Prooxidant

Biological Antioxidant Models. Tea extracts, tea polyphenol fractions, and purified catechins have all been shown to be effective antioxidants in biologically-based model systems. A balance between oxidants and antioxidants is critical for maintenance of homeostasis. Imbalances between free radicals and antioxidants may be caused by an increased production of free radicals or decreased effectiveness of the antioxidants within the reaction system. These imbalances can be caused by the radicals overwhelming the antioxidants within the system, or by an excess of antioxidants leading to a prooxidant functionaHty (105—118). When antioxidant defense systems are consistently overwhelmed by oxidative reactions, significant damage can... 

Ascorbic acid also forms soluble chelate complexes with iron (142—145). It seems ascorbic acid has no effect on high iron levels found in people with iron overload (146). It is well known, in fact, that ascorbic acid in the presence of iron can exhibit either prooxidant or antioxidant effects, depending on the concentration used (147). The combination of citric acid and ascorbic acid may enhance the iron load in aging populations. Iron overload may be the most important common etiologic factor in the development of heart disease, cancer, diabetes, osteoporosis, arthritis, and possibly other disorders. The synergistic combination of citric acid and ascorbic acid needs further study, particularly because the iron overload produced may be correctable (147). 

Where there is a danger of contamination of a hydrocarbon polymer with such ions it is common practice to use a chelating agent which forms a complex with the metal. It is, however, important to stress that a chelating agent which effectively slows down oxidation initiated by one metal ion may have a prooxidant effect with another metal ion. Table 7.5 summarises some work by... 

T24.Dyhukt, J. M., Ankarcrona, M., Burkitt, M.,. Sjbholm,. 4.,. Strom, K., Orrenius, S., and Nico-tera, P. (1994). Different prooxidant levels stimulate grownh, trigger apoptosis, or produce necrosis of insulin-secreting RINm5F cells./. Biol. Chem. 269, 30553-30560. 

PALOZZA p (1998) Prooxidant actions of carotenoids in biological systems , Nutr Rev, 56, 257. 

CAO G, SOFIC E and prior r l (1997) Antioxidant and prooxidant behavior of flavonoids structure-activity relationships . Free Rad Biol Med, 22 (5), 749-60. 

Olive oil Elenolic acid derivatives bonded to tyrosol are main components (400 ppm provides protection, in low concentration prooxidative) Cooked tuna in brine Less protection of cooked tuna in oil Medina et al., 1999... 

ARUOMA o I (1996) Assessment of potential prooxidant and antioxidant actions, 73, 1617-25. 

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. ... 

It is possible that dietary flavonoids participate in the regulation of cellular function independent of their antioxidant properties. Other non-antioxidant direct effects reported include inhibition of prooxidant enzymes (xanthine oxidase, NAD(P)H oxidase, lipoxygenases), induction of antioxidant enzymes (superoxide dismutase, gluthathione peroxidase, glutathione S-transferase), and inhibition of redox-sensitive transcription factors. 

Young, A.J. and Lowe, G.M., Antioxidant and prooxidant properties of carotenoids. Arch. Biochem. Biophys., 385, 20, 2001. 

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

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. 

Two main mechanisms by which a carotenoid can become a prooxidant have been proposed and reviewed ... 

Carotenoid reactions with ROS or RNS (reactive nitrogen species) would generate prooxidative products. ... 

Interestingly, early examples of carotenoid autoxidation in the literature described the influence of lipids and other antioxidants on the autoxidation of carotenoids." " In a stndy by Budowski et al.," the influence of fat was fonnd to be prooxidant. The oxidation of carotenoids was probably not only cansed by molecnlar oxygen bnt also by lipid oxidation products. This now well-known phenomenon called co-oxidation has been stndied in lipid solntions, in aqueons solntions catalyzed by enzymes," and even in food systems in relation to carotenoid oxida-tion." The inflnence of a-tocopherol on the antoxidation of carotenoids was also stndied by Takahashi et al. ° who showed that carotene oxidation was snppressed as... 

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. 

Truscott, T.G., Beta-carotene and disease a suggested prooxidant and anti-oxidant mechanism and speculations concerning its role in cigarette smoking, J. Photochem. Photobiol. B, 35, 233, 1996. 

Bowry, V.W. and Stocker, R. (1993). Tocopherol-mediated peroxidation. The prooxidant effect of vitamin E on the radical-initiated oxidation of human low-density lipoprotein. J. Am. Chem. Soc. 115, 6029-6044. 

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