Oxidation carotenoids

Seventy naturally occurring carotenoid epoxides have been referenced and 43 of them have been fully characterized. These compounds can be formally considered oxidation products as defined above, but they first have the status of carotenoids. They are indeed found in vivo and are possibly biosynthesized from the corresponding non-oxidized carotenoids. If carotenoids containing epoxide functions have been found in humans, the epoxidation reaction has not yet been proven to occur in humans. 

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). 

The structures of four of the synthetic carotenoids (beta-carotene, canthaxanthin, beta-apo-8 -carotenol, beta-apo-8 -carotenoic acid) are shown in Fig. 8.2. By virtue of their conjugated double bond structure, they are susceptible to oxidation but formulations with antioxidants were developed to minimize oxidation. Carotenoids are classified as oil soluble but most foods require water soluble colorants thus three approaches were used to provide water dispersible preparations. These included formulation of colloidal suspensions, emulsification of oily solutions, and dispersion in suitable colloids. The Hoffman-LaRoche firm pioneered the development of synthetic carotenoid colorants and they obviously chose candidates with better technological properties. For example, the red canthaxanthin is similar in color to lycopene but much more stable. Carotenoid colorants are appropriate for a wide variety of foods.10 Regulations differ in other countries but the only synthetic carotenoids allowed in foods in the US are beta-carotene, canthaxanthin, and beta-8-carotenol. 

In step , a Qs near the outer surface accepts an electron from Q of Q -P-C. In step , the radical anion Qs " accepts a proton from the nearby aqueous phase, forming a semiquinone Qs H. Step indicates diffusion ofQs H aaoss the membrane toward the interior aqueous phase where the oxidizing carotenoid moiety is located, and there becomes reoxidized to form Qs H [step ]. Evidence for this reaction is provided by the diminished lifetime ofthe radical cation in the presence ofQs, as mentioned above. The protonated quinone shuttle releases a proton [step ] and then diffuses toward the exterior region [step ] and completes the cycle. 

The structure of Thiothece-478 has been revised to (32) and, owing to a change in the u.v. spectrum, its name is also revised to Thiothece-474. A more oxidized carotenoid, Thiothece-484 (33), is an ester of okenone. 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

Since GAs as diterpenes share many intermediates in the biosynthetic steps leading to other terpenoids, eg, cytokinins, ABA, sterols, and carotenoids, inhibitors of the mevalonate (MVA) pathway of terpene synthesis also inhibit GA synthesis (57). Biosynthesis of GAs progresses in three stages, ie, formation of / Akaurene from MVA, oxidation of /-kaurene to GA 2" hyde, and further oxidation of the GA22-aldehyde to form the different GAs more than 70 different GAs have been identified. 

An important function of certain carotenoids is their provitamin A activity. Vitamin A may be considered as having the stmcture of half of the P-carotene molecule with a molecule of water added at the end position. In general, all carotenoids containing a single unsubstituted P carotene half have provitamin A activity, but only about half the activity of P carotene. Provitamin A compounds are converted to Vitamin A by an oxidative enzyme system present in the intestinal mucosa of animals and humans. This conversion apparendy does not occur in plants (see Vitamins, VITAMIN a). 

In nature, vitamin A aldehyde is produced by the oxidative cleavage of P-carotene by 15,15 - P-carotene dioxygenase. Alternatively, retinal is produced by oxidative cleavage of P-carotene to P-apo-S -carotenal followed by cleavage at the 15,15 -double bond to vitamin A aldehyde (47). Carotenoid biosynthesis and fermentation have been extensively studied both ia academic as well as ia iadustrial laboratories. On the commercial side, the focus of these iavestigations has been to iacrease fermentation titers by both classical and recombinant means. 

Rich sources of vitamin A include dairy products such as milk cheese, butter, and ice cream. Eggs as well as internal organs such as the Hver, kidney, and heart also represent good sources. In addition, fish such as herring, sardines, and tuna, and in particular the Hver oil from certain marine organisms, are excellent sources. Because the vitamin A in these food products is derived from dietary carotenoids, vitamin A content can vary considerably. Variation of vitamin A content in food can also result from food processing and in particular, oxidation processes (8). 

Fertile sources of carotenoids include carrots and leafy green vegetables such as spinach. Tomatoes contain significant amounts of the red carotenoid, lycopene. Although lycopene has no vitamin A activity, it is a particularly efficient antioxidant (see Antioxidants). Oxidation of carotenoids to biologically inactive xanthophyUs represents an important degradation pathway for these compounds (56). 

Zeaxanthin (135) was synthesized from the salt (133) and the dialdehyde (134) in 1,2-epoxybutane, a reagent superior to ethylene oxide particularly for polyenedialdehydes. The same salt was also used to prepare /3-cryptoxanthin and zeinoxanthin. Phenolic carotenoids from Strep-tomyces mediolani and 1,2-dihydro- and l,2,r,2 -tetrahydro-lycopene have also been obtained by conventional olefin synthesis. 

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. 

Because the carotenoids favour hydrophobic domains they are generally localised in the membranes and lipoproteins of animal cells. In this location they can influence the oxidation of membrane lipids and prevent the passage of free radicals from one cellular compartment to another. Thus, DNA in the nucleus is protected from intracellularly generated ROS by (at least) the nuclear membrane and from extracellular ROS by a number of membranes. Should ROS reach the nucleus, base oxidation can occur. The base most susceptible to oxidation is guanine, although all other bases can also be affected. The cell has the ability to detect damaged bases, excise them. 

As has already been stated, the carotenoids are lipophilic and are therefore absorbed and transported in association with the lipoprotein particles. In theory, this fortuitous juxtaposition of lipid and carotenoid should confer protection on the lipid through the antioxidant properties of the carotenoid. No doubt some antioxidant protection is afforded by the presence of the carotenoids derived from the diet. However, with one or two exceptions, human supplementation studies have not supported a role for higher dose carotenoid supplements in reducing the susceptibility of the low-density lipoproteins to oxidation, either ex vivo or in vivo (Wright et al, 2002 Hininger et al, 2001 Iwamoto et al, 2000). 

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. 

It is well known that excessive intake of P-carotene may lead to carotenodermia (yellow skin), and it is undoubtedly the case that some carotenoid is directly lost via the skin or through photo-oxidation in the skin. As far as is known the carotenoids are not cytotoxic or genotoxic even at concentrations up to 10 times the normal plasma concentration which may cause carotenodermia. However, they are associated with amenorrhoea in girls who may be consuming bizarre diets and, in long-term supplementation studies, with an increase in lung cancer (The Alpha-tocopherol, Beta-carotene Cancer Prevention Study Group, 1994). 

ASTLEY s B, ELLIOTT R M, ARCHER D B and souTHON s (2002) Increased cellular carotenoid levels reduce the persistance of DNA single strand breaks following oxidative chaUange. Nutrition and Cancer. In press. 

KHACHiK F BEECHER G R and GOLi M B (1992) Separation and identification of carotenoids and their oxidation products in the extracts of human plasma. Anal Chem. 64(18) 2111-22. 

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