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Detoxication of paraoxon by rat liver homogenate and serum carboxylesterases and A‐esterases*
Author(s) -
Tang Jun,
Chambers Janice E.
Publication year - 1999
Publication title -
journal of biochemical and molecular toxicology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.526
H-Index - 58
eISSN - 1099-0461
pISSN - 1095-6670
DOI - 10.1002/(sici)1099-0461(1999)13:5<261::aid-jbt6>3.0.co;2-0
Subject(s) - paraoxon , carboxylesterase , metabolite , parathion , chemistry , esterase , incubation , chromatography , hydrolysis , biochemistry , enzyme , active metabolite , phosphorylation , organophosphate , acetylcholinesterase , biology , pesticide , agronomy
Paraoxon, the active metabolite of parathion, can be detoxified through a noncatalytic pathway by carboxylesterases and a catalytic pathway by calcium‐dependent A‐esterases, producing p‐nitrophenol as a common metabolite. The detoxication patterns of carboxylesterases and A‐esterases were investigated in vitro in the present study with a high tissue concentration (75 mg/mL rat liver homogenate or 50% rat serum solution) to more closely reflect enzyme concentrations in intact tissues. A final paraoxon concentration of 3.75 μM was used to incubate with liver homogenates or serum solutions for 5 seconds or 3, 5, 15, or 25 minutes; also 0.625, 1.25, 2.5, 3.125, 3.75, or 5.0 μM paraoxon (final concentration) was incubated with liver homogenates or serum solutions for 15 minutes. Phenyl saligenin cyclic phosphate and EDTA were used to inhibit carboxylesterases and A‐esterases, respectively. Significant amounts of p‐nitrophenol were generated with or without either inhibitor during a 15 minute incubation with paraoxon from low (0.625 μM) to high (5.0 μM) concentrations. The amount of p‐nitrophenol generated via carboxylesterase phosphorylation was greater than via A‐esterase–mediated hydrolysis in the initial period of incubation or when incubating with a low concentration of paraoxon. Plateau shape curves of p‐nitrophenol concentration versus time or paraoxon concentration indicated that carboxylesterase phosphorylation was saturable. When incubated for long time intervals or with high concentrations of paraoxon, more p‐nitrophenol was generated via A‐esterase–mediated hydrolysis than from carboxylesterase phosphorylation. The ratio of paraoxon concentration to tissue amount used in in vitro assays of this study was equivalent to dosing a rat with toxicologically relevant dosages. These in vitro data suggest that both carboxylesterases and A‐esterases detoxify paraoxon in vivo; carboxylesterases may be an important mode of paraoxon detoxication in initial exposures to paraoxon or parathion before they become saturated, whereas A‐esterases may contribute to paraoxon detoxication in repeated exposures to paraoxon or parathion because they will not become inhibited and will remain catalytically active unlike the carboxylesterases. The importance of carboxylesterases in detoxication of paraoxon was verified by an in vivo study. In rats pretreated with tri‐o‐tolyl phosphate, an in vivo carboxylesterase inhibitor, brain acetylcholinesterase was significantly inhibited after intravenous exposure to parathion. No significant inhibition of brain acetylcholinesterase was observed in rats pretreated with corn oil. © 1999 John Wiley & Sons, Inc. J Biochem Toxicol 13: 261–268, 1999