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.
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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