FIG. 1.

Endogenous sources of oxidative stress. Four sources of oxidative stress that represent normal metabolic pathways are represented. The NADPH oxidase is a membrane-bound enzyme complex that generates superoxide. Xanthine oxidase is an enzyme important for uric acid formation, which also generates superoxide as a byproduct of its function. Flavoenzymes are a diverse group of enzymes that are involved in numerous biologic processes and include many monooxidases. Mitochondrial electron transport generates superoxide primarily through complex I and III. Superoxide dismutases inactivate superoxide but generate hydrogen peroxide, which can give rise to hydroxyl radical in the presence of transition metals.