Flavonoids as antioxidant agents: Importance of their interaction with biomembranes

Flavonoids, a group of phenolic compounds widely occurring in the plant kingdom, have been reported to possess strong antioxidant activity. In the present study, four flavonoids (quercetin, hesperetin, naringenin, rutin), chosen according to their structural characteristics, were tested in two different in vitro experimental models: (1) Fe2+-induced linoleate peroxidation (Fe2+-ILP), by detection of conjugated dienes; and (2) autooxidation of rat cerebral membranes (ARCM), by using thiobarbituric acid for assay of free malondialdehyde production. The results obtained were also interpreted in the light of flavonoid interactions, studied by differential scanning calorimetry, with dipalmitoylphosphatidylcholine (DPPC) vesicles as a biological membrane model. The antilipoperoxidant activity of the flavonoids tested can be classified as follows: rutin > hesperetin > quercetin much greater-than naringenin in the Fe2+-ILP test; quercetin > rutin much greater-than hesperetin > naringenin in the ARCM test. Quercetin, hesperetin, and naringenin interacted with DPPC liposomes causing different shifts, toward lower values, of the main transition peak temperature (Tm) typical for DPPC liposomes; however, no change in Tm of DPPC dispersion was observed in the presence of rutin. The hypothesis will be discussed that flavonoid capacity to modify membrane-dependent processes, such as free-radical-induced membrane lipoperoxidation, is related not only to their structural characteristics but also to their ability to interact with and penetrate the lipid bilayers.