Glutathione-dependent oxidoreductase that facilitates the maintenance of mitochondrial redox homeostasis upon induction of apoptosis by oxidative stress. Involved in response to hydrogen peroxide and regulation of apoptosis caused by oxidative stress. Acts as a very efficient catalyst of monothiol reactions because of its high affinity for protein glutathione-mixed disulfides. Can receive electrons not only from glutathione (GSH), but also from thioredoxin reductase supporting both monothiol and dithiol reactions. Efficiently catalyzes both glutathionylation and deglutathionylation of mitochondrial complex I, which in turn regulates the superoxide production by the complex. Overexpression decreases the susceptibility to apoptosis and prevents loss of cardiolipin and cytochrome c release. The 2Fe-2S present in the homodimer leads to inactivation of the enzyme. The 2Fe-2S may serve as a redox sensor: the presence of one-electron oxidants or reductants leading to the loss of the 2Fe-2S cluster, subsequent monomerization and activation of the enzyme (By similarity). Monomer; active form. Homodimer; inactive form. The homodimer is probably linked by 1 2Fe-2S cluster (By similarity). Widely expressed. Highly expressed in testis, and at much lower level in kidney and brain. During development, it is expressed at highest level at 11 dpc. Belongs to the glutaredoxin family.