Exhaled Breath Condensate: A Promising Source for Biomarkers of Lung Disease

Figure 3

Reactive oxygen and nitrogen species and redox relevant molecules in EBC. Exhaled nitric oxide (NO) is derived from L-arginine by enzyme nitric oxide synthase (NOS). NO can combine with superoxide (•O₂⁻) to form peroxynitrite (ONOO^-). ONOO^- induces nitrosation of tyrosine (Tyr) residues and forms 3-nitrotyrosine (3-NT). NO can also react with thiols to form S-nitroso thiols (RS-NO). The end-products of NO are nitrite (NO₂⁻) or nitrate (NO₃⁻). •O₂⁻ is one of major reactive oxygen species generated from NADPH oxidase (NOX) or mitochondrial electron transfer chain. •O₂⁻ is converted to hydrogen peroxide (H₂O₂) by superoxide dismutases (SOD). H₂O₂ can be converted to the highly reactive hydroxyl radical (•OH), which is catalyzed by Fe²⁺ (Fenton reaction). H₂O₂ can be removed by thiol-specific antioxidant enzymes to form water.