Acetylcholinesterase inhibition carbamates

Pralidoxime is administered by intravenous infusion, 1-2 g given over 15-30 minutes. In spite of the likelihood of aging of the phosphate-enzyme complex, recent reports suggest that administration of multiple doses of pralidoxime over several days may be useful in severe poisoning. In excessive doses, pralidoxime can induce neuromuscular weakness and other adverse effects. Pralidoxime is not recommended for the reversal of inhibition of acetylcholinesterase by carbamate inhibitors. Further details of treatment of anticholinesterase toxicity are given in Chapter 58. 

Tike the OP insecticides, the mode of action of the carbamates is acetylcholinesterase inhibition with the important difference that the inhibition is more rapidly reversed than with OP compounds. 

Although OPPs and carbamates exhibit very similar modes of action in various animal species, i.e, acetylcholinesterase inhibition in the CNS with resulting paralysis—there is an important difference between the two classes of pesticides. Carbamates do not require metabolic conversion prior to exhibiting their toxicity. Furthermore the enzyme activity may at times be rapidly regenerated by reversal of inhibition. The kinetics of the inhibition (carbamoylation) reaction have been well studied in it electrophilic carbamoyl moieties form covalent bonds with enzyme esteratic sites. This is followed by carbamate transfer of an acidic group to the site to yield the acetylated enzyme complex (ref. 176). 

Weinstock M, et al. Acetylcholinesterase inhibition by novel carbamates a kinetic and nuclear magnetic resonance study. In Multidisciplinary Approaches to Cholinesterase Functions. Velan B (Ed.). Plenum Press, New York, 1992,... 

Table 10.5 Bimolecular rate constants for the inhibition of acetylcholinesterase by carbamate and organophosphorus insecticides in fall armyworm adults...

Methomyl exerts toxicity by inhibiting acetylcholinesterase. As with other carbamate insecticides, acetylcholinesterase inhibition is much less persistent than with organophosphate intoxication. 

Table 2. Bimolecular rate constants for acetylcholinesterase inhibition by organophosphates and a carbamate in resistant (R) and susceptible (S) insects...

Acetylcholinesterase. Altered acetylcholinesterase less sensitive to organophosphorus and carbamate insecticides has been observed in a wide variety of insects and mites (51). Acetylcholinesterase inhibiting insecticides phosphorylate or carbamylate the serine residue in the active site of the enzyme preventing vital catalysis of acetylcholine. Resistance due to reduced sensitivity to inhibition of this target enzyme has been found in house fly, mosquitoes, green rice leafhopper, and both phytophagous and predacious species of mites. 

There is also a global screening method for carbamate/OPPs using a test based on acetylcholinesterase inhibition. 

Carbamate poisoning produces reversible acetylcholinesterase inhibition, and spontaneous recovery of enzyme activity may occur within several hours, making these tests less usehl. 

Eckert S, Eyer P, Worek F (2007) Reversible inhibition of acetylcholinesterase by carbamates or huperzine A increases residual activity of the enzyme upon soman challenge. Toxicology 233(1-3) 180-186. doi 10.1016/j.tox.2006.09.012... 

Carbamates - Structure activity relationships have been studied with respect to toxicity and anticholinesterase activity. 9 The mechanism of acetylcholinesterase inhibition by the carbamates is not entirely clear. 

FIGURE 5.46 Interaction of the serine hydroxyl residue in the catalytically active site of acetylcholinesterase enzyme with esters of organophosphates or carbamates. The interaction leads to binding of the chemical with the enzyme, inhibition of the enzyme, inhibition of acetylcholine hydrolysis, and thus accumulation of acetylcholine in the synapses. 

Enzymes can be used not only for the determination of substrates but also for the analysis of enzyme inhibitors. In this type of sensors the response of the detectable species will decrease in the presence of the analyte. The inhibitor may affect the vmax or KM values. Competitive inhibitors, which bind to the same active site than the substrate, will increase the KM value, reflected by a change on the slope of the Lineweaver-Burke plot but will not change vmax. Non-competitive inhibitors, i.e. those that bind to another site of the protein, do not affect KM but produce a decrease in vmax. For instance, the acetylcholinesterase enzyme is inhibited by carbamate and organophosphate pesticides and has been widely used for the development of optical fiber sensors for these compounds based on different chemical transduction schemes (hydrolysis of a colored substrate, pH changes). 

The above-mentioned system has also been used for the indirect CL determination of some carbamate and organophosphorous pesticides that inhibit acetylcholinesterase. Acetylcholinesterase in solution or immobilized on methacrylate beads is coupled to immobilized choline oxidase and peroxidase [46],... 

Although bicyclophosphates do not inhibit acetylcholinesterase, they exhibit a synergistic toxic effect with materials that do. Individuals who have had previous exposure to cholinesterase inhibitors such as nerve agents and commercial organophosphate or carbamate pesticides may be at a greater risk from exposure to bicyclophosphates. 

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