QUERCETIN IS RECOVERED IN HUMAN PLASMA AS CONJUGATED DERIVATIVES WHICH RETAIN ANTIOXIDANT PROPERTIES

Quercetin is one of the most abundant flavonoids in the human diet. Th is study aimed to determine the plasma concentrations of quercetin in 10 healthy volunteers after the consumption of a complex meal rich in plant products. Quercetin was determined in plasma (2 h before, and 3, 7 and 20 h after the meal), and in a duplicated portion of the meal b y HPLC analysis with an electrochemical detection. The amount of inges ted quercetin,vas estimated to be 87 mg, Before the meal, quercetin co ncentration in hydrolyzed plasmas ranged from 28 to 142 nM. A marked i ncrease was observed 3 h after the meal in all subjects, with a mean c oncentration of 373 nM (S.E.M. = 61), After 7 h, quercetin concentrati on in hydrolyzed plasmas decreased and after 20 h basal levels mere fo und again. The antioxidant capacities of quercetin, 3'-O-methylquercet in, and of some of their conjugated derivatives were compared by the m easurement of the conjugated dienes resulting from the Cu2+-induced ox idation of human LDL, 3'-O-Methylquercetin and conjugated derivatives of quercetin significantly prolonged the lag phase, but the magnitude of their effect was about half that of the aglycone. (C) 1998 Federati on of European Biochemical Societies.