ToxSci Advance Access published online on December 15, 2008
Toxicological Sciences, doi:10.1093/toxsci/kfn244
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Mechanism of Ethylbenzene-Induced Mouse-Specific Lung Tumor: Metabolism of Ethylbenzene by Rat, Mouse and Human Liver and Lung Microsomes
Toxicology & Environmental Research and Consulting, The Dow Chemical Company, Bldg 1803, Midland, MI 48674
Corresponding Author: Shakil A. Saghir, M.Sc., M.S.P.H, Ph.D., D.A.B.T., Senior Toxicology Specialist, Toxicology & Environmental Research and Consulting, The Dow Chemical Company, 1803 Building, Midland, MI 48674; Email: ssaghir{at}dow.com; Phone: (989) 636-8708; Fax: (989) 638-9863
Received September 2, 2008; revision received November 6, 2008; accepted November 12, 2008
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Abstract |
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This study was conducted to determine species differences in the metabolism of ethylbenzene (EB) in liver and lung. EB (0.22-7.0 mM) was incubated with mouse, rat and human liver and lung microsomes and the formation of 1-phenylethanol (1PE), acetophenone (AcPh), 2-ethylphenol (2EP), 4-ethylphenol (4EP), 2,5-ethylquinone (2,5EQ), and 3,4-ethylquinone (3,4EQ) were measured. Reactive metabolites (2,5-dihydroxyethylbenzene-GSH [2EP-GSH] and 3,4-dihydroxyethylbenzene-GSH [4EP-GSH]) were monitored via glutathione (GSH) trapping technique. None of the metabolites were formed at detectable levels in incubations with human lung microsomes. Percent conversion of EB to 1PE ranged from 1% (rat lung; 7.0 mM EB) to 58% (mouse lung; 0.22 mM EB). More 1PE was formed in mouse lung than in mouse liver microsomes, while formation of 1PE by rat liver and lung microsomes was similar. Metabolism of EB to 1PE was in the order of mouse > rat > human. Formation of AcPh was roughly an order of magnitude lower than 1PE. Conversion of EB to ring-hydroxylated metabolites was much lower (0.0001% [4EP-GSH; rat lung] to 0.6% [2EP-GSH; mouse lung]); 2EP-GSH was typically 10-fold higher than 4EP-GSH. Formation of 2EP-GSH was higher by lung (highest by mouse lung) than liver microsomes and the formation of 2EP-GSH by mouse liver microsomes was higher than rat and human liver microsomes. Increasing concentrations of EB did lead to a decrease in amount of some formed metabolites. This may indicate some level of substrate- or metabolite-mediated inhibition. High concentrations of 2EP and 4EP were incubated with microsomes to investigate further their oxidation to ethylcatechol (ECat) and ethylhydroquinone (EHQ). Conversion of 2EP to EHQ ranged from 6 to 9% by liver (mouse > human > rat) and from 0.1 to 18% by lung microsomes (mouse >> rat >> human). Conversion of 4EP to ECat ranged from 2 to 4% by liver (mouse > human 
rat) and from 0.3 to 7% by lung microsomes (mouse >> rat >>human). Although ring-oxidized metabolites accounted for a relatively small fraction of overall EB metabolism, its selective elevation in mouse lung microsomes is nonetheless consistent with the hypothesized mode of action for observed preferential toxicity of EB to the lung in this species.
Key Words: Ethylbenzene; Liver and Lung microsomes; Rat; Mouse and Human microsomal metabolism; 1-phenylethanol; acetophenone; 2-ethylphenol; 4-ethylphenol; ethylquinone; ethylhydroquinone and ethylcatechol.