Biotransformation of the chemopreventive agent 2′,4′,4-trihydroxychalcone (isoliquiritigenin) by UDP-glucuronosyltransferases

2',4',4-Trihydroxychalcone (isoliquiritigenin), a chalcone found in licorice root and shallots, exhibits antioxidant, estrogenic, and antitumor activities. To complement our previous studies concerning the phase 1 metabolism of isoliquiritigenin, the phase 2 transformation of isoliquiritigenin by human hepatocytes and pooled human liver microsomes (HLMs) was investigated using liquid chromatography/tandem mass spectrometry and UV absorbance. Five glucuronides were detected corresponding to monoglucuronides of isoliquiritigenin and liquiritigenin, but no sulfate conjugates were observed. The UDP-glucuronosyltransferases (UGTs) involved in the formation of the major glucuronide conjugates were identified using recombinant human UGTs in combination with liquid chromatography/mass spectrometry. UGT1A1 and UGT1A9 were the major enzymes responsible for the formation of the most abundant conjugate, isoliquiritigenin 4'-O-glucuronide (MG5), with Km values of 4.30 +/- 0.47 and 3.15 +/- 0.24 mu M, respectively. UGT1A1 and UGT1A10 converted isoliquiritigenin to the next most abundant phase 2 metabolite, isoliquiritigenin 2'-O-glucuronide (MG4), with K-m values of 2.98 +/- 0.8 and 25.8 +/- 1.3 mu M, respectively. In addition, isoliquiritigenin glucuronides MG4 and MG5 were formed by pooled human intestine and kidney microsomes, respectively. Based on the in vitro determination of a 25.3-min half-life for isoliquiritigenin when incubated with HLMs, the intrinsic clearance of isoliquiritigenin was estimated to be 36.4 ml/min/kg. These studies indicate that isoliquiritigenin will be conjugated rapidly in the liver to form up to five monoglucuronides.