Post-Translational Chemical Modification of Amyloid-beta Peptides by 4-Hydroxy-2-Nonenal

Background: The extraction and quantification of amyloid-beta (A beta) peptides in brain tissue commonly uses formic acid (FA) to disaggregate A beta fibrils. However, it is not clear whether FA can disaggregate post-translationally modified A beta peptides, or whether it induces artifact by covalent modification during disaggregation. Of particular interest are A beta peptides that have been covalently modified by 4-hydroxy-2-nonenal (HNE), an oxidative lipid degradation product produced in the vicinity of amyloid plaques that dramatically accelerates the aggregation of A beta peptides. Objective: Test the ability of FA to disaggregate A beta peptides modified by HNE and to induce covalent artifacts. Methods: Quantitative liquid-chromatography-tandem-mass spectrometry of monomericA beta peptides and identify covalently modified forms. Results: FA disaggregated ordinary A beta fibrils but also induced the time-dependent formylation of at least 2 residue side chains in A beta peptides, as well as oxidation of its methionine side chain. FA was unable to disaggregate A beta peptides that had been covalently modified by HNE. Conclusion: The inability of FA to disaggregate A beta peptides modified by HNE prevents FA-based approaches from quantifying a pool of HNE-modified A beta peptides in brain tissue that may have pathological significance.