Carotenoids antioxidant action

The ability of carotenoids to act as antioxidants is closely related to their long-chain conjugated polyene structures (see Section 2.2 in Chapter 2). Two main types of antioxidant actions can be distinguished singlet oxygen quenching and reactions with radicals. The first mechanism occurs in vivo in plants and has been extensively studied in vitro. Reactions with radicals of different types have also been extensively studied in vitro under different conditions but their occurrence in vivo is still a matter of discussion. 

As mentioned earlier, physiological concentrations of carotenoids in vivo are in the micromolar range, mainly because of limited bioavailabiUty. Also, the antioxidant efficiencies of carotenoids after absorption are probably limited. Concentrations before absorption are much higher and can justify possible antioxidant actions in vivo. To test this hypothesis, Vulcain et al. developed an in vitro system of lipid peroxidation in which the oxidative stress is of dietary origin (metmyoglobin from meat) and different types of antioxidants (carotenoids, phenols) are tested. 

Colorant containing annatto and Ca caseinate as carrier mixed with water to be added directly to cheese milk yielding uniform colored cheese mass Water-dispersible beadlet of p-carotene is mixed with oil to attein composition that remains stable even in presence of polyphosphates and with antioxidant action even in absence of ascorbic acid Blending carotenoid pigment and soybean fiber (wifii tomato juice) as effective ingredient for dispersion stability... 

Liebler, D.C. et al.. Antioxidant actions of beta-carotene in liposomal and microsomal membranes role of carotenoid-membrane incorporation and alfa-tocopherol, Arch. Biochem. Biophys., 338, 244, 1997. 

In most assays designed to study antioxidant action of carotenoids, the effects of carotenoids were followed for a relatively short periods of time, while carotenoids were still present at substantial concentrations. Carotenoids, such as [1-carotene, lutein, and zeaxanthin, undergo rapid degradation upon exposure to oxidants or irradiation with ultraviolet and visible light (Ojima et al 1993 Siems et al 1999, 2005). 

Rozanowska, MB, Bigaj, J, Boulton, ME, Czuba-Pelech, B, Landrum, J, Rozanowski, B, and Zareba, M, 2004b. Uptake of carotenoids and their antioxidant action in ARPE-19 cells in culture. Invest Ophthalmol Vis Sci 45, U531. 

Carotenoids, present in fruits and vegetables, are widely believed to protect human health. In particular, some epidemiological studies have correlated the intake of carotenoid-rich fruits and vegetables with protection from some forms of cancer (4,5). This action is probably due to the carotenoids antioxidant properties rather than to their vitamin A activity. 

As this paper is prepared for a special volume on bioactive natural products, a brief overview on bioactivity of carotenoids is included, divided into known functions of carotenoids and to their actions on biological systems. Charged carotenoid species are involved in the function of carotenoids in photosynthesis and may turn out to be relevant unstable intermediates in other biological contexts, including antioxidant action. 

Carotenoids as Radical Scavengers—Mechanism of Antioxidant Action... 

Burton, W.G. Antioxidant action of carotenoids. British Journal of Nutrition, 119, 109-111. 1988. 

A large body of scientific evidence suggests that carotenoids scavenge and deactivate free radicals both in vitro and in vivo. It has been reported that their antioxidant action is determined by (1) electron transfer reactions and the stability of the antioxidant free radical (2) the interplay with other antioxidants and (3) their structure and the oxygen pressure of the microenvironment. Moreover, the antioxidant activity of carotenoids is characterized by literature data for (1) their relative rate of oxidation by a range of free radicals, or (2) their capacity to inhibit lipid peroxidation in multilamellar liposomes. ... 

To nnderstand the mechanism of antioxidant activity of the carotenoids it is also important to analyze the oxidation prodncts that are formed during their action as antioxidants. A relationship between prodnct-forming oxidation reactions to carotenoid antioxidant effects has been additionally... 

Packer, L., Antioxidant action of carotenoids-in vitro and in vivo protection against oxidation of hnman LDL, Ann. N. Y. Acad. Sci., 691,48,1993. 

Carotenoids react with free radicals, they deactivate them and thus act as antioxidants. They exhibit antioxidant effects in proportion to their concentration in systems containing Upids and also in vivo. The mechanism of the antioxidant action of carotenoids differs... 

The presence of oxygenated functional groups also modifies the bioavailability of these compounds. It has been demonstrated recently that some ketocarotenoids are more rapidly absorbed and metabolized than other carotenes such as, for instance, lycopene. These xanthophylls do not present provitamin A activity, but their antioxidant action is more effective than that of p-carotene. The incorporation of the carotenoid pigments into cell structures is affected by the pigment stmcture and the presence of functional groups that may modify the interaction with other molecules. Such stmcture, as mentioned above, determines the effectiveness of the pigment s action. 

It is beyond the scope of this chapter to do more than touch on this area of carotenoid research. Suffice it to say that in addition to 02 quenching, carotenoids also interfere with radical-initiated reactions, particularly with those that result in lipid peroxidation. Several reviews on this topic have appeared [12, 16, 17] and newer results suggest that this protection is unlike that seen with classical dietary antioxidants such as a-tocopherol. Burton and Ingold first pointed out the unique antioxidant effects of (3-carotene [18] and recent studies of the in vitro antioxidant activities of (3-carotene and other carotenoids demonstrate excellent activity, in some cases showing a more powerful antioxidant action than a-tocopherol [19]. 

The studies referred to above use LDL in an in vitro assay, but the ultimate antioxidant test is whether these compounds work in animals or humans. There are very few examples of an in vivo antioxidant action of a carotenoid in humans. The best recent examples come from studies with children suffering from cystic fibrosis who are known to have relatively low levels of carotenoids in their serum. These patients are supplemented with large amounts of vitamin E, but still show significantly elevated levels of malondialdehyde in their serum. Two groups have supplemented patients with this disease with p-carotene, and in each case, they reported a decrease in the malondialdehyde levels [27, 20] as well as increased resistance of LDL to oxidant stress [27]. 

Fruifs and vegetables also contain ofher bioactive substances such as polyphenols (including well-known pigments anthocyanins, flavonols) and non-provitamin A carotenoids (mainly lycopene, lutein, and zeaxanthin) that may have protective effects on chronic diseases. Polyphenols and carotenoids are known to display antioxidant activities, counteracting oxidative alterations in cells. Besides these antioxidant properties, these colored bioactive substances may exert other actions on cell signaling and gene expression. 

In the Unites States, the daily intake of 3-carotene is around 2 mg/day Several epidemiological studies have reported that consumption of carotenoid-rich foods is associated with reduced risks of certain chronic diseases such as cancers, cardiovascular disease, and age-related macular degeneration. These preventive effects of carotenoids may be related to their major function as vitamin A precursors and/or their actions as antioxidants, modulators of the immune response, and inducers of gap-junction communications. Not all carotenoids exert similar protective effects against specific diseases. By reason of the potential use of carotenoids as natural food colorants and/or for their health-promoting effects, research has focused on better understanding how they are absorbed by and metabolized in the human body. 

Antioxidant and Prooxidant Actions and Stabilities of Carotenoids In Vitro and In Vivo and Carotenoid Oxidation Products... 

Results obtained in in vivo and ex vivo experiments are of various types. Some studies have found positive effects of the consumption of carotenoids or foods containing carotenoids on the markers of in vivo oxidative stress, even in smokers. Other studies demonstrated no effects of carotenoid ingestion on oxidative stress biomarkers of lipid peroxidation. " It should be noted that for studies using food, the activity observed may also be partly due to other antioxidant molecules in the food (phenols, antioxidant vitamins) or to the combination of actions of all the antioxidants in the food. 

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

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