Oxidatively damaged low density lipoprotein (LDL) may play an important role in atherogenesis, but the physiologically relevant pathways have proved difficult to identify. Mass spectrometric quantification of stable compounds that result from specific oxidation reactions represents a powerful approach for investigating such mechanisms. Analysis of protein oxidation products isolated from atherosclerotic lesions implicates tyrosyl radical, reactive nitrogen species, and hypochlorous acid in LDL oxidation in the human artery wall. These observations provide chemical evidence for the reaction pathways that promote LDL oxidation and lesion formation in vivo.— Heinecke, J. W. Mass spectrometric quantification of amino acid oxidation products in proteins: insights into pathways that promote LDL oxidation in the human artery wall. FASEB J. 13, 1113–1120 (1999)