The side-chain sulfur of methionine is exceptionally susceptible towards oxidation, which may lead to inactivation and degradation of proteins affected. Methionine sulfoxide reductases A and B reduce the two stereoisomers of methionine sulfoxide (MetO), MetSO and MetRO, respectively, in a thioredoxin-dependent manner. Thus, they repair oxidized proteins. Since the oxidation of surface-exposed Met does not necessarily lead to inactivation, as shown for E. coli glutamine synthetase, cyclic oxidation by cellular oxidants followed by reduction by MSRs may also contribute to inactivation of free radicals.
MSRA and MSRB enzymes from Jena Bioscience constitute useful tools for oxidative stress research. Furthermore, MSRs can be utilized for the reduction of Met-containing peptides and proteins oxidized during synthesis or purification, with DTT efficiently substituting the thioredoxin system.