Photochemically-induced protein tyrosine nitration in vitro and in cellula by 5-methyl-1,4-dinitro-1H-imidazole (DNI): synthesis and biochemical characterization

Dimethyl labeling of N-terminal amines allows unambiguous identification of protein crosslinks

Protein crosslinks induced through either deliberate enzymatic oxidation or reactive oxidants (oxidative eustress/distress), are associated with multiple human pathologies including atherosclerosis, Alzheimer's and Parkinson's diseases. In many cases, the nature of the crosslinks, their position(s) either within (intramolecular) or between (intermolecular) polypeptide chains, and concentrations are unclear. Although limited data are available from specific antibodies, detailed characterization of protein crosslinks is often performed by mass spectrometric analysis of peptides from proteolytic digestion. Such analyses are challenging due to the low concentration of these species, and the complexity of their fragment ion spectra when compared to non-crosslinked species. We hypothesized that highly efficient and specific chemical amine labeling of the two N-termini in crosslinked peptides (compared to the single N-terminus of linear peptides), using "light" and "heavy" isotope-labelled reagents would facilitate identification, validation and quantification of crosslinks. This method was compared to a previous enzyme-catalyzed 18O C-terminal carboxylate labeling approach. N-terminal amine dimethyl labeling is shown to have major advantages over the 18O-approach including high labeling yields (92-100 %) and well-defined mass spectrometric isotope distribution patterns. This approach has allowed identification of novel dityrosine crosslinks between pair of tyrosine (Tyr, Y) residues in photo-oxidized β-casein (Y195-Y195, Y195-Y208, Y208-Y208), and α-synuclein exposed to nitrosative stress (Y39-Y39, Y39-Y125, Y39-Y133, Y133-Y136). This approach is also applicable to disulfide bond mapping, with 15 of 17 disulfides in serum albumin readily detected. These data indicate that dimethyl labeling is a highly versatile and efficient approach for the site-specific identification of oxidation- and nitration-induced crosslinks in proteins.

Revisiting the role of 3-nitrotyrosine residues in the formation of alpha-synuclein oligomers and fibrils

Nitration of tyrosine residues in alpha-synuclein (a-syn) has been detected in different synucleinopathies, including Parkinson's disease. The potential role of 3-nitrotyrosine formation in a-syn, as an oxidative post-translational modification, is still elusive. In this work, we generated well-characterized tyrosine nitrated a-syn monomers and studied their capability to form oligomers and fibrils. We constructed tyrosine to phenylalanine mutants, containing a single tyrosine residue, a-syn mutant Y(125/133/136)F and Y(39/125/133)F) and assessed the impact in a-syn biophysical properties. Nitrated wild-type a-syn and the Y-F mutants, with one 3-nitrotyrosine residue in either the protein's N-terminal or C-terminal region, showed inhibition of fibril formation but retained the capacity of oligomer formation. The inhibition of a-syn fibrillation occurs even when an important amount of unmodified a-syn is still present. We characterized oligomers from both nitrated and non-nitrated forms of the wild-type protein and the mutant forms obtained. Our results indicate that the formation of 3-nitrotyrosine in a-syn could induce an off-pathway oligomer formation which may have an important impact in the development of synucleinopathies.