REACTIVE NITROGEN INTERMEDIATES PROMOTE LOW-DENSITY-LIPOPROTEIN OXIDATION IN HUMAN ATHEROSCLEROTIC INTIMA

Oxidized low density lipoprotein (LDL) may be of central importance in triggering atherosclerosis. One potential pathway involves the produc tion of nitric oxide (NO) by vascular wall endothelial cells and macro phages. NO reacts with superoxide to form peroxynitrite (ONOO-), a pot ent agent of LDL oxidation in vitro. ONOO- nitrates the aromatic ring of free tyrosine to produce 3-nitrotyrosine, a stable product. To expl ore the role of reactive nitrogen species such as ONOO- in the pathoge nesis of vascular disease, we developed a highly sensitive and specifi c method involving gas chromatography and mass spectrometry to quantif y 3-nitrotyrosine levels in proteins. In vitro studies demonstrated th at 3 nitrotyrosine was a highly specific marker for LDL oxidized by ON OO-. LDL isolated from the plasma of healthy subjects had very low lev els of 3-nitrotyrosine (9 +/- 7 mu mol/mol of tyrosine). In striking c ontrast, LDL isolated from aortic atherosclerotic intima had 90-fold h igher levels (840 +/- 140 mu mol/mol of tyrosine). These observations strongly support the hypothesis that reactive nitrogen species such as ONOO- form in the human artery wall and provide direct evidence for a specific reaction pathway that promotes LDL oxidation in vivo. The de tection of 3-nitrotyrosine in LDL isolated from vascular lesions raise s the possibility that NO, by virtue of its ability to form reactive n itrogen intermediates, may promote atherogenesis, counteracting the we ll established anti-atherogenic effects of NO.