Assessment method for deamidation in proteins using carboxylic acid derivatization-liquid chromatography-tandem mass spectrometry

Abstract

An analytical method for the degree of protein deamidation has been developed by using carboxy group derivatization and liquid chromatography-tandem mass spectrometry (LCMS/MS). The fragment peptides (LGEYGFQNALIVR and YNGVFQECCQAEDK) obtained by digesting bovine serum albumin (BSA) with trypsin and their asparagine deamidated peptides (LGEYGFQDALIVR and YDGVFQECCQAEDK) were selected as model peptides, and their carboxy groups were derivatized with ethylamine. This derivatization enabled a clear distinction between natural peptides and deamidated peptides by mass, allowing for facile distinction by LCMS/MS before and after deamidation. Good linearity was confirmed for four peptides used in this study via isotope dilution mass spectrometry, showing that protein deamidation can be evaluated by the present method. To confirm the validity of this method for the evaluation of deamidation, natural peptides and deamidated peptides were mixed in arbitrary ratios, and degree of deamidation in these solution was analyzed. This confirmed that accurate evaluation was possible at deamidation degree values of ca. 10 %, 5 %, 2.5 %, and 1 %. Additionally, an accelerated storage test of BSA demonstrated that the deamidation of asparagine at position 404 of BSA progressed by 4 % in 9 weeks at 40 °C and pH 8 in the dark, and that the deamidation process can be traced over time.