Rapid characterization of amyloid-β side-chain oxidation by tandem mass spectrometry and the scoring algorithm for spectral analysis

Purpose. Amyloid-β (Aβ) is a self-aggregating protein found in senile plaques in Alzheimer's disease (AD) brain and is thought to play a major role in the disease process. Oxidative stress may be a predominant cause of the formation of these Aβ aggregates. This study aims at identifying possible sites of copper-catalyzed oxidation of Aβ1-40 using liquid chromatography tandem mass spectrometry (LC/MS/MS) and scoring algorithm for spectral analysis (SALSA). Traditionally, identification of post-translational modifications by tandem mass spectrometric analysis requires users to inspect manually thousands of MS/MS spectra, which can be a tedious and time-consuming process. With the use of SALSA, users can automatically search for post-translational modifications based on the spacing of the m/z values associated with the ion series of an amino acid sequence. Methods. Aβ1-40 was subjected to copper-catalyzed oxidative stress. LC/MS/MS and SALSA analyses were used to determine the sites of post-translational modification within the tryptic fragments. Results. Oxidation was found to occur preferentially at the histidine residues His13 and His14 and at the methionine residue (Met35) of Aβ1-40. Conclusions. The combination of LC/MS/MS and SALSA searches could dramatically improve the efficiency and accuracy of determining the specific sites of oxidation of in vitro, copper-oxidized Aβ1-40 as well as other oxidized proteins.