Characterization of human UDP-glucuronosyltransferases responsible for glucuronidation and inhibition of norbakuchinic acid, a primary metabolite of hepatotoxicity and nephrotoxicity component bakuchiol in Psoralea corylifolia L.

Norbakuchinic acid (NBKA) is the most abundant metabolite of bakuchiol (a hepatotoxicity and nephrotoxicity component in Psoralea corylifolia L.) in plasma and urine. The present study aimed to identify human UDP-glucuronosyltransferase (UGT) isoforms involved in NBKA glucuronidation, and to assess the inhibition potential of NBKA against recombinant UGTs. NBKA glucuronidation was determined using human liver microsomes (HLM) and human intestine microsomes (HIM) as well as expressed UGTs. The mechanisms of NBKA-UGT interactions were explored through kinetic characterization and modeling. As a result, two glucuronides (G1 and G2) were detected by HLM and HIM. G2 was dominantly produced with CLint values of 3.21 and 0.96 μL min−1 mg−1 in HLM and HIM, respectively. Furthermore, based on the relative activity factor (RAF) approach, UGT1A9 and 2B7 represented 5.98 and 46.16% of G2 in HLM, respectively. In addition, G2 was both significantly correlated with propofol-glucuronidation (r = 0.681, p = 0.015) and AZT-glucuronidation (r = 0.832, p = 0.0008). Also, NBKA glucuronidation showed marked species differences. Moreover, NBKA displayed broad-spectrum inhibition against human UGTs. Kinetic analyses showed that inhibition of these UGTs followed competitive (UGT1A1), mixed (UGT1A3, 1A6, 1A10, 2B7, 2B15 and 2B17) and non-competitive (UGT1A4, 1A7, 1A8 and 1A9) mechanisms, with Ki values ranging from 2.48 to 20.31 μM. Among them, NBKA exhibited moderate inhibitory effects against UGT1A1, 1A3, 1A4, 1A7, 1A8, 2B15 and 2B17 (both IC50 and Ki values lower than 8 μM). In contrast, the effect of NBKA was activating for UGT2B10. Taken altogether, these findings together demonstrate that NBKA has a potent and broad-spectrum inhibitory effect against most human UGTs and thus suggest that much caution should be exercised when high-dose BK is co-administered with UGT substrates.