The chemical origins of nitrated tyrosine residues (NT) formed in proteins during a variety of pathophysiological conditions remain controversial. Although numerous studies have concluded that NT is a signature for peroxynitrite (ONOO⁻) formation, other works suggest the primary involvement of peroxidases. Because metal homeostasis is often disrupted in conditions bearing NT, the role of metals as catalysts for protein nitration was examined. Cogeneration of nitric oxide (NO) and superoxide (O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document}), from spermine/NO (2.7 μM/min) and xanthine oxidase (1–28 μM O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document}/min), respectively, resulted in protein nitration only when these species were produced at approximately equivalent rates. Addition of ferriprotoporphyrin IX (hemin) to this system increased nitration over a broad range of O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document} concentrations with respect to NO. Nitration in the presence of superoxide dismutase but not catalase suggested that ONOO⁻ might not be obligatory to this process. Hemin-mediated NT formation required only the presence of NO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document} and H₂O₂, which are stable end-products of NO and O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document} degradation. Ferrous, ferric, and cupric ions were also effective catalysts, indicating that nitration is mediated by species capable of Fenton-type chemistry. Although ONOO⁻ can nitrate proteins, there are severe spatial and temporal constraints on this reaction. In contrast, accumulation of metals and NO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document} subsequent to NO synthase activity can result in far less discriminate nitration in the presence of an H₂O₂ source. Metal catalyzed nitration may account for the observed specificity of protein nitration seen under pathological conditions, suggesting a major role for translocated metals and the labilization of heme in NT formation.