Activation of 3-Mercaptopyruvate Sulfurtransferase by Glutaredoxin Reducing System

Glutaredoxin (EC 1.15–1.21) is known as an oxidoreductase that protects cysteine residues within proteins against oxidative stress. Glutaredoxin catalyzes an electron transfer reaction that donates an electron to substrate proteins in the reducing system composed of glutaredoxin, glutathione, glutathione reductase, and nicotinamide-adenine dinucleotide phosphate (reduced form). 3-mercaptopyruvate sulfurtransferase (EC 2.8.1.2) is a cysteine enzyme that catalyzes transsulfuration, and glutaredoxin activates 3-mercaptopyruvate sulfurtransferase in the reducing system. Interestingly, even when glutathione or glutathione reductase was absent, 3-mercaptopyruvate sulfurtransferase activity increased, probably because reduced glutaredoxin was partly present and able to activate 3-mercaptopyruvate sulfurtransferase until depletion. A study using mutant Escherichia coli glutaredoxin1 (Cys(14) is the binding site of glutathione and was replaced with a Ser residue) confirmed these results. Some inconsistency was noted, and glutaredoxin with higher redox potential than either 3-mercaptopyruvate sulfurtransferase or glutathione reduced 3-mercaptopyruvate sulfurtransferase. However, electron-transfer enzymatically proceeded from glutaredoxin to 3-mercaptopyruvate sulfurtransferase.