We investigated the relationship between the chem- ical structure of acetylcholinesterase (AChE; EC 3.1.1.7) reactivators and their potency in reactivat- ing this enzyme, after prior inhibition by VX (O- ethyl-S-(2-diisopropylaminoethyl)-methylthiophos- phonate), tabun, sarin, and cyclosarin. The oximes, pralidoxime (2-PAM), HI-6 [1-(2-hydroxyimi- nomethylpyridinium)-3-(4-carbamoylpyridinium)- 2-oxa-propane dichloride], obidoxime and HS-6 [1- (2-hydroxyiminomethylpyridinium)-3-(3-car- bamoylpyridinium)-2-oxa-propane dichloride] were used as representatives of the group of AChE reacti- vators. Rat brain AChE was used as the appropriate source of the enzyme. Our results confirm that there is no single broad-spectrum oxime suitable for the treatment of poisoning with all highly toxic organophosphorus agents. Keywords: Acetylcholinesterase; Reactivation; Inhibition; 2-PAM; HI-6; HS-6; Obidoxime; Nerve agents; Organophosphorus compounds INTRODUCTION The interaction of acetylcholinesterase (AChE; EC 3.1.1.7) with organic phosphorus compounds results in the formation of enzymatically and physiologically inactive phosphorylated enzyme (Kuca et al., 2003a; Luo et al., 2003). Organophosphate-inhibited AChE is irreversibly inhibited, but its activity can be restored by treatment with some nucleophilic agents, the most potent of these being quaternary derivatives of 2- and 4-pyridinealdoxime (Kassa 2002; Pang et al., 2003). Due to the fact that AChE plays an important physio- logical role in the cholinergic nervous system, its inhi- bition (particularly at neuromuscular junctions) is a life-threatening event (Marrs, 1993; Taylor, 1996). Compounds that are capable of dephosphorylating organophosphate-inhibited AChE and restoring its physiological activity are known as "acetyl- cholinesterase reactivators", and these are valuable antidotes with therapeutic efficacy against organophos- phate toxicity (Tuarová et al., 1999; Kassa 2002). All AChE reactivators are powerful nucleopholic agents, which can cleave the P-O bond formed by the reaction of a serine hydroxyl (on AChE) with the inhibiting reactive organophosphorus compound. From a chemical point of view, most of the AChE reactiva- tors can be characterized as mono- or bisquaternary pyridinium salts with one or two aldoxime groups at the pyridinium rings at positions two or four. Recently used reactivators of AChE are not sufficiently efficacious in reactivating AChE inhibited by each nerve agent (Worek et al., 1997; 1998; Kuca et al., 2003b). Owing to this fact, there needs to be continued development of new AChE reactivators, capable of reactivating AChE inhibited by all nerve agents. In our studies we have been interested in developing a universal AChE reactivator - able to reactivate inhibited AChE, regardless of the nerve agent used. The aim of the currect study was to determine the reactivation capability of four pyridinium oximes - 2-PAM [prali- doxime; 1-methyl-2-hydroxyiminomethylpyridinium chloride], HI-6 [1-(2-hydroxyiminomethylpyridinium)- 3-(4-carbamoylpyridinium)-2-oxa-propane dichloride], HS-6 [1-(2-hydroxyiminomethylpyridinium)-3-(3-car- F.P. Graham Publishing Co. Neurotoxicity Research, 2004, VOL. 6(7,8). pp. 565-570 Reactivation of Organophosphate-inhibited Acetylcholinesterase by Quaternary Pyridinium Aldoximes KAMIL KUCA a, *, JIRÍ PATOCKA a,b , JIRÍ CABAL a AND DANIEL JUN a,c a Purkyne Medical Military Academy, Department of Toxicology, Hradec Králové, Czech Republic; b South Bohemia University, Faculty of Health and Social Studies, Department of Radiology and Toxicology, Ceské Budejovice, Czech Republic; c Department of Pharmaceutical Botany and Ecology, Charles University in Prague, Faculty of Pharmacy in Hradec Králové, Hradec Králové, Czech Republic. kucakam@pmfhk.cz (Received 18 July 2004; Revised 29 November 2004; In final form 29 November 2004) *Corresponding author. Tel.: +420 973 251 523; FAX: +420 495 518 094; E-mail: kucakam@pmfhk.cz, kucakam@seznam.cz ISSN 1029 8428 print/ ISSN 1476-3524 online. © 2004 FP Graham Publishing Co., www.NeurotoxicityResearch.com