Formation and disposition of diethylphosphoryl-obidoxime, a potent anticholinesterase that is hydrolyzed by human paraoxonase (PON1).
The potential of pyridinium-4-aldoximes, such as obidoxime, to reactivate diethylphosphorylated acetylcholinesterases is not fully exploited due to the inevitable formation of phosphoryloximes (POX) with high anticholinesterase activity. Mono(diethylphosphoryl) obidoxime (DEP-obidoxime) was isolated for the first time showing remarkable stability under physiological conditions (half-life 13.5min; pH 7.1; 37 degrees C). The half-life was considerably extended to 20h at 0 degrees C, which facilitated the preparation and allowed isolation by HPLC. The structure was confirmed by mass spectrometry and the degradation pattern. DEP-obidoxime decomposed by an elimination reaction forming the intermediate nitrile that hydrolyzed mainly into the pyridone and cyanide. The intermediates were prepared and confirmed by mass spectroscopy. DEP-Obidoxime was an extremely potent inhibitor of human acetylcholinesterase approaching a second-order rate constant of 10(9)M(-1)min(-1) (pH 7.4; 37 degrees C). The nitrile and the pyridone were still good reactivators. In the presence of human plasma DEP-obidoxime was hydrolyzed into parent obidoxime. Calcium-dependence and sensitivity towards chelators, substitution pattern by other divalent cations and protein-modifying agents all pointed to human paraoxonase (hPON1) as the responsible protein with POX-hydrolase activity. Subjects, probably belonging to the homozygous (192)arginine subtype, were virtually devoid of POX-hydrolase activity while a highly purified hPON1 of the homozygous (192)glutamine subtype exhibited particularly high POX-hydrolase activity. Two parathion-poisoned patients with high and low POX-hydrolase activity responded well and poorly, respectively, to obidoxime treatment although the former patient had higher plasma paraoxon levels than the poor responder. Hence, the POX-hydrolase associated PON1 subtype may be another contributor that modulates pyridinium-4-aldoxime effectiveness.
Kiderlen
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Walther-Straub-Institute of Pharmacology and Toxicology, Ludwig-Maximilians-University, Goethestrasse 33, 80336 Munich, Germany.
Eyer
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P
Worek
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F
D043303:Aryldialkylphosphatase / Q000737:chemistry* / Q000302:isolation & purification / Q000502:physiology*; D002800:Cholinesterase Inhibitors / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D006868:Hydrolysis; D009768:Obidoxime Chloride / Q000737:chemistry / Q000493:pharmacokinetics*; D010091:Oximes / Q000737:chemistry / Q000493:pharmacokinetics*; D013045:Species Specificity
D002800:Cholinesterase Inhibitors; D010091:Oximes; C118528:diethylphosphoryloxime; D009768:Obidoxime Chloride; D043303:Aryldialkylphosphatase; C468406:PON1 protein, human