Nitrotyrosine (NO2Tyr) formation is a hallmark of acute and chronic inflammation and has been detected in a wide variety of human pathologies. However, the mechanisms responsible for this posttranslational protein modification remain elusive. While NO2Tyr has been considered a marker of peroxynitrite (ONOO-) formation previously, there is growing evidence that heme-protein peroxidase activity, in particular neutrophil-derived myeloperoxidase (MPO), significantly contributes to NO2Tyr formation in vivo via the oxidation of nitrite (NO2-) to nitrogen dioxide (·NO2). Coronary arteries from a patient with coronary artery disease, liver and lung tissues from a sickle cell disease patient, and an open lung biopsy from a lung transplant patient undergoing rejection were analyzed immunohistochemically to map relative tissue distributions of MPO and NO2Tyr. MPO immunodistribution was concentrated along the subendothelium in coronary tissue and hepatic veins as well as in the alveolar epithelial compartment of lung tissue from patients with sickle cell disease or acute rejection. MPO immunoreactivity strongly colocalized with NO2Tyr formation, which was similarly distributed in the subendothelial and epithelial regions of these tissues. The extracellular matrix protein fibronectin (FN), previously identified as a primary site of MPO association in vascular inflammatory reactions, proved to be a major target protein for tyrosine nitration, with a strong colocalization of MPO, NO2Tyr, and tissue FN occurring. Finally, lung tissue from MPO-/- mice, having tissue inflammatory responses stimulated by intraperitoneal zymosan administration, revealed less subendothelial NO2Tyr immunoreactivity than tissue from wild-type mice, confirming the significant role that MPO plays in catalyzing tissue nitration reactions. These observations reveal that (i) sequestration of neutrophil-derived MPO in vascular endothelial and alveolar epithelial compartments is an important aspect of MPO distribution and action in vivo, (ii) MPO-catalyzed NO2Tyr formation occurs in diverse vascular and pulmonary inflammatory pathologies, and (iii) extracellular matrix FN is an important target of tyrosine nitration in these inflammatory processes. © 2002 Elsevier Science Inc.