Increased liver-specific expression of catalase in transgenic mice

Paradoxical Roles of Antioxidant Enzymes: Basic Mechanisms and Health Implications

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are generated from aerobic metabolism, as a result of accidental electron leakage as well as regulated enzymatic processes. Because ROS/RNS can induce oxidative injury and act in redox signaling, enzymes metabolizing them will inherently promote either health or disease, depending on the physiological context. It is thus misleading to consider conventionally called antioxidant enzymes to be largely, if not exclusively, health protective. Because such a notion is nonetheless common, we herein attempt to rationalize why this simplistic view should be avoided. First we give an updated summary of physiological phenotypes triggered in mouse models of overexpression or knockout of major antioxidant enzymes. Subsequently, we focus on a series of striking cases that demonstrate "paradoxical" outcomes, i.e., increased fitness upon deletion of antioxidant enzymes or disease triggered by their overexpression. We elaborate mechanisms by which these phenotypes are mediated via chemical, biological, and metabolic interactions of the antioxidant enzymes with their substrates, downstream events, and cellular context. Furthermore, we propose that novel treatments of antioxidant enzyme-related human diseases may be enabled by deliberate targeting of dual roles of the pertaining enzymes. We also discuss the potential of "antioxidant" nutrients and phytochemicals, via regulating the expression or function of antioxidant enzymes, in preventing, treating, or aggravating chronic diseases. We conclude that "paradoxical" roles of antioxidant enzymes in physiology, health, and disease derive from sophisticated molecular mechanisms of redox biology and metabolic homeostasis. Simply viewing antioxidant enzymes as always being beneficial is not only conceptually misleading but also clinically hazardous if such notions underpin medical treatment protocols based on modulation of redox pathways.

Catalase transgenic mice: characterization and sensitivity to oxidative stress

The role of catalase in the antioxidant defense system was studied using transgenic mice [Tg(CAT)] harboring a human genomic clone containing the entire human CAT gene. Catalase activity was 2-fold higher in the tissues of hemizygous [Tg(CAT)(+/o)] mice and 3- to 4-fold higher in the tissues of homozygous [Tg(CAT)(+/+)] mice compared to wild type mice. The human CAT transgene was expressed in a tissue-specific pattern that was similar to the endogenous catalase gene. The levels of other major antioxidant enzymes were not altered in the tissues of the transgenic mice. Hepatocytes and fibroblasts from the Tg(CAT)(+/+) mice were more resistant to hydrogen peroxide-induced cell death but were more sensitive to paraquat and TNFalpha toxicity. Fibroblasts from the Tg(CAT)(+/+) mice showed reduced growth rate in culture without treatment and reduced colony-forming capability after gamma-irradiation compared to fibroblasts from wild type mice. In addition, the Tg(CAT)(+/+) animals were more sensitive to gamma-irradiation.