A free radical is a molecule containing an unpaired electron.
Free radicals are unstable and reactive.
In 1969 Joe M. McCord, Ph.D. and Irwin Fridovich, Ph.D. discovered an enzyme that catalyzes the elimination of a free radical form of oxygen, O 2 .- , called superoxide . Until this discovery it was assumed that biological systems were too mild-mannered to produce such reactive chemicals. The existence of an enzyme to get rid of free radicals implied that cells must be able to produce the radicals. The only cells found not to contain SOD were anaerobic bacteria, which cannot grow in the presence of oxygen. SOD provided a very useful tool for the study of oxygen-derived free radicals.