Page 452 - The Toxicology of Fishes

432 The Toxicology of Fishes

OH O H
OC N CH 3 OC N CH 3

OCH(CH 3) 2

CARBARYL PROPOXUR

CH 3 O
S C CH N O C NH
CH 3 CH 3
CH 3
ALDICARB
FIGURE 9.4 Examples of N-methylcarbamate (carbaryl and propoxur) and N-methylcarbamoyl oxime (aldicarb) carbamate
insecticides.

Mechanisms of Cholinesterase Inhibitor Neurotoxicity
The neurotoxic mechanism for organophosphorus and carbamate insecticides is inhibition of acetylcho-
linesterase (AChE) in the CNS and PNS (Figure 9.5) (reviewed in Ecobichon, 1996; Taylor, 2001). AChE
is responsible for hydrolyzing the neurotransmitter acetylcholine; inhibition of the esterase causes
accumulation of acetylcholine in synapses and excessive stimulation of muscarinic and nicotinic receptors
(Figure 9.5B). Increased levels of acetylcholine overstimulate nicotinic and muscarinic receptors in the
CNS, PNS, and neuromuscular junctions, causing a wide range of signs of poisoning. These signs may
include changes in body temperature, heart rate, blood pressure, muscle twitching, or tremors. Death,
although rare, is usually due to cessation of respiration due to anticholinesterase effects in both the CNS
and PNS. Although organophosphates and carbamates both target acetylcholinesterase, nicotine acts as
a direct cholinergic agonist rather than by cholinesterase inhibition; nicotine directly binds nicotinic
receptors to hyperstimulate cholinergic neurons.
Both organophosphosphorus and carbamate compounds inhibit AChE activity by acting as pseudo-
substrates. Unlike the natural substrate, these compounds remain in the active site of the enzyme for
much longer periods of time, thereby preventing the enzyme from hydrolyzing its natural substrate,
acetylcholine (Aldridge and Reiner 1969). Inhibition of AChE activity by organophosphorus insecti-
cides is primarily a function of the electrophilicity of the phosphorus atom, with increased electro-
philicity (increased partial positive charge) associated with greater rates of bonding to the negatively
charged oxygen in the serine hydroxyl group. Thus, for organophosphorus insecticides, the binding
of the organophosphorus compound to AChE is reasonably fast. The hydrolysis of the organophos-
phorus esters, however, is very slow, leading to an accumulation of phosphorylated AChE. Phospho-
rylated AChE is, for all practical purposes, irreversibly inhibited due to the very low reversibility of
the enzyme activity.
Recovery of active AChE activity is dependent on the synthesis of new enzyme. In some cases,
phosphorylated enzymes undergo an aging reaction when an alkoxy or aryloxy R group is dealkylated
or dearylated and results in a negatively charged monoalkyl or aryl enzyme. In such instances, the
hydrolysis step is no longer possible; thus, the enzyme is irreversibly altered. Kinetic studies on the
inhibition of AChE activity, including aging and recovery, have been reported in a variety of fish species
(Carr et al., 1995; Johnson and Wallace, 1987; Straus and Chambers, 1995; Wallace and Herzberg, 1988).
Typically, recovery of AChE activity in fish is slower than that observed in mammals.
Inhibition of AChE activity by carbamate insecticides is a function of the carbamylation of the enzyme.
The kinetics of this inhibition are slightly different from that observed with organophosphorus insecti-
cides. Hydrolysis of carbamates (i.e., decarbamylation), while significantly slower than that observed
for acetylcholine, is much more rapid than that observed with most organophosphorus compounds; thus,
although AChE inhibition elicited by an organophosphorus pesticide is, in effect, irreversible, carbamate
inhibition of esterase activity may be reversed. Furthermore, aging of the carbamylated enzyme does
not occur.