Correlation of systemic superoxide dismutase deficiency to airflow obstruction in asthma

Abstract

Rationale: Increased oxidative stress and decreased superoxide dismutase (SOD) activity in the asthmatic airway are correlated to airflow limitation and hyperreactivity. We hypothesized that asthmatic individuals with higher levels of oxidative stress may have greater loss of SOD activity, which would be reflected systemically in loss of circulating SOD activity and clinically by development of severe asthma and/or worsening airflow limitation.

Methods: To investigate this, serum SOD activity and proteins, the glutathione peroxidase/glutathione antioxidant system, and oxidatively modified amino acids were measured in subjects with asthma and healthy control subjects.

Results: SOD activity, but not Mn-SOD or Cu,Zn-SOD protein, was lower in asthmatic serum as compared with control, and activity loss was significantly related to airflow limitation. Further, serum SOD activity demonstrated an inverse correlation with circulating levels of 3-bromotyrosine, a posttranslational modification of proteins produced by the eosinophil peroxidase system of eosinophils. Exposure of purified Cu,Zn-SOD to physiologically relevant levels of eosinophil peroxidase-generated reactive brominating species, reactive nitrogen species, or tyrosyl radicals in vitro confirmed that eosinophil-derived oxidative pathways promote enzyme inactivation.

Conclusion: These findings are consistent with greater oxidant stress in asthma leading to greater inactivation of SOD, which likely amplifies inflammation and progressive airflow obstruction.

Figure 1.

Cu,Zn-SOD (superoxide dismutase) protein, Mn-SOD protein, and total SOD activity in serum of control subjects (n = 20), subjects with nonsevere asthma (n = 74), and subjects with severe asthma (n = 40). Subjects with asthma have decreased SOD activity compared with control subjects (analysis of variance [ANOVA], p = 0.001), and SOD activity levels of subjects with severe and nonsevere asthma are similarly reduced. There is no significant difference in Mn-SOD (ANOVA, p = 0.416) or Cu,Zn-SOD levels among nonsevere, severe asthma, or control groups (ANOVA, p = 0.165), although subjects with asthma as a group tend to have more Cu,Zn-SOD than do control subjects (t test, p = 0.057).

Figure 2.

Evaluation of eGPx protein, GPx activity, and total glutathione (reduced glutathione [GSH] plus oxidized glutathione [GSSG]) in serum of control subjects (n = 20), individuals with nonsevere asthma (n = 74), and individuals with severe asthma (n = 40). eGPx protein, GPx activity, and total serum GSH are not significantly different among the groups (all p > 0.05).

Figure 3.

Analysis of SOD activity and proteins in individuals with asthma, based on airflow limitation. SOD activity is significantly lower in individuals with asthma with FEV₁ lower than 60% of predicted (ANOVA, p = 0.005; %FEV₁ < 60, n = 19; %FEV₁ between 60 and 80, n = 36; %FEV₁ > 80, n = 59). Mn-SOD protein is not different among the groups (ANOVA, p > 0.05), whereas Cu,Zn-SOD protein is significantly different among the groups, with higher levels in individuals with asthma with %FEV₁ < 60 or > 80% (ANOVA, p = 0.03).

Figure 4.

Evaluation of serum eGPx protein, GPx activity, and total glutathione in individuals with asthma, based on airflow limitation. Levels of eGPx protein and GPx are similar among the groups (all p > 0.1), whereas GSH tends to be higher in subjects with asthma with severe airflow limitation (p = 0.08).

Figure 5.

Correlations of serum SOD activity with airflow (%FEV₁, FEV₁/FVC, and ΔFEV₁). SOD activity is directly correlated with %FEV₁ (R = 0.312, p < 0.001) and FEV₁/FVC (R = 0.296, p < 0.001), whereas SOD activity is inversely correlated with hyperresponsiveness, as determined by change in FEV₁ after β-agonist administration (180 μg of albuterol; R = –0.334, p = 0.001).

Figure 6.

Analysis of SOD activity corrected for atopy. Individuals with asthma and control individuals with allergies have lower levels of serum SOD activity, but no significant difference was found among nonatopic and atopic individuals within the same group (control subjects: nonatopic, n = 11; atopic, n = 6; subjects with nonsevere asthma: nonatopic, n = 18; atopic, n = 56; subjects with severe asthma: nonatopic, n = 10; atopic, n = 30; all p > 0.1).

Figure 7.

Loss of specific Cu, Zn-SOD activity occurs after protein is exposed to eosinophil peroxidase–generated reactive nitrogen species (RNS), reactive brominating species (RBS), or tyrosyl radicals (·Tyr) in vitro (p = 0.001).