In enzymology, a sulfiredoxin () is an enzyme that catalyzes the chemical reaction peroxiredoxin-(S-hydroxy-S-oxocysteine) + ATP + 2 R-SH peroxiredoxin-(S-hydroxycysteine) + ADP + phosphate + R-S-S-R The 3 substrates of this enzyme are peroxiredoxin-(S-hydroxy-S-oxocysteine), ATP, and a thiol, whereas its 4 products are peroxiredoxin-(S-hydroxycysteine), ADP, phosphate, and a disulfide. This enzyme is involved in antioxidant metabolism by re-activating peroxiredoxins, which are a group of peroxidases, when these enzymes are inhibited by over-oxidation. This enzyme belongs to the family of oxidoreductases, specifically those acting on a sulfur group of donors with other, known, acceptors. The systematic name of this enzyme class is peroxiredoxin-(S-hydroxy-S-oxocysteine):thiol oxidoreductase [ATP-hydrolysing; peroxiredoxin-(S-hydroxycysteine)-forming]. Other names in common use include Srx1, sulphiredoxin, and peroxiredoxin-(S-hydroxy-S-oxocysteine) reductase. Function The sulfur atom in the side-chain of the amino acid cysteine can exist in several different oxidation states. The most reduced of these is as a thiol group (Cys-SH). Oxidation of cysteine produces cystine, which is one half of a disulfide bond (Cys-S-S-Cys). These lower oxidation states of cysteine (disulfides) are readily reversible, but higher oxidation states, such as sulfinic acid (Cys-SOOH), were once considered irreversible, biologically speaking. This view changed with the discovery of sulfiredoxin, an enzyme that can reduce sulfinic acid back to thiol, in an ATP-dependent manner. Additional work suggests that it plays a role in resolving mixed disulfide bonds. Initially discovered in yeast, sulfiredoxin is conserved in all eukaryotes, including mammals. In a perfect example of how multiple gene names can confuse the field, sulfiredoxin (Srxn1) was already known as a gene of unknown function, cloned by differential display of an in vitro model of tumorigenesis, and termed “Neoplastic progression 3/Npn3” although nothing about its actual function was reported. As a result, in most mouse microarray studies, sulfiredoxin is termed neoplastic progression 3, and typically classified as “cancer related” or “other” rather than as “antioxidant”. Npn3/Srxn1 is upregulated by an exceptionally large fold-magnitude in microarray studies of oxidative stress. Npn3/Srxn1 is induced up to 32-fold by D3T (liver), 12-fold by CdCl2, (liver), 4- to 10-fold by paracetamol (liver) and 3.3-fold by paraquat (heart). A survey of the GEO database also indicates a large induction of Npn3/Srxn1 is observed in injury to the lung by hyperoxia (data set GDS247, ID# 102780_at) or phosgene (GDS1244, 1451680_at). That Npn3 and Sxrn1 are synonyms of the same gene has not been pointed out in any of the 15 papers written on Srxn1 since