Toxicity from OP insecticides is primarily through inhibition of
acetylcholinesterase (AChE) enzyme. The enzymatic inhibition reaction is shown in
Figure 1. The first step is a complex formation between enzyme and OP, followed by
covalent bonding between OP with AChE (abbreviated E) at hydroxyl group (OH) of
the active-site serine. The second step involves the loss of a leaving group (X) which
takes with it a hydrogen atom from the enzyme. The product of this second step is a
phosphorylated enzyme. From here, the phosphorylated enzyme can be hydrolyzed
by adding water to regenerate the original enzyme but this dephosphorylation with
water (spontaneous reactivation) is very slow when the substrate is an OP. Thus
phosphorylated enzyme can undergo dealkylation, which is termed ageing. This
ageing involves covalent cleavage of a bond with in the phosphorylated enzyme, and
the formation of a negative charge which stabilizes it and results in irreversible
enzyme inhibition. Since AChE enzyme can’t hydrolyze acetylcholine (ACh), the
neurotransmitter saturates the receptors and accumulates in the synapses, causing