Sulfiredoxin protein is critical for redox balance and survival of cells exposed to low steady-state levels of H2O2.

Sulfiredoxin (Srx) is an enzyme that catalyzes the reduction of cysteine sulfinic acid of hyperoxidized peroxiredoxins (Prxs). Having high affinity toward H2O2, 2-Cys Prxs can efficiently reduce H2O2 at low concentration. We previously showed that Prx I is hyperoxidized at a rate of 0.072% per turnover even in the presence of low steady-state levels of H2O2. Here we examine the novel role of Srx in cells exposed to low steady-state levels of H2O2, which can be achieved by using glucose oxidase. Exposure of low steady-state levels of H2O2 (10-20 µm) to A549 or wild-type mouse embryonic fibroblast (MEF) cells does not lead to any significant change in oxidative injury because of the maintenance of balance between H2O2 production and elimination. In contrast, loss-of-function studies using Srx-depleted A549 and Srx-/- MEF cells demonstrate a dramatic increase in extra- and intracellular H2O2, sulfinic 2-Cys Prxs, and apoptosis. Concomitant with hyperoxidation of mitochondrial Prx III, Srx-depleted cells show an activation of mitochondria-mediated apoptotic pathways including mitochondria membrane potential collapse, cytochrome c release, and caspase activation. Furthermore, adenoviral re-expression of Srx in Srx-depleted A549 or Srx-/- MEF cells promotes the reactivation of sulfinic 2-Cys Prxs and results in cellular resistance to apoptosis, with enhanced removal of H2O2. These results indicate that Srx functions as a novel component to maintain the balance between H2O2 production and elimination and then protects cells from apoptosis even in the presence of low steady-state levels of H2O2.

Sulfiredoxin Translocation into Mitochondria Plays a Crucial Role in Reducing Hyperoxidized Peroxiredoxin III.

The mitochondria are the major intracellular source of reactive oxygen species (ROS), which are generated during cellular respiration. The role of peroxiredoxin (Prx) III, a 2-Cys Prx family member, in the scavenging of mitochondrial H(2)O(2) has recently been emphasized. While eliminating H(2)O(2), Prx can become overoxidized and inactivated by modifying the active cysteine into cysteine sulfinic acid (Cys-SO(2)H). When 2-Cys Prxs are inactivated in vitro, sulfiredoxin (Srx) reduces the cysteine sulfinic acid to cysteines. However, whereas Srx is localized in the cytoplasm, Prx III is present exclusively in the mitochondria. Although Srx reduces sulfinic Prx III in vitro, it remains unclear whether the reduction of Prx III in cells is actually mediated by Srx. Our gain- and loss-of-function experiments show that Srx is responsible for reducing not only sulfinic cytosolic Prxs (I and II) but also sulfinic mitochondrial Prx III. We further demonstrate that Srx translocates from the cytosol to mitochondria in response to oxidative stress. Overexpression of mitochondrion-targeted Srx promotes the regeneration of sulfinic Prx III and results in cellular resistance to apoptosis, with enhanced elimination of mitochondrial H(2)O(2) and decreased rates of mitochondrial membrane potential collapse. These results indicate that Srx plays a crucial role in the reactivation of sulfinic mitochondrial Prx III and that its mitochondrial translocation is critical in maintaining the balance between mitochondrial H(2)O(2) production and elimination.