Process for eliminating N-terminal methionine

This invention relates to a method for the efficient removal, from peptides (including proteins) or salts thereof which possess an optionally oxidized N-terminal methionine residue or diketone of said methionine residue, of the N-terminal methionine residue or the diketone of said methionine residue, in the presence of acetic acid and sodium formate, formic acid and sodium formate, or formic acid and sodium acetate; and to a method for manufacturing peptides or salts thereof which do not possess an optionally oxidized N-terminal methionine residue or diketone of said methionine residue. A method for removing from a peptide the diketone of the methionine residue, and a method for manufacturing a peptide or salt thereof which does not possess an N-terminal methionine residue, characterized by having a peptide or salt thereof which possesses a diketone of the optionally oxidized N-terminal methionine residue react with 3,4-diaminobenzoic acid or a salt thereof in the presence of acetic acid and sodium formate, formic acid and sodium formate, or formic acid and sodium acetate.