Identification and characterization of human UDP-glucuronosyltransferases responsible for xanthotoxol glucuronidation

Abstract

1. Xanthotoxol is a furanocoumarin that possesses many pharmacological activities and in this study its in vitro glucuronidation was studied.

2. Xanthotoxol can be rapidly metabolized to a mono-glucuronide in both human intestine microsomes (HIM) and human liver microsomes (HLM); the structure of the metabolite was confirmed by NMR spectroscopy.

3. Reaction phenotyping with 12 commercial recombinant human UGTs, as well as with the Helsinki laboratory UGT1A10 that carry a C-terminal His-tag (UGT1A10-H), revealed that UGT1A10-H catalyzes xanthotoxol glucuronidation at the highest rate, followed by UGT1A8. The other enzymes, namely UGT1A3, UGT1A1, UGT1A6, UGT1A10 (commercial), and UGT2B7 displayed moderate-to-low reaction rates.

4. In kinetic analyses, HIM exhibited much higher affinity for xanthotoxol, along with high V_max and mild substrate inhibition, whereas the kinetics in HLM was biphasic. UGT1A1 (high K_m value), UGT1A10-H (low K_m value), and UGT1A8 exhibited mild substrate inhibition.

5. Considering the above findings and the current knowledge on UGTs expression in HIM, it is likely that UGT1A10 is mainly responsible for xanthotoxol glucuronidation in the human small intestine, with some contribution from UGT1A1. In the liver, this reaction is mainly catalyzed by UGT1A1 and UGT2B7.

6. Glucuronidation appears to be the major metabolic pathway of xanthotoxol in human.

Keywords:

• Intestinal glucuronidation
• UDP-glucuronosyltransferases
• xanthotoxol

Acknowledgements

This work was supported by the National Basic Research Program of China [2013CB531805], Nature Science Foundation of China [81273590, 81473181, 81573501, 81503152, and 81403003], and in Finland by the Sigrid Juselius Foundation [No. 4704583] and the Magnus Ehrnroth Foundation [4704555].

Declaration of interest

The authors state no conflicts of interest.