Catechol metabolites of zeranol and 17β-estradiol: A comparative in vitro study on the induction of oxidative DNA damage and methylation by catechol-O-methyltransferase

alpha-Zearalanol (alpha-ZAL, zeranol) is a highly estrogenic macrocyclic beta-resorcylic acid lactone, which is used as a growth promotor for cattle in various countries. We have recently reported that alpha-ZAL and its major metabolite zearalanone (ZAN) are hydroxylated at the aromatic ring by microsomes from human liver in vitro, thereby forming two catechol metabolites each. Thus, the oxidative metabolism of alpha-ZAL and ZAN resembles that of the endogenous steroidal estrogens 17 beta-estradiol (E2) and estrone (E1), which also give rise to two catechols each. As these catechol metabolites are believed to mediate the carcinogenicity of E2 and E1 by causing oxidative DNA damage and DNA adducts, their methylation by catechol-O-methyltransferase (COMT) is an important inactivation pathway. Here we report that hepatic microsomes from five species generate catechol metabolites of alpha-ZAL and ZAN, the highest amounts being formed by human liver microsomes, followed by rat, mouse, steer and swine. The microsomal extracts and the individual catechols of alpha-ZAL, ZAN, E2 and E1 were found to induce oxidative DNA damage, as measured by the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine in a cell-free system. The ranking of pro-oxidant activity was 15-HO-ZAN>15-HO-alpha-ZAL approximate to 4-HO-E2/E1 approximate to 2-HO-E2/E1 > 13-HO-ZAN > 13-HO-alpha-ZAL With respect to the rate of methylation by human hepatic COMT, the ranking was 2-HO-E2/E1 4-HO-E2/E1 > 15-HO-alpha-ZAL/ZAN 13-HO-alpha-ZAL/ZAN. Thus, some catechol metabolites of alpha-ZAL and ZAN are better pro-oxidants and poorer substrates of COMT than the catechols of E2 and E1. These findings warrant further investigations into the genotoxic potential of alpha-ZAL, which may constitute another biological activity in addition to its well-known estrogenicity. (C) 2012 Elsevier Ireland Ltd. All rights reserved.