PLASMA METABOLITES OF QUERCETIN AND THEIR ANTIOXIDANT PROPERTIES

Quercetin is one of the most widely distributed flavonoids present in fruits and vegetables. The present experiments were performed on rats adapted for 3 wk to a semipurified diet supplemented with 0.2% quercet in. The major part of the circulating metabolites of quercetin (91.5%) are glucurono-sulfo conjugates of isorhamnetin (3'-O-methyl quercetin ; 89.1 +/- 2.1 mu M) and of quercetin (14.7 +/- 1.7 mu M); the minor p art (8.5%) is constituted by glucuronides of quercetin and its methoxy lated forms (9.6 +/- 2.3 mu M). Conjugated dienes formation, resulting from Cu2+-catalyzed oxidation of rat very low density lipoproteins low density lipoproteins (LDL), was effectively inhibited in vitro by conjugated metabolites of quercetin. These metabolites appeared to be four times more potent than trolox in inhibiting LDL oxidation. Moreov er, the plasma from rats adapted to a diet containing 0.2% quercetin e xhibited a total antioxidant status markedly higher than that of contr ol rats (+ 60%). This study shows that ubiquitous quercetin is conjuga ted in vivo, yielding metabolites that exhibit antioxidant properties. Thus the health benefits of flavonoids in foods can be due to the ant ioxidant properties of their metabolites.