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Encyclopedia of Physical Science and Technology EN010B-472 July 16, 2001 15:41

Natural Antioxidants In Foods 341

it to inactivate metals in the lipid phase. Chelator activity III. ANTIOXIDANT ENZYMES
is pH dependent with a pH below the pK a of the ion-
izable groups resulting in protonation and loss of metal A. Superoxide Anion
binding activity. Chelator activity is also decreased in the
Superoxide anion is produced by the addition of an elec-
presence of high concentrations of other chelatable non-
tron to molecular oxygen. Superoxide anion can promote
prooxidative metals (e.g., calcium), which will compete
oxidative reactions by (1) reduction of transition metals
with the prooxidative metals for binding sites.
to their more prooxidative state, (2) promotion of metal
release from proteins, (3) through the pH dependent for-
mation of its conjugated acid which can directly catalyze
B. Metal-Binding Proteins
lipid oxidation, and (4) through its spontaneous dismu-
The reactivity of prooxidant metals in biological tissues tation into hydrogen peroxide. Due to the ability of su-
are mainly controlled by proteins. Metal binding pro- peroxide anion to participate in oxidative reactions, the
teins in foods include transferrin(blood plasma), phosvitin biological tissues from which foods originate will contain
(egg yolk), lactoferrin (milk), and ferritin (animal tissues). superoxide dismutase (SOD).
Transferrin, phosvitin, and lactoferrin are structurally sim- Two forms of SOD are found in eukaryotic cells, one in
ilar proteins consisting of a single polypeptide chain with the cytosol and the other in the mitochondria. Cytosolic
a molecular weight ranging from 76,000–80,000. Trans- SOD contains copper and zinc in the active site. Mito-
ferrin and lactoferrin each bind two ferric ions, whereas chondrial SOD contains manganese. Both forms of SOD
phosvitin has been reported to bind three. Ferritin is a mul- catalyze the conversion of superoxide anion (O 2 )tohy-
−
tisubunit protein (molecular weight of 450,000) with the drogen peroxide by the following reaction.
capability of chelating up to 4500 ferric ions. Transferrin,
+
phosvitin, lactoferrin, and ferritin inhibit iron-catalyzed 2O 2 + 2H → O 2 + H 2 O 2 .
−
lipid oxidation by binding iron in its inactive ferric state
and, possibly, by sterically hindering metal/peroxide in-
teractions. Reducing agents (ascorbate, cysteine, and su-
B. Catalase
peroxide anion) and low pH can cause the release of iron
from many of the iron-binding proteins, resulting in an Hydroperoxides are important oxidative substrates be-
acceleration of oxidative reactions. Copper reactivity is cause they decompose via transition metals, irradiation,
controlled by binding to serum albumin, ceruloplasmin, and elevated temperatures to form free radicals. Hydro-
and the skeletal muscle dipeptide, carnosine. gen peroxide exists in foods due to its direct addition (e.g.,
aseptic processing operations) and by its formation in bi-
ological tissues by mechanisms including the dismutation
C. Phytic Acid of superoxide by SOD and the activity of peroxisomes.
Lipid hydroperoxides are naturally found in virtually all
Phytic acid or myoinositol hexaphophate is one of the pri-
food lipids. Removal of hydrogen and lipid peroxides from
mary metal chelators in seeds where it can be found at con- biological tissues is critical to prevent oxidative damage.
centrations ranging from 0.8–5.3% (Fig. 2). Phytic acid is Therefore, almost all foods originating from biological tis-
not readily digested in the human gastrointestinal tract sues contain enzymes that decompose peroxides into com-
but can be digested by dietary plant phytases and by phy- pounds less susceptible to oxidation. Catalase is a heme-
tases originating from enteric microorganisms. Phytate is containing enzyme that decomposes hydrogen peroxide
highly phosphorylated, thus, allowing it to form strong by the following reaction.
chelates with iron, with the resulting iron chelates having
lower reactivity. The antioxidant properties of phytic acid 2H 2 O 2 → 2H 2 O + O 2 .
are thought to help minimize oxidation in legumes and
cereal grains as well as in foods that may be susceptible to
oxidation in the digestive tract. Phytic acid has been cited C. Ascorbate Peroxidase
as a preventative agent in iron-mediated colon cancer. Al- Hydrogen peroxide in higher plants and algae may also
though phytate may be beneficial toward colon cancer, it be decomposed by ascorbate peroxidase. Ascorbate per-
should be noted that it can potentially have deleterious oxidase inactivates hydrogen peroxide in the cytosol and
health effects because of its ability to dramatically de- chloroplasts by the following mechanism.
crease the bioavailability of minerals including iron, zinc,
and calcium. 2 ascorbate+H 2 O 2 → 2 monodehydroascorbate+2H 2 O.