Insecticides acetylcholinesterase inhibitors

Insects are very sensitive to fluorophosphonates, so that the compound parathion was synthesised and used as an insecticide soon after the Second World War. However, it entered the food chain and eventually found its way into mammals and caused death. An important breakthrough occurred with the synthesis of malathion, an insecticide which has high toxicity to insects, where it is converted to malaoxon, a potent acetylcholinesterase inhibitor. However, malathion is much less toxic to mammals, since it is readily detoxified (Appendix 3.8). 

Selective bioactivation (toxification) is illustrated in the case of the insecticide malathion (3.35). This acetylcholinesterase inhibitor is desulfurized selectively to the toxic malaoxon, but only by insect and not mammalian enzymes. Malathion is therefore relatively nontoxic to mammals (LDjg = 1500 mg/kg, rat p.o.). Higher organisms rapidly detoxify malathion by hydrolyzing one of its ester groups to the inactive acid, a process not readily available to insects. This makes the compound doubly toxic to insects since they cannot eliminate the active metabolite. 

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. 

Remember too that it was the research of insecticides that led to the discovery of the organophosphorus acetylcholinesterase inhibitors by Schrader at the Bayer laboratory. The stndy of their mechanism of action has shown that they act by acylation of a serine hydroxyl in the catalytic site of the enzyme. This was one of the first examples describing a molecular mechanism for an enzymatic inhibition. 

Thiophosphate insecticides are protoxins, as they are unable to phosphorylate the serine until after the first liver passage, during which the thiono group is replaced by an 0x0 group. Therefore, they are called indirect acetylcholinesterase inhibitors. 

Carbofuran Metabolite Assay. Rapid detection of enhanced degradation of carbofuran in soil can be accomplished in two different manners. The first approach is to measure the actual amount of carbofuran remaining after application and compare this value with what might be expected to remain after a selected time. This concept has been used by FMC in the form of a test ticket that colorimetrically determines the amount of carbofuran based on inhibition of acetylcholinesterase. The diagnostic kit is manufactured by Enzy-Tek of Lenexa, KS (20). A disadvantage of this method is lack of specificity due to the ability of the test reagents to react with other soil insecticides and their metabolites that are also acetylcholinesterase inhibitors. 

Metrifonate, which was originally introduced as an organic phosphate insecticide, is a pro-drug for dichlorovos,10 a potent acetylcholinesterase inhibitor (Chapter 8) to which it is nonenzymatically metabolized spontaneously in vitro even at neutral pH (Eq. 7.11). The drug s clinical application is exclusively against S. hematobium infections. Although some effectiveness against other schistosomes exists, this is not achieved at safe doses. 

A. Classification and Prototypes The three major classes of insecticides are the chlorinated hydrocarbons (DDT and its analogs), acetylcholinesterase inhibitors (carbamates, organophos-phates), and the botanical agents (nicotine, rotenone, pyrethrum alkaloids). 

Acetylcholinesterase inhibitors has been a productive approach in the design of insecticides starting with the phosphate esters such as malathion and continuing on to carbamate esters such as carbaryl. For the latter, leads came from two carbamate reversible cholinesterase inhibitors used in medicine, the natural product physostigmine and the synthetic derivative, neostigmine. 

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. 

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). 

Many phosphorus derivatives function as irreversible inhibitors of acetylcholinesterase, and are thus potentially toxic. These include a range of organophosphorus insecticides, such as malathion and parathion, and nerve gases such as sarin. 

Parathion is one of a class of phosphorothionate triesters widely used as insecticides. These compounds exert their toxic effects in insects and mammals by inhibiting the enzyme acetylcholinesterase. The phosphorothionates, in general, are relatively poor inhibitors of acetylcholinesterase but are converted by the cytochrome P-450-containing monooxygenase enzyme systems in insects and mammals to the corresponding phosphate triesters that are potent inhibitors of this enzyme. 

It Is well known that phosphorothlonate insecticides such as parathlon (, 0-diethyl p-nitrophenyl phosphorothloate) and malathion [0, -dimethyl -(l,2 -dlcarbethoxy)ethyl phosphoro-dithioate] are Intrinsically poor inhibitors of acetylcholinesterase and in vivo activation to the respective anticholinesterases paraoxon and malaoxon is required before animals exposed to the phosphorothionates are intoxicated. Since metabolic activation is essential to the biological activity of these thiono sulfur-containing organophosphorus insecticides, compounds of this type may be considered as propesticides or, more specifically, prolnsectlcldes. 

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