The primary molecular mechanism of action of
the OP pesticides is inhibition of AChE activity, a
widely distributed serine esterase (Ecobichon, 1996).
AChE occurs throughout the central and peripheral
nervous system of vertebrates and its normal
physiological action is to hydrolyze the
neurotransmitter ACh, so that activation of
cholinergic receptors is transient. AChE hydrolyses
ACh into choline and acetic acid and is responsible
for the removal of the neurotransmitter ACh from the
synaptic cleft through hydrolysis (Habig and
DiGiulio, 1991). ACh is the primary neurotransmitter
in the sensory and neuromuscular systems in most
species. Activity of AChE system is vital to normal
behavior and muscular function and represents a
prime target on which some toxicants can exert a
detrimental effect. Once bound, organophosphorus
compounds are considered irreversible inhibitors, as
recovery usually depends on new enzyme synthesis
(Habig and DiGiulio, 1991).