The inhibitory effects of nor-oleanane triterpenoid saponins from Stauntonia brachyanthera towards UDP-glucuronosyltransferases

The inhibition of UDP-glucuronosyltransferases (UGTs) by herbal components might be an important reason for clinical herb-drug interaction (HDI). The inhibitory effects on UGTs via nor-oleanane triterpenoid saponins, which were the bioactive ingredients from Stauntonia brachyanthera, a traditional Chinese folk medicines with highly biological values, were evaluated comprehensively with recombinant UGT isoforms as enzyme source and a nonspecific substrate 4-methylumbelliferone (4-MU) as substrate. The results showed that there are seven compounds, 2, 3, 4, 8, 9, 13 and 14, respectively, exhibited potential inhibitions towards UGT1A1, UGT1A3 and UGT1A10 among all 23 compounds isolated from the plants. The IC50 values were 17.1 μM, 13.5 μM, 9.5 μM, 15.7 μM, 16.3 μM, 1.1 μM, and 0.3 μM, respectively. Data fitting using Dixon and Lineweaver-Burk plots demonstrated that the inhibition of UGT1A10, UGT1A1 and UGT1A3 was best fit to noncompetitive type and competitive, respectively. The inhibition kinetic parameter (Ki) was calculated to be 39 μM, 17 μM, 3.3 μM, 10 μM, 9.3 μM, 0.19 μM, and 0.016 μM, respectively. All these experimental data suggested that HDI might occur when compounds containing herbs were co-administered with drugs which mainly undergo UGTs-mediated metabolism.

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