Received for publication April 15, 2008.
Revised July 22, 2008.
Accepted for publication July 23, 2008.

Biotransformation of the chemopreventive agent isoliquiritigenin by UDP-glucuronosyltransferases

Abstract

Isoliquiritigenin (2’,4’,4-trihydroxychalcone), a chalcone found in licorice root and shallots, exhibits antioxidant, estrogenic, and anti-tumor 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 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 LC-MS. UGT1A1 and UGT1A9 were the major enzymes responsible for the formation of the most abundant conjugate, isoliquiritigenin 4'-O-glucuronide (MG5) with K_m values of 4.30 ± 0.47 µM and 3.15 ± 0.24 µM, 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 µM and 25.8 ± 1.3 µM, respectively. In addition, the 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 human liver microsomes, 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.

Key words: glucuronidation, hepatocytes, liver microsomes, metabolite identification, metabolite kinetics, microsomes, UDP glucuronyltransferases