The reactivation by oximes of phosphonylated acetylcholinesterase: the possible erroneous interpretation of reactivating potency

Determination of acetylcholinesterase activity by the Ellman assay: a versatile tool for in vitro research on medical countermeasures against organophosphate poisoning

Inhibition of acetylcholinesterase (AChE) is the main mechanism of action of organophosphorus compounds (OP), and AChE reactivators (oximes) are at present the only causal therapeutic approach. Being the key target of OP toxicity, AChE may serve as a valuable tool for diagnosis of OP exposure as well as for the investigation of the kinetics of interactions between OP and oximes. At present, the rapid, simple, and cheap spectrophotometric Ellman assay is widely used for diagnosis, therapeutic monitoring and in vitro kinetic investigations. Application of the assay for investigation of the interactions between AChE, inhibitors, and oximes requires the consideration of potential matrix effects (e.g. hemoglobin), side reactions (e.g. oximolysis of substrate) and other determinants (e.g. pH, temperature). By taking these factors into account, the Ellman assay allows the precise and reproducible determination of kinetic constants as a basis for the understanding of toxic OP effects and for the development of improved therapies against poisoning by OP. In addition, advanced applications of the Ellman assay, for example, in a dynamic in vitro model for the real-time activity determination of membrane-bound AChE, enables the proper investigation of relevant tissue, primarily respiratory muscle, and extends the applicability of this method.

Reactivation of organophosphate-inhibited human AChE by combinations of obidoxime and HI 6in vitro

Highly toxic organophosphorus-type (OP) chemical warfare agents (nerve agents) and OP pesticides may be used by terrorists and during military conflicts emphasizing the necessity for the development of effective medical countermeasures. The standard treatment with atropine and acetylcholinesterase (AChE) reactivators (oximes) is considered to be ineffective with certain nerve agents due to low oxime efficacy. Despite research over decades none of the oximes has turned out to be a broad spectrum reactivator to cover the whole range of potential threat agents. The prospective oxime HI 6 is a weak reactivator of tabun- and pesticide-inhibited AChE, while the established oxime obidoxime mainly lacks efficacy with cyclosarin-inhibited enzyme. In order to investigate the feasibility of combining obidoxime and HI 6, human AChE inhibited by sarin, cyclosarin, VX, tabun and paraoxon was reactivated by these oximes either alone or in combination. Two major findings of this study were that a combination of HI 6 and obidoxime did not impair reactivation, compared with HI 6 or obidoxime alone, but broadened the spectrum compared with the individual oximes. By using different oxime concentrations a combination of oxime doses may be suggested which could be an alternative to individual obidoxime or HI 6 autoinjectors.

Direct reaction of oximes with crotylsarin, cyclosarin, or VX in vitro

The direct reaction of seven pyridinium oximes with the organophosphorus compounds (OPCs) crotylsarin, cyclosarin, and VX was studied by spectrophotometry. This method allows to quantify different parameters: (a) the half-life times (t (1/2)) of the oxime-OPC reactions on the basis of the changes in the absorption at the zwitterion (betaine) peak maximum, (b) the first- and second-order rate constants (k (1), k (2)), and (c) the maximum reaction velocities (v (max)). The results of the study show that the reaction velocity of the nerve agents with any of the oximes investigated decreased in the order crotylsarin > cyclosarin > VX. The comparison of the reaction rates of the three therapeutically used oximes (2-PAM, obidoxime, HI 6) with the respective OPC gave the highest rate for crotylsarin and cyclosarin with obidoxime and to a similar degree with HI 6, while in the case of VX the most reactive oxime was HI 6. The reaction velocity of the nerve agents with the monopyridinium oxime 2-PAM was lower as compared to the bispyridinium oximes (obidoxime, HI 6). The results obtained with the two sarin analogues indicate that the direct reaction with 2-PAM, obidoxime, or HI 6 could be used for non-corrosive decontamination purposes, especially, if sensitive biological surfaces like skin, mucous membranes, or wounds are considered. However, in view of the concentrations of nerve agents and oximes, which could be expected during OPC poisoning in man, the maximum reaction velocities would not be high enough to contribute markedly to the detoxication of nerve agents in vivo.

Phosphylation kinetic constants and oxime-induced reactivation in acetylcholinesterase from fetal bovine serum, bovine caudate nucleus, and electric eel

Kinetic constants for selected phosphonate and phosphinate inhibitors of fetal bovine serum acetylcholinesterase (FBS AChE; EC 3.1.1.7), bovine caudate nucleus AChE (BCN AChE), and eel AChE have been determined. Oxime reactivation of the phosphylated enzymes has also been evaluated. In general, a rank order with respect to organophosphorus compound (OP) inhibition of the enzymes was observed: soman (pinacolyl methylphosphonofluoridate) was found to be the most potent inhibitor, and 4-nitrophenyl methyl(phenyl)phosphinate (PMP) the least potent. On average the bimolecular rate constant for soman inhibition of eel AChE was nearly twofold greater (9.3 x 10(7) M-1 s-1) than that for FBS AChE (5.5 x 10(7) M-1 s-1) and nearly fourfold greater than that for BCN AChE (2.2 x 10(7) M-1 s-1). In addition, 4-nitrophenyl chloromethyl(phenyl)phosphinate (CPMP) inhibition of eel AChE on average was nearly 10-fold greater than FBS AChE and three orders of magnitude greater than BCN AChE. The oxime HI-6 reactivated soman phosphonylated enzymes to a considerably greater extent than other oximes, and FBS AChE was notably more responsive to HI-6 than to other oximes. The individual mean values of the ki for each inhibitor in each class (phosphonate or phosphinate) were different with respect to each AChE, which may be a reflection of differences in enzyme configuration, whereas the general rank order of inhibitor potency within each class, reflected by the ki, was similar with respect to each AChE, which may be related to similar active centers. In general, oxime potency and some rank order varied with each inhibitor and with each AChE, although there was some similarity in oxime rank order between the two mammalian AChEs. Overall, the data support the selection of FBS AChE as the enzyme of choice for in vitro testing of OP inhibitors and reactivators.

Oxime-induced reactivation of acetylcholinesterase inhibited by organophosphinates

The comparative potency of oximes for reactivation of inhibited eel acetylcholinesterase (AChE) in vitro is dependent on the organophosphinate inhibitor. Some of the data, dealing with a reference organophosphonate, support the conclusion of other investigators that the oxime potency order is also dependent on the inhibiting phosphonate. This work was done to identify more clearly the nature of phosphinylated AChE with regard to oxime reactivation potency and the potential of phosphinates as pretreatment drugs to protect AChE against organophosphonate poisoning. We have determined the reactivation potency of four oximes--2-PAM, HI-6, TMB-4 and toxogonin--against four phosphinates--4-nitrophenyl methyl(phenyl)phosphinate (PMP), 4-nitrophenyl chloromethyl(phenyl)phosphinate (CPMP), 4-nitrophenyl trifluoromethyl(phenyl)phosphinate and 4-nitrophenyl bis(2-thienyl)phosphinate. For comparison, the phosphonate sarin (GB, isopropyl methylphosphonofluoridate) was included. Incubation of the inhibited enzyme (I-AChE) at 25 degrees C was with 0.30 microM oxime for PMP, 3.0 microM oxime for sarin and CPMP and 100 microM oxime for the two remaining phosphinates. AChE activity was assayed spectrophotometrically for 3.0 min at 272.5 nm at 25 degrees C in 0.10 M MOPS buffer (pH 7.60) using phenyl acetate as substrate. When sarin was the inhibitor (0% spontaneous recovery after a 2-h incubation), the order of oxime reactivation was 2-PAM (46%) greater than or equal to toxogonin (33%) = TMB-4 (31%) greater than HI-6 (9%) after 2-h incubations. For PMP (12% spontaneous recovery after a 2-h incubation) the oxime order was toxogonin (67%) greater than TMB-4 (53%) greater than 2-PAM (40%) after 2-h incubations.(ABSTRACT TRUNCATED AT 250 WORDS)