The nature of heme/iron-induced protein tyrosine nitration

Abstract

Recently, substantial evidence has emerged that revealed a very close association between the formation of nitrotyrosine and the presence of activated granulocytes containing peroxidases, such as myeloperoxidase. Peroxidases share heme-containing homology and can use H₂O₂ to oxidize substrates. Heme is a complex of iron with protoporphyrin IX, and the iron-containing structure of heme has been shown to be an oxidant in several model systems where the prooxidant effects of free iron, heme, and hemoproteins may be attributed to the formation of hypervalent states of the heme iron. In the current study, we have tested the hypothesis that free heme and iron play a crucial role in NO₂-Tyr formation. The data from our study indicate that: (i) heme/iron catalyzes nitration of tyrosine residues by using hydrogen peroxide and nitrite, a reaction that revealed the mechanism underlying the protein nitration by peroxidase, H₂O₂, and NO \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}; (ii) H₂O₂ plays a key role in the protein oxidation that forms the basis for the protein nitration, whereas nitrite is an essential element that facilitates nitration by the heme(Fe), H₂O₂, and the NO \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*} system; (iii) the formation of a Fe(IV) hypervalent compound may be essential for heme(Fe)-catalyzed nitration, whereas O \begin{equation*}{\mathrm{_{2}^{{\bullet}-}}}\end{equation*} (ONOO⁻ formation), ^•OH (Fenton reaction), and compound III are unlikely to contribute to the reaction; and (iv) hemoprotein-rich tissues such as cardiac muscle are vulnerable to protein nitration in pathological conditions characterized by the overproduction of H₂O₂ and NO \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}, or nitric oxide.