Oxidized proteins, which have undergone oxidative modifications, are labeled with labeling reagents and quantified by
mass spectrometry. In this process, a first labeling
reagent, which is capable of reacting with the oxidized proteins, and a second labeling
reagent, which has the same
chemical structure as the first labeling
reagent and in which at least part of the component atoms is substituted by an
isotope of the atom concerned, are employed as the labeling reagents. The oxidized proteins labeled with the first labeling reagent and the oxidized proteins labeled with the second labeling reagent are mixed together and subjected to
mass spectrometry, with mixing ratios varied. As the labeling reagents, are raised here 2,4-dinitrophenylhydrazine and 2,4-dinitrophenylhydrazine in which a
carbon atom on the
phenyl group has been substituted by a stable
isotope (13C), for example.