Glutathione peroxidase catalyzes the reaction:

2 GSH + H2O2 <=> GSSG + 2H2O,

where GSH refers to reduced glutathione and GSSG is the oxidized form of glutathione in which two molecules are joined by sulfhydryl bonds.

Glutathione peroxidase thus protects against oxidative damage by reducing the reactive oxygen species, H2O2 to water. The enzyme is interesting in that it contains the rare amino acid, selenocysteine.