Nerve agent-AChE complex

The destruction of acetylcholine by AChE is a very rapid reaction. Approximately 104 molecules of acetylcholine are hydrolyzed per second by a single AChE molecule. In contrast, hydrolysis of the nerve agent-AChE complex is much slower than hydrolysis of the acetylated enzyme that is produced by complexation with the normal substrate, ACh. The result is that the activity of AChE is inhibited for a prolonged period. 

The use of the subchronic rat study for developing an oral RfD for GD is complicated by the fact that rodents have a mnch lower RBC-AChE activity level compared to hnmans (ElUn, 1981, see Table 1). By itself, this could cause rats to be relatively more sensitive than hnmans to anticholinesterase compounds however, the lower RBC-ChE activity may be offset by the presence of aliesterase in rat blood. Aliesterase, which is not present in hnmans (Cohen et al., 1971), is known to bind to and thereby rednce the toxicity of cholinesterase inhibitors (Fonnnm and Sterri, 1981). Other species differences, snch as the rates of aging of the nerve agent-ChE complex, the rates of synthesis of plasma cholinesterase in the liver, and the levels of AChE in various parts of the nervous system (see Ivanov et al., 1993) may also resnlt in differences in species sensitivities. There are insufficient data to determine the relative snsceptibilities of humans and rodents to GD therefore, for the pnrpose of this assessment, the EPA method will be followed which assumes that humans may be as mnch as ten times more sensitive to a chemical than laboratory animals. 

The oxime HI-6 with atropine is reasonably effective against soman regardless of the choice of experimental animals while currently used oximes (pralidoxime and obidoxime) seem to be practically uneffective to protect mammals poisoned with supralethal dose of soman (Table 4). Presented data confirm that soman appears to be one of the most resistant nerve agent to the antidotal treatment because of the rapid aging of soman-phosphonylated AChE and the existence of a soman depot in the poisoned organisms (31, 54, 55). The soman-AChE complexes age very quickly and this fact prevents the oxime-induced reac-... 

The ageing process consists of the monodealkylation of the AChE-nerve agent complex. The loss of an alkyl group produces a conformational change that results in the formation of a very stable agent-enzyme complex which is then resistant to spontaneous hydrolysis and reactivation by oximes. The rate of ageing is dependent... 

However, even erythrocyte AChE measurements cannot be expected to be a perfect surrogate for the nervous tissue enzyme this is because pharmacokinetic factors may result in differential access of the inhibitor to the red cell and to neural structures. A further consideration is that, where nerve agents react with the enzyme to produce a phosphonylated structure that does not spontaneously reactivate, red cells of mammals lack the protein synthetic capability to synthesize new AChE. By contrast, in nervous tissue, after inhibition by OPs whose enzyme-inhibitor complex with AChE does not readily reactivate, activity may reappear relatively quickly. Thus, Wehner et al (1985) observed approximately 30% recovery after 24 h in di-isopropylfluorophosphate (DFP)-treated mouse CNS reaggregates, which was clearly due to synthesis de novo of AChE. Another consideration in the interpretation of butyrylcholinesterase activity measurements is that the normal range is relatively wide, rendering interpretation in individual patients difficult unless the results of previous estimations in the patient are available (Swami-nathan and Widdop, 2001). 

The OP insecticides, as discussed elsewhere in this book, have as a primary mechanism of acute toxicity the inhibition of the critical and widespread nervous system enzyme AChE. However, the anticholinesterase (anti-ChE) potencies do not correspond with the acute toxicity levels (Chambers et a ., 1990), indicating that metabolism is an important factor in determining the overall toxicity level. The OP insecticides evolved from the chemical technologies of World War II, which were used to develop the anti-ChE nerve agents. The OP inseeticidc.s bear some chemical resemblance to these nerve agents but are generally less toxic, often require metabolic activation to display anti-ChE activity and therefore are slower to act, and usually have more complex chemical substituents. 

The mechanism of AChE inhibition for the all OP and nerve agents is practically the same—the inhibition via phosphorylation or phosphonylation of the esteratic site of AChE. However, reactivation of inhibited AChE by oximes is different for different nerve agents phosphorylated but reactivata-ble AChE is changed to a nonreactivatable complex. The half-times for this reaction described as dealkylation (F5) are different for various OP/nerve agents (B3, Bll). 

Hornberg, A., A.-K. Tunemalm and F. Ekstrom, Biochemistry, 46, 2007, 4815-4825. (Grystal structures of AChE complexed with VX and other nerve agents)... 

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