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Metabolic Detoxification: Implications for Thresholds
Franz Oesch
Institute of Toxicology, University of Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany
Maria Elena Herrero
Institute of Toxicology, University of Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany
Jan Georg Hengstler
Institute of Toxicology, University of Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany
Matthias Lohmann
Institute of Toxicology, University of Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany
Michael Arand
Institute of Toxicology, University of Mainz, Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany
The fact that chemical carcinogenesis involves single, isolated, essentially irreversible molecular events as discrete steps, several of which must occur in a row to finally culminate in the development of a malignancy, rather suggests that an absolute threshold for chemical carcinogens may not exist. However, practical thresholds may exist due to saturable pathways involved in the metabolic processing, especially in the metabolic inactivation, of such compounds. An important example for such a pathway is the enzymatic hydrolysis of epoxides via epoxide hydrolases, a group of enzymes for which the catalytic mechanism has recently been established. These enzymes convert their substrates via the intermediate formation of a covalent enzyme-substrate complex. Interestingly, the formation of the intermediate proceeds faster by orders of magnitude than the subsequent hydrolysis, ie, the formation of the terminal product. Under normal circumstances, this does not pose a problem, since the microsomal epoxide hydrolase (mEH), the epoxide hydrolases with the best documented importance in the metabolism of carcinogens, is highly abundant in the liver, the organ with the highest capacity to metabolically generate epoxides. Computer simulation provides evidence that the high amount of mEH enzyme is favorable for the control of the steady-state level of a substrate epoxide and can keep it extremely low. However, once the mEH is titrated out under conditions of extraordinarily high epoxide concentration, the epoxide steady-state level steeply rises, leading to a sudden burst of the genotoxic effect of the noxious agent. This prediction of the computer simulation is nicely supported by experimental work. V79 Chinese hamster cells that we have genetically engineered to express human mEH at about the same level as that observed in human liver are completely protected from any measurable genotoxic effect of the model compound styrene oxide (STO) up to a dose of 100 µM in the cell culture medium (toxicokinetic threshold). In V79 cells that do not express mEH, STO leads to the formation of DNA strand breaks in a dose-dependent manner with no toxicokinetic threshold observable. Above 100 µM, the genotoxic effect of STO in the mEH-expressing cell line parallels the one in the parental cell line. Thus, the saturable protection from STO-induced strand breaks by mEH represents a typical example of a practical threshold. However, it must be pointed out that even in the presence of protective amounts of mEH, a minute but definite level of STO is present that does not contribute sufficiently to the strand break formation to overcome the background noise of the detection procedure. As pointed out above, absolute thresholds probably do not exist in chemical carcinogenesis.
Key Words: Epoxide hydrolase •  /β hydrolase fold • ester intermediate • epoxy compounds metabolism • V79 Chinese hamster lung fibroblasts • DNA damage • genotoxicity
References
- Arand M., Grant DF, Beetham JK, Friedberg T., Oesch F., Hammock BD (1994). Sequence similarity of mammalian epoxide hydrolases to the bacterial haloalkane dehalogenase and other related proteins—Implication for the potential catalytic mechanism of enzymatic epoxide hydrolysis. FEBS Lett 338: 251-256.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Arand M., Müller F., Mecky A., Hinz W., Urban P., Pompon D., Kellner R., Oesch F. (1999). Catalytic triad of microsomal epoxide hydrolase: Replacement of Glu404 with Asp leads to a strongly increased turnover rate. Biochem J 337: 37-43.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Arand M., Wagner H., Oesch F. (1996). Asp333, Asp495, and His 523 form the catalytic triad of rat soluble epoxide hydrolase. J Biol Chem 271: 4223-4229.[Abstract/Free Full Text]
- Doehmer J., Dogra S., Friedberg T., Monier S., Adesnik M., Glatt HR, Oesch F. (1988). Stable expression of rat cytochrome P-450IIB 1 cDNA in Chinese hamster cells (V79) and the metabolic activation of aflatoxin B1. Proc Natl Acad Sci USA 85: 5769-5773.[Abstract/Free Full Text]
- Hammock BD, Pinot F., Beetham JK, Grant DF, Arand ME, Oesch F. (1994). Isolation of a putative hydroxyacyl enzyme intermediate of an epoxide hydrolase. Biochem Biophys Res Commun 198: 850-856.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Herrero ME, Arand M., Hengstler JG, Oesch F. (1997). Recombinant expression of human microsomal epoxide hydrolase protects V79 Chinese hamster cells from styrene oxide—But not from ethylene oxide-induced DNA strand breaks. Environ Mol Mutagen 30: 429-439.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Lacourciere GM, Armstrong RN (1993). The catalytic mechanism of microsomal epoxide hydrolase involves an ester intermediate. J Am Chem Soc 115: 10466-10467.[CrossRef][Web of Science]
- Lacourciere GM, Armstrong RN (1994). Microsomal and soluble epoxide hydrolases are members of the same family of C-X bond hydrolase enzymes. Chem Res Toxicol 7: 121-124.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Laughlin LT, Tzeng H-F. Lin S., Armstrong RN (1998). Mechanism of microsomal epoxide hydrolase. Semifunctional site-specific mutants affecting the alkylation half-reaction. Biochemistry 37: 2897-2904.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Müller F., Arand M., Frank H., Seidel A., Hinz W., Winkler L., Hänel K., Blee E., Beetham JK, Hammock BD, Oesch F. (1997). Visualization of a covalent intermediate between microsomal epoxide hydrolase, but not cholesterol epoxide hydrolase, and their substrates. Eur J Biochem 245: 490-496.[Web of Science][Medline]
[Order article via Infotrieve]
- N.N. (1991). Guidelines for the evaluation of chemicals for carcinogenicity. Rep Health Soc Subj (Lond) 42: 1-80.
- N.N. (1994). Risk assessment of carcinogenic chemicals in the Netherlands. Regul Toxicol Pharmacol 19: 14-30.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Oesch F. (1973). Mammalian epoxide hydrases: Inducible enzymes catalysing the inactivation of carcinogenic and cytotoxic metabolites derived from aromatic and olefinic compounds. Xenobiotica 3: 305-340.[Web of Science][Medline]
[Order article via Infotrieve]
- Ollis DL, Cheah E., Cygler M., Dijkstra B., Frolow F., Franken SM, Harel M., Remington SJ, Silman I., Schrag J., Sussman JL, Verschueren Khg, Goldman A. (1992). The /β hydrolase fold. Protein Eng 5: 197-211.[Abstract/Free Full Text]
- Purchase IF, Auton TR (1995). Thresholds in chemical carcinogenesis. Regul Toxicol Pharmacol 22: 199-205.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Thomas H., Timms CW, Oesch F. (1990). Epoxide hydrolases: Molecular properties, induction, polymorphisms and function. In: Frontiers in Biotransformation, Vol 2, Ruckpaul K, Rein H (eds). Akademie-Verlag, Berlin, pp 278-337.
- Tzeng H-F., Laughlin LT, Armstrong RN (1998). Semifunctional site-specific mutants affecting the hydrolytic half-reaction of microsomal epoxide hydrolase. Biochemistry 37: 2905-2911.[CrossRef][Web of Science][Medline]
[Order article via Infotrieve]
- Tzeng H-F., Laughlin LT, Lin S., Armstrong RN (1996). The catalytic mechanism of microsomal epoxide hydrolase involves reversible formation and rate-limiting hydrolysis of the alkyl-enzyme intermediate. J Am Chem Soc 118: 9436-9437.[CrossRef][Web of Science]
Toxicologic Pathology, Vol. 28, No. 3,
382-387 (2000)
DOI: 10.1177/019262330002800305
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