UGT1A10 Is a High Activity and Important Extrahepatic Enzyme: Why Has Its Role in Intestinal Glucuronidation Been Frequently Underestimated?

The aim of this work was to highlight a considerable and broad problem in UGT1A10 activity assessment that has led to underestimation of its role in intestinal glucuronidation of drugs and other xenobiotics. The reason appears to be poor activity of the commercial UGT1A10 that is used by many laboratories, and here we have tested it by comparison with our recombinant His-tagged UGT1A10 (designated as UGT1A10-H), both expressed in insect cells. The glucuronidation rates of morphine, estradiol, estrone, SN-38, diclofenac, 4-methylumbelliferone, 7-amino-4-methylcoumarin, N-(3-carboxypropyl)-4-hydroxy-1,8-naphthalimide, and bavachinin were assayed. The results revealed that the activity of commercial UGT1A10 was low, very low, and in the cases of morphine, estrone, 7-methyl-4-aminocoumarin, and bavachinin it was below the detection limit. On the other hand, under the same conditions, UGT1A10-H exhibited high glucuronidation rates toward all these compounds. Moreover, using estradiol, morphine, and estrone, in the presence and absence of suitable inhibitors, nilotinib or atractylenolide I, it was demonstrated that UGT1A10-H, but not the commercial UGT1A10, provides a good tool to study the role of native UGT1A10 in the human intestine. The results also suggest that much of the data in the literature on UGT1A10 activity may have to be re-evaluated.

Identification and characterization of naturally occurring inhibitors against UDP-glucuronosyltransferase 1A1 in Fructus Psoraleae (Bu-gu-zhi).

As an edible traditional Chinese herb, Fructus psoraleae (FP) has been widely used in Asia for the treatment of vitiligo, bone fracture and osteoporosis. Several cases on markedly elevated bilirubin and acute liver injury following administration of FP and its related proprietary medicine have been reported, but the mechanism in FP-associated toxicity has not been well investigated yet. This study aimed to investigate the inhibitory effects of FP extract and its major constituents against human UDP-glucuronosyltransferase 1A1 (UGT1A1), the key enzyme responsible for metabolic elimination of bilirubin. To this end, N-(3-carboxy propyl)-4-hydroxy-1,8-naphthalimide (NCHN), a newly developed specific fluorescent probe for UGT1A1, was used to evaluate the inhibitory effects of FP extract or its fractions in human liver microsomes (HLM), while LC-UV fingerprint and UGT1A1 inhibition profile were combined to identity and characterize the naturally occurring inhibitors of UGT1A1 in FP. Our results demonstrated that both the extract of FP and five major components of FP displayed evident inhibitory effects on UGT1A1 in HLM. Among these five identified naturally occurring inhibitors, bavachin and corylifol A were found to be strong inhibitors of UGT1A1 with the inhibition kinetic parameters (Ki) values lower than 1 µM, while neobavaisoflavone, isobavachalcone, and bavachinin displayed moderate inhibitory effects against UGT1A1 in HLM, with the Ki values ranging from 1.61 to 9.86µM. These findings suggested that FP contains natural compounds with potent inhibitory effects against human UGT1A1, which may be one of the important reasons for triggering FP-associated toxicity, including elevated bilirubin levels and liver injury.