Biology of disease: free radicals and tissue injury.

A free radical is any molecule that has an odd number of electrons. Free radicals, which can occur in both organic (i.e., quinones) and inorganic molecules (i.e., O(2)), are highly reactive and, therefore, transient. Free radicals are generated in vivo as by products of normal metabolism. They are also produced when an organism is exposed to ionizing radiation, to drugs capable of redox cycling, or to xenobiotics that can form free radical metabolites in situ. Cellular targets at risk from free radical damage depend on the nature of the radical and its site of generation. In this review we survey cellular sources of free radicals and the reactions they can undergo and discuss cellular defenses and adaptive mechanisms.