A method for preparing a methionine containing peptide or protein for analysis by mass spectrometry by incubating the methionine containing peptide or protein with a heterologous methionine sulfoxide reductase A (MsrA), a heterologous methionine sulfoxide reductase B (MsrB), or a heterologous methionine sulfoxide reductase AB (MsrAB) enzyme results in the reduction of an oxidized methionine containing peptide or protein.

A method for preparing a methionine-containing peptide or protein for analysis by mass spectrometry includes incubating the methionine-containing peptide or protein with a heterologous methionine sulfoxide reductase A (MsrA), MsrB, or MsrAB enzyme resulting in the reduction of oxidized methionine-containing peptides or proteins.