Figure 1. Reaction of organophosphate pesticides with acetylcholinesterase.

A dimethylphosphorylated organophosphate pesticide (methylparaoxon) inhibits acetylcholinesterase (AChE) by phosphorylating the serine hydroxyl group at the enzyme’s active site (reaction 1). This reaction occurs very quickly. Active AChE is subsequently regenerated by a hydroxyl ion attacking the phosphorylated serine residue, removing the phosphate moiety and releasing active enzyme (reaction 2). This regenerative process, however, is much slower than inhibition, requiring hours to days to occur (spontaneous reactivation t½ ∼ 0.7h for dimethyl and 31h for diethyl compounds). While in the inactive state, the enzyme is prone to ‘ageing’ (reaction 3) in which one alkyl side chain of the phosphoryl moiety is removed non-enzymatically, leaving a hydroxyl group in its place. ‘Aged’ AChE with its negatively charged phosphate can no longer be attacked by a negatively charged nucleophile, i.e. OH^. or an oximate group, and regeneration is no longer possible. This reaction occurs considerably faster with enzymes that have been inhibited by dimethylated pesticides (t½ ∼ 3.7h) than those inhibited by diethylated pesticides (t½ ∼ 31h). The slower the regenerative process, the greater the quantity of inactive AChE available for ageing. Pralidoxime catalyses the regeneration of active AChE by exerting a nucleophilic attack on the phosphoryl group, transferring it from the enzyme to itself. By speeding up reaction 2, it reduces the quantity of inactive AChE available for ageing. However, because ageing occurs more rapidly with dimethylated pesticides, pralidoxime is only useful before about 12h with dimethylated enzyme. In trials of pralidoxime, the identification of the poison taken, in particular its dimethyl/diethyl status, is therefore essential. (Data from refs. ¹⁴,¹⁸).