ABSTRACT:The dietary polyphenol resveratrol (RES) exists as cis-and transisomers with known stereospecific and stereoselective glucuronidation at the 3 and 4 positions by distinct UGT1A isoforms. We examined cis-RES glucuronidation in various protein sources. UGT1A6 or UGT1A1 genotype-dependent cis-or trans-RES glucuronidation, respectively, was further determined. cis-RES exhibited partial substrate inhibition in UGT1A6 Supersomes and human embryonic kidney 293 cells overexpressing genetically variant UGT1A6 alleles. Cells expressing UGT1A6*4 had the highest activity with a V max of 612 ؎ 27.36 nmol/min/mg, followed by UGT1A6*3. The *2 allozyme had a higher V max (1.6-fold) and K m (1.9-fold) than *1. In 51 human liver samples genotyped for UGT1A6, four alleles (frequencies) were identified as *1 (0.58), *2 (0.36), *3 (0.01), and *4 (0.05), leading to assignment of the following genotypes (frequencies): *1/*1 (0.29), *1/*2 (0.45), *1/*3 (0.02), *1/*4 (0.10), and *2/*2 (0.14). Up to 5-fold variability in trans-RES glucuronidation was observed in individual liver samples. In livers stratified by UGT1A6 genotype, a significant difference in cis-RES glucuronidation activity (p < 0.05) was seen between the *2 variants compared with homozygous *1 livers. The trans-RES glucuronidation was quantitated in a human liver bank genotyped for the UGT1A1 TATA box repeat polymorphism. There was no significant difference for formation of trans-RES 3-O-glucuronide. We were surprised to find that trans-RES 4-O-glucuronide formation was higher in livers with the 7/7 genotype compared with 6/6 and 6/7 (p < 0.05). In conclusion, cis-RES glucuronidation exhibited atypical partial substrate inhibition kinetics in vitro. Whereas cis-RES glucuronidation varied with UGT1A6 genotypes, the UGT1A1*28 polymorphism did not explain variability in trans-RES glucuronidation.Resveratrol (3,4Ј,5-trihydroxystilbene, RES) is a polyphenol of plant origin found in grapes, peanuts, and red wine as the cis-and trans-isomers. RES is known to be both glucuronidated and sulfated. RES glucuronidation in humans is both regioselective and stereospecific (Aumont et al., 2001) and is mediated by members of the UDP glucuronosyltransferase (UGT) superfamily of enzymes. Several UGT isoforms exist, and their distinct but sometimes overlapping substrate specificity has been shown (Tukey and Strassburg, 2000). Their organ-and tissue-specific expression and variant polymorphic nature have also been documented.The major UGT isoforms involved in RES metabolism are UGT1A1, UGT1A9, and UGT1A6 with minor contributions by UGT1A10, UGT1A7, UGT1A8, and UGT1A3. UGT1A1 catalyzes the glucuronidation of trans-RES preferentially at the 3 position, yielding the major 3-O-glucuronide (R3G) metabolite, and less so at the 4Ј position to give the minor metabolite [trans-RES 4Ј-O-glucuronide (R4ЈG)]. UGT1A6 selectively glucuronidates the cis-isomer of RES at the 3 position to give the major metabolite [cis-RES 3-O-glucuronide (cis-R3G)], whereas UGT1A9 has been shown to glucuronidate bo...