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1. Glutathione
Glutathione (Fig. 2) is a tripeptide (γ -glutamyl-cysteinylglycine)
where cysteine can be in either the reduced or
oxidized glutathione state. Reduced glutathione inhibits
lipid oxidation directly by interacting with free radicals
to form a relatively unstable sulfhydryl radical or
by providing a source of electrons, which allows glutathione
peroxidase to enzymically decompose hydrogen
and lipid peroxides. Total glutathione concentrations in
muscle foods range from 0.7–0.9 ug/kg. Oral administration
of 3.0 of glutathione to seven healthy adults did not
result in any increases in plasma glutathione or cysteine
concentrations after 270 minutes. The bioavailability of
glutathione in rats has also been reported to be low. Lack
of, or low absorption of, glutathione may be due to the
hydrolysis of the tripeptide by gastrointestinal protease.
2. Lipoic Acid
Lipoic (thioctic) acid (Fig. 2) is a thiol cofactor for many
plant and animal enzymes. In biological systems, the two
thiol groups of lipoic acid are found in both reduced (dihydrolipoic
acid) and oxidized forms (lipoic acid). Both the
oxidized and reduced forms of the molecule are capable
of acting as antioxidants through their ability to quench
singlet oxygen, scavenge free radicals, chelate iron, and,
possibly, regenerate other antioxidants such as ascorbate
and tocopherols. Lipoic and dehydrolipoic acids can protect
LDL, erythrocytes, and cardiac muscle from oxidative
damage.
Although lipoic acid has been found in numerous biological
tissues, reports on its concentrations in foods are
scarce. Lipoic acid is detectable in wheat germ (0.1 ppm)
but not in wheat flour. It has been detected in bovine liver
kidney and skeletal muscle. Oral administration of lipoic
acid (1.65 g/kg fed) to rats for five weeks resulted in elevated
levels of the thiol in liver, kidney, heart, and skin.
When lipoic acid was added to diets lacking in vitamin E,
symptoms typical of tocopherol deficiency were not observed
suggesting that lipoic acid acts as an antioxidant in
vivo. However, lipoic acidwas not capable of recycling vitamin
E in vivo, as determine by the fact that α-tocopherol
concentrations are not elevated by dietary lipoic acid in vitamin
E deficient rats.
D. Carotenoids
Carotenoids are a chemically diverse group (>600 different
compounds) of yellow to red colored polyenes consisting
of 3–13 conjugates double bonds and in some cases,
six carbon hydroxylated ring structures at one or both ends
of the molecule. β-Carotene is the most extensively studied
carotenoid antioxidant (Fig. 2). β-Carotene will react
with lipid peroxyl radicals to form a carotenoid radical.
Whether this reaction is truly antioxidative seems to depend
on oxygen concentrations, with high oxygen concentrations
resulting in a reduction of antioxidant activity.
The proposed reason for loss of antioxidant activity with
increasing oxygen concentrations involves the formation
of carotenoid peroxyl radicals that autoxidizes into additional
free radicals. Under conditions of low oxygen tension,
the carotenoid radical would be less likely to autooxidize
and thus could react with other free radicals thereby
forming nonradical species with in a net reduction of radical
numbers.
The major antioxidant function of carotenoids in foods
is not due to free radical scavenging but instead is through
its ability to inactivate singlet oxygen. Singlet oxygen
is an excited state of oxygen in which two electrons in
the outer orbitals have opposite spin directions. Initiation
of lipid oxidation by singlet oxygen is due to its electrophillic
nature, which will allow it to add to the double
bonds of unsaturated fatty acids leading to the formation
of lipid hydroperoxides. Carotenoids can inactivate singlet
oxygen by both chemical and physical quenching.
Chemical
quenching results in the direct addition of singlet
oxygen to the carotenoid, leading to the formation
of carotenoid breakdown products and loss of antioxidant
activity.Amore effective antioxidative mechanism of
carotenoids is physical
quenching. The most common energy
states of singlet oxygen are 22.4 and 37.5 kcal above
ground state. Carotenoids physically quench singlet oxygen
by a transfer of energy from singlet oxygen to the
carotenoid, resulting in an excited state of the carotenoid
and ground state, triplet oxygen. Harmless transfer of energy
from the excited state of the carotenoid to the surrounding
medium by vibrational and rotational mechanisms
then takes place. Nine or more conjugated double
bonds are necessary for physical quenching, with the presence
of six carbon oxygenated ring structures at the end
the molecule increasing the effectiveness of singlet oxygen
quenching.
In foods, light will activate chlorophyll, riboflavin, and
heme-containing proteins to high energy excited states.
These photoactivated molecules can promote oxidation
by direct interactions with an oxidizable compounds to
produce free radicals, by transferring energy to triplet oxygen
to form singlet oxygen or by transfer of an electron to
triplet oxygen to form the superoxide anion. Carotenoids
inactivate photoactivated sensitizers by physically absorbing
their energy to form the excited state of the carotenoid
that then returns to the ground state by transfer of energy
into the surrounding solvent.
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