Glucuronidation of 1′-hydroxyestragole (1′-HE) by human UDP-glucuronosyltransferases UGT2B7 and UGT1A9

Abstract

Estragole (4-allyl-1-methoxybenzene) is a naturally occurring food flavoring agent found in basil, fennel, bay leaves, and other spices. Estragole and its metabolite, 1′-hydroxyestragole (1′-HE) are hepatocarcinogens in rodent models. Recent studies from our laboratory have shown that glucuronidation of 1′-HE is a major detoxification pathway for estragole and 1′-HE, accounting for as much as 30% of urinary metabolites of estragole in rodents. Therefore, this study was designed to investigate the glucuronidation of 1′-HE in human liver microsomes in vitro and identify the specific uridine diphosphate glucuronosyltransferase (UGT) isoforms responsible for 1′-HE glucuronidation. The formation of the glucuronide of 1′-HE (1′-HEG) followed atypical kinetics, and the data best fit to a Hill equation, resulting in apparent kinetic parameters of Km = 1.45 mM, Vmax = 164.5 pmoles/min/mg protein, and n = 1.4. There was a significant intersubject variation in 1′-HE glucuronidation in 27 human liver samples, with a CV of 42%. A screen of cDNA expressed UGT isoforms indicated that UGT2B7 (83.94 ± 0.188 pmols/min/mg), UGT1A9 (51.36 ± 0.72 pmoles/min/mg , and UGT2B15 (8.18 ± 0.037 pmoles/min/mg) were responsible for 1′-HEG formation. Glucuronidation of 1′-HE was not detected in cells expressing UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, and UGT1A10. 1′-HE glucuronidation in 27 individual human liver samples significantly (p < 0.05) correlated with the glucuronidation of other UGT2B7 substrates (morphine and ibuprofen). These results imply that concomitant chronic intake of therapeutic drugs and dietary components that are UGT2B7 and/or UGT1A9 substrates may interfere with estragole metabolism. Our results also have toxicogenetic significance, as UGT2B7 is polymorphic and could potentially result in genetic differences in glucuronidation of 1′-HE and, hence, toxicity of estragole.