Comparison of nitration and oxidation of tyrosine in advanced human carotid plaque proteins

The importance of reactive nitrogen species in atherosclerosis remains poorly understood, despite the semi-quantitative evidence for the presence of 3-nitrotyrosine provided by immuno-histochemical staining studies. At this time, there appear to be no data describing the prevalence of nitration relative to oxidation in atherosclerotic plaque proteins. The present study used 3-nitrotyrosine and dityrosine as markers of nitration and oxidation respectively to examine the relative abundance of each process. Substantial methodological improvements were required to overcome problems associated with sensitivity and artefactual production of 3-nitrotyrosine when quantified by GLC-MS. It was shown that careful selection of hydrolysis vessel, sample reduction and the use of the oxazolinone derivative provided sample stability and exquisite sensitivity. Using these methods, it was observed that the frequency of nitration was 92 +/- 15 mumol/ mol of tyrosine (0.01%). Dityrosine was present at 1.5 +/- 0.14 mmol/mol of tyrosine (0.30 %) using HPLC/fluorescence; thus nitration accounted for approx. 3 % of the tyrosine modifications measured. Given that other modifications of tyrosine are known to occur in carotid plaque proteins, the contribution of nitration to the total pool of modified tyrosine is very small. However, the possibility of metabolic processes or chemical agents modifying 3-nitrotyrosine to secondary oxidation products remains an alternative explanation for the low levels demonstrated in this study.