Hydrogen peroxide-scavenging properties of normal human airway secretions

To examine the antioxidant capacity of normal human airway secretions and to characterize its molecular components, tracheal lavages were obtained from eight patients intubated for elective surgery and free of lung disease. These samples (20 microl, approximately 6.8 microg of protein) scavenged 0.57 +/- 0.09 nmol of added 0.96 nmol hydrogen peroxide (H2O2) within 10 minutes at room temperature (n = 8). The scavenging activity was inhibited 60 +/- 4% by azide (an inhibitor of heme-containing peroxidases and catalase) and 42 +/- 9% by dapsone (an inhibitor of lactoperoxidase). Mercaptosuccinic acid (an inhibitor of glutathione peroxidase) did not significantly inhibit H2O2 scavenging by these secretions. Fourfold diluted secretions showed only nonenzymatic scavenging activity, but the addition of thiocyanate to these samples (0.4 mM; substrate for lactoperoxidase) restored their ability to scavenge H2O2. The addition of reduced glutathione (8 microM) only enhanced nonenzymatic scavenging activity. These data provide evidence that multiple enzymatic and nonenzymatic systems coexist in human airway secretions that contribute to H2O2 scavenging. It appears, however, that H2O2 is mainly consumed by the lactoperoxidase system.