Only relatively recently has importance of free radicals gained widespread acceptance. This may be the result of the compounds' inherent intangibility. They are generally short lived, hard to work with, and difficult to study. Regardless though, it is now confirmed that low levels of free radicals are normally produced as a consequence of metabolic activity. It is also known that the compounds serve as potent biochemical mediators in a number of physiologic roles. However, their great catabolic potential does require focus and control. Unrestrained free radicals can significantly damage biological systems. To counteract such deleterious effects, defenses have evolved. Under normal circumstances, animal cells typically maintain an intricate balance between the generation of free radicals and antioxidant defenses.
Free radicals can be defined as any "chemical species possessing an unpaired electron" (1:482). These compounds generally contain oxygen and are highly reactive. They can be positively charged, negatively charged, or electrically neutral. Because of their unpaired electrons, free radicals may also be thought of as molecular fragments. To name a few, hydroxyl, peroxy, hypochlorite, superoxide, and alkoxy radicals are all considered to be free radicals; moreover, there are carbon centered and sulphur centered free radicals as well. While hydrogen peroxide and singlet oxygen are not free radicals, they are "reactive oxygen species" and are also highly capable of causing cellular damage (2:441).
There are perhaps three primary mechanisms by which free radicals may be produced. The first involves homolytic cleavage of a covalent body in a normal molecule (X:Y > X. + Y.). This generally produces two molecular fragments, each of which retains one of the originally paired electrons. In addition, free radicals may also be formed by either the addition (A + e > A .) or loss (A - e > A .) of a ...