Regulatory Antioxidant Enzymes

Enzymes play an important role in the production of radicals and their metabolism. Techniques to measure pro-oxidant conditions that generate radicals and end-products are described in various chapters throughout this book. The major defense enzymes are superoxide dismutase (SOD), which converts the superoxide radical to hydrogen peroxide: (H
2
O
2
), catalase (CAT), selenium (SE)-dependent glutathione peroxidase (GSHPx), and leukocytic myeloperoxidase, which degrade inorganic and lipid hydroperoxides formed by interaction with reactive oxygen species and glutathione-S-transferase (GST; which also has peroxidase activity, but is selenium-independent) (
1
). The activity of GSHPx is coupled to glutathione reductase (GSSG-R), which maintains reduced glutathione (GSH) levels (
2
). Enzyme activity can be decreased by negative feedback from excess substrate or from damage by oxidative modification (
3
).