Reactivation Acetylcholinesterase reactivators

A two-site immunometric assay of undecapeptide substance P (SP) has been developed. This assay is based on the use of two different antibodies specifically directed against the N- and C-terminal parts of the peptide (95). Affinity-purified polyclonal antibodies raised against the six amino-terminal residues of the molecule were used as capture antibodies. A monoclonal antibody directed against the carboxy terminal part of substance P (SP), covalently coupled to the enzyme acetylcholinesterase, was used as the tracer antibody. The assay is very sensitive, having a detection limit close to 3 pg/mL. The assay is fiiUy specific for SP because cross-reactivity coefficients between 0.01% were observed with other tachykinins, SP derivatives, and SP fragments. The assay can be used to measure the SP content of rat brain extracts. 

Hansen ME, Wilson BW. 1999. Oxime reactivation of RBC acetylcholinesterases for biomonitoring. Arch Environ Contam Toxicol 37 283-289. 

This process of aging is believed to be critical in the development of delayed neuropathy, after NTE has been phosphorylated by an OP (see Chapter 10, Section 10.2.4). It is believed that most, if not all, of the B-esterases are sensitive to inhibition by OPs because they, too, have reactive serine at their active sites. It is important to emphasize that the interaction shown in Fignre 2.11 occurs with OPs that contain an oxon group. Phosphorothionates, which contain instead a thion group, do not readily interact in this way. Many OP insecticides are phosphorothionates, but these need to be converted to phosphate (oxon) forms by oxidative desulfuration before inhibition of acetylcholinesterase can proceed to any significant extent (see Section 2.3.2.2). 

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... 

Thioesters play a paramount biochemical role in the metabolism of fatty acids and lipids. Indeed, fatty acyl-coenzyme A thioesters are pivotal in fatty acid anabolism and catabolism, in protein acylation, and in the synthesis of triacylglycerols, phospholipids and cholesterol esters [145], It is in these reactions that the peculiar reactivity of thioesters is of such significance. Many hydrolases, and mainly mitochondrial thiolester hydrolases (EC 3.1.2), are able to cleave thioesters. In addition, cholinesterases and carboxylesterases show some activity, but this is not a constant property of these enzymes since, for example, carboxylesterases from human monocytes were found to be inactive toward some endogenous thioesters [35] [146], In contrast, allococaine benzoyl thioester was found to be a good substrate of pig liver esterase, human and mouse butyrylcholinesterase, and mouse acetylcholinesterase [147],... 

HPMA copolymers are water-soluble biocompatible polymers, widely used in anticancer drug delivery (reviewed in Reference [22]). HPMA copolymers containing reactive groups at side-chain termini were previously used for the modification of trypsin [23], chymotrypsin [23,24], and acetylcholinesterase [25]. The modification dramatically increased the acetylcholinesterase survival in the blood stream of mice and the thermostability of modified enzymes when compared to the native proteins. However, the modification involved multipoint attachment of the copolymers to the substrates, which may cause crosslinking. To modify proteins or biomedical surfaces by one point attachment, semitelechelic polymers should be used. 

Another drug with a high incidence of hepatotoxicity is the acetylcholinesterase inhibitor tacrine. Binding of reactive metabolites to liver tissue correlated with the formation of a 7-hydroxy metabolite [13], highly suggestive of a quinone imine metabolite as the reactive species. Such a metabolite would be formed by further oxidation of 7-hydroxy tacrine (Figure 8.11). 

Even more reactive towards acetylcholinesterase are the organophosphorus derivatives developed as chemical warfare nerve agents, e.g. sarin. Such compounds react readily with the enzyme and form very stable addition intermediates. It is unusual to see fluoride as a leaving group, as in sarin, but its presence provides a huge inductive effect, thus accelerating the initial nucleophilic addition step (see also Section 13.7). 

Therapeutic measures include 1. administration of atropine in high dosage to shield muscarinic acetylcholine receptors and 2. reactivation of acetylcholinesterase by obidoxime, which successively binds to the enzyme, captures the phosphate residue by a nucleophilic attack, and then dissociates from the active center to release the enzyme from inhibition. 

Hobbiger, F. Reactivation of phosphorylated acetylcholinesterase. In Koelle, G.B., ed. Cholinesterases and Anticholinesterase Agents. Handb. Exp. Pharmakol. 15 921-988, 1963. Berlin Springer-Verlag. 

Wilson, I.B., and Glnsburg, S. A powerful reactivator of alkylphosphate-inhibited acetylcholinesterase. Blochlm. Blophys. Acta. 18 168-170, 1955. 

In dogs poisoned with soman (Intravenously at 30 pg/kg) and treated with I at 104 mg/kg (Intravenously 31/2 min after soman), the large dose of I stopped aging of Inhibited cholinesterase and reactivated 24.0% and 35.6% of the red-cell and diaphragm cholinesterase activities, respectively. It failed to reactivate brain cholinesterase. Indeed, the brain acetylcholinesterase activity after the treatment with 1 was lower than that just before the injection of I. The last finding indicates the inability of I to cross the blood-brain barrier in significant quantities. 

Erdmann and Engelhardl23,124 reported that in vitro IV was a more potent reactivator of acetylcholinesterase inhibited by DFP or... 

Hobblger, F. 1963. Reactivation of phosphonylated acetylcholinesterase. In, Koelle, G.B. (ed) 1963 Cholinesterases and Anticholinesterase Agents, Berlin-Gottingen-Heidelberg, Sprlnger-Verlag, pp. 921-988. 

The first suggestion of a practical form of antidotal therapy came in 1949 from Hestrin, who found that acetylcholinesterase (AChE) catalyzed the formation of acetohydroxamlc acid when incubated with sodium acetate and hydroxylamine. Critical in vitro studies in the next decade led to the development of a practical approach to therapy. The crucial concept in these studies was the recognition that the compound formed when AChE reacted with a phosphorus ester was a covalent phosphoryl-enzyme Intermediate similar to that formed in the hydrolysis of acetylcholine. 3 Wilson and colleagues, beginning in 1951, demonstrated that AChE inhibited by alkyl phosphate esters (tetraethyl pyrophosphate, TEPP) could be reactivated by water, but that free enzyme formed much more rapidly in the presence of hydroxylamine. 0 21 Similar results... 

Wilson, I.B. Acetylcholinesterase. XIII. Reactivation of alkyl phosphate-inhibited enzyme. J. Biol. Chem. 199 113-120, 1952. 

Wilson, I.B., and Ginsburg, S. Reactivation of acetylcholinesterase inhibited by alkylphosphates. Arch. Biochem. 54 569-571, 1955. 

Thus, the phosphorylated oxime can itself be a potent cholinesterase inhibitor. Wilson and Ginsburg advanced this concept when they noticed incomplete reactivation of acetylcholinesterase by 2-PAM and suggested that the phosphorylated oxime reinhibited the enzyme (according to Reaction 2). 

Oxime reactivators (R-CH N0H) are weak acids that partly Ionize at biologic pH. This property allows them to function as nucleophiles and displace organophosphate moieties from inhibited acetylcholinesterase. It also makes them vulnerable to decomposition by other mechanisms in the body. 

It causes reactivation of the phosphorylated acetylcholinesterase enzyme. After administration, it is metabolised in liver. 

Dewair M, Baur X, Mauermayer R. 1983. Inhibition of acetylcholinesterase by diisocyanates and its spontaneous reactivation. Int Arch Occup Environ Health 52 257-261. 

It is important to know that the inhibition of acetylcholinesterase by OPs is through an attack on the relatively positive phosphorus atom by the hydroxyl group of a serine residue at the enzyme s site of action. Electron withdrawing substitutions within the OP tend to make the phosphorus more positive and, therefore, more reactive. Unfortunately, this type of substitution also makes the compound less stable hydrolytically. The discovery and development of OP insecticides has always been a balance between activity against the enzyme of the insect, selectivity in comparison with mammalian systems and stability within the insect. The binding of OPs to acetylcholinesterase is often irreversible. Typical OP insecticides are shown in Figure 3.3. 

Metabolites that are less reactive than suicide inhibitors may impact more distant enzymes, within the same cell, adjacent cells, or even in other tissues and organs, far removed from the original site of primary metabolism. For example, organopho-sphates (OPs), an ingredient in many pesticides, are metabolized by hepatic CYPs to intermediates, which, when transported to the nervous system, inhibit esterases that are critical for neural function. Acetylcholinesterase (AChE) catalyzes the hydrolysis of the ester bond in the neurotransmitter, acetylcholine, allowing choline to be recycled by the presynaptic neurons. If AChE is not effectively hydrolyzed by AChE in this manner, it builds up in the synapse and causes hyperexcitation of the postsynaptic receptors. The metabolites of certain insecticides, such as the phos-phorothionates (e.g., parathion and malathion) inhibit AChE-mediated hydrolysis. Phosphorothionates contain a sulfur atom that is double-bonded to the central phosphorus. However, in a CYP-catalyzed desulfuration reaction, the S atom is... 

Figure 7.49 General scheme for acetylcholinesterase action. R may be equal to C or P. If R=P, then (R3) is present. The group OR, may be replaced by SR, giving R,SH on hydrolysis (reaction 1). IF R=P, reaction 2 is very slow, giving inactivated enzyme. The rate of hydrolysis or reactivation depends on the nature of R2 and R3.

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