No abstract

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188). 1215 Jefferson Davis Highway. Suite 1204. Arlington. VA 22202-4302 Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

The ability of carboxylesterase (CaE) to act as a bioscavenger to provide protection against organophosphorus (OP) compounds has been demonstrated in several animal models. To further evaluate the effectiveness of CaE as a bioscavenger, the specificity and stoichiometry of the detoxication of OP compounds by rat plasma CaE were examined. The specificity of CaE was evaluated by determining the bimolecular rate constants for inhibition (k(i)) of CaE by a variety of OP compounds. CaE exhibited a broad specificity for neutral OP compounds with k(i) > 10(6) M(-1) x min(-1) for paraoxon, sarin, soman, diisopropyl fluorophosphate, and diphenyl p-nitrophenyl phosphinate. CaE exhibited poor reactivity (k(i) < 10(4) M(-1) x min(-1)) with cationic OP compounds, such as echothiophate, VX, and iso-OMPA. The stoichiometry of CaE detoxication of OP compounds was evaluated by determining the rates of enzyme reactivation and ageing of OP-inhibited CaE. CaE exhibited no ageing after inhibition by any of the OP compounds, including soman. However, OP-inhibited CaE did exhibit spontaneous reactivation with reactivation rates that decreased as the size of the OP increased (i.e., VX > sarin > soman). The pH dependence of the spontaneous reactivation of sarin-inhibited CaE suggested that its reactivation was dependent on an amino acid residue with a pK(a) of 6.1, which is probably a histidine that is highly conserved in CaE but not in other esterases.

The reactivation of nerve agent-inhibited acetylcholinesterase (AChE) by oxime is the most important step in the treatment of nerve agent poisoning. Since the evaluation of nerve agent antidotes cannot be conducted in humans, results from animal experiments are extrapolated to humans. Guinea pig is one of the animal models that is frequently used for conducting nerve agent antidote evaluations. Several investigations have demonstrated that the efficacy of an oxime primarily depends on its ability to reactivate nerve agent-inhibited AChE. If the in vitro oxime reactivation of nerve agent-inhibited animal AChE is similar to that of human AChE, it is likely that the results of an in vivo animal study will reliably extrapolate to humans. Therefore, the goal of this study was to compare the reactivation of guinea pig and human AChEs inhibited by six different G and V type nerve agents. Reactivation kinetic studies with five mono- and bis-pyridinium oximes showed that oxime reactivation of nerve agent-inhibited human AChE in most cases was faster than guinea pig AChE. The most significant enhancement was observed in the reactivation of human AChE inhibited by nerve agents containing bulky side chains GF, GD, and VR, by H-series oximes HLo-7, HI-6, and ICD-585. In these cases, species-related differences observed between the two AChEs, based on the second-order reactivation rate constants, were 90- to over 400-fold. On the other hand, less than 3-fold differences were observed in the rates of aging of nerve agent-inhibited guinea pig and human AChEs. These results suggest that the remarkable species-related differences observed in the reactivation of nerve agent-inhibited guinea pig and human AChEs were not due to differences in the rates of aging. These results also suggest that guinea pig may not be an appropriate animal model for the in vivo evaluation of oxime therapy.