In Vitro Antioxidant versus Metal Ion Chelating Properties of Flavonoids: A Structure-Activity Investigation

Natural flavonoids such as quercetin, (+) catechin and rutin as well as four methoxylated derivatives of quercetin used as models were investigated to elucidate their impact on the oxidant and antioxidant status of human red blood cells (RBCs). The impact of these compounds against metal toxicity was studied as well as their antiradical activities with DPPH assay. Antihemolytic experiments were conducted on quercetin, (+) catechin and rutin with excess of Fe, Cu and Zn (400 mu M), and the oxidant (malondialdehyde, carbonyl proteins) and antioxidant (reduced glutathione, catalase activity) markers were evaluated. The results showed that Fe and Zn have the highest prooxidant effect (37 and 33% of hemolysis, respectively). Quercetin, rutin and (+) catechin exhibited strong antioxidant properties toward Fe, but this effect was decreased with respect to Zn ions. However, the Cu showed a weak antioxidant effect at the highest flavonoid concentration (200 mu M), while a prooxidant effect was observed at the lowest flavonoid concentration (100 mu M). These results are in agreement with the physico-chemical and antiradical data which demonstrated that binding of the metal ions (for FeNTA: (+) Catechin, KLFeNTA = 1.0(7) x 10(6) M-1 > Rutin, KLFeNTA = 2.0(9) x 10(5) M-1 > Quercetin, K-LFeNTA = 1.0(7) x 10(5) M-1 > Q35OH, K-LFeNTA = 6.3(8.7) x 10(4) M-1 > Quercetin3'4'OH and Quercetin 3OH, K-LFeNTA similar to 2 x 10(4) M-1) reflects the (anti) oxidant status of the RBCs. This study reveals that flavonoids have both prooxidant and antioxidant activity depending on the nature and concentration of the flavonoids and metal ions.