There are five types of endogenous antioxidants: superoxide dismutase, catalase, glutathione peroxidase, α-lipoic acid, and coenzyme Q₁₀.
The first three are the most important, because the body can produce more of them when needed.
Superoxide Dismutase (SOD)
The most common SODs are based on Cu and Zn ions.
They catalyze the conversion of the toxic superoxide radical (O₂·⁺) into oxygen and hydrogen peroxide.
Cu²⁺-SOD + O₂·⁻ → Cu⁺-SOD + O₂
Cu⁺-SOD + O₂·⁻ + 2H⁺ → Cu²⁺-SOD + H₂O₂
O₂·⁻ is produced as a by-product of oxygen metabolism and causes many types of cell damage. Hydrogen peroxide is also damaging, but less so, and is degraded by other enzymes such as catalase.
Thus, SOD is an important antioxidant defense in nearly all living cells exposed to oxygen.
Catalase catalyzes the decomposition of hydrogen peroxide to water and oxygen.
2H₂O₂ → 2H₂O + O₂
Glutathione Peroxidase (Gpx)**
GPx reduces lipid hydroperoxides (LOOH) to their corresponding alcohols and to reduce free hydrogen peroxide to water.
There are eight different forms of Gpx. The two important ones are Gpx1, which reduces H₂O₂, and Gpx4, which reduces LOOH.
The main reaction that glutathione peroxidase catalyzes is:
2GSH + H₂O₂ → GS–SG + 2H2O
where GSH represents reduced monomeric glutathione, and GS–SG represents glutathione disulfide.
The diagram below shows how SOD, catalase, and Gpx work together to remove superoxide and hydrogen peroxide from the body.