Antioxidant properties of cystic fibrosis sputum

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Abstract
Oxidative stress is a likely contributor to the pathogenesis of cystic fibrosis (CF) lung disease. However, hydrogen peroxide (H2O2), a physiological oxidant, is not elevated in CF exhalates. H2O2may be neutralized by antioxidants in CF airway secretions. The H2O2-detoxifying capacity of CF airway secretions, obtained via sputum induction, was studied in an in vitro H2O2cytotoxicity model. 16HBE14o- cells were exposed to H2O2in culture medium containing either 0 or 10% fetal bovine serum (FBS) or 10% CF sputum supernatant (extracted without use of dithiothreitol). The efficiency of H2O2neutralization was estimated by measuring intracellular oxidant levels (dihydrorhodamine 123) after 2 h and cell viability (propidium iodide) after 24 h of H2O2exposure. Furthermore, the presence of reduced thiols (DTNB assay) and reduced glutathione (recycling assay) in CF sputum samples was evaluated. CF sputum extracts completely prevented intracellular oxidant accumulation seen in cells incubated with H2O2in both control media (i.e., 0 or 10% FBS). Furthermore, CF sputum abolished cell death in 16HBE14o- cells exposed to up to 1 mM H2O2. In contrast, there was 100% cytotoxicity in cells exposed to 600 μM H2O2in both control media. The H2O2-detoxifying potential of CF sputum was sustained after catalase and heme peroxidases were inactivated by sodium azide, which does not affect glutathione peroxidase. In addition, reduced protein thiols were found in abundance in CF sputum. In conclusion, CF sputum is capable to neutralize H2O2and abundant reduced thiols and/or glutathione peroxidase are fully sufficient to detoxify H2O2.