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.
Hydroperoxides are important oxidative substrates because
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.
C. Ascorbate Peroxidase
Hydrogen peroxide in higher plants and algae may also
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
|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
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.