Method of analyzing oxidation state of methionine in protein sample

A method of analyzing an oxidation state of a methionine residue in a protein sample while accurately reflecting an in vivo oxidation state of the methionine residue, comprising:

• (i) inducing a reaction between a first protein sample A having a methionine residue with an oxidation state to be analyzed, and hydrogen peroxide H218O2 having oxygen atoms labeled by the isotope 18O, to obtain a first modified protein sample A′ in which said oxidation state of said methionine residue is stabilized in order to protect from any further oxidation until it is subjected to a measurement operation, and subjecting said first modified protein sample A′ to a measurement to quantify an oxidation degree of said methionine residue in said first modified protein sample A′;
• (ii) inducing a reaction between a second protein sample B having a methionine residue with an oxidation state to be analyzed, and hydrogen peroxide H218O2 having oxygen atoms labeled by the isotope 18O to obtain a second modified protein sample B′ in which said oxidation state of said methionine residue is stabilized in order to protect from any further oxidation until it is subjected to a measurement operation, and subjecting said second modified protein sample B′ to a measurement to quantify an oxidation degree of said methionine residue in said second modified protein sample B′;
• and
• (iii) comparing between respective measurement results in said operation (i) and said operation (ii).