Method for assessing protein identity and stability

A method of confirming the amino acid sequence of a pegylated interleukin-10 (PEG-IL-10), comprising the steps of:

• a) fragmenting a test PEG-IL-10 with a glutamyl endopeptidase (Endo Glu-C), to yield a test plurality of peptides;
• b) adding a reducing agent in an amount sufficient to reduce the disulfide bonds of the test PEG-IL10;
• (c) separating peptide members of the test plurality of peptides by chromatography;
• (d) analyzing said separated peptide members using ultraviolet absorption spectroscopy to provide a test PEG-IL-10 chromatogram;
• and
• (e) comparing the test PEG-IL-10 chromatogram to a reference standard chromatogram, said reference standard chromatogram generated by Endo Glu-C digestion of a reference standard PEG-IL-10, adding a reducing agent in an amount sufficient to reduce the disulfide bonds of the reference standard PEG-IL-10, separating peptide members of the reference standard PEG-IL-10 by chromatography, and analyzing said separated peptide members using ultraviolet absorption spectroscopy to provide the reference standard PEG-IL10 chromatogram;
• wherein the amino acid sequence of the test PEG-IL-10 is confirmed by the substantial equivalency of the retention time of the peaks of the test PEG-IL-10 chromatogram and the reference standard PEG-IL10 chromatogram.