New covalent modifications of phosphatidylethanolamine by alkanals: mass spectrometry based structural characterization and biological effects

The pathophysiology of numerous human disorders, such as atherosclerosis, diabetes, obesity and Alzheimer's disease, is accompanied by increased production of reactive oxygen species (ROS). ROS can oxidatively damage nearly all biomolecules, including lipids, proteins and nucleic acids. In particular, (poly)unsaturated fatty acids within the phospholipid (PL) structure are easily oxidized by ROS to lipid peroxidation products (LPP) carrying reactive carbonyl groups. Carbonylated LPP are characterized by high in vivo toxicity due to their reactivity with nucleophilic substrates (Lys-, Cys-and His-residues in proteins or amino groups of phosphatidylethanolamines [PE]). Adducts of unsaturated LPP with PE amino groups have been reported before, whereas less is known about the reactivity of saturated alkanals - which are significantly increased in vivo under oxidative stress conditions - towards nucleophilic groups of PLs. Here, we present a study of new alkanal-dipalmitoyl-phosphatidylethanolamine (DPPE) adducts by MS-based approaches, using consecutive fragmentation (MS(n)) and multiple reaction monitoring techniques. At least eight different DPPE-hexanal adducts were identified, including Schiff base and amide adducts, six of which have not been reported before. The structures of these new compounds were determined by their fragmentation patterns using MS(n) experiments. The new PE-hexanal adducts contained dimeric and trimeric hexanal conjugates, including cyclic adducts. A new pyridine ring containing adduct of DPPE and hexanal was purified by HPLC, and its biological effects were investigated. Incubation of peripheral blood mononuclear cells and monocytes with modified DPPE did not result in increased production of TNF-α as one selected inflammation marker. However, incorporation of modified DPPE into 1,2-dipalmitoleoyl-sn-phosphatidylethanolamine multilamellar vesicles resulted in a negative shift of the transition temperature, indicating a possible role of alkanal-derived modifications in changes of membrane structure.