Glutathione peroxidase 1 (GPx1) is the most abundant version, found in the cytoplasm of nearly all mammalian tissues, whose preferred substrate is hydrogen peroxide.

Glutathione peroxidase 4 (GPx4) has a high preference for lipid hydroperoxides; it is expressed in nearly every mammalian cell, though at much lower levels.

The selenenic acid is then converted back to the selenol by a two step process that begins with reaction with GSH to form the GS-SeR and water.

However, Gpx1−/− mice develop cataracts at an early age and exhibit defects in muscle satellite cell proliferation.

A direct assay by linking the peroxidase reaction with glutathione reductase with measurement of the conversion of NADPH to NADP is widely used.

[citation needed] In one study, the activity of glutathione peroxidase along with other antioxidant enzymes such as superoxide dismutase and catalase was not associated with coronary heart disease risk in women.

[19] One study has suggested that glutathione peroxidase and superoxide dismutase polymorphisms play a role in the development of celiac disease.