Lipid Antioxidants: More than Just Lipid Radical Quenchers

Phenolic antioxidants are well-known to inhibit lipid oxidation by quenching radicals. This slows propagation and is a key component of antioxidant action. However, total antioxidant effects reach far beyond early radicals. Phenolic compounds also inhibit initiation by binding metals and quenching singlet oxygen, cause shifts in oxidation pathways by blocking some reactions of lipid radicals, and alter product distributions by diverting hydroperoxides and complexing with lipid oxidation products. To complicate matters further, phenolic compounds modify total system oxidation by reacting with proteins that themselves quench lipid radicals and complex lipid products, and it is not currently known whether this action interferes with or augments lipid oxidation. Both phenol and quinone forms are active. Importantly, these multiple interventions of phenolic antioxidants are missed when only lipid hydroperoxides are analyzed. This chapter looks beyond radical quenching and outlines antioxidant effects at all three stages of lipid oxidation-initiation, propagation, and termination-and on total system oxidation.