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A disulfide intermediate is required for the reduction of methionine sulfoxide reductase by thioredoxin

The methionine sulfoxide reductase (Msr) enzymes catalyze the reduction of protein‐bound methionine sulfoxide (Met(O)) to methionine and have been shown to have an important role in protecting organisms against oxidative stress. The two families of Msr, MsrA and MsrB, reduce the S and R enantiomers of Met(O), respectively. Thioredoxin (Trx) has been considered to be the in vivo reductant for the Msr enzymes, based on studies with MsrA. Recent studies from this lab have shown that Trx has low efficacy with human MsrB2 and MsrB3 (hMsrB2 and hMsrB3) and that reductants such as thionein, the reduced, metal‐free form of metallothionein, and certain selenium compounds have much greater activity in vitro with these enzymes. Structurally, hMsrB2 and hMsrB3, unlike bovine MsrA and E. coli MsrA and MsrB, lack an additional cysteine that can form a disulfide bond with the cysteine at the catalytic site. The results suggest that Trx is an efficient reducing agent only when the oxidized Msr enzyme contains a disulfide bond. To test this, mutants of bovine MsrA were engineered lacking additional cysteines. These MsrA mutants also show poor in vitro activity with Trx but are reduced by selenium compounds and thionein. Studies to identify the normal in vivo reducing system for hMsrB2 and hMsrB3 are in progress.