a readily displaceable group and R and R' are various combinations of alkyl, alkoxy, alkylthio, and amido moieties. Specific examples illustrating the effect of AChE inhibition on the selective toxicity of organophosphorus esters for insects and mammals are also presented.
The inactivation ofacetylcholinesterase (AChE) by organophosphorus esters has been demonstrated to be the result of an actual chemical reaction between the enzyme and the phosphorus compound (O'Brien, 1960; Heath, 1961; Fukuto, 1957). The overall inhibition process, leading ultimately to the formation of a covalently bonded phosphorylated enzyme, may be depicted by the equations in Fig. 1-i.e., the ester and enzyme first combine toform a complex and this is followed by phosphorylation. The model for the active site of AChE proposed by Krupka (1964) is used here for the purpose of illustration. In this scheme, B is a basic group (histidine imidazole nitrogen), OH is a serine hydroxyl, HA is an acidic group (tyrosine hydroxyl), and S is the anionic site. The anionic site, whose normal function is to attract thetrimethylammonium moiety of the natural substrate acetylcholine (ACh) to the active site, may be visualized as a flexible pouch in which resides a negative charge. In certain cases a group in the organophosphorus estere.g., R in the 3-phenyl position-may interact with the anionic site and either aid or hinder the inhibition process. By using different kinetic methods, the bimolecular inhibition constant ke,the equilibrium or affinity constant for enzymeinhibitor-complex formation Ka, and the phosphoryl* This study was supported in part by US Public Health Service Research Grant FD 00239 from the Food & Drug Administration, Rockville, Md., USA, and by the Rockefeller Foundation, New York, USA. 1 Professor of Entomology and Professor of Chemistry, University of California, Riverside, Calif., USA.ation constant kp may be determined (Aldridge & Davison, 1952; Main, 1964). According to this mechanism of inhibition, phosphorylation of AChE takes place by serine hydroxyl attack on the phosphorus atom, the reaction being catalysed by the basic and acidic moieties in the active site. The phosphorylated enzyme thus obtained is unable to catalyse the hydrolysis of ACh. The relation between thechemical structure of organophosphorus esters and the inactivation of AChE has been studied extensively. These studies have shown that the anticholinesterase activity of organophosphorus esters depends largely on the reactivity of the ester. For example, in pioneering work by Aldridge & Davison (1952) the inhibition of erythrocyte AChE by paraoxon and related substituted-phenyl diethyl phosphates wasshown to take place in a bimolecular manner and a direct relationship was established between the inhibition constant ke and rates of solvolysis of these esters in phosphate buffer. Subsequently, it was demonstrated from a study of a larger series of substitutedphenyl diethyl phosphates that the inhibition of fly-head AChE by these compounds was related to the effect imposed by the substituent onthe lability of the P-O-phenyl bond as estimated by Hammett's a constants, shifts in P-O-phenyl stretching frequencies, and hydrolysis rates (Fukuto & Metcalf, 1956). Fig. 2 shows the correlation between Hammett's cr constant, a parameter that provides an estimate of the electron-withdrawing or -donating properties of the substituent, and the logarithm
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T. R. FUKUTO

Fig. 1...