Aging serine esterase

FIGURE 57.9. Inhibition and aging of serine esterases by diisopropylphosphorofluoridate (DFP). The active site serine is organophosphorylated in the inhibition step. Aging results in net loss of an isopropyl group to yield the monoisopropylphosphoryl esterase. This mode of inhibition and aging has been established for acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and neuropathy target esterase catalytic domain (NEST) (Kropp and Richardson, 2007). 

Tyrosine adducts were found in guinea pigs and marmosets poisoned with sarin, soman, cyclosarin, and tabun. VX, which is less reactive than other WNAs, formed an adduct in human plasma in vitro only at high concentrations (Williams et al., 2007 Black, 2010 Black and Read, 2013). Tyrosine adducts were less sensitive than ChE as biomarkers with respect to exposure levels but were more stable and did not undergo an aging reaction such as OP binding to serine esterases (Read et al.,... 

Considerahons of probable mechanisms of aging for mipafox-inhibited serine esterases suggest that mass spectrometry studies would support a deprotonation mechanism for NTE or NEST and hydrolytic P—N bond scission for AChE and BChE. As summarized in Figure 63.10, these expectahons were borne out for human recombinant NEST, which was used as a surrogate for NTE (Kropp et al., 2004) and BChE, respectively (Kropp and Richardson, 2007). However, mipafox-inhibited AChE gave the surprising result, confirmed by... 

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). 

The inhibited esterase can also undergo aging, e.g. via net loss of the R-group to yield the negatively charged phos-phonyl adduct on the active site serine of the enzyme. Aging is characterized by a first-order rate constant, fcj, and the operational definition of this reaction is the time-dependent... 

Hydrolysis. Carboxylesterases are frequently one of the major factors in OP resistance. In some insects, for instance the house fly (28), there are highly substrate specific esterases which attack only one or a very few molecules. "Malathionase", the prominent esterase responsible for many cases of malathion resistance, is highly specific for malathion. It cleaves one or both of the ethyl ester groups leaving malathion mono- or diacid (29). This enzyme is a true serine carboxylesterase that is inhibited by malaoxon (28) and does not hydrolyze any of the phosphoester bonds. In Anopheles stephensi from Pakistan, the malathion resistance decreased with adult age, but there was no concommittant decrease in general esterase activity as measured with 1- and 2-naphthylace-tate as model substrates (301. other mosquitoes have a carboxylesterase with broad substrate specificity that is associated with resistance (31-331. As mentioned above, the green peach aphid has a carboxylesterase, E4, with broad substrate specificity that sequesters toxicants (24). 

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