Highly reactive, free radical-containing molecules in cells or tissues can initiate oxidative chain reactions resulting in oxidative damage to DNA, proteins, and lipids. Pro-oxidant molecules can have free radicals or they can catalyze or initiate reduction/oxidation (redox) reactions that result in the production of free radicals. Enzymes that can catalyze free radical-producing redox reactions include certain hydroxylases, oxidases, oxygenases, peroxidases and synthases. Because unregulated oxidative reactions can result in severe cellular damage, cells utilize anti-oxidant molecules to inhibit excess oxidation that can lead to oxidative stress. Anti-oxidants can be reducing agents or enzymes that catalyze redox reactions resulting in the removal of free radicals. Several classes of enzymes catalyze anti-oxidant reactions including select dehydrogenases, dismutases, glutaredoxins, hydroxylases, oxygenases, peroxidases, reductases, synthases, and thioredoxins.