Identification of quercetin 3-O-beta-D-glucuronide as an antioxidative metabolite in rat plasma after oral administration of quercetin

The potential beneficial effect of dietary quercetin (3,3',4',5,7-pentahydr oxyflavone) has attracted much attention in relation to the prevention of c ardiovascular disease. It is generally recognized that dietary quercetin is subject to metabolic conversion resulting in conjugated forms during absor ption and circulation. However, no quercetin conjugates have yet been ident ified from biological fluids or tissues. In the present study, we isolated and characterized two quercetin conjugates from the plasma of quercetin-adm inistered rats. The blood plasma was collected from 26 rats 30 min after or al administration of quercetin (250 mg/kg body weight), concentrated, disso lved in 2% acetic acid aqueous solution (pH 2.65), and extracted with ethyl acetate. Two compounds (P2, P3) were obtained from the extract by repeated reversed-phase HPLC. On the other hand, two quercetin glucuronides were sy nthesized chemically and identified as quercetin 3-O-beta -D-glucuronide (Q 3GA) and quercetin 4'-O-beta -D-glucuronide (Q4'GA). as determined from FAB MS, H-1- and C-13-NMR, and HMBC data. The retention times of P2 and P3 in t he HPLC chromatogram corresponded to those of Q3GA and Q4'GA, respectively. FABMS data demonstrated that P2 and P3 are quercetin monoglucuronides. H-1 -NMR data for P2 were completely in agreement with those for Q3GA. P2 was t herefore identified as Q3GA. This is, to our knowledge, the first evidence that Q3GA accumulates in vivo after oral administration of quercetin. Q3GA is likely to act as an effective antioxidant in blood plasma low-density li poprotein, because this conjugated metabolite was found to possess a substa ntial antioxidant effect on copper ion-induced oxidation of human plasma lo w-density lipoprotein as well as 1,1-diphenyl-2-picrylhydrazyl radical-scav enging activity. (C) 2001 Elsevier Science Inc.