Structural aspects of antioxidant activity of flavonoids

Flavonoids, a group of naturally occurring antioxidants and iron chelators, might be used as cardioprotective agents in doxorubicin-induced cardiotoxicity, which is believed to be caused by the formation of oxygen free radicals. To investigate the underlying molecular mechanism, we tested a large group of flavonoids from all major structural subclasses on their ability to inhibit doxorubicin (enzymatically)-induced and Fe2+/ascorbate (nonenzymatically)-induced microsomal lipid peroxidation (LPO) and to chelate Fe2+. In addition, we measured half peak oxidation potentials (Ep/2). LPO inhibition data gave a good qualitative correlation with the oxidation potentials. Most flavonoids tested chelated Fe2+, but there were large differences in the chelating capacity. For good scavenging activity, a catechol moiety on ring B is required. The 3-OH moiety can function as a chelation site and can also be oxidized. The 3-OH group in combination with a C2 C3 double bond, increases the scavenging activity. Fe2+ chelation only plays a role in the LPO inhibition by less active scavengers. Chelation can then raise the activity to the level of the most active scavengers, possibly by site-specific scavenging. It can be concluded that Ep/2 values and iron chelating activity can almost completely describe the LPO inhibiting behaviour of the flavonoids.