Peroxiredoxins (PRxs) play a role in protecting protein free thiol groups against oxida-tive damage and thioredoxin-dependent peroxidase activity. This report describes the characteristics of the fourth member of the mammalian PRxs, PRx TV. Rat PRx TV produced in Sf21 cells by a baculovirus expression system has two bands with different electrophoretic mobilities, 31 and 27 kDa [Matsumoto et al. (1999) FEBS Lett. 443, 246-250]. The 27-kDa PRx TV lacks the NHj-terminal 36 amino acids which correspond to a predicted leader peptide, which is required for secretion from cells. Thus, the 31-kDa form is probably a precursor form, and the 27-kDa form, a secretable form which is enzymatically active. Pulse-chase experiments of PRx IV-transfected COS-1 cells showed that PRx IV is processed within 10 min and released from cells. The secretable form contains both reduced and oxidized forms. The reduced form binds to both a heparin affinity column and human umbilical vein endothelial cells, while the oxidized form does not. The equilibrium dissociation constants, KD, for heparin and heparan sulfate as judged by surface plasmon resonance experiments were 19 and 870 nM, respectively. The secretable form corresponds to the major bands found in most tissues, as evidenced by immunoblot analysis. Within cells, secretable form was largely localized on the endo-plasmic reticulum, as judged by colocalization with calreticulin. Moreover, PRx TV has glutathione-dependent peroxidase activity in addition to thioredoxin-dependent activity. These data indicate that PRx TV is a secretable protein and may exert its protective function against oxidative damage by scavenging reactive oxygen species in the extracellular space.