SOD and another important antioxidant enzyme set, the glutathione system, work within the cell. By contrast, circulating biochemicals such as uric acid and ceruloplasmin react with free radicals in the intercellular spaces and bloodstream. The substances that plants create to fight free radicals can help the human body do the same thing. Thus, as a second line of defense, the body makes use of many standard vitamins and other nutrients to quench the oxy radicals' thirst for electrons. Among the many substances used are Vitamins C and E, beta-carotene, and bioflavonoids.
Self repair The body also has systems to repair or replace damaged building blocks of cells.
These systems are rapid and thorough. For example, the system for repairing damage to DNA and other nucleic acids is particularly elaborate and efficient, with various specialized enzymes that locate damaged areas, snip out ruined bits, replace them with the correct sequence of molecules, and seal up the strand once again. Every aspect of the cell receives similar attention. Most protein constituents in the cell, for example, are completely replaced every few days. Scavenger enzymes break used and damaged proteins into their component parts for reuse by the cell.