Evaluation of MALDI-TOF/TOF Mass Spectrometry Approach for Quantitative Determination of Aspartate Residue Isomerization in the Amyloid-β Peptide.

Immunoprecipitation (IP) combined with MALDI-TOF mass spectrometry is a powerful instrument for peptide and protein identification in biological samples. In this study, the analytical capabilities of MALDI-TOF/TOF mass spectrometry for relative quantitation of isoAsp7 in Aβ(1-42) and Aβ(1-16) were investigated. The possibility of quantitative determination of isoAsp7 in Aβ(1-42) with the detection limit as low as 2 pmol has been demonstrated. The same approach was applied for a shorter peptide Aβ(1-16) and resulted in enhanced accuracy (± 3.2%), and lower detection limit (50 fmol). Pilot experiments with artificial cerebrospinal fluid and mouse brain tissue were performed and showed that the proposed IP-MALDI-TOF/TOF approach could be applied for measuring isoAβ content in biological fluids and tissues. Additionally, it was shown that 6E10 anti-amyloid antibodies might affect the accuracy of the amyloid-β quantitation in the presence of the isomerized peptide.