Effect of Glycyrrhiza glabra roots and glycyrrhizin on the glucuronidation in rats.

As an approach to elucidate the possible in vivo interaction of synthetic drugs and herbs which are frequently used in combination in Asia, the effect of Glycyrrhiza glabra on the metabolism of acetaminophen (AAP) was examined in male Sprague-Dawley rats. The pretreatment of the methanol extract of Glycyrrhiza glabra roots (Glycyrrhizae Radix, GR, 1 g/kg, p.o.) for 6 days significantly increased the cumulative biliary (156%) and urinary (132%) excretions of AAP-glucuronide conjugate within 120 min after the administration of AAP (150 mg/kg, i.v.) without affecting thioether and sulfate conjugates. These findings suggest that GR might enhance the glucuronidation pathway of AAP. In order to study the effect of GR on the glucuronidation in rat liver, we examined enzymatic activity of p-nitrophenol UDP-glucuronosyltransferase (UGT), which is also called UGT1A, and intracellular concentrations of hepatic UDP-glucuronic acid, upon the administration of GR (1 g/kg, p.o.) or glycyrrhizin (23 mg/kg, p.o.), a major component of GR, for 6 days. GR and glycyrrhizin caused increases in specific activities of UGT1A by 111% and 96%, respectively. Concentration of UDP-glucuronic acid was increased 257% by GR and 484% by glycyrrhizin. These data indicate that GR and glycyrrhizin activated glucuronidation and thus suggest the possibility that GR may influence detoxification of xenobiotics in rat liver. Using the p-nitrophenol UGT1A1 cDNA as a probe, we found that the activation of UGT1A by GR was not due to the induction of mRNAs for the enzyme.