Aromatic hydroxylation and catechol formation: A novel metabolic pathway of the growth promotor zeranol

Abstract

α-Zearalanol (α-ZAL, zeranol) is a macrocyclic resorcylic acid lactone, which is highly estrogenic and used as a growth promotor for cattle in various countries. Little is known about the phase I metabolism of α-ZAL. We now report that α-ZAL and its major metabolite zearalanone (ZAN) are extensively monohydroxylated at the aromatic ring by microsomes from human liver in vitro. This novel pathway leads to catechols, the chemical structures of which were unambiguously established by the use of deuterium-labeled α-ZAL and ZAN, and by the synthesis of authentic standards. The aromatic hydroxylation of α-ZAL is almost exclusively mediated by the human cytochrome P450 (hCYP) 1A2 isoform. The catechol metabolites of α-ZAL and ZAN are unstable and readily oxidized to quinones, which could be detected among the metabolites of α-ZAL and ZAN generated by human hepatic microsomes and hCYP1A2. Furthermore, the quinone metabolites are able to form covalent adducts with N-acetylcysteine (NAC), as several of such adducts were found in microsomal incubations fortified with NAC. Aromatic hydroxylation of α-ZAL was also observed with bovine, porcine and rat hepatic microsomes. Further studies are needed to demonstrate the catechol pathway of α-ZAL in vivo and to assess its toxicological significance.