Catalase is an important enzyme in the elimination of H
2O
2 from tissues
(Fig. 1) . Catalase activity has been reported in experimental corneal animal models,
54 55 56 but this is the first report of catalase activity in human corneas. In this study, expression of catalase mRNA levels along with its enzyme activity was increased significantly in KC corneas. In a lens epithelial cell line conditioned to survive H
2O
2 exposure, the major antioxidant enzyme upregulated was catalase.
39 42 Similarly, increased levels of catalase were found in H
2O
2-stressed Chinese hamster fibroblasts, A549 human lung adenocarcinoma cells and U87MG glioblastoma cells.
57 58 59 It is suggested that catalase and not GPX is the major antioxidant enzyme responsible for H
2O
2 degradation.
39 40 41 Because H
2O
2 induces catalase expression, it is likely that KC corneas have elevated levels of this ROS, which could account for some of the oxidative damage associated with KC corneas.
9 Furthermore, under experimental conditions, H
2O
2 can inhibit SOD,
60 which may play a role in the lower SOD3 activities found in KC corneas.
8 Moreover, the cytotoxic by-products of oxidative stress, that is malondialdehyde (MDA) and peroxynitrite, are present in KC corneas.
9 MDA and peroxynitrite are formed from hydrogen peroxide and superoxide, respectively, both highly reactive oxygen species. Taken together with our findings of increased catalase levels, we believe these data support our hypothesis that ROS production and oxidative stress are important elements in KC.