What are Free Radicals?
Rafael Radi
Departamento de Bioquímica and Center for Free Radical and Biomedical
Research, Facultad de Medicina, Universidad de la República, Avda. General
Flores 2125, Montevideo, Uruguay.
Free radicals are molecules that contain an unpaired electron in their external
orbital, and as such, usually possess important chemical reactivity and are shortlived. A variety of free radicals are generated in biological systems, by enzymatic
and non-enzymatic mechanisms. Some, but not all, of these radicals species are
oxidizing intermediates including the highly reactive hydroxyl radical (•OH)
which participates in oxidative damage to biomolecules. Other radicals such as
nitric oxide (•NO) have selective reactivity and participate in signaling processes.
An array of enzymatic and non-enzymatic mechanisms participate in the
catabolism of free radicals and oxidants. A premier example is constituted by the
family of superoxide dismutases (SOD), transistion metal-containing proteins that
detoxify cellular and extracellular superoxide radicals (O2•─). Free radical
processes include lipid peroxidation and protein tyrosine nitration. Peroxides,
including hydrogen peroxide and peroxynitrite, are not free radicals themselves,
but participate in oxidation reactions and intermediates in free radical pathways.
The chemistry and consequences of free radical formation and reactions in
biological systems will be addressed.