Abstract
A-esterases are calcium-dependent hydrolases that can detoxify the active metabolites (oxons) of organophosphorus insecticides such as chlorpyrifos and parathion. A-esterases from rat liver have previously been shown to hydrolyze chlorpyrifos-oxon but not paraoxon at low substrate concentrations. Two A-esterases were extracted by ammonium sulfate fractionation from solubilized rat liver microsomes followed by gel filtration chromatography and preparative scale isoelectric focusing. The proteins displayed similar characteristics and were difficult to separate; both had similar high molecular mass and isoelectric point range and exhibited A-esterase activity toward high and low concentrations of chlorpyrifos-oxon and high concentrations of paraoxon. Sufficient amounts of the higher molecular mass protein were obtained for kinetic studies, which yielded a Km of 0.93 mM toward high concentrations of chlorpyrifos-oxon and a Vmax of 369 nmoles product formed/mg protein-min. The protein hydrolyzed phenyl acetate, chlorpyrifos-oxon and paraoxon, suggesting that arylesterase and A-esterase activities are attributable to the same liver protein(s). Assays of purified protein and kinetic studies of microsomes suggested that the activity toward high (320 μM) and low (≤10−5 M) concentrations of chlorpyrifos-oxon are due to the same protein(s), and that the activity toward low concentrations of chlorpyrifos-oxon is attributable to both a higher affinity and a higher Vmax (but primarily the latter) for chlorpyrifos-oxon than for paraoxon, which is not detectably hydrolyzed at low concentrations. The higher A-esterase activity with chlorpyrifos-oxon than paraoxon may be a major determinant in the observed lower acute toxicity of chlorpyrifos than parathion.
Footnotes
-
Send reprint requests to: Dr. Janice E. Chambers, Center for Environmental Health Sciences, College of Veterinary Medicine, Box 9825, Mississippi State University, Mississippi State, MS 39762-9825.
-
↵1 This research was partially funded by National Institutes of Health Grant R01 ES04394. J.E.C. also acknowledges the support of NIH Research Career Development Award KO4 ES00190 and the Burroughs Wellcome Toxicology Scholar Award. Partial support was also provided by the College of Veterinary Medicine and the Mississippi Agricultural and Forestry Experiment Station, and is MAFES Publication J9245 under Project MISV-3403, and Center for Environmental Health Sciences publication no. 84.
-
↵2 Current address: Biochemistry Department, 221 Hansen Building, Purdue University, West Lafayette, IN 47907.
- Abbreviations:
- OP
- organophosphorus
- Pxn
- paraoxon
- Cpxn
- chlorpyrifos-oxon
- AChE
- acetylcholinesterase
- PA
- phenyl acetate
- PSIEF
- preparative scale isoelectric focusing
- SDS-PAGE
- sodium dodecyl sulfate polyacrylamide gel electrophoresis
- EDTA
- disodium ethylenediaminetetraacetic acid
- 2-NAc
- 2-naphthylacetate
- HDL
- high density lipoprotein
- Received November 10, 1997.
- Accepted May 7, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|