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Aristolochic acids (AAs) are naturally occurring nitro phenanthrene carboxylic acids primarily found in plants of the Aristolochiaceae family. Aristolochic acid D (AAD) is a major constituent in the roots and rhizomes of the Chinese herb Xixin (the roots and rhizomes of Asarum heterotropoides F. Schmidt), which is a key material for preparing a suite of marketed Chinese medicines. Structurally, AAD is nearly identical to the nephrotoxic aristolochic acid I (AAI), with an additional phenolic group at the C-6 site. Although the nephrotoxicity and metabolic pathways of AAI have been well-investigated, the metabolic pathway(s) of AAD in humans and the influence of AAD metabolism on its nephrotoxicity has not been investigated yet. To identify the major metabolites of AAD in human tissues and to characterize AAD O -glucuronidation kinetics in different enzyme sources, as well as to explore the influence of AAD O -glucuronidation on its nephrotoxicity. The O -glucuronide of AAD was biosynthesized and its chemical structure was fully characterized by both 1H-NMR and 13C-NMR. Reaction phenotyping assays, chemical inhibition assays, and enzyme kinetics analyses were conducted to assess the crucial enzymes involved in AAD O -glucuronidation in humans. Docking simulations were performed to mimic the catalytic conformations of AAD in human UDP-glucuronosyltransferases (UGTs), while the predicted binding energies and distances between the deprotonated C-6 phenolic group of AAD and the glucuronyl moiety of UDPGA in each tested human UGT isoenzyme were measured. The mitochondrial membrane potentials (MMP) and reactive oxygen species (ROS) levels in HK-2 cells treated with either AAI, or AAD, or AAD O -glucuronide were tested, to elucidate the impact of O -glucuronidation on the nephrotoxicity of AAD. AAD could be rapidly metabolized in human liver and intestinal microsomes (HLM and HIM, respectively) to form a mono-glucuronide, which was purified and fully characterized as AAD-6- O -β-D-glucuronide (AADG) by NMR. UGT1A1 was the predominant enzyme responsible for AAD-6- O -glucuronidation, while UGT1A9 contributed to a lesser extent. AAD-6- O -glucuronidation in HLM, HIM, UGT1A1 and UGT1A9 followed Michaelis-Menten kinetics, with the K m values of 4.27 μM, 9.05 μM, 3.87 μM, and 7.00 μM, respectively. Docking simulations suggested that AAD was accessible to the catalytic cavity of UGT1A1 or UGT1A9 and formed catalytic conformations. Further investigations showed that both AAI and AAD could trigger the elevated intracellular ROS levels and induce mitochondrial dysfunction and in HK-2 cells, but AADG was hardly to trigger ROS accumulation and mitochondrial dysfunction. Collectively, UGT1A-catalyzed AAD 6- O -glucuronidation represents a crucial detoxification pathway of this naturally occurring AAI analogs in humans, which is very different from that of AAI. Aristolochic acid D (AAD), an abundant naturally occurring aristolochic acid derivative isolated from the Chinese herb Xixin (the roots and rhizomes of Asarum heterotropoides F. Schmidt), can be rapidly converted into AAD-6- O -β-D-glucuronide (AADG) in human tissue preparations. UGT1A-catalyzed AAD 6- O -glucuronidation offers a crucial detoxification pathway of this aristolochic acid derivative in humans, which is very different from that of the nephrotoxin aristolochic acid I (AAI). [Display omitted] • AAD- O -glucuronidation was a major metabolic pathway of AAD in humans. • The glucuronide was biosynthesized and characterized as AAD-6- O -β-D-glucuronide. • UGT1A1 was the predominant enzyme responsible for AAD-6- O -glucuronidation. • AAD-6- O -glucuronidation offers a crucial detoxification pathway of AAD in humans. [ABSTRACT FROM AUTHOR]

Methylophiopogonanone A (MOA) is an abundant homoisoflavonoid in the Chinese herb Ophiopogonis Radix. Recent investigations revealed that MOA inhibited several human cytochrome P450 enzymes (CYPs) and stimulated OATP1B1. However, the inhibitory effects of MOA on phase II drug‐metabolizing enzymes, such as human UDP‐glucuronosyltransferases (hUGTs), have not been well investigated. Herein, the inhibition potentials of MOA on hUGTs were assessed. The results clearly demonstrated that MOA dose‐dependently inhibited all tested hUGTs including UGT1A1 (IC50 = 1.23 μM), one of the most important detoxification enzymes in humans. Further investigations showed that MOA strongly inhibited UGT1A1‐catalysed NHPH‐O‐glucuronidation in a range of biological settings including hUGT1A1, human liver microsomes (HLM) and HeLa cells overexpressing UGT1A1. Inhibition kinetic analyses demonstrated that MOA competitively inhibited UGT1A1‐catalysed NHPH‐O‐glucuronidation in both hUGT1A1 and HLM, with Ki values of 0.52 and 1.22 μM, respectively. Collectively, our findings expanded knowledge of the interactions between MOA and human drug‐metabolizing enzymes, which would be very helpful for guiding the use of MOA‐related herbal products in clinical settings. [ABSTRACT FROM AUTHOR]