Quantitative measurement of deamidation in lens βB2-crystallin and peptides by direct electrospray injection and fragmentation in a Fourier transform mass spectrometer

Purpose: Deamidation of lens crystallins and specific deamidation sites have been suggested to be associated with aging and cataracts. However, these studies have been hindered by the lack of suitable quantitative methods of measurement of protein deamidation. We demonstrate herein a method to quantitatively measure deamidation of proteins and peptides without prior sample preparation or separation in order to directly compare the amidated and deamidated forms. We have tested the hypothesis that the 19 mDa mass defect that distinguishes deamidated peptides and proteins from the ordinary natural isotopic species can be utilized for quantitative measurement of their rate and extent of deamidation. The measurement technique used was ion cyclotron resonance Fourier transform mass spectrometry (FTMS), alone with no prior sample preparation or separation. The amidated and deamidated species were recombinantly expressed human eye lens βB2-crystallins and the peptides GlyIleAsnAlaGly and GlyAsnAsnAsnGly. FTMS measurements of lens proteins from a 1-month-old human donor were also carried out. Methods: Wild type and mutant human eye lens βB2-crystallins with Gln₁₆₂ replaced by Glu₁₆₂ were produced in bacteria, and GlyIleAsnAlaGly and GlyAsnAsnAsnGly were synthesized by Merrifield solid-phase peptide synthesis. The peptides were deamidated in pH 7.4, 37.00 °C, 0.15 M Tris-HC1 aqueous solution for 18 successive time intervals before analysis. Mutant and wildtype βB2-crystallin solutions at various compositional percentages were mixed and analyzed. The peptides were introduced by electrospray ionization and immediately analyzed in the ion cyclotron resonance (ICR) Fourier transform mass analyzer. Two mass defect analysis procedures were demonstrated for the proteins. In the first, βB2-crystallin was introduced into the mass spectrometer by electrospray ionization and the +29 isotopic group was selectively introduced into the ICR mass analyzer, where 14 residue and 18 residue laser-induced fragments were separated and the extent of deamidation determined by mass defect analysis. In the second, βB2-crystallin was introduced into the mass spectrometer by electrospray ionization and the entire sample was fragmented by collision ionization before introduction into the ICR mass analyzer, where 14 residue fragments were separated and the extent of deamidation determined by mass defect analysis. Results: The βB2-crystallin mass spectra showed a good quantitative dependence upon extent of deamidation. Direct injection by electrospray ionization followed by ion selection and laser fragmentation or by collision fragmentation produced fragments of amidated and deamidated βB2-crystallin that were appropriate for FTMS quantitative analysis. The two peptides exhibited the expected four deamidation rate curves with acceptable precision. Conclusions: Mass defect FTMS quantitative analysis of protein deamidation, as reported for the first time herein and illustrated with βB2-crystallin, should prove quite useful. This procedure omits gel separation, chromatography, enzymatic digestion, derivatization, and other procedures that currently add cost and time while degrading quantitative comparison of the amidated and deamidated forms. Mass defect FTMS is also well suited to quantitative deamidation rate studies of peptides. The substantial potential significance of this technique is evident, as example, for lens crystallins where it makes possible quantitative studies of age and disease-dependent deamidation that have heretofore been very difficult. This technique should allow convenient and reliable identification and quantitative measurement of specific deamidation sites that may play a role in aging and cataracts.