Table 3.

GPx isoenzymes in mammalian cells

Type	Structure	Distribution	Function
GPx1 (cytosolic GPx;cGPx)	Homotetramer; contains a single selenocysteine residue in each of four identical subunits	Abundant in cytosol of erythrocytes, kidney, liver or lung	Selenium-dependent, Ubiquitously distributed (Chu et al., 2004)
GPx2 (gastrointestinal GPx; GI-GPx)	Homotetramer; selenocysteine at active site 40 of the protein sequence	Abundant in the epithelium of the whole gastrointestinal tract	Selenium-dependent (Yan and Chen, 2006)
GPx3 (plasma/extracellualar GPx;pGPx)	A glycosylated homotetramer of 23 kDa subunits	The only extracellular isoform of GPxs; a secretrd protein into blood plasma; also expressed in the kidney, lung, heart, placenta	Selenium-dependent, Extracellular peroxidase (Olson et al., 2010)
GPx4 (phospholipid hydro- peroxide GPx;PHGPx)	Monomer; selenocysteine at active site 73)	In most tissue both in cytosol and associated with membranes	Selenium-dependent, protect phospholipid, inactive struc- tural capsule of epididymal spermatozoa (Imai and Nakagawa, 2003)
GPx5 (epididymal androgerelated protein or secretory GPx)	221 amino acids	In epididymis; secreted protein.	Selenium-independent (Vernet et al., 1999)