Antioxidants: The Basics-what they are and how to Evaluate them

Words such as “antioxidant” and “oxidative stress” are difficult to define. The term “antioxidant” used in the literature is restricted to chain-breaking antioxidant inhibitors of lipid peroxidation. Food scientists frequently equate antioxidants to inhibitors of lipid peroxidation because they use antioxidants to prevent rancidity. Free radicals generated in vivo damage proteins, DNA, and other molecules in addition to lipids. A broader definition is: an antioxidant is any substance that when present at low concentrations compared to those of an oxidizable substrate delays or prevents oxidation of that substrate. Mechanisms of antioxidant action can include: removal of O2, scavenging reactive oxygen/nitrogen species or their inhibiting ROS/RNS formation, binding metal ions needed for catalysis of ROS generation, and up regulation of endogenous antioxidant defenses. This chapter emphasizes the importance of the source of stress and the target (“oxidizable substrate”) measured; but also mentions the cases that the definition does not include. When ROS/RNS are generated in vivo, many antioxidants come into play. The chapter discusses the antioxidants known, or proposed, to be important in aerobic organisms. There is outline on some approaches to the characterization of direct antioxidant activity and discussion on biologically relevant ROS/RNS—superoxide and hydrogen peroxide, hydroxyl radical, peroxyl radicals, lipid peroxidation, hypochlorous acid, heme proteins/peroxides, peroxynitrite, and singlet oxygen—measuring their scavenging, their reactions, some inhibitions on radical formation and other properties. Some approaches have been listed to help prove that a given compound act as an antioxidant in vivo, including two approaches for putative antioxidants. Specific assays are being developed to measure rates of oxidative damage to protein, DNA, and lipid. Steady-state and total body oxidative damage to these targets can now be approximated, providing a tool to examine the effects of “antioxidants” in vivo.