Glutaredoxin (Grx), also known as thiol transferase, is a small heat-stable oxidoreductase. Grxs form part of the glutaredoxin system, comprising NADPH, GSH and glutathione reductase, which transfers electrons from NADPH to glutaredoxins via GSH. First recovered in E.coli as GSH-dependent hydrogen donors for ribonucleotide reductase, Grx catalyzes GSH-disulfide oxido-reductase via two redox-active cysteine residues. The active sequence (Cys-Pro-Tyr-Cys) is conserved in a variety of species. The 12 kDa dithiol protein has a role in reduction of mixed disulfides in cells exposed to oxidative stress. Has a glutathione-disulfide oxidoreductase activity in the presence of NADPH and glutathione reductase. Reduces low molecular weight disulfides and proteins.