Glutathione peroxidase: is an enzyme belonging to the class of oxidoreductases, which catalyzes the following reaction:

2 glutathione + H₂O₂ ⇄ glutathione disulfide + 2 H₂O

and has an antioxidant action in the human body.

Glutathione from a biochemical point of view, is defined as a false tripeptide, because despite being composed of three amino acids (cysteine, glycine and glutamic acid), there is a canonical peptide bond and an abnormal peptide bond. Glutathione owes its antioxidant properties, again to the SH group of cysteine it contains. From a quantitative point of view, glutathione is the most abundant antioxidant present in our cells and logically if it has an antioxidant function, it means that it can be present both in reduced form (GSH) and in oxidized form (GSSG). GSH means is present the group -SH of cysteine, able to give up an electron. GSSG means that glutathione is oxidized because the -SH group has been lost and a disulfide bond (SS) has been formed between two glutathione molecules that have given up two electrons to two free radicals stabilizing them. The transition from the reduced to the oxidized form represents the glutathione cycle. Crucial to the antioxidant activity of glutathione is the enzyme glutathione reductase, which uses NADPH acting as a coenzyme to convert GSSG back to GSH so that it can function as an antioxidant. Glutathione reductase has NADPH as its coenzyme. The cofactor generally is inorganic and does not participate in the reaction, the coenzyme does.