Antioxidant Enzymes

A. Superoxide Anion
Superoxide anion is produced by the addition of an electron to molecular oxygen. Superoxide anion can promote oxidative reactions by (1) reduction of transition metals to their more prooxidative state, (2) promotion of metal release from proteins, (3) through the pH dependent formation of its conjugated acid which can directly catalyze lipid oxidation, and (4) through its spontaneous dismutation into hydrogen peroxide. Due to the ability of superoxide anion to participate in oxidative reactions, the biological tissues from which foods originate will contain superoxide dismutase (SOD).

Two forms of SOD are found in eukaryotic cells, one in the cytosol and the other in the mitochondria. Cytosolic SOD contains copper and zinc in the active site. Mitochondrial SOD contains manganese. Both forms of SOD catalyze the conversion of superoxide anion (O₂₋) to hydrogen peroxide by the following reaction.

2H2 + 2H+ → O2 + O2O2.

B. Catalase
Hydroperoxides are important oxidative substrates because they via transition metals, irradiation, and elevated temperatures to form free radicals. Hydrogen peroxide exists in foods due to its direct addition (e.g., aseptic processing operations) and by its formation in biological tissues by mechanisms including the dismutation of superoxide by SOD and the activity of peroxisomes. Lipid hydroperoxides are naturally found in virtually all food lipids. Removal of hydrogen and lipid peroxides from biological tissues is critical to prevent oxidative damage. Therefore, almost all foods originating from biological tissues contain enzymes that decompose peroxides into compounds less susceptible to oxidation. Catalase is a hemecontaining enzyme that decomposes hydrogen peroxide by the following reaction.

2H2O2 → H2O + O2.

C. Ascorbate Peroxidase
Hydrogen peroxide in higher plants and algae may also be decomposed by ascorbate peroxidase. Ascorbate peroxidase inactivates hydrogen peroxide in the cytosol and chloroplasts by the following mechanism.

2 ascorbate + H2O2 → 2 monodehydroascorbate + 2H2O.

Two ascorbate peroxidase isozymes have been described that differ in molecular weight (57,000 versus 34,000), substrate specificity, pH optimum, and stability.

D. Glutathione Peroxidase
Many foods also contain glutathione peroxidase. Glutathione peroxidase differs from catalase in that it decomposes both lipid and hydrogen peroxides. GSH-Px is a selenium-containing enzyme that catalyzes hydrogen or lipid (LOOH) peroxide reduction using reduced glutathione (GSH):

H2O2 + 2GSH—2H2O + GSSG

or,

LOOH + 2GSH—LOH + H2O + GSSG,

where GSSG is oxidized glutathione and LOH is a fatty acid alcohol. Two types of GSH-Px exist in biological tissues, of which one shows high specificity for phospholipid hydroperoxides.

E. Antioxidant Enzymes in Foods
Antioxidant enzyme activity in foods can be altered in rawmaterials and finished products. Antioxidant enzymes differ in different genetic strains and at different stages of development in plant foods. Heat processing and food additives (e.g., salt and acids) can inhibit or inactivate antioxidant enzyme activity. Dietary supplementation of selenium can be used to increase the glutathione peroxidase activity of animal tissues. These factors suggests that technologies could be developed to increase natural levels of antioxidant enzymes in raw materials and/or minimize their loss of activity during food processing operations.