Other Oxidized Carotenes

Research on carotenoids and cardiovascular disease (CVD) stems from the discovery that the etiology of this disease involves oxidative processes that may be slowed by exogenous antioxidants. One of the best understood processes contributing to development of CVD is the oxidation of low-density lipoprotein (LDL). When LDL becomes oxidized, it is readily taken up by foam cells in the vascular endothelium where it contributes to the development of atherosclerotic lesion. Enhancement of the oxidative stability of LDL may also prevent other oxidative steps involved in clinical expression of coronary disease (e.g., myocardial infarction) and possibly steps not related to LDL oxidation. There is optimism about the potential role of P-carotene in prevention of CVD... 

A variety of C4q carotenes do not yield vitamin A on oxidation and these variously have altered cyclic groups or no cyclic, groups at all and can be variously oxidized or reduced. Good examples are the widespread lutein (X —(IP)2—(PI)2—X ) (yellow) and the non-cyclic carotenes lycopene ( /, /-carotene the orange-red colour of tomatoes and other fruits), -carotene (7,8,7, 8 -tetrahydro- / V"carotenei yellow) and lyc.oxanthin ( /, /-caroten-16-0I yellow). 

In contrast to B Carotene, 7o is reported to be very stable toward 02 oxidation. For this reason the unique, stable 1,4-quinone methides could enjoy general use as I-O2 quenchers. Since 68 can be formed easily in the substrate protected by by other oxidative routes (117), (and similar compounds, e.g., JO ) may exert their protective effect in the polymer by a dual mechanism first, by I02 quenching, and second, by retarding the 2 oxidation of phenol 66, thus saving it for other, probably more Important, free-radical Inhlbltlve processes. 

Ionones are metabolites of the corresponding carotenoids. As long ago as 1910, Richard WiUstatter [53] observed the oxidation of carotene. It could be shown experimentally that carotene reacted photochemically with oxygen in the absence of a sensitiser to produce ) -ionone and other oxidation products. [54] Also, thermolysis of -carotene produces ji-ionone in significant amounts. [55]... 

Exempt colorants are made up of a wide variety of organic and inorganic compounds representing the animal, vegetable, and mineral kingdoms. Some, like -carotene and 2inc oxide, are essentially pure factory-produced chemicals of definite and known composition. Others, including annatto extract, cochineal extract, caramel, and beet powder are mixtures obtained from natural sources and have somewhat indefinite compositions. 

The antioxidant activities of carotenoids and other phytochemicals in the human body can be measured, or at least estimated, by a variety of techniques, in vitro, in vivo or ex vivo (Krinsky, 2001). Many studies describe the use of ex vivo methods to measure the oxidisability of low-density lipoprotein (LDL) particles after dietary intervention with carotene-rich foods. However, the difficulty with this approach is that complex plant foods usually also contain other carotenoids, ascorbate, flavonoids, and other compounds that have antioxidant activity, and it is difficult to attribute the results to any particular class of compounds. One study, in which subjects were given additional fruits and vegetables, demonstrated an increase in the resistance of LDL to oxidation (Hininger et al., 1997), but two other showed no effect (Chopra et al, 1996 van het Hof et al., 1999). These differing outcomes may have been due to systematic differences in the experimental protocols or in the populations studied (Krinsky, 2001), but the results do indicate the complexity of the problem, and the hazards of generalising too readily about the putative benefits of dietary antioxidants. 

The mechanisms of the metabolism and excretion of P-carotene are not clear, other than the identification of a number of partially oxidised intermediates found in plasma (Khachik et al., 1992). It is assumed that the carotenoids are metabolised in a manner analogous to the P-oxidation of fatty acids although there is no evidence for this. 

The second pathway is the eccentric cleavage that occurs at double bonds other than the central 15,15 -double bond of the P-carotene molecule to produce different products called P-apocarotenals with various chain lengths. Because only trace amounts of apocarotenals were detected in vivo from tissues of animals fed P-carotene and these compounds can be formed non-enzymatically from P-carotene auto-oxidation, the existence of this pathway was controversial until recently. The identification of P-carotene 9, 10 -oxygenase (BC02), which acts specifically at the 9, 10 double bond of P-carotene to produce P-apo-lO -carotenal and P-ionone, provided clear evidence of the eccentric cleavage pathway in vivo. Lycopene was also reported as a substrate for BC02 activity. 

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... 

Another study showed that a mixture of oxidative metabolites of P-carotene, but not P-carotene, was able to increase the binding of benzo[a]pyrene to DNA. Other mixtures of P-carotene cleavage products have been shown to induce oxidative stress in vitro,exert cytotoxic and genotoxic effects, and inhibit gap junction intercellular communications. It has been suggested that these detrimental effects could possibly occur in vivo following the intake of high doses of carotenoids. 

The lag-phase measurement at 234 nm of the development of conjugated dienes on copper-stimulated LDL oxidation is used to define the oxidation resistance of different LDL samples (Esterbauer et al., 1992). During the lag phase, the antioxidants in LDL (vitamin E, carotenoids, ubiquinol-10) are consumed in a distinct sequence with a-tocopherol as the first followed by 7-tocopherol, thereafter the carotenoids cryptoxanthin, lycopene and finally /3-carotene. a-Tocopherol is the most prominent antioxidant of LDL (6.4 1.8 mol/mol LDL), whereas the concentration of the others 7-tocopherol, /3-carotene, lycopene, cryptoxanthin, zea-xanthin, lutein and phytofluene is only 1/10 to 1/300 of a-tocopherol. Since the tocopherols reside in the outer layer of the LDL molecule, protecting the monolayer of phospholipids and the carotenoids are in the inner core protecting the cholesterylesters, and the progression of oxidation is likely to occur from the aqueous interface inwards, it seems reasonable to assign to a-tocopherol the rank of the front-line antioxidant. In vivo, the LDL will also interact with the plasma water-soluble antioxidants in the circulation, not in the artery wall, as mentioned above. 

The literature contains other examples of the chemical oxidation of carotenoids that aim to mimic oxidation processes that potentially occur in vivo. For example, hypochlorous acid, an oxidant produced by polymorphonuclear leukocytes during inflammatory processes, was shown to oxidatively cleave [3-carotene into apocarotenals and shorter chain compounds (Sommerburg et al. 2003). 

Moreover in the retina, iron is a cofactor of a number of other enzymes, including nitric oxide synthase, (i-carotene monooxygenase, and RPE65-isomerohydrolase converting all-tranx-retinol to 11 -m -retinol in the visual cycle. 

One possible mechanism responsible for cooperative action of antioxidants is reduction of a semi-oxidized carotenoid by another antioxidant. Carotenoid cation radicals can be reduced, and therefore recycled to the parent molecule, by a-tocopherol, ascorbate, and melanins (Edge et al., 2000b El-Agamey et al., 2004b) (Figure 15.5). Interestingly, lycopene can reduce radical cations of other carotenoids, such as astaxanthin, (3-carotene, lutein, and zeaxanthin (Edge et al., 1998). 

Antioxidants in fruits and vegetables including vitamin C and (3-carotene reduce oxidative stress on bone mineral density, in addition to the potential role of some nutrients such as vitamin C and vitamin K that can promote bone cell and structural formation (Lanham-New 2006). Many fruits and vegetables are rich in potassium citrate and generate basic metabolites to help buffer acids and thereby may offset the need for bone dissolution and potentially preserve bone. Potassium intake was significantly and linearly associated with markers of bone turnover and femoral bone mineral density (Macdonald and others 2005). 

Carrots (Daucus carota) are excellent sources of (3-carotene and vitamin A, although they have been reported to exert low antioxidant activity compared to some other vegetables (Al-Saikhan and others 1995 Cao and others 1996 Ramarathnam and others 1997 Vinson and others 1998 Beom and others 1998). However, boiling carrots for 30 min significantly improved their antioxidant activity toward coupled oxidation of (3-carotene and linolenic acid (Gazzani and others 1998). 

Fresh peppers are excellent sources of vitamins A and C, as well as neutral and acidic phenolic compounds (Howard and others 2000). Levels of these can vary by genotype and maturity and are influenced by growing conditions and processing (Mejia and others 1988 Howard and others 1994 Lee and others 1995 Daood and others 1996 Simmone and others 1997 Osuna-Garcia and others 1998 Markus and others 1999 Howard and others 2000). Peppers have been reported to be rich in the provitamin A carotenoids (3-carotene, a-carotene, and (3-cryptoxanthin (Minguez-Mosquera and Hornero-Mendez 1994 Markus and others 1999), as well as xanthophylls (Davies and others 1970 Markus and others 1999). Bell peppers have been shown to exert low antioxidant activity (Al-Saikhan and others 1995 Cao and others 1996 Vinson and others 1998) or may even act as pro-oxidants (Gazzani and others 1998). 

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