Cholinesterase insecticides

Moreover organophosphoric acid esters have found application as insecticides (e.g. Parathion). Some derivatives are highly toxic to man (e.g. Sarin, Soman). The organophosphonates act as inhibitors of the enzyme cholinesterase by phosphorylating it. This enzyme is involved in the proper function of the parasympathetic nervous system. A concentration of 5 x 10 g/L in the air can already cause strong toxic effects to man. 

Cholinesterase inhibitor (anti-cholinesterase, ChEI) is a chemical that prevents cholinesterases (ChEs) from breaking down. ACh, which consequently increases the level and duration of action of this neurotransmitter. ChEIs such as organophosphates (esters of phosphoric acid) and carbamates (esters of carbamic acid) - serve as insecticides, pesticides, warfare agents and drugs. 

Diagnosis of organophosphate poisoning (including methyl parathion) can be confirmed by evaluation of serum (plasma) cholinesterase and erythrocyte cholinesterase. However, cholinesterase inhibition is not specific for organophosphates. For example, carbamate insecticides also result in cholinesterase inhibition, which is usually transitory. Erythrocyte cholinesterase measurement is a specific test for... 

Brock A. 1991. Inter and intraindividual variations in plasma cholinesterase activity and substance concentration in employees of an organophosphorus insecticide factory. Br J Ind Med 48 562-567. 

McConell R, Cedillo E, Keifer M, et al. 1992. Monitoring organophosphate insecticide-exposed workers for cholinesterase depression New technology for office or field use. J Occup Med 34 34-37. 

Moeller HC, Rider JA. 1959. The effects of various organic phosphate insecticides on RBC and plasma cholinesterase in humans [Abstract]. Fed Proc 18 424. 

Pope CN, Chakraborti TK, Chapman ML, et al. 1991. Comparison of in vivo cholinesterase inhibition in neonatal and adult rats by three organophosphorothioate insecticides. Toxicology 68 51-61. 

The inhibition of brain cholinesterase is a biomarker assay for organophosphorous (OP) and carbamate insecticides (Chapter 10, Section 10.2.4). OPs inhibit the enzyme by forming covalent bonds with a serine residue at the active center. Inhibition is, at best, slowly reversible. The degree of toxic effect depends upon the extent of cholinesterase inhibition caused by one or more OP and/or carbamate insecticides. In the case of OPs administered to vertebrates, a typical scenario is as follows sublethal symptoms begin to appear at 40-50% inhibition of cholinesterase, lethal toxicity above 70% inhibition. 

Devonshire, A.L., Byrne, G.D., and Moores, G.D. et al. (1998). Biochemical and molecular characterisation of insecticide sensitive acetylcholinesterase in resistant insects. In Structure and Function of Cholinesterases and Related Proteins, Doctor, B.P, Quinn, D.M., Rotundo, R.L. and Taylor, P. (Eds.) New York Plenum Press, 491 96. 

Grue, C.E., Hart, A.D.M., andMineau, P. (1991). Biological consequences of depressed brain cholinesterase activity in wildlife. In Mineau, P. (Ed.) Cholinesterase Inhibiting Insecticides— Their Impact on Wildlife and the Environment, 151-210. Amsterdam Elsevier. 

Oxime carbamates are generally applied either directly to the tilled soil or sprayed on crops. One of the advantages of oxime carbamates is their short persistence on plants. They are readily degraded into their metabolites shortly after application. However, some of these metabolites have insecticidal properties even more potent than those of the parent compound. For example, the oxidative product of aldicarb is aldicarb sulfoxide, which is observed to be 10-20 times more active as a cholinesterase inhibitor than aldicarb. Other oxime carbamates (e.g., methomyl) have degradates which show no insecticidal activity, have low to negligible ecotoxicity and mammalian toxicity relative to the parent, and are normally nondetectable in crops. Therefore, the residue definition may include the parent oxime carbamate (e.g., methomyl) or parent and metabolites (e.g., aldicarb and its sulfoxide and sulfone metabolites). The tolerance or maximum residue limit (MRL) of pesticides on any food commodity is based on the highest residue concentration detected on mature crops at harvest or the LOQ of the method submitted for enforcement purposes if no detectable residues are found. For example, the tolerances of methomyl in US food commodities range from 0.1 to 6 mg kg for food items and up to 40 mg kg for feed items. ... 

Azinphos-methyl is unstable in basic conditions and can degrade to benzazimide, hydroxymethyl-benzazimide, mercaptomethylbenzazimide or bis(benzazimide-A/-methyl) sulfide Azinphos-methyl is an organophosphorus insecticide used to control chewing and sucking mites and insects such as aphids and scale. Azinphos-methyl has both contact and stomach action. Azinphos-methyl is a cholinesterase inhibitor and interferes with the nervous system... 

Carbamates are used as insecticides, nematocides, fungicides, and herbicides the toxicity of carbamate insecticides is similar to that of OP compounds and is based on the inhibition of ACHE. Also, carbamate metabolites may inhibit ACHE but are usually weaker inhibitors than the unchanged compound. Cholinesterase inhibition caused by carbamates is labile, of short duration, and rapidly reversible in fact, the half-life of the inhibited enzymes ranges between some minutes and 2 to 3 hours for RBC-ACHE and is on the order of some minutes for PCHE. Accumulation of cholinesterase activity on repeated exposures, as observed with OP compounds, does not occur with... 

Occupational exposure to carbamate insecticides may be monitored by measuring RBC-ACHE and/or PCHE. However, given the low cholinesterase inhibition levels and the short time duration of this effect, ACHE inhibition can generally be used as a biomarker of exposure only when exposure levels are high. Three sequential samples are recommended to establish an individual baseline before exposure. In exposed workers, blood sampling and analysis should be carried out soon after the end of exposure (WHO, 1986). 

B. Bucur, M. Dondoi, A. Danet, and J.L. Marty, Insecticide identification using a flow injection analysis system with biosensors based on various cholinesterases. Anal. Chim. Acta 539, 195-201 (2005). 

Chlorinated hydrocarbons Cholinesterase inhibitors Botanical insecticides... 

Famphur use on livestock is contraindicated for less than 3-month-old calves animals stressed from castration, dehorning, or overexcitement and sick or convalescent animals. Brahman and Brahman crossbreeds are less tolerant of cholinesterase-inhibiting insecticides than other breeds, and Brahman bulls are especially sensitive and should not be treated with famphur. Cattle should not be slaughtered for at least 35 days after treatment with famphur. 

Mineau, P. (ed.). 1991. Cholinesterase-inhibiting insecticides. Their impact on wildlife and the environment. Chemicals in Agriculture. Vol. 2. Elsevier, Amsterdam, The Netherlands. 348 pp. 

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