Acetylcholinesterase carbamates

Butyrylcholinesterase occurs in the liver and at the motor endplates in muscle fibers and at synapses together with aeetylcholinesterase (Silver 1974). It is estimated that approximately 15% of total cholinesterase activity in the nervous system is due to the nonspecific cholinesterase activity in some of the white matter (Ecobichon and Joy 1982), and the synthesis of this nonspecific cholinesterase occurs in the liver. Several organophosphorus compounds can react and phosphorylate acetylcholinesterase carbamate ester compounds can carbamylate the enzyme, so both can inhibit the acetylcholinesterase. 

Mode of Action. All of the insecticidal carbamates are cholinergic, and poisoned insects and mammals exhibit violent convulsions and other neuromuscular disturbances. The insecticides are strong carbamylating inhibitors of acetylcholinesterase and may also have a direct action on the acetylcholine receptors because of their pronounced stmctural resemblance to acetylcholine. The overall mechanism for carbamate interaction with acetylcholinesterase is analogous to the normal three-step hydrolysis of acetylcholine however, is much slower than with the acetylated enzyme. 

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. 

The carbamate insecticide aldicarb (Figure 2.13) that exerts its effect by inactivating acetylcholinesterase is metabolized by a flavin monooxygenase from rainbow trout to the sulfoxide, which is a more effective inhibitor (Schlenk and Buhler 1991). 

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). 

Absorbance- and reflectance-based measurements are widespread, as there are many enzymatic reaction products or intermediates that are colored or if not, can react with the appropriate indicator. Sensors using acetylcholinesterase for carbamate pesticides detection are an example of indirect optical fiber biosensors. This enzyme catalyses the hydrolysis of acetylcholine with concomitant decrease in pH41 ... 

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. 

Cambon, C., C. Declume, and R. Derache. 1979. Effect of the insecticidal carbamate derivatives (Carbofuran, Pirimicarb and Aldicarb) on the activity of acetylcholinesterase in tissues from pregnant rats and fetuses. Toxicol. Appl. Pharmacol. 49 203-208. 

Carbamates Organic chemical compounds that can be neurotoxic by competitively inhibiting acetylcholinesterase binding to acetylcholine. 

The N-methyl carbamate esters cause reversible carbamylation of the acetylcholinesterase enzyme, allowing accumulation of acetylcholine, the neuromediator substance, at parasympathetic neuroeffector junctions (muscarinic effects), at... 

Rivastigmine Transdermal (Exelon Patch) [Cholinesterase Inhibitor/Anti-Alzheimer Agent] Uses MUd/mod Alzheimer and Parkinson Dz dementia Action Acetylcholinesterase inhibitor Dose Initial 4.6-mg patch/d applied to back, chest, upper arm, T 9.5 mg after 4 wk if tolCTated Caution [ ] Sick sinus synd, conduction defects, asthma, COPD, urinary obst, Sz Contra Hypersensitivity to rivastigmine, other carbamates Disp Transdermal patch 5 cm (4.6 mg/24 h), 10 cm (9.5 mg/24 h) SE NA /D EMS See Rivastigmine OD See Rivastigmine... 

Smulders CJ et al Selective effects of carbamate pesticides on rat neuronal nicotinic acetylcholine receptors and rat brain acetylcholinesterase. Toxicol Appl Pharmacol 2003 193 139. [PMID 14644616]... 

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. 

Carbamate Insecticides. Carbamate insecticides interact with acetylcholinesterase in exactly the same way as OPs, with the hydroxyl group in the serine at the enzyme s active site attacking the carbamate residue in the insecticide. However, the binding to the active site is reversible. Typical carbamate insecticides are shown in Figure 3.4. 

The major action resulting from human exposure to diazinon is the inhibition of cholinesterase activity (refer to Section 2.4 for discussion). Two pools of cholinesterases are present in human blood acetylcholinesterase in erythrocytes and serum cholinesterase (sometimes referred to as pseudocholinesterase or butyrlcholinesterase) in plasma. Acetylcholinesterase, present in human erythrocytes, is identical to the enzyme present in neural tissue (the target of diazinon action) while serum cholinesterase has no known physiological function. Inhibition of both forms of cholinesterase have been associated with exposure to diazinon in humans and animals (Coye et al. 1987 Edson and Noakes 1960 Soliman et al. 1982). Inhibition of erythrocyte, serum, or whole blood cholinesterase may be used as a marker of exposure to diazinon. However, cholinesterase inhibition is a common action of anticholinesterase compounds such as organophosphates (which include diazinon) and carbamates. In addition, a wide variation in normal cholinesterase values exists in the general population, and there are no studies which report a quantitative... 

Irreversible inhibition - This type of inhibition involves the stable combination of an enzyme with some species so that the only way to have enzyme activity is to produce more enzyme. Specific examples of irreversible inhibition will be covered in Part II of the course in the section on the nervous system. Organophosphate and carbamate insecticides are the prime examples of such inhibitors when they combine with the enzyme acetylcholinesterase. 

Insecticidal carbamates also inhibit the enzyme acetylcholinesterase by transferring a carbamoyl group to the active hydroxyl. However, they differ from the phosphates in that they inhibit the enzyme reversibly and so a better fit at the active site is required for high activity. In consequence, a narrower range of structures is active. The chemistry, biochemistry, metabolism and toxicology of carbamate insecticides have been thoroughly reviewed (B-76MI10702). 

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