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

Abstract

Glucuronidation constitutes an important pathway in the phase II metabolism of the mycotoxin zearalenone (ZEN) and the growth promotor α-zearalanol (α-ZAL, zeranol), but the enzymology of their formation is yet unknown. In the present study, ZEN, α-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, α-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.

Invited review: Diagnosis of zearalenone (ZEN) exposure of farm animals and transfer of its residues into edible tissues (carry over)

“…RALs are obviously preferentially conjugated with glucuronic acid by liver microsomes at C-14 rather than C-16 which is supposedly due to the strong hydrogen bonding of the hydroxyl group at C-16 with the carbonyl group at C-1 (Pfeiffer et al, 2010a). Glucuronidation at the aliphatic C-7 was only of minor importance and did not occur in ZEN and ZAN (no OH-group at C-7).…”

“…A comparison of the activities of hepatic UDP-glucuronosyltransferases of various species as measured in vitro by using liver microsomes would suggest marked differences (Pfeiffer et al, 2010a). The porcine liver microsomes were characterized by the highest UDPglucuronosyltransferase activity while those of cattle varied between approximately 30 and 50%, and of humans between 10 and 20% relative to the porcine enzyme activity.…”

“…Glucuronidation at the aliphatic C-7 was only of minor importance and did not occur in ZEN and ZAN (no OH-group at C-7). Moreover, large species differences were not identified (Pfeiffer et al, 2010a).…”

Biotransformation of zearalenone and zearalenols to their major glucuronide metabolites reduces estrogenic activity

“…It has been demonstrated in vitro using steer, pig, rat and human hepatic microsomes and human intestinal microsomes that ZEN and its metabolites are glucuronidated in humans and animals in the intestine and liver, preferably at the sterically unhindered 14-hyroxyl group resulting in e.g. ZEN-14-O-glucuronide (Pfeiffer et al, 2010). Human microsomes presented lower activity than those of other species and the activity in the liver was higher than that of the intestine.…”

“…The aromatic glucuronides carrying the glucuronic acid moiety at C-14 or C-16 are more polar than the aliphatic glucuronide carrying the glucuronic acid moiety at C-7 (Pfeiffer et al, 2010). It has been demonstrated in vitro using steer, pig, rat and human hepatic microsomes and human intestinal microsomes that ZEN and its metabolites are glucuronidated in humans and animals in the intestine and liver, preferably at the sterically unhindered 14-hyroxyl group resulting in e.g.…”