Carbamates reversible inhibition

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). 

Based on features of their interaction with ChE, anti-cholinesterase substances are clas-sified as reversible and irreversible inhibitors. Referred to the reversible anti-cholinesterase substances are quaternary ammonium compounds and aminoformic acid esters OPC are referred to the irreversible anti-cholinest-erase substances. At the same time, a great amount of anti-cholinesterase substances, carbamates in particular, though induce transitional reversible inhibition of ChE, are irreversible reagents. They are destroyed on the enzyme surface. 

Carbamates Aldicarb. carbaiyl, carbofuran Reversible inhibition of red blood cell acety Icholi neslerase and plasma cholinesterase... 

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... 

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. 

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... 

Pharmacologically, carbofuran inhibits cholinesterase, resulting in stimulation of the central, parasympathetic, and somatic motor systems. Sensitive biochemical tests have been developed to measure cholinesterase inhibition in avian and mammalian brain and plasma samples and are useful in the forensic assessment of carbamate exposure in human and wildlife pesticide incidents (Bal-lantyne and Marrs Hunt and Hooper 1993). Acute toxic clinical effects resulting from carbofuran exposure in animals and humans appear to be completely reversible and have been successfully treated with atropine sulfate. However, treatment should occur as soon as possible after exposure because acute carbofuran toxicosis can be fatal younger age groups of various species are more susceptible than adults (Finlayson et al. 1979). Carbofuran labels indicate that application is forbidden to streams, lakes, or ponds. In addition, manufacturers have stated that carbofuran is poisonous if swallowed, inhaled, or absorbed through the skin. Users are cautioned not to breathe carbofuran dust, fumes, or spray mist and treated areas should be avoided for at least 2 days (Anonymous 1971). Three points are emphasized at this juncture. First, some carbofuran degradation... 

The carbamate insecticides also inhibit AChE. The mechanism of inhibition is similar, but the reaction is reversible. 

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. 

Carbary , a widely used methyl carbamate, is a pseudosubstrate of acetylcholinesterase that reacts 105 to 106 times more slowly than do normal substrates. The carbamoylated enzyme formed is not as stable as the phosphorylated enzymes and the inhibition is reversible. 

Carbamates are substituted esters of carbamic acid (NH2COOH) with aliphatic or aromatic substituents on the oxygen and nitrogen atoms. Carbamate insecticides have an aryl or oxime N-methylcarbamate structure, and their mode of action is based on the inhibition of the enzyme acethylcholine esterase (1). However, this inhibition is reversible, and recovery from sublethal doses occurs rapidly. Some carbamate fungicides have a dithio, bisdithio, or benzimidazole carbamate basic structure, and dithiocarbamate fungicides inhibit the enzyme aldehyde deshydro-genase (2). The herbicides have an /V-alkylthiocarbamate or A-phenylcarbamate structure and interfere with photosynthetic activity or affect meristematic activity or lipid metabolism (3). Representative structures of carbamate pesticides are shown in Fig. 1. 

Enzymatic techniques have also been employed in the analysis of these compounds. The toxicity of carbamate insecticides is due to the inhibition of the enzyme acetylcholine esterase, so the determination of these compounds can be achieved by enzyme inhibition (2,83,119), bioassay (118,167), or enzyme-linked immunosorbent assay (ELISA) (168-171). In the detection of carbamates by fluorimetric enzyme inhibition, the effluent from a reversed-phase chromatographic column was incubated with cholinesterase, which was introduced via a postcolumn reagent delivery pump. Then, the resulting partially inhibited cholinesterase was reacted with N-methyl indoyl acetate to produce a fluorophore and a reduction in the baseline fluorescence (172). 

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). 

Bendiocarb is absorbed through all the normal routes of exposure (oral, dermal, and inhalation), but dermal absorption is especially rapid. Carbamates generally are excreted rapidly and do not accumulate in mammalian tissue. If exposure does not continue, cholinesterase inhibition and its symptoms reverse rapidly. In nonfatal cases, the illness generally lasts less than 24 hours.7 Bendiocarb is moderately toxic to birds. The LD50 in mallard ducks is 3.1 mg/kg, and in quail is 19 mg/kg.8 Bendiocarb is moderately to highly toxic to fish. The LC50 (96 hours) for bendiocarb in rainbow trout is 1.55 mg/L.2... 

Like other carbamates, propoxur can inhibit the action of cholinesterase and disrupt nervous system function. Depending on the severity of exposure, this effect may be short-term and reversible. 

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