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Antioxidants, nature and chemistry

Submitted by Dr. Tamer Fouad, M.D.

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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