Acetylcholinesterase inhibitors Organophosphates

Another important example is the nicotinic acetylcholine receptor, which is activated by the agonist nicotine causing muscular fibrillation and paralysis. Indirect effects can also occur. For example, organophosphates and other acetylcholinesterase inhibitors increase the amount of acetylcholine and thereby overstimulate the receptor, leading to effects in a number of sites (see chap. 7). Alternatively, botulinum toxin inhibits the release of acetylcholine and causes muscle paralysis because muscular contraction does not take place (see chap. 7). 

As well as excitable cells, other cell types such as liver cells may also have receptors, which can be affected by chemicals, although usually with less dramatic results. Thus, liver cells have oci adrenergic receptors, activation of which causes metabolic effects such as increased glycogenolysis and increased intracellular Ca2+. The muscarinic receptor in exocrine gland cells is influenced by acetylcholine, hence excess secretions occur with acetylcholinesterase inhibitors such as organophosphates (see chap. 7). 

The toxicity of TEPP to humans and other mammals is very high it has a toxicity rating of 6, supertoxic. TEPP is a very potent acetylcholinesterase inhibitor. (The inhibition of acetylcholinesterase by organophosphate insecticides is discussed in Section 18.7.)... 

Organophosphate insecticides with the P=S group are oxidatively desulfurated by cytochrome P450 monooxygenases of insects to their corresponding P=0 analogs. This reaction results in activation (increased toxicity), because the product, P=0, binds more tightly to the acetylcholinesterase than the parent compound and, thus, to more potent acetylcholinesterase inhibitors. For example, parathion is oxidatively desulfurated to paraoxon. 

The mode of action of the carbamate insecticides is similar to that of the organophosphates. As shown in Figure 7.15, the reaction yields a carbamylated AChE, followed by decarbamylation via hydrolysis. Carbamates also attack the CNS system, and the symptoms of intoxication are similar to those with the organophosphates. However, unlike the organophosphates, decarbamylation of acetylcholinesterase is rapid, typically in minutes, and therefore carbamate insecticides are regarded as reversible acetylcholinesterase inhibitors. 

The pesticides most frequently responsible for equine poisonings are the organophosphate, carbamate, and chlorinated hydrocarbon insecticides. Both the organo-phosphates and the carbamates are acetylcholinesterase inhibitors and present clinical pictures similar to those seen in food-producing animals. Affected horses salivate and sweat profusely and have muscle incoordination and ataxia. The chlorinated hydrocarbons are strong CNS stimulants affected horses become hyperalert, then excited, and, in severe cases, develop convulsions. In almost all instances, the mode of horses being exposed to pesticides is topical. 

Occasionally, compounds with distinctive modes of action are better modeled apart from the general case. An example of such compounds are the acetylcholinesterase inhibitors. These compounds are very specific in their inhibition of serine enzymes. In the instance of predicting Daphnia EC, it was found that the organophosphates were outliers that biased the regression and were better removed from the general model and treated separately. Another class of specialized models are those grouped by chemical class. These have proven popular because of their relative simplicity but the datasets upon which they are built are usually small. 

Parathion Organophosphate acetylcholinesterase inhibitor prototype used as insecti-... 

Buccafusco, J. J., and R. S. Aronstam. 1986. Clonidine protection from the toxicity of soman, an organophosphate acetylcholinesterase inhibitor in themonse. J. Pharmacol. Exp. Ther. 239(1) 43-47. 

Lorke/ D.E., Hasan, M.Y., Nurulain, S.M., et al., 2012. Acetylcholinesterase inhibitors as pretreatment before acute exposure to organophosphates assessment using methyl-paraoxon. CNS Neurol. Disord. Drug Target 11,1052-1060. 

Carbamates are also acetylcholinesterase inhibitors, but their action on the enzyme system generally seems to be more reversible than that of the organophosphates, and they are less toxic to vertebrates. They have the general structure 25.5 and important members of the family are shown in Figure 25.3. Like the organophosphates, these are relatively quickly broken down in the environment. 

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). 

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. 

A. M. Shafferman, C. Kronman, Y. Flashner, S. Leitner, H. Grosfeld, A. Ordenthch, Y. Gozes, S. Cohen, N. Ariel, D. Barak, M. Harel, I. Silman, J. L. Sussman, B. Velan, Mutagenesis of Human Acetylcholinesterase , J. Biol. Chem. 1992, 267, 17640-17648 A. Ordenthch, D. Barak, C. Kronman, N. Ariel, Y. Segall, B. Velan, A. M. Shafferman, The Architecture of Human Acetylchohnesterase Active Center Probed by Interactions with Selected Organophosphate Inhibitors , J. Biol. Chem. 1995, 271, 11953-11962. 

Phosphinates are a class of organophosphorus compounds, the metabolism of which has received less attention than that of phosphates (see above) or phosphorothioates and P-halidc compounds (see below). Many phosphinates are rapid but transient inhibitors of acetylcholinesterase and carboxyl-esterases. And like organophosphates and phosphonates, phosphinates are substrates of arylesterases (EC 3.1.1.2). This is exemplified by 4-nitrophen-yl ethyl(phenyl)phosphinate (9.62), whose (-)-enantiomer was hydrolyzed by rabbit serum arylesterase almost 10 times faster than the (+)-enantiomer [133],... 

Irreversible inhibitors combine or destroy a functional group on the enzyme so that it is no longer active. They often act by covalently modifying the enzyme. Thus a new enzyme needs to be synthesized. Examples of irreversible inhibitors include acetylsal-icyclic acid, which irreversibly inhibits cyclooxygenase in prostaglandin synthesis. Organophosphates (e.g., malathion, 8.10) irreversibly inhibit acetylcholinesterase. Suicide inhibitors (mechanism-based inactivators) are a special class of irreversible inhibitors. They are relatively unreactive until they bind to the active site of the enzyme, and then they inactivate the enzyme. 

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