Table 1.

Summary of the major chemical species associated with oxidative stress in the eye.

Chemical Species	Source(s)	Downstream Reaction(s)	Ref.
Superoxide (O2·−)	The reaction of O2 with enzymes in the electron transport chain in the mitochondria generated via a single-electron transfer; enzymatic and non-enzymatic biosynthetic pathways; produced by neutrophils	Reacts with O2·− and H2O to generate H2O2 and O2	[13,14,15]
Hydroxyl radical (·OH)	The reaction of H2O2 with iron or copper (Fenton reaction); may also be generated as a byproduct of the exposure of water molecules to ionizing radiation	Reacts with deoxyguanosine residues of DNA to form 8-hydroxy-2-deoxyguanosine; also reacts with deoxycytidine and deoxyadenosine, among others	[16,17]
Hydrogen peroxide (H2O2)	The reaction of O2·− molecules mediated via superoxide dismutase and non-enzymatically; may also be generated as a byproduct of normal catalytic oxidative processes mediated via oxidases	May be converted by myeloperoxidase or other enzymes containing Fe2+ or react with UV light to form hydroxyl radical (OH·); involved in downstream signaling pathways, such as platelet-derived growth factor signaling	[18,19]
Malondialdehyde (MDA) (CH2(CHO)2)	Produced by lipid peroxidation of polyunsaturated fatty acids	Reacts with deoxyguanosine of DNA to form 8-hydroxy-2-deoxyguanosine; also reacts with deoxyadenosine residues; may also react with lysine residues on proteins to form secondary oxidation products	[20,21]
4-Hydroxynonenal (4-HNE) (CH3(CH2)4CH(OH)CH=CH(CHO))	Produced by lipid peroxidation of polyunsaturated fatty acids or linoleic or arachidonic side chains	Reacts with lysine on proteins to form carbonylated side chains, increasing the hydrophobicity of modified proteins; also involved in downstream signaling pathways, including activation of glutamate-cysteine ligase expression	[22,23]