Interaction With Other Dietary Antioxidants

The potential for the interaction of carotenoids with other antioxidants is discussed in detail in Chapter 3 and only an outline is given here. Truscott (72) first proposed a plausible mechanism for the interaction of vitamins C and E with b-carotene whereby the carotenoid molecule repairs the vitamin E radical [reaction (8)] and the resulting carotenoid cation radical is, in turn, repaired by vitamin C [reactions (9) and (10)]. An additive response has been observed for b-carotene and vitamin E, but a synergistic response was only seen when vitamin C was also present (73). If this model is correct then the reduction in the levels of vitamin C in the plasma of smokers compared with nonsmokers (74) is of significance as the repair of any b-carotene radical cations formed would be impaired. A xanthophyll such as zeaxanthin whose conjugated system spans the membrane (see above) would, in theory, be able to interact much more effectively with both lipid- and water-soluble antioxidants than carotenes such as b-carotene

Figure 1 Variation of the rate constant for the decay of singlet oxygen with carotenoid concentration for zeaxanthin in DPPC liposomes (69). At concentrations up to ~45 mM the response is linear with a quenching rate constant of 2.3 x s . With increasing zeaxanthin concentration the rate progressively decreases and is completely lost at -70 mM.

and lycopene because the carotene radical cation would first have to migrate from the hydrophobic core to the membrane surface to interact with vitamin C, for example. The relatively long lifetime and polarity of the carotenoid radical cation may permit this (27). Vitamin C has been shown to protect both the carotenoid and vitamin E pools in LDLs from Cu2+-mediated oxidative damage (75). The synergistic protection afforded by carotenoids and other co-antioxidants is dependent on a balance between these components and changes in the concentration of any one of these might disturb this balance, reducing antioxidant effectiveness. An increase in carotenoid content, for example, may result in the formation of carotenoid cation radicals or adducts at a level beyond which the tocopherol/ascorbate pool can effectively repair, resulting in pro-oxidant effects.

Carotenoid-carotenoid interactions have rarely been considered, and the vast majority of studies have focused instead on single compounds in isolation. This belies the natural in vivo state in which a heterogeneous stage of a range of xanthophylls and carotenes coexist. A synergistic response between different carotenoids has been reported (76), with a combination of lutein and lycopene proving to be most effective against 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN)-induced oxidation in multilamellar liposomes. The potential for electron transfer between different carotenoids [reaction (11)] has been examined (27,37). Lycopene is a strong reducing agent but astaxanthin weak. While lycopene was able to reduce the radical cations of lutein and zeaxanthin, b-carotene could not.

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