Daidzein, one of the major soy isoflavones, has a number of beneficial bioactivities for human health. It is mainly metabolized into 7- and/or 4'-glucuronides by UDP-glucuronosyltransferase (UGT) enzymes in mammals, including humans. The present study was conducted to examine the regioselective glucuronidation of daidzein at the 7- and 4'-hydroxyl groups in the liver and intestinal microsomes of humans, monkeys, rats, and mice. Daidzein glucuronidation activities at substrate concentrations of 1.0-200 µM were assessed, and Eadie-Hofstee plots were constructed. The kinetics for 7- and 4'-glucuronidation in the liver microsomes fit the Michaelis-Menten model, except for an atypical model for 7-glucuronidation in rats and a biphasic model for 4'-glucuronidation in monkeys. These kinetics in the intestinal microsomes followed the Michaelis-Menten model, except for a biphasic model for 7-glucuronidation in mice. The CLint values for 7-glucuronidation were in the order of monkeys (49) ≫ rats (5.3) > humans (1.0) > mice (0.7) for liver microsomes, and rats (2.4) ≥ monkeys (2.2) > humans (1.0) ≥ mice (0.8) for intestinal microsomes. On the other hand, the CLint values for 4'-glucuronidation were in the order of monkeys (4.0) > mice (1.0) ≈ humans (1.0) > rats (0.4) for liver microsomes, and humans (1.0) ≫ monkeys (0.08) ≥ mice (0.07) > rats (0.05) for intestinal microsomes. These results demonstrated that the metabolic abilities of UGT enzymes toward daidzein in the liver and intestines markedly differed among humans, monkeys, rats, and mice, and suggest that species and regioselective differences are closely associated with the bioactivities of soy isoflavones.
Keywords: Daidzein; Glucuronidation; Intestinal microsomes; Liver microsomes; UDP-glucuronosyltransferase (UGT).