p-Hydroxyphenylacetaldehyde, an aldehyde generated by myeloperoxidase, modifies phospholipid amino groups of low density lipoprotein in human atherosclerotic intima

Oxidation of low density lipoprotein (LDL) may be of critical importance in the pathogenesis of atherosclerosis. Recent studies suggest that oxidized phospholipids render LDL atherogenic, However, both the structures and the physiologically relevant pathways for the formation of modified phospholipi ds in oxidized LDL remain poorly understood. We previously showed that p-hy droxyphenylacetaldehyde (pHA) is the major product of L-tyrosine oxidation by the myeloperoxidase/hydrogen peroxide/chloride system of phagocytes, In the current studies, we demonstrate that this reactive aldehyde targets the aminophospholipids of LDL in vitro and in vivo. Activated human neutrophil s generated pHA-ethanolamine, the reduced adduct of pHA with the amino grou p of phosphatidylethanolamine, on LDL phospholipids by a reaction that requ ired myeloperoxidase, H2O2, and L-tyrosine, The cellular system could be re placed by HOCl and L-tyrosine but not by a wide variety of other oxidation systems, indicating that pHA-ethanolamine is a specific marker for covalent modification of aminophospholipids by myeloperoxidase. To determine whethe r aldehydes modify aminophospholipids in vivo, we quantified levels of pHA- ethanolamine in acid hydrolysates of reduced lipid extracts through isotope dilution gas chromatography/mass spectrometry, Circulating LDL contained u ndetectable levels of pHA-modified phospholipid (<0.1 mmol/mol), In contras t, the concentration of pHA-ethanolamine in LDL isolated from human atheros clerotic lesions was strikingly elevated (4.5 mmol/mol), Collectively, thes e results demonstrate a novel, myeloperoxidase-based mechanism for modifyin g the amino group of LDL phospholipids. They also offer the first evidence that myeloperoxidase may damage LDL lipids in Ditto, raising the possibilit y that aldehyde-modified aminophospholipids play a role in inflammation and vascular disease.