Table 1.

Examples of free radicals and their biological relevance.

Radical	Reaction	Biology/function	Refs.
Superoxide O2·−	O2 + e− → O2·−	Mainly produced by the reaction of O2 with an escaped electron from mitochondria; also produced by xanthine oxidase, lipoxygenase, cyclooxygenase and NADPH dependent oxygenase.	[3, 6, 7]
Hydrogen peroxide H2O2	2O2·− + 2H+ → 2H2O2 + O2	An intermediate detoxification of O2·− by SOD; comparatively low intrinsic toxicity and biological half-life make H2O2 well suited to act as an intracellular signaling molecule; involved in remodeling the structure of cells and activation of transcription factors.	[8–12]
Hypochlorous acid HOCl	H+ + Cl− + H2O2 → HOCl+H2O	Formed by myeloperoxidase reaction of H+, Cl−, and H2O2; can terminate bacterial DNA replication by destroying DNA anchoring at the membrane.	[13]
Hydroxyl radical HO·	HOCl+ O2·− → HO·+O2 + Cl− HOCl+ Fe2+ → HO·+Fe3+ + Cl− HOCl+ Cu+ → HO·+Cu2+ + Cl− H2O2+ Fe2+ → HO·+ Fe3+ + OH− H2O2+ Cu+ → HO·+ Cu2+ + OH− H2O2+ O2·− → HO·+O2 + Cl−	Produced spontaneously by HOCl with O2·− or metal ions; also produced from H2O2 through Fenton reactions. Because of its high reactivity, short half-life and irreversible modification of macromolecules, HO· has high biological toxicity.	[3] [14]
Nitric oxide NO	L-arginine+O2+NADPH → L-citrulline+NO+NADP++e−	Synthesized enzymatically by NOS; can function as a free radical scavenger as it has a long half-life compared with O2·− and HO·. At normal physiological concentrations, NO is an intracellular messenger for guanylate cyclase and protein kinases. NO conjugates with GSH.	[2, 15–17]
Peroxynitrite ONOO−	NO + O2·− → ONOO−	In cells with high NO cells (e.g. stimulated leukocytes), reaction can be faster than the dismutation of O2·− by SOD, then ONOO− can undergo hemolysis to form HO·.	[2]
Nitrogen dioxide NO2	ONOO− + H+ → HO·+NO2 2NO+ O2 → 2NO2	Increased formation of ONOO− can lead to autohomolysis into HO· and NO2; NO2 can also be produced by direct oxidation of NO by O2.	[2, 18]