Identification and Characterization of Human UDP-glucuronosyltransferases Responsible for the In Vitro Glucuronidation of Salvianolic Acid A

Glucuronidation is an important pathway in the elimination of salvianolic acid A (Sal A); however the mechanism of UDP-glucuronosyltransferases (UGTs) in this process remains to be investigated. In this study, the kinetics of Sal A glucuronidation by pooled human liver microsomes (HLMs), pooled human intestinal microsomes (HIMs) and 12 recombinant UGT isozymes were investigated. The glucuronidation of Sal A can be shown both in HLMs and HIMs with K-m values of 39.84 +/- 3.76 and 54.04 +/- 4.36 mu M, respectively. Among the 12 human UGTs investigated, UGT1A1 and UGT1A9 were the major isoforms that catalyzed the glucuronidation of Sal A (K-m values of 29.72 +/- 2.20 and 24.40 +/- 2.60 mu M). UGT1A9 showed the highest affinity of Sal A glucuronidation. Furthermore, a significant correlation between Sal A glucuronidation and propofol glucuronidation (a typical UGT1A9 substrate) was observed. The chemical inhibition study showed that the IC50 for phenylbutazone inhibition of Sal A glucuronidation was 50.3 +/- 4.3 and 39.4 +/- 2.9 mu M by HLMs and UGT1A9, respectively. Mefenamic acid inhibited Sal A glucuronidation in UGT1A1 and HLMs with IC50 values of >200 and 12.4 +/- 2.2 mu m, respectively.