Comparison of quercetin and dihydroquercetin: Antioxidant-independent actions on erythrocyte and platelet membrane

We investigated the effects of two flavonoids quercetin and dihydroquercetin (DHQ), which have different solubilities and antioxidant capacities, on hemolysis and platelet aggregation in human blood. Exposure of human red blood cells (RBCs) to free radicals generated by 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) for 2 h resulted in 63.5 ± 3.9% hemolysis (vehicle: 0.3 ± 0.4%). Pre-incubation of RBCs with lipid-soluble quercetin and water-soluble DHQ for 30 min significantly reduced the AAPH-induced hemolysis to 3.6 ± 1.5% and 32.5 ± 5.6% respectively. In contrast, quercetin and DHQ were similarly effective in reducing phospholipase C-induced hemolysis (37.2 ± 9.1% and 45.4 ± 10.0% versus vehicle 75.7 ± 5.2%, P < 0.001). Pre-incubation with quercetin, but not DHQ, inhibited the aggregation of platelets by adenosine diphosphate. DHQ was more potent than quercetin in inhibiting superoxide produced by xanthine oxidase. These results suggest that the antihemolytic effects of flavonoids may not be directly mediated by removal of free radicals and may likely be due to their interaction with cell membrane.