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Antioxidants, nature and chemistry
Submitted by Dr. Tamer Fouad, M.D.
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Antioxidants are substances that
protect other chemicals of the body from damaging oxidation
reactions |
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Beta Carotene
Source and Nature:
Carotenoids are pigmented micronutrients
present in fruits and vegetables.
Carotenoids are precursors of vitamin A
and have antioxidant effects. While over 600 carotenoids have been found in the
food supply, the most common forms are alpha-carotene, beta-carotene, lycopene,
crocetin, canthaxanthin, and fucoxanthin. Beta-carotene is the most widely
studied. It is composed of two molecules of vitamin A (retinol) joined together.
Dietary beta-carotene is converted to retinol at the level of the intestinal
mucosa.
Mechanisms of Action:
The antioxidant function of
beta-carotene is due to its ability to quench singlet oxygen, scavenge free
radicals and protect the cell membrane lipids from the harmful effects of
oxidative degradation (Krinsky and Deneke,
1982; Santamaria et al. 1991). The quenching involves a physical reaction in
which the energy of the excited oxygen is transferred to the carotenoid, forming
an excited state molecule (Krinsky, 1993).
Quenching of singlet oxygen is the basis for beta-carotene's
well known therapeutic efficacy in erythropoietic protoporphyria (a
photosensitivity disorder) (Matthews-Roth,
1993). The ability of beta-carotene
and other carotenoids to quench excited oxygen, however, is limited, because the
carotenoid itself can be oxidized during the process (autoxidation). Burton and
Ingold (Burton and Ingold, 1984) and
others have shown that beta-carotene
autoxidation in vitro is dose-dependent and dependent upon oxygen
concentrations. At higher concentrations, it may function as a pro-oxidant and
can activate proteases.
In addition to singlet oxygen,
carotenoids are also thought to quench other oxygen free radicals. It is also
suggested that beta carotene might react directly with the peroxyl radical at
low oxygen tensions; this may provide some synergism to vitamin E which reacts
with peroxyl radicals at higher oxygen tensions (Cotgreave
et al. 1988).
b-carotene
(CAR) + LOO˙
®
LOO-CAR˙
LOO-CAR˙+
LOO˙®
LOO-CAR-OOL
Carotenoids also have been reported to
have a number of other biologic actions, including immuno-enhancement;
inhibition of mutagenesis and transformation; and regression of premalignant
lesions.
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