Sarin reactivity

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). 

The kinetic scheme for the reactivation of OP-inhibited AChE is depicted in equation 20 for pyridinium-based aldoximes, where R = CH3 or an alkoxy group, and = alkyl or aryl. The nerve agents that generate the covalent OP-AChE conjugates are those with R = CH3 and R = ethyl(VX), isopropyl (sarin), cyclohexyl (cyclosarin) and pinacolyl (soman). For tabun-inhibited AChE, R = Af,A-dimethylamido and R = ethyl. 

The pX a (6.5-8.2) and nucleophilicity of MINA, 44, and of a series of aliphatic oximes derived from it, were found to be consistent with their ability to reactivate AChE inhibited by the nerve agents, sarin and VX. Yet, despite their ability to significantly reactivate AChE in the brains of sarin-intoxicated rats, these aliphatic oximes are not used as antidotes for treatment of OP poisoning in humans this is presumably due to their poor stability in aqueous solution and to their rapid clearance from the circulation. 

A recent study has revisited the reactivity of oximate nucleophiles toward detoxification of sarin, soman and DFP, using a fluoride-selective electrode to kinetically monitor decomposition of the neurotoxin . Results are shown in Table 1. 

The oxime-induced reactivation of organophosphorus-inhibited AChE has been modeled recently through the Density Functional Theory (DFT) approach. Two possible computed reactivation pathways of Sarin-inhibited AChE adduct by formoximate anion are shown in Scheme The two-step mechanism (Scheme 7B) is favored by the authors. 

PAM I seems to be less effective than IV In reactivating brain cholinesterase after its inactivation by paraoxon or other OP compounds.There was early evidence that 2-PAM I produced more reactivation of cholinesterase in the pontomedullary region and the area postrema that had been inhibited by paraoxon than in the cerebellum and the cerebral cortex TO and that it could prevent the appearance of grand mal-llke discharges in EEGs of rabbits after doses of sarin that evoked such discharges in rabbits not protected... 

In the final study to be mentioned in regard to the possible penetration of pyrldinium oximes into brain, anesthetized, atropi-nized cats were given intravenous injections of sarin at 27/tg/kg. 12 Thirty minutes later, to allow clearance of unreacted sarin from the tissues, some cats received saline injections into one common carotid artery and others received similar injections of 1 at 15 mg/kg. The cholinesterase activity of cerebral cortex was measured. In animals given 1, nearly 20% of the cholinesterase in their cerebral cortices that had been inhibited by sarin was calculated to have been reactivated by the oxime. 

Experimental measurement (6, 8) of the solvolysis of Sarin and DFP in the presence of 1 to 1 diamine-metal chelates, such as those listed in Table I, under varying solution conditions showed that the catalytic effect was not proportional to, or a simple function of, the total metal chelate species in solution. A detailed analysis of the variation of rate with composition of the solutions indicated (4, 7) the presence of hydroxo and dihydroxo mononuclear forms of the chelate compound, as well as a binuclear //,-dihydroxo species. If the possible reactivities of all catalytic species are taken into consideration, the rate expression would have the form ... 

Application of the fluoride-reactivation method to serum samples of victims from the Tokyo subway attack, and of the Matsumoto incident, yielded, sarin concentrations in the range of 0.2-4.1ng/ml serum (44). Evidently, these casualties had been exposed to an organophosphate with the formula i-PrO(CH3)P(0)X, presumably with X = F (sarin). 

Harris, L.W., Heyl, W.C., Stitcher, D.L., Broomfield, C.A. (1978). Effects of l,l-oxydimethylene-bis-(4-tert-butyl pyridinium chloride) (SAD-128) and decamethonium on reactivation of soman- and sarin-inhibited cholinesterase by oximes. Biochem. Pharmacol. 27 757-61. 

Astrocyte activation or reactive gliosis involves proliferation, recruitment to the site of injury (Walton et al, 1999), and release of numerous cytotoxic agents including proteolytic enzymes, cytokines, complement proteins, reactive oxygen intermediates, NMDA-like toxins, and nitric oxide (Weldon et al, 1998). Sarin exposure activated astrocytes... 

The presence of sarin, soman, and VX in the brain demonstrates the necessity for antidotes, especially AChE reactivators, to be capable of passing the blood-brain barrier, representing a current scientific challenge (Lorke et al, 2008 Okuno et al, 2008). 

Interaction of CarbE with nerve agents follows a kinetic of first order characterized by inhibition of CarbE at the active site serine residue described by a bimolecular rate constant, ki (Maxwell and Brecht, 2001). For noncharged nerve agents (e.g. sarin and soman) the ki of rat serum CarbE was found to be >10 M min whereas cationic substrates (e.g. VX) are converted with poor reactivity (ki < 10" M min ). This specificity is explained by the electrostatic characteristics of the large active site containing only a few cation-II bonding and anionic residues (Maxwell and Brecht, 2001 Satoh and Hosokawa, 2006). 

Adams, T.K., Capacio, B.R., Smith, J.R., Whalley, C.E., Korte, W.D. (2004). The application of the fluoride reactivation process to the detection of sarin and soman nerve agent exposure in biological samples. Drug Chem. Toxicol. 27 77-91. 

Sarin is metabohzed to IMPA, which slowly undergoes further hydrolysis to the very stable MPA. IMPA also forms in the course of spontaneous reactivation of sarin-inhibited CarbEs and ChEs. IMPA has low oral toxicity in rats and mice, but it produces mild skin irritation in rabbits. 

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