Participates in various redox reactions through the reversible oxidation of its active center dithiol to a disulfide and catalyzes dithiol-disulfide exchange reactions (PubMed:11013257, PubMed:20673832). By modifying the redox status of targeted proteins, induces changes in their structure and activity (PubMed:19360125, PubMed:20673832). Reduces oxidized glutathione (GSSG), thereby acting as a backup for the glutathione redox system (PubMed:11013257). Reduces nitroglutathione (GSNO), a compound involved in the transport of nitric oxide (NO) (PubMed:11013257). Also reduces oxidative stress by detoxifying hydrogen peroxide, tert-butyl hydroperoxide and cumene hydroperoxide (PubMed:14962358). Activates ornithine aminotransferase OAT by reducing a disulfide bond in the substrate binding loop, thereby enhancing the affinity of OAT for its substrates (PubMed:20673832). May reduce S-adenosyl-L-homocysteine hydrolase SAHH (PubMed:19360125). E(0) is -270 mV (at pH 7.4 and 25 degrees Celsius). The disulfide bond between Cys-30 and Cys-33 acts as a redox-active center and is reduced by thioredoxin reductase TRXR. Belongs to the thioredoxin family.