Characterization of nitrophospholipid-peptide covalent adducts by electrospray tandem mass spectrometry: a first screening analysis using different instrumental platforms

Abstract

Lipids are well‐known targets of reactive nitrogen species and this reaction leads to the formation of nitrated lipids that have been associated with anti‐inflammatory and cytoprotective effects. Nitro‐fatty acids (NO2‐FA) are highly electrophilic compounds that can form covalent adducts with proteins, leading to the formation of lipoxidation adducts, which modulate the protein structure and function. Nitrated phospholipids (NO2‐PL) have been detected recently in biological samples, but their biological effects are unknown, although similarly to what has been described for nitrated lipids, it has been hypothesized that they may react with peptides and proteins. In this study, in vitro biomimetic assays are used to synthetize adducts of nitrated POPC (NO2POPC), already detected in biological samples, and GSH peptide. The formation of NO2POPC‐GSH adducts is studied by ESI‐MS and MS2, using both low and high energy CID in different MS platforms: a LXQ linear ion trap, a Q‐TOF 2, and a Q‐Exactive Hybrid Quadrupole‐Orbitrap. Typical product ions observed under MS2 conditions are modified b, y, and C ions bearing NO2POPC covalently linked that unequivocally confirms the presence of the lipid‐peptide adduct. Typical loss of HNO2 is only observed in the MS2 of the mono‐charged precursor ions, [M+H]+. Product ions at m/z 184 or neutral loss of 183 Da are assigned as typical fragmentations that confirm the presence of the phosphatidylcholine. In summary, the characterization of nitro PL‐peptide adducts by MS and MS2 allows the identification of the structure and specific MS2 reporter ions to be used to pinpoint these adductions in biological systems.