Enzyme organophosphorus esters

All the enzymes discussed above belong to the class of dimetalloenzymes. In this context, it should be mentioned that serine-type hydrolases are irreversibly inhibited by organophosphorus esters, among them highly toxic chemical warfare agents. However, in some cases, for example of human butyrylcholi-noesterase, the inhibited enzyme could be reactivated by proper mutations." Moreover, such mutahons were found to confer phosphotriesterase activity in this... 

Organophosphorus esters are known to react with a serine hydroxyl group in the active site of the acetylcholinesterase protein (Ecobichon 1991 Murphy 1986). Some organophosphorus esters (e.g., diisopropyl fluorophosphate, [DFP]) bind irreversibly, while others bind in a slowly reversible fashion, thereby leading to a slow reactivation (dephosphorylation) of the enzyme. A process known as "aging" has also been described in which reversibly bound compounds are changed with time to moieties that are essentially irreversibly... 

Toxicity The agents GA, GB, and VX are rapidly acting, lethal nerve agents. They directly affect the nervous system and are toxic as liquids and vapors. They are organophosphorus esters that inhibit acetylcholinesterase, an enzyme that prevents the accumulation of the neurotransmitter acetylcholine at the nerve synapses. When too much acetylcholine is present, convulsion and death may result. 

Fid. 1. An organophosphorus ester associates with the serine-hydroxyl moiety at the acctylcholinestcra.se active site (A) to form a reversible complex that i.s ihen inhibited via phosphorylation. The rates of a.ssociation and phosphorylation arc governed by and k. The phosphorylalcd enzyme Is then either reactivated via hydrolysis or becomes aged (irreversibly inhibited) via dcallQtlaiion (B>. 

Organophosphorus esters that inhibit neurotoxic esterase, an enzyme present in the brain and peripheral nerves, are associated with delayed neurotoxicity (i.e., peripheral axonopathy). Although inhibition of neurotoxic esterase may not play a primary role in the development of the nerve lesions, it serves as a marker for potential damage. Chickens are often used as test animals for this effect. 

Chem. Res. Toxicol. (8) Aldridge, W. N., and Reiner, R. E. (1972) Enzyme Inhibitors as Substrates Interactions of Esterases with Esters of Organophosphorus and Carbamic Acids. North-Holland Publishing, Amsterdam. 

Inhibition of the cholinesterase enzymes depends on blockade of the active site of the enzyme, specifically the site that binds the ester portion of acetylcholine (Fig. 7.48). The organophosphorus compound is thus a pseudosubstrate. However, in the case of some compounds such as the phosphorothionates (parathion and malathion, for example), metabolism is necessary to produce the inhibitor. 

Aldridge, W.N. Reiner, E. "Enzyme Inhibitors as Substrates Interaction of Esterases with Esters of Organophosphorus and Carbamic Acids" North Ho Hand-Amsterdam, 1975. 

Carboxylamidase activity toward p-nitroacetanilide has been detected in different insect species from the orders Lepidoptera, Orthoptera, and Dictyoptera. The carboxylamidase from fall army worm larvae has been purified. The purified enzyme is a monomer with a molecular weight of 59,000-60,000 Da. The enzyme is inhibited by the hydrolase inhibitors paraoxon, triphenyl phosphate, eserine, and phenylmethylsulfonyl fluoride, showing I50 values of 4.7 iM, 0.2 mM, 16 iM, and 90 iM, respectively. Activity is also completely inhibited by the organophosphorus insecticides profenfos and dichlorvos at 0.1 mM. The enzyme is active toward other amides, such as acetanilide and phenacetin, and various a- and p-naphtholic esters. Based on the purification factor, substrate specificity, and sensitivity to hydrolase inhibitors, the carboxylamidase appears to be different from carboxy-lesterases in the fall army worm (Yu and Nguyen, 1998). 

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. 

The phosphorylation of the serine hydroxyl group of cholinesterase by organophosphorus compounds inhibits the activity of this enzyme [192]. This phosphorylation is reversible when the phosphoryl ester so formed is of the dialkoxy type, but becomes irreversible when dealkylation takes place. Wilson [15] found nycleophilic attack of the dialkyl phosphorylated enzyme with hydroxylamine to result in regeneration of the enzyme activity. It has been suggested that some oximes act initially by forming a complex with the phosphorylated enzyme [193-195]. Later work [196] showed the incorporation of a quaternary nitrogen into the nucleophilic molecule (for example A-methylpyridinium-2-aldoxime salts 2-PAM P-2-S prali-... 

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