Inhibition of rat brain and human red cell acetylcholinesterase by thiocarbamate herbicides

Thiocarbamates are a major class of herbicides that were used extensively in the agricultural industry. Toxicological evaluation showed molinate caused reproductive impairment in male rats, whilst others produced behavioural effects at high doses. Rats dosed with molinate either as a single large oral dose of 100 mg/kg or as multiple doses of 50 mg/kg for 7 days produced inhibition of brain acetylcholinesterase (AChE). Molinate and other thiocarbamate herbicides undergo metabolism to form sulphoxides that can carbamoylate thiol's such as glutathione and proteins. We have chemically synthesised the sulphoxide and sulphone metabolites of six thiocarbamate herbicides and examined their ability to inhibit rat brain and human red cell AChE in vitro. Parent thiocarbamates were inactive, whilst the sulphoxides produced inhibition with IC₅₀'s in the 1-10 mM range, the sulphone metabolites were the most active with IC₅₀'s for molinate, pebulate, EPTC and vernolate in the μM range. Inhibition was both time- and dose-dependent with biomolecular rate constants for the inhibition of the human red cell enzyme of 0.3 × 10² and 2.0 × 10² M^-1 min^-1 for molinate sulphoxide and sulphone, respectively. No recovery of enzyme activity, with either enzyme, was seen following dilution of the inhibitor to a concentration that does not inhibit the enzyme for up to 24 h at 25°C at pH 7.4. The metabolites of these thiocarbamate herbicides are rather poor inhibitors of AChE when compared to the organophosphorus ester, paraoxon or the monomethylcarbamate, eserine. Unlike eserine the inhibition produced by the thiocarbamates is irreversible.