Glucuronidation of zearalenone, zeranol and four metabolites in vitro: Formation of glucuronides by various microsomes and human UDP-glucuronosyltransferase isoforms

Glucuronidation constitutes an important pathway in the phase II metabolism of the mycotoxin zearalenone (ZEN) and the growth promotor alpha-zearalanol (alpha-ZAL, zeranol), but the enzymology of their formation is yet unknown. In the present study, ZEN, alpha-ZAL and four of their major phase I metabolites were glucuronidated in vitro using hepatic microsomes from steer, pig, rat and human, intestinal microsomes from humans, and eleven recombinant human UDP-glucuronosyltransferases (UGTs). After assigning chemical structures to the various glucuronides by using previously published information, the enzymatic activities of the various microsomes and UGT isoforms were determined together with the patterns of glucuronides generated. All six compounds were good substrates for all microsomes studied. With very few exceptions, glucuronidation occurred preferentially at the sterically unhindered phenolic 14-hydroxyl group. UGT1A1, 1A3 and 1A8 had the highest activities and gave rise to the phenolic glucuronide, whereas glucuronidation of the aliphatic hydroxyl group was mostly mediated by UGT2B7 with low activity. Based on these in vitro data, ZEN, alpha-ZAL and their metabolites must be expected to be readily glucuronidated both in the liver and intestine as well as in other extrahepatic organs of humans and various animal species.