Abstract
The Fusarium toxin zearalenone (ZEN) is a frequent contaminant of corn and other grains world‐wide and known for its high estrogenic activity. In a NTP carcinogenicity study, ZEN was found to increase the incidence of hepatocellular adenomas in female mice and of pituitary adenomas in both female and male mice. The mechanism of ZEN tumorigenicity is unknown to date and may be related to the hormonal activity. However, the recent disclosure of a novel pathway in the metabolism of ZEN opens up another possibility: ZEN was found to readily undergo aromatic hydroxylation when incubated with hepatic microsomes from humans and various other mammalian species, and two catechol metabolites were unequivocally identified in our laboratory (Pfeiffer et al., Mol. Nutr. Food Res. 53, 1123–1133, 2009). Thus, the phase I metabolism of ZEN resembles that of the steroidal estrogens 17β‐estradiol (E2) and estrone (E1), which also form catechol metabolites. There is increasing evidence that these catechol metabolites contribute to the well‐established carcinogenicity of E2 and E1 by acting as genotoxins after further activation to quinones (Cavalieri and Rogan, Ann. N. Y. Acad. Sci. 1089, 286–301, 2006; Bolton and Thatcher, Chem. Rev. Toxicol. 21, 93–101, 2008). A similar mechanism is conceivable for the carcinogenicity of ZEN. In order to obtain evidence for the formation of reactive quinone metabolites from ZEN, it was the aim of the present study to trap such quinones as mercapturic acids in microsomal incubations fortified with N‐acetylcysteine (NAC).
Incubations (0.4 mL of 0.1 M phosphate buffer pH 7.4) containing human hepatic microsomes (0.4 mg protein), ZEN (0.1 mM) and a NADPH‐generating system were conducted for 10 min at 37°C for the formation of catechol metabolites. 0.1 mL of a 40 mM NAC solution in water was then added and incubation continued for another 30 min. Thereafter, 0.5 mL of 0.7 M glycine/HCl buffer pH 1.2 was added and the NAC adducts were extracted with 2 × 0.5 mL ethyl acetate. Analysis of the extract by LC‐MS/MS revealed the presence of at least two NAC adducts, which were identified as the N‐acetylcysteinyl derivatives of one of the catechol metabolites of ZEN and of its microsomal reductive metabolite alpha‐zearalenol by comparison with authentic synthetic standards.
This is the first report to show that reactive quinone metabolites of ZEN are formed in vitro. The toxicological sequelae of this pathway and its in vivo relevance should now be studied.
Citation Information: Cancer Prev Res 2010;3(1 Suppl):A38.
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