Molecular dissection of cholinesterase domains responsible for carbamate toxicity

Carbamate compounds marked for their cholinesterase (ChE) inhibition are widely used as therapeutics and as insecticides. Groups of closely related carbamate molecules provide an important tool in the understanding of the domains responsible for binding these ligands to ChEs. Comparative inhibition profiles were derived for five N-methyl carbamates, mostly carbofuran derivatives, differing in length and branching of their hydrocarbonic chain towards human erythrocyte acetylcholinesterase (H.AChE), human serum butyrylcholinesterase (H.BChE) in its normal form or in a mutant form containing the point mutation Asp₇₀→Gly, and Drosophila nervous system ChE. Carbofuran was more toxic to all three ChEs than any of the other derivatives, with IC50 values which differed by more than 1000-fold. Drosophila ChE appeared to be most sensitive to all of the examined carbamates, and H. AChE was consistently more sensitive than H. BChE. Morever, inhibition efficiency for H. BChE decreased more effectively than it did for H. AChE with increased length and complexity of the side chain, indicating less flexible carbamate binding site in BChE as compared with AChE. The Asp₇₀→Gly mutation had no apparent effect on H. BChE inhibition by N-methyl carbamates, suggesting that the Asp₇₀ domain localized near the rim of the active site groove is not important in carbamate binding. Comparison of the carbamate IC50 values with published LD₅₀ values demonstrated correlation between the in vivo toxicity and inhibition of BChE by carbamates, suggesting a biological in addition to scavenging importance for BChE in mammals. Pinpointing different domains characteristic of carbamate binding in each member of the ChE family can thus shed light on the variable toxicity of these inhibitors to insects and mammals, predict the toxicity of yet untested inhibitor molecules and help in designing novel and improved ChE inhibitors.