Site-specific Nitration of Apolipoprotein A-I at Tyrosine 166 Is Both Abundant within Human Atherosclerotic Plaque and Dysfunctional

Background: The functional importance of apolipoprotein A-I (apoA-I) nitration at tyrosine 166 (Tyr(166)) in vivo is controversial. Results: Nitrotyrosine 166-apoA-I accounts for 8% of apoA-I within human atheroma, is not HDL-associated, and is functionally impaired. Conclusion: Buoyant density ultracentrifugation of HDL can lead to erroneous results, particularly with modified apoA-I forms. Significance: Detection and quantification of nitrotyrosine 166-apoA-I may provide insights into a pathophysiological process within the artery wall. We reported previously that apolipoprotein A-I (apoA-I) is oxidatively modified in the artery wall at tyrosine 166 (Tyr(166)), serving as a preferred site for post-translational modification through nitration. Recent studies, however, question the extent and functional importance of apoA-I Tyr(166) nitration based upon studies of HDL-like particles recovered from atherosclerotic lesions. We developed a monoclonal antibody (mAb 4G11.2) that recognizes, in both free and HDL-bound forms, apoA-I harboring a 3-nitrotyrosine at position 166 apoA-I (NO2-Tyr(166)-apoA-I) to investigate the presence, distribution, and function of this modified apoA-I form in atherosclerotic and normal artery wall. We also developed recombinant apoA-I with site-specific 3-nitrotyrosine incorporation only at position 166 using an evolved orthogonal nitro-Tyr-aminoacyl-tRNA synthetase/tRNA(CUA) pair for functional studies. Studies with mAb 4G11.2 showed that NO2-Tyr(166)-apoA-I was easily detected in atherosclerotic human coronary arteries and accounted for approximate to 8% of total apoA-I within the artery wall but was nearly undetectable (>100-fold less) in normal coronary arteries. Buoyant density ultracentrifugation analyses showed that NO2-Tyr(166)-apoA-I existed as a lipid-poor lipoprotein with <3% recovered within the HDL-like fraction (d = 1.063-1.21). NO2-Tyr(166)-apoA-I in plasma showed a similar distribution. Recovery of NO2-Tyr(166)-apoA-I using immobilized mAb 4G11.2 showed an apoA-I form with 88.1 +/- 8.5% reduction in lecithin-cholesterol acyltransferase activity, a finding corroborated using a recombinant apoA-I specifically designed to include the unnatural amino acid exclusively at position 166. Thus, site-specific nitration of apoA-I at Tyr(166) is an abundant modification within the artery wall that results in selective functional impairments. Plasma levels of this modified apoA-I form may provide insights into a pathophysiological process within the diseased artery wall.