Mass spectrometry analysis of photo-induced methionine oxidation of a recombinant human monoclonal antibody

Oxidation of methionine (Met) residues of a recombinant fully human monoclonal antibody after exposure to light was investigated and compared with chemically induced oxidation using tert-butyl-hydroperoxide (tBHP). Met256 and Met432 in the Fc region in the samples exposed to light or incubated with tBHP were oxidized. The Fc mass spectra of the antibody exposed to light showed mainly peaks with a molecular weight (MW) increase of 32 Da, however the sample treated with tBHP showed peaks with increase of only 16 Da. These results suggested that either oxidation of one Met residue (either Met256 or Met432) catalyzed the oxidation of the second Met residue on the same heavy chain (HC) or Met residues of one HC were preferentially oxidized when the antibody was exposed to light, while Met256 and Met432 were randomly oxidized when the antibody was incubated with tBHP.