Formation of Nitric Oxide–Derived Oxidants by Myeloperoxidase in Monocytes

Abstract

—Protein nitration and lipid peroxidation are implicated in the pathogenesis of atherosclerosis; however, neither the cellular mediators nor the reaction pathways for these events in vivo are established. In the present study, we examined the chemical pathways available to monocytes for generating reactive nitrogen species and explored their potential contribution to the protein nitration and lipid peroxidation of biological targets. Isolated human monocytes activated in media containing physiologically relevant levels of nitrite (NO 2 − ), a major end product of nitric oxide ( • NO) metabolism, nitrate apolipoprotein B-100 tyrosine residues and initiate LDL lipid peroxidation. LDL nitration (assessed by gas chromatography–mass spectrometry quantification of nitrotyrosine) and lipid peroxidation (assessed by high-performance liquid chromatography with online tandem mass spectrometric quantification of distinct products) required cell activation and NO 2 − ; occurred in the presence of metal chelators, superoxide dismutase (SOD), and scavengers of hypohalous acids; and was blocked by myeloperoxidase (MPO) inhibitors and catalase. Monocytes activated in the presence of the exogenous • NO generator PAPA NONOate ( Z -[ N -{3-aminopropyl}- N -{ n -propyl}amino]diazen-1-ium-1,2-diolate) promoted LDL protein nitration and lipid peroxidation by a combination of pathways. At low rates of • NO flux, both protein nitration and lipid peroxidation were inhibited by catalase and peroxidase inhibitors but not SOD, suggesting a role for MPO. As rates of • NO flux increased, both nitrotyrosine formation and 9-hydroxy-10,12-octadecadienoate/9-hydroperoxy-10,12-octadecadienoic acid production by monocytes became insensitive to the presence of catalase or peroxidase inhibitors, but they were increasingly inhibited by SOD and methionine, suggesting a role for peroxynitrite. Collectively, these results demonstrate that monocytes use distinct mechanisms for generating • NO-derived oxidants, and they identify MPO as a source of nitrating intermediates in monocytes.

Nitric oxide mediates neurodegeneration and breakdown of the blood-brain barrier in tPA-dependent excitotoxic injury in mice

“…This nTyr increase, which is not observed in tPA -/-mice (Fig. 1A,B), although usually attributed to ONOO -, is not completely specific for it, since the nitrite-hemeperoxidase pathway can also result in tyrosine nitration (Chao et al, 1992;Hazen et al, 1999). Scavenging ONOO -by FeTMPyP attenuates the neurotoxic properties of KA and reduces the levels of oxidation (Fig.…”

Activated antigen-presenting cells select and present chemically modified peptides recognized by unique CD4 T cells

“…Certainly the T cells induced by immunizing with peroxynitrite HEL or peptides also reacted with the activated APC. However, products derived from iNOS and myeloperoxidase show nitrating activity (24,25,46,47). A final point is that modifications are not taking place in a single vesicular compartment; they took place in both HEL, which is processed in a late vesicular compartment, and the 48-62 peptide, processed in a recycling early vesicle (48).…”

Nitric oxide, oxidants, and protein tyrosine nitration

“…However, after polymorphonuclear cell degranulation, MPO can be taken up by nonphagocytic cells, and a strong codistribution of MPO and 3-nitrotyrosine is observed in different inflammatory processes (63). Excess • NO can also modulate the hemeperoxidase-mediated nitration; indeed, • NO readily reacts with resting state MPO as well as with compounds I and II, and therefore peroxidases may serve as catalytic sinks of • NO and influence nitration yields (64).…”