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Mammalian tissues express three immunologically distinct peroxiredoxin (Prx) proteins (Prx I, II, and III), which are the products of distinct genes. With the use of recombinant proteins Prx I, II, and III, all have now been shown to possess peroxidase activity and to rely on Trx as a source of reducing equivalents for the reduction of H 2 O 2 . Prx I and II are cytosolic proteins, whereas Prx III is localized in mitochondria. Transient overexpression of Prx I or II in cultured cells showed that they were able to eliminate the intracellular H 2 O 2 generated in response to growth factors. Moreover, the activation of nuclear factor B (NF B) induced by extracellularly added H 2 O 2 or tumor necrosis factor-␣ was blocked by overproduction of Prx II. These results suggest that, together with glutathione peroxidase and catalase, Prx enzymes likely play an important role in eliminating peroxides generated during metabolism. In addition, Prx I and II might participate in the signaling cascades of growth factors and tumor necrosis factor-␣ by regulating the intracellular concentration of H 2 O 2 .We have previously purified a 25-kDa thioredoxin peroxidase (TPx) 1 from Saccharomyces cerevisiae that reduces hydroperoxides with thioredoxin (Trx) as an immediate electron donor (1-7). A data base search revealed more than 40 proteins from a wide variety of species that show sequence similarity to yeast TPx (8,9). These homologous proteins were named the peroxiredoxin (Prx) family (8, 9). They were not termed the TPx family because not all members use Trx as the hydrogen donor (10, 11). 2 The Prx family includes 12 mammalian proteins that were identified without reference to peroxidase activity but rather in association with a variety of diverse cellular functions including proliferation, differentiation, and immune response (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). Whether any of these mammalian Prx proteins possess peroxidase activity and, if so, the identity of the immediate electron donors both remain unknown. As will be published elsewhere, the mammalian Prx members can be divided into three distinct groups (Prx I, II, and III) on the basis of their amino acid sequences and immunological properties. 3

The active-site cysteine of peroxiredoxins is selectively oxidized to cysteine sulfinic acid during catalysis, which leads to inactivation of peroxidase activity. This oxidation was thought to be irreversible. However, by metabolic labeling of mammalian cells with 35 S, we show that the sulfinic form of peroxiredoxin I, produced during the exposure of cells to H 2 O 2 , is rapidly reduced to the catalytically active thiol form. The mammalian cells' ability to reduce protein sulfinic acid might serve as a mechanism to repair oxidatively damaged proteins or represent a new type of cyclic modification by which the function of various proteins is regulated.