Prooxidant activity, carotenoids

It has been established that carotenoid structure has a great influence in its antioxidant activity for example, canthaxanthin and astaxanthin show better antioxidant activities than 3-carotene or zeaxanthin. 3- 3 3-Carotene also showed prooxidant activity in oil-in-water emulsions evaluated by the formation of lipid hydroperoxides, hexanal, or 2-heptenal the activity was reverted with a- and y-tocopherol. Carotenoid antioxidant activity against radicals has been established. In order of decreasing activity, the results are lycopene > 3-cryptoxanthin > lutein = zeaxanthin > a-carotene > echineone > canthaxanthin = astaxanthin. ... 

Many reviews have been written about the antioxidant activities of carotenoids. Some also describe prooxidant activities. - In consequence, only selected points about this very broad subject will be presented in the first part of this chapter. Linked to these properties and important for food nutritional value is the stability of caro-... 

A molecule that has a prooxidant effect can be dehned as a molecule that can react with reactive oxygen species (ROS) to form compounds more deleterious to biomolecules than the ROS alone. Possible prooxidant activity of carotenoids was for the first time mentioned by Burton and Ingold. Since then, many other examples of loss of antioxidant activity or prooxidant activity have been illustrated and reviewed in the literature. Increasing oxygen partial pressure (PO2) and/or carotenoid concentration can convert a carotenoid from antioxidant to prooxidant. Thus, depending on the environment, the same molecule can exert either antioxidant or prooxidant activity. ... 

Various types of cell-based in vitro studies have shown that carotenoids can exert prooxidant effects under certain conditions. Most of these studies show in fact decreases in antioxidant efficacy of carotenoids with increasing carotenoid concentration examples of true prooxidant effects are rarer. It is also important to pay attention to the experimental conditions and their biological relevance. Indeed, carotenoids have sometimes been proven to (I) exert prooxidant activity in an atmosphere of pure oxygen, (2) never occur in vivo, or (3) appear in concentrations that they would never reach in vivo. 

Astaxanthin has exhibited potent antioxidant, immunomodulating, and enzyme-inducing properties, all of which suggest a potential role for this carotenoid in the prevention of cancer. Moreover, its unique stractiual properties and its lack of prooxidant activity make it a prime candidate for further investigation. More research is needed on the absorption and metabolism of this promising anticancer agent in humans and on its interactions with other carotenoids and vitamins in the human system. 

These contradictory results, showing evidence of both antioxidant and prooxidant activity, may suggest that the antioxidant behaviour of carotenoids is closely related to their own oxidation. Studies of the kinetics of carotenoid oxidative degradation under different conditions, as well as of the stmctures of the products formed, have allowed the mechanism of the oxidation process to be clarified (Yanishilieva et al, 1998). 

It has been found (Polyakov et al. 2001c) that when carotenoids are involved in a reaction cycle with the participation of iron as Fe2+, an increase of the total radical yield or a prooxidant effect will occur and will increase with decreasing carotenoid oxidation potential and its scavenging activity. The mechanism of the participating carotenoid is shown in Scheme 9.4 (Polyakov et al. 2001c). 

The concentrations of carotenoids and the level of oxygen they are exposed to can also influence their antioxidant activities. At low oxygen partial pressures, diverse carotenoids effectively inhibit in vitro oxidation reactions, and their antioxidative abilities increase with increasing carotenoid concentration." " As oxygen levels are increased, however, their antioxidant potential typically decreases." " Certain carotenoids, notably P-carotene but also lycopene, exhibit unusual behavior beyond a threshold carotenoid concentration, they actually decrease in antioxidant ability with increasing carotenoid concentration, and this effect is further exacerbated at high oxygen levels." This prooxidant... 

The carotenoid activity during oxidation is strongly influenced by the oxygen pressure (PO2) of the experimental conditions. Kiokias and Oreopoulou have shown that certain natural carotenoid mixtures (paprika, bixin and tomato, and palm-oil preparations) inhibited the azo-initiated oxidation of sunflower oil-in-water emulsions (operated rapidly under low pOj) in terms of both primary and secondary oxidation products. However, other studies " concluded that carotenoids not only did not inhibit aerial lipid autoxidation (high PO2) but even exerted a prooxidant character, a phenomenon also observed at high carotenoid concentrations that could be due mainly to a more increased formation of carotene-peroxyl radicals, promoting the propagation of autoxidation. 

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