Figure 5. Protective effect of N-acetylcysteine and vitamin C against hydrogen peroxide–induced elevation of superoxide anion production in Graves’ ophthalmopathy orbital fibroblasts. After pretreatment of Graves’ ophthalmopathy (GO) orbital fibroblasts (n=7) with 200 μM N-acetylcysteine (NAC) or 500 μM vitamin C (VitC) for 1 h, followed by the addition of 6.25 μM hydrogen peroxide (H₂O₂) for 24 h, the intracellular levels of the superoxide anions were determined with dihydroethidine staining with flow cytometry. The mean values of the superoxide anions from the GO orbital fibroblasts are shown in the histogram. The H₂O₂-induced intracellular levels of the superoxide anions in the GO orbital fibroblasts were significantly abolished by pretreatment with 200 μM NAC or 500 μM VitC. The data are presented as mean ± standard deviation of the results from three independent experiments. (p<0.0005, and p<0. 01 versus the indicated group.)