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Mechanisms and Biomarkers (WG 4) page 16
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tocopheroxyl radical (Buettner, 1993). The ascorbyl radical is reduced back to ascorbic acid
by enzymatic systems using NADH or GSH as co-factors. The ascorbyl radical may have a
strong prooxidant activity in the presence of metal ions leading to the production of
superoxide anion and hydrogen peroxide. This property is largely used to induce peroxidation
reaction in vitro by using ascorbate-iron mixture. Fortunately, in normal physiological
conditions, transition metals are sequestered by many proteins as mentioned above.
Carotenoids are coloured pigments found in vegetables such as carrot, spinach or sweet
potatoes. Some carotenes (ß-carotene) are precursors of vitamin A and many carotenes
including ß-carotene, lutein and lycopene are chain breaking antioxidants and singlet oxygen
quenchers in vitro (Palozza et al., 1992). For instance ß-carotene may react with peroxyl
radical (ROO°) to form a carotenoid radical species or a resonance-stabilized carbon-centered
radical. This latter stabilisation is authorized by the presence of conjugated double bonds able
to disssipate the energy of the unpaired electron. However, the antioxidant activity of
carotenoids may become prooxidant depending on the level of oxygen present (Palozza,
1998). For high oxygen tension (pO2), ß-carotene may react with oxygen and transform into a
peroxyl radical (ßCar-OO°) capable of acting as prooxidant and undergoing autooxidation.
The formation of such radicals have been demonstrated only by indirect in vitro observation
where ß-carotene was used at high concentration and with oxygen tension not likely to occur
in body tissues. However, such a mechanism may be a basis for increased lipid oxidised
products in liver of mice fed a high level of ß-carotene and exposed to methyl mercuric
chloride (Andersen and Andersen, 1993) and the prooxidant activity of ß-carotene in rats fed
vitamin E deficient diet (Lomnitsky et al., 1991). Thus carotene may function as part of the
human antioxidant defence system but may also have prooxidant properties. Epidemiologic
studies have shown that people having a high intake of carotene rich foods had lower rates of
cancer and heart diseases than control populations (Gaziano et al., 1995). However,
carotenoids rich plant foods are also rich in other protective components such as vitamin C,
folate or fibers which may contribute to that observation. In a recent epidemiological study
(Kohlmeier et al., 1997), it was reported that α-carotene, ß-carotene and lycopene each
appeared to be protective against myocardial infarction risk. Other studies reported possible
beneficial effects of lycopene on prostate cancer (Giovanucci et al., 1995) particularly when
lycopene was provided through processed tomatoes. Indeed, lycopene is mainly found in
tomatoes and tomato products and it accounts for about 50 % of carotenoids in human plasma
and is concentrated in the testis (Gerster, 1997). Data on ß-carotene are more controversial. A
recent placebo-controlled study in women healthy volunteers clearly show that low carotenoid