Phosphonylated Acetylcholinesterase as Transition State Analogs: The Anticholinesterase Properties of Halomethylphosphonates

Abstract

The CH₃P(O) moiety of methylphosphonates of the type CH₃P(O) (OR) X,I, may play an important role in the effective binding to the appropriate hydrophobic patch in the active site of acetylchlinesterase (AChE), in analogous manner to the acetyl residue, CH₃C(O), of the substrate acetylcholine which determines substrate specificity. Since the substitution of halide atom for hydrogen in I is expected to introduce electronic, steric and hydrophobic changes, it was interesting to study the effect of such a substitution on both the inhibition of AChE and the stabilily of the phosphonylated enzyme. A significant decrease in the stability of the enzyme conjugates, YCH₂P(O)?(OisoPr)O?AChE (Y=Cl, Br, I) was observed in terms of an increase in the rate constants of the spontaneous and induced reactivations as well as the aging process relative to the non-substituted molecule where Y=H. The electron withdrawal effect of the halogen atom alone, cannot explain the changes in the anti-ChE properties of the halomethylphosphonates when compared to the non-substituted inhibitor. These results are in accord with the view that the phosphonylated enzyme may be considered as a transition-state analog for the hydrolysis of an acylated enzyme(Ashani and Green in “Studies in Organic Chemistry”, pp 169–188, Vol 10, Elsevier, 1981). It is further concluded that in contrast to the OP-AChE conjugates the OP themselves cannot be rationalized in terms of transition-state analogs.