Figure 1. Comparison of methionine oxidation profiles before and after spiking peptide into human serum, reductive alkylation and trypsinization.

Trypsin-digested E. coli β-galactosidase (2.5 μg/μl serum) was added to human serum as a monitor peptide and subsequently processed by reductive alkylation and trypsinization. Representative extracted ion chromatograms corresponding to the peptides LAVMLR (a,b) and MSGIFR (c,d) of the tryptic E. coli β-galactosidase peptide before (a,c) and after (b,d) spiking, reductive alkylation and trypsinization, are presented. Smaller peaks with an earlier retention time, corresponding to LAVMLR or MSGIFR peptides containing methionine sulfoxide (designated as (O) above M) were magnified 10-fold and are shown above the original peaks.