Table 1: Locations and properties of antioxidants.

Enzymatic antioxidants located in mitochondria and cytosol

Glutathione peroxidase (GSH)Removal of H2O2, hydroperoxides
Superoxide dismutase (SOD)Catalytic removal of O2
Catalase (CAT)Catalytic reduction of H2O2 to H2O

Nonenzymatic antioxidants located in cell membrane, exogenous dietary source
Vitamin E (α tocopherol)Chain-breaking antioxidant
β-caroteneScavenger of ROS, singlet O2 quencher
Co-enzyme QRegenerates vitamin E

Compounds that reduce the availability of transition metals, Fenton reactions
TransferrinSequesters iron and copper ions
LactoferrinSequesters iron at lower pH
AlbuminSequesters heme and copper
Ceruloplasmin (ferroxidase)Scavenges superoxide radical, binds copper ions

Scavengers, products of metabolism, exogenous dietary source
BilirubinScavenges peroxyl radical
Uric acidScavenges hydroxyl radical
Vitamin C (ascorbic acid)Scavenges hydroxyl radical, recycles vitamin E

Thiol group donors
Reduced glutathione (GSSH)Binds free radicals, SH group oxidized to disulfide group (GSSG)
α-lipoic acidRecycles vitamin C, glutathione substitute