RP-HPLC analysis of purified oxidized KNI-272 (S-methyl cysteine sulfoxide) before and after treatment with MsrA. Oxidized KNI-272 was purified by RP-HPLC, incubated in the presence of 20 mM DTT, and then treated with or without MsrA as described in Materials and Methods. After treatment, the sample was analyzed by RP-HPLC. Curves: ——, without MsrA; - - -, with MsrA. The locations of oxidized and unoxidized KNI-272 are indicated.