An antioxidant can be described in simple terms as anything that can delay or prevent oxidation of a susceptible substrate. Our antioxidant system is complex, however, and consists of various intracel-lular and extracellular, endogenous and exogenous, and aqueous and lipid-soluble components that act in concert to prevent ROS formation (preventative antioxidants), destroy or inactivate ROS that are formed (scavenging and enzymatic antioxidants), and terminate chains of ROS-initiated peroxidation of biological substrates (chain-breaking antioxi-dants). In addition, metals and minerals (such as selenium, copper, and zinc) that are key components of antioxidant enzymes are often referred to as antioxidants.
There are many biological and dietary constituents that show 'antioxidant' properties in vitro. For an antioxidant to have a physiological role, however, certain criteria must be met.
1. The antioxidant must be able to react with ROS found at the site(s) in the body where the putative antioxidant is found.
2. Upon interacting with a ROS, the putative anti-oxidant must not be transformed into a more reactive species than the original ROS.
3. The antioxidant must be found in sufficient quantity at the site of its presumed action in vivo for it to make an appreciable contribution to defense at that site: if its concentration is very low, there must be some way of continuously recycling or resupplying the putative antioxidant.
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