Methionine Sulfoxide Reductase (MSR)
MSR repairs protein damage caused by reactive oxygen species that escaped the first line of antioxidant defense. In the scalp, MSR can fix oxidized amino acids, like methionine, in a damaged tyrosinase protein so that the enzyme can keep catalyzing the first key step in the synthesis of melanin. The crucial role of MSR is not limited to the scalp. Oxidation of proteins by reactive oxygen species is associated with aging, oxidative stress, and many diseases. Protein-bound methionine residues are among the most susceptible to oxidation by reactive oxygen species (ROS), resulting in formation of methionine sulfoxide residues Met(O), but this deleterious modification can be repaired by methionine sulfoxide reductase which catalyzes the thioredoxin-dependent reduction of free and protein-bound Met(O) to methionine.