DIETARY QUERCETIN GLYCOSIDES - ANTIOXIDANT ACTIVITY AND INDUCTION OF THE ANTICARCINOGENIC PHASE-II MARKER ENZYME QUINONE REDUCTASE IN HEPALCLC7 CELLS

It has recently been shown by Hollman et al. (Am. J. Clin, Nutr., 62, 1276-1282) that flavonoid glycosides are preferentially absorbed from dietary onions compared to the flavonoid aglycone, In the light of thi s, we have compared the bioactivities of the two most abundant flavono id glycosides that we have purified from onions (quercetin-3,4'-digluc oside and quercetin-4'-glucoside) to the quercetin aglycone, and also to the more commonly studied commercially-available flavonoid glycosid es, rutin (quercetin-3-rutinoside) and isoquercitrin (quercetin-3-gluc oside). Quercetin aglycone was the most effective inducer of the antic arcinogenic phase II marker enzyme, quinone reductase (QR), in mouse H epalclc7 cells. Of the glycosides, only quercetin-4'-glucoside was abl e to induce QR activity in this assay. Inhibition of NADPH/iron- and a scorbate/iron-induced lipid peroxidation of human liver microsomes, an d the Trolox C-equivalent antioxidant capacity (TEAC), were also measu red, The 4'-glycosylation dramatically decreased activity in the 'anti oxidant' assays, whereas 3-substitutions produced much smaller changes . These results show that the preferentially-absorbed quercetin glycos ides in onions have markedly different biological properties compared with the aglycone.