Chemical structure and properties

All carotenoids may be derived from the acylic C₄₀H₅₆ structure having a long central chain of conjugated double bonds (Pfander, Reference PFANDER1992) and they can be described by the general formula C₄₀H₅₆O_n. Where n is 0-6 hydrocarbons (n=0) these compounds are called carotenes, whereas oxygenated carotenoids (n=1-6) are termed xanthophylls (Castenmiller and West, Reference CASTENMILLER and WEST1998). The basic structure of canthaxanthin (CX) is shown in Figure 1. Carotenoids are based upon the same C40 isoprenoid skeleton, which is modified by cyclisation, addition, elimination, rearrangement and substitution (Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997).

Figure 1

Chemical structure of Canthaxanthin .

Molecular mechanisms of carotenoid action in animals

It is well known that carotenes are precursors of vitamin A and this function has been well defined. However, less than 10% of known carotenoids can be converted to vitamin A, and clearly-defined roles for non-provitamin carotenoids have still to be definitively established (Thurnham and Northrop-Clewes, Reference THURNHAM and NORTHROP-CLEWES1999; Faulks and Southon, Reference FAULKS and SOUTHON2005), but evidence is emerging for several important functions. These include:

• • antioxidant activities (Krinsky, Reference KRINSKY1989a; Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997; Edge et al., Reference EDGE, MCGARVEY and TRUSCOTT1997; Moller et al., Reference MOLLER, BIARD, BLOUNT, HOUSTON, NINNI, SAINO and SURAI2000; Surai et al., Reference SURAI, SPEAKE and SPARKS2001; Surai, Reference SURAI2002);

• • the promotion of cell differentiation (Zhang et al., Reference ZHANG, COONEY and BERTRAM1991; Zhang et al., Reference ZHANG, COONEY and BERTRAM1992; Rock et al., Reference ROCK, KUSLUSKI, GALVEZ and ETHIER1995);

• • regulation of cell proliferation (Krinsky, Reference KRINSKY1992; Bertram and Borthiewicz, Reference BERTRAM and BORTHIEWICZ1995);

• • regulation of intracellular communication via gap junctions (Sies ans Stahl, 1997; Stahl and Sies, Reference STAHL and SIES2005);

• • regulation of cellular levels of the detoxifying enzymes (De Flora, 1999);

• • enhancement of immune function (Bendich, Reference BENDICH1991; Hughes, Reference HUGHES1999; Moller et al., Reference MOLLER, BIARD, BLOUNT, HOUSTON, NINNI, SAINO and SURAI2000);

• • natural colorants providing coloration to birds, reptiles, amphibians, fish and various invertebrates (Surai, Reference SURAI2002);

• • cell membrane stabilisers in molluscs (Surai, Reference SURAI2002).

These functions are considered to be responsible for the various health-promoting properties of carotenoids. However, the physiological requirement for carotenoids in avian species is not yet established and more work in this field is needed to understand how carotenoids fulfil any essential nutritional function in poultry.

Antioxidant properties of carotenoids

The antioxidant potential of carotenoids was first described in 1932 (Monoghan and Schmitt, Reference MONOGHAN and SCHMITT1932). The discovery by Foote and Denny (Reference FOOTE and DENNY1968) that carotenoids, such as β-carotene, lycopene, zeaxanthin, lutein and CX, could quench singlet oxygen ¹O₂ was an important advance in understanding the effectiveness of carotenoid pigments in preventing damage within photobiological systems (Foote et al., Reference FOOTE, CHANG and DENNY1970). Sixteen years later, Burton and Ingold (Reference BURTON and INGOLD1984) proposed the mechanism of quenching lipid radicals within biological membranes by carotenoids. In recent years an important role of carotenoids in biological systems as antioxidants has received substantial attention (Krinsky, Reference KRINSKY1989a; Edge et al., Reference EDGE, MCGARVEY and TRUSCOTT1997; Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997; Bast et al., Reference BAST, HAENEN, VAN DEN BERG and VAN DEN BERG1998; Surai, Reference SURAI2002; Stahl and Sies, Reference STAHL and SIES2003).

The antioxidant properties of carotenoids include scavenging singlet oxygen and peroxyl radicals (Krinsky, Reference KRINSKY1989b; Terao et al., Reference TERAO, BOEY, OJIMA, NAGAO, SUZUKI and TAKAMA1992), sulphur radicals (Chopra et al., Reference CHOPRA, WILLSON and THURNHAM1993) as well as thiyl, sulphonyl and NO₂ radicals (Everett et al., Reference EVERETT, DENNIS, PATEL, MADDIX, KUNDU and WILSON1996) and provide protection for lipids against superoxide and hydroxyl radical attack (Krinsky and Deneke, Reference KRINSKY and DENEKE1982). The mechanism of protection of biological systems against damage due to ¹O₂ by carotenoids includes both a physical component as well as a chemical reaction between the carotenoid and the reactive oxygen molecule (Krinsky, Reference KRINSKY1989a). The deactivation of ¹O₂ by carotenoids results predominantly in physical quenching, a process involving transfer of reactive energy from ¹O₂ to the carotenoids and resulting in the formation of ground state oxygen ³O₂ and triplet reactive carotenoid ³Car* (Stahl and Sies, Reference STAHL and SIES1993). In real terms, this means that, instead of participating in further chemical reactions, the carotenoid returns to its ground state, dissipating its energy by interaction with the surrounding solvent. Therefore, carotenoids can actively quench singlet oxygen (^IO₂) and prevent lipid peroxidation caused by singlet oxygen and can intercept the propagation step of lipid peroxidation in vitro (Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997).

The physical quenching reaction involves the transfer of energy from high-energy state molecules, such as ¹O₂, to the carotenoid (CAR) with a formation of the carotenoid triplet (Bast et al., Reference BAST, HAENEN, VAN DEN BERG and VAN DEN BERG1998):

¹O₂ + CAR = ³O₂ + ³CAR

In the subsequent reaction the carotenoid dissipates its energy as heat and returns to its basic state:

³CAR = CAR + heat

Since the carotenoids remain intact during physical quenching of ¹O₂ or reactive compounds, they can be reused several fold in quenching cycles. Among the various carotenoids, xanthophylls as well as carotenes have proven to be efficient quenchers of singlet oxygen interacting with reaction rates that approach diffusion control (Foote and Denny, Reference FOOTE and DENNY1968; Conn et al., Reference CONN, SCHALCH and TRUSCOTT1991). The efficacy of carotenoids for physical quenching is related to the number of conjugated double bonds present in the molecule which determines their lowest triplet energy level. In this way, one molecule of β-carotene is able to quench 1000 molecules of singlet oxygen before it reacts chemically and forms products (Bast et al., Reference BAST, HAENEN, VAN DEN BERG and VAN DEN BERG1998; Krinsky, Reference KRINSKY1998). Maximum protection is afforded by carotenoids which have nine or more double bonds (Krinsky, Reference KRINSKY1989a) and CX is one such effective carotenoids. When reactions between ¹O₂ and carotenoids takes place through chemical scavenging, oxidative products of carotenoids are formed, but this is considered to be a very minor side reaction (Edge et al., Reference EDGE, MCGARVEY and TRUSCOTT1997) and the antioxidant impact of this chemical reaction is negligible.

Carotenoids are able to react with a range of free radicals (R*) and in this case three possible mechanisms are considered:

• • electron transfer with a formation of carotenoid radical cation:

R* + CAR = R^- + CAR*⁺

• • addition reaction with the formation of a carotenoid-adduct radical which can react with another radical to form a non-radical product:

ROO* + CAR = ROO-CAR*

ROO-CAR* + ROO* = ROO-CAR-ROO

• • hydrogen abstraction with a formation of the neutral carotenoid radical:

R* + CAR(H) = RH + CAR*

In accordance with widely accepted views, the addition reaction and/or hydrogen abstraction are the more probable reactions that occur between free radicals and carotenoids (Kennedy and Liebler, Reference KENNEDY and LIEBLER1991; Reference KENNEDY and LIEBLER1992).

The relative reduction potentials of a variety of carotenoids have been established by monitoring the reaction of carotenoid radical anion (CAR1(*-)) with another carotenoid (CAR2) in hexane and benzene (Edge et al., Reference EDGE, EL-AGAMEY, LAND, NAVARATNAM and GEORGE TRUSCOTT2007). This work illustrated that the presence of a carbonyl group causes the reducing ability to decrease. Indeed, the radical cations are strong oxidising agents and the authors have shown that the radical anions are very strong reducing agents.

The chemical reactions between radical species and carotenoids should result in certain products, which include epoxy, hydroxy and carbonyl derivatives of the original molecules. When CX was reacted with peroxyl radicals generated by thermolysis of 2,2‘-azobis(2,4-dimethylvaleronitrile) (AMVN) in benzene (Yamauchi and Kato, Reference YAMAUCHI and KATO1998), the peroxyl radical addition occurred during the AMVN-initiated peroxidation of methyl linoleate. In contrast to the action of other antioxidants such as vitamin E, where reactions with free radicals involves electron or hydrogen transfer, the above products appear to be formed by radical addition to the carotenoid molecule (Canfield et al., Reference CANFIELD, VALENZUELA and FORAGE1992).

Evaluation of the antioxidant activity of carotenoids

The antioxidant activity of carotenoids in vitro has been characterised by using different methodological approaches (Soffers et al., Reference SOFFERS, VAN HAANDEL, BOERSMA, TYRAKOWSKA, LAANE and RIETJENS1999) including their ability to scavenge various radicals in solutions, their relative rate of oxidation by a range of free radicals, and their capacity to inhibit lipid peroxidation in multilamelar liposomes.

The comparative mechanisms and relative rates of nitrogen dioxide, thiyl and sulphonyl radical scavenging by such carotenoids as lycopene, lutein, zeaxanthin, astaxanthin and CX have been determined by pulse radiolysis (Mortensen et al., Reference MORTENSEN, SKIBSTED, SAMPSON, RICE-EVANS and EVERETT1997). Under experimental conditions, all carotenoids react with the NO₂^* radical via electron transfer to generate the carotenoid radical cation. In marked contrast, the glutathione and 2-mercaptoethanol thiyl radicals react via a radical addition process to generate carotenoid-thiyl radical adducts. The sulphonyl radical undergoes both radical addition, and electron abstraction. The mechanism and rate of scavenging is strongly dependent on the nature of the oxidising radical species, but much less dependent on the carotenoid structure.

The peroxyl trapping activity of carotenoids was shown to be in the following order: astaxanthin= CX >>β-carotene = zeaxanthin (Terao, Reference TERAO1989), astaxanthin > CX > β-carotene > zeaxanthin (Jorgensen and Skibsted, Reference JORGENSEN and SKIBSTED1993). In contrast, when the interaction with the stable radical cation (ABTS+) was used to evaluate the antioxidant activity of carotenes and xanthophylls, the ranking was as follows: lycopene >> β-carotene = β-cryptoxanthin > lutein = zeaxanthin = α-carotene>echinenone>>astaxanthin= CX (Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997) which was similar to that reported by Miller et al. (Reference MILLER, SAMPSON, CANDEIAS, BRAMLEY and RICE-EVANS1996).

Several studies have looked at carotenoids, mainly β-carotene and CX, as inhibitors of LDL oxidation (Carpenter et al., Reference CARPENTER, VAN DER VEEN, HIRD, DENNIS, DING and MITCHINSON1997). The order of carotenoid oxidation in LDL exposed to Cu²⁺ was as follows: lycopene > β-cryptoxanthin > lutein/zeaxanthin > α and β-carotene (Esterbauer et al., Reference ESTERBAUER, GEBICKI, PUHL and JURGENS1992). Similarly, as a result of carotenoid interaction with the ABTS+ radical cation, the order of carotenoid oxidation was lycopene > β-carotene > lutein > CX = astaxanthin (Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997). It has been shown that CX can protect liposomes against Cu+-initiated lipid peroxidation (Rengel et al., Reference RENGEL, DIEZ-NAVAJAS, SERNA-RICO, VEIGA, MUGA and MILICUA2000). The ability of xanthophylls (CX, zeaxanthin, and astaxanthin) as chain-breaking antioxidants was investigated in peroxyl radical-mediated peroxidation of phosphatidylcholine (PC) liposomes under atmospheric conditions using lipid-soluble and water-soluble radical generators (Lim et al., Reference LIM, NAGAO, TERAO, TANAKA, SUZUKI and TAKAMA1992). These xanthophylls retarded the chain propagation reaction of phosphatidylcholine hydroperoxides (PC-OOH) formation, although their activities to trap chain-carrying peroxyl radical were much less than that of α-tocopherol. When peroxidation was initiated by a water soluble initiator (AAPH) the order of antioxidant activity changed: astaxanthin=zeaxanthin>>β-carotene>CX (Lim et al., Reference LIM, NAGAO, TERAO, TANAKA, SUZUKI and TAKAMA1992).

When an electrochemical method of antioxidant activity evaluation was applied to pure compounds, the order of antioxidant activity was as follows: lycopene > β-carotene > zeaxanthin > α-carotene > β-cryptoxanthin > lutein > α-tocopherol > astaxanthin > CX (Buratti et al., Reference BURATTI, PELLEGRINI, BRENNA and MANNINO2001). Methylene blue plus visible light in the presence of oxygen, induced lipid peroxidation in rat liver microsomes, as assessed by the formation of thiobarbituric acid reactive substances (TBARS), lipid hydroperoxides and the loss of membrane-bound enzymes (Kamat and Devasagayam, Reference KAMAT and DEVASAGAYAM1996). Protective effects were observed with natural antioxidants such as CX, β-carotene, lipoic acid, glutathione, α-tocopherol and, to a lesser extent, ascorbic acid.

In a detailed study, the ability of carotenoids to protect egg-yolk phosphatidylcholine lipids against oxidation by peroxyl radicals generated from azo-initiators was evaluated (Woodall et al., Reference WOODALL, BRITTON and JACKSON1997). In a homogeneous organic solution, β-ring carotenoids showed a correlation between a protective effect and the rate of carotenoid destruction. The reactivity and protective ability of the 4,4'-diketocarotenoids, astaxanthin and CX was less than β-carotene and zeaxanthin. It is necessary to underline that yellow xanthophylls and β-carotene have the highest rates of oxidation, with the ketocarotenoids and violaxanthin degrading at lower rates (Perez-Galvez and Minguez-Mosquera, Reference PEREZ-GALVEZ and MINGUEZ-MOSQUERA2001).

The ability of astaxanthin and CX as chain-breaking antioxidants was studied in Cu²⁺-initiated peroxidation of phosphatidyl choline large unilamellar vesicles (Rengel et al., Reference RENGEL, DIEZ-NAVAJAS, SERNA-RICO, VEIGA, MUGA and MILICUA2000). Both carotenoids increased the lag period that preceeds the maximum rate of lipid peroxidation, although astaxanthin showed stronger activity. Differential scanning calorimetry assays demonstrated that, when incorporated, xanthophylls interact with the lipid matrix, becoming interspersed among the phospholipid molecules. It is interesting that, in HL 60 cells, lutein and CX scavenged phenoxyl radical faster than β-carotene or lycopene (Tyurin et al., Reference TYURIN, CARTA, TYURINA, BANNI, DAY, CORONGIU and KAGAN1997).

Using a model system based on brain homogenate oxidation, antioxidant activities of lutein, zeaxanthin, CX, lycopene, β-cryptoxanthin and β-carotene were clearly shown (Surai et al., Reference SURAI, SPEAKE, NOBLE, KUCHMISTOVA and IONOV1995). At concentrations, comparable with those in tissues of chickens fed on diets low (1-5 ppm) in CX supplementation, CX (10 ppm in the incubation medium) showed antioxidant activities in terms of prevention of MDA formation, comparable to vitamin E. The ability of several dietary carotenoids to quench singlet oxygen in a model membrane system (unilamellar DPPC liposomes) has been investigated. Singlet oxygen was generated in both the aqueous and the lipid phase, with quenching by a particular carotenoid independent of the site of generation. Lycopene and β-carotene exhibit the fastest singlet oxygen quenching rate constants with lutein the least efficient. The other carotenoids, astaxanthin and CX, were intermediate (Cantrell et al., Reference CANTRELL, MCGARVEY, TRUSCOTT, RANCAN and BÖHM2003).

By using cell-free systems, it was found that carotenoids could scavenge superoxide anion generated by xanthine/xanthine oxidase system. Their ability to scavenge the superoxide anion decreased in the order of CX > bixin > lutein > β-carotene (Zhao et al., Reference ZHAO, HAN, ZHAO, HIROTA, HOY and XIN1998). CX also showed a scavenging effect for the superoxide anion generated from irradiation of riboflavin. Bleaching of β-carotene, bixin and CX by peroxynitrite resulted in the increasing absorption between 290 and 365 nm and the diminishing absorption between 400 and 500 nm.

Electron transfer to the radical cations of β-carotene, zeaxanthin, CX, and astaxanthin from each of the three acid/base forms of the diphenolic isoflavonoid daidzein and its C-glycoside puerarin, as studied by laser flash photolysis in homogeneous methanol/chloroform (1/9) solution, was found to depend on the carotenoid structures involved and, more significantly, on the deprotonation degree of the isoflavonoids (Han et al., Reference HAN, CHEN, TIAN, ZHANG and SKIBSTED2010). Electron transfer from isoflavonoids to the carotenoid radical cation, as formed during oxidative stress, was faster for astaxanthin and CX than for the other carotenoids, which may relate to astaxanthin and CX more effective antioxidative properties and in agreement with their higher electron accepting index.

The antioxidant activity of β-carotene and oxygenated carotenoids lutein, CX, and astaxanthin was investigated during spontaneous and peroxyl-radical-induced cholesterol oxidation (Palozza et al., Reference PALOZZA, BARONE, MANCUSO and PICCI2008). Cholesterol oxidation, measured by the generation of 7-keto-cholesterol (7-KC), was evaluated in a heterogeneous solution with cholesterol, AAPH, carotenoids solubilised in tetrahydrofuran in water, and in a homogeneous solution of chlorobenzene, with AIBN as a pro-oxidant. The formation of 7-KC was dependent on temperature and on cholesterol and pro-oxidant concentrations. All the carotenoids tested, exhibited significant antioxidant activity by inhibiting spontaneous, AAPH- and AIBN-induced formation of 7-KC, although the overall order of efficacy of these compounds was astaxanthin > CX > lutein = β-carotene.

It is necessary to mention that different model systems have been employed to test antioxidant properties of carotenoids including cultured hepatocytes, normal and tumour thymocytes, kidney fibroblasts, embryonic hippocampal cultures, embryo fibroblast ovary cells, primary cultures of chicken embryo fibroblasts, leukaemia HL-60 cells, monocyte-macrophages, cultured Ito cells, LDL in different systems, Salmonella typhimurium, pigmented yeast Rhodotorula mucilaginosa and liver microcosms. In most cases, β-carotene was tested in the model systems as an antioxidant. But other carotenoids have been studied as well, including lutein, lycopene, α-carotene, CX, astaxanthin, zeaxanthin (Surai, Reference SURAI2002). In the model systems, different stress factors were used including CCl₄, tret-butyl hydroperoxide, UV light, ethanol, paraquat, T-2 toxin, aflatoxin B1, H₂O₂, adriamycin, peroxynitrous acid and CuSO₄. In many cases the accumulation of TBARS was used to monitor lipid peroxidation. In addition, lipid hydroperoxide formation, cholesteryl ester hydroperoxide formation, sister chromatid exchanges and DNA breaking were monitored. Under the conditions of the model systems the carotenoids confirmed their antioxidative protective effects preventing lipid peroxidation, decreasing cytotoxicity or decreasing DNA breaking. Thus, in vitro experiments using different modelling systems clearly showed an antioxidative protective effect for various carotenoids and the ranking of their antioxidant activity depends on the model system used. In general, CX was shown to be quite effective as an antioxidant (Surai, Reference SURAI2002; Palozza et al., Reference PALOZZA, BARONE, MANCUSO and PICCI2008), as measured in terms of prevention TBARS formation, lipid hydroperoxide formation, cholesteryl ester hydroperoxide formation, sister chromatid exchanges and DNA breaking.

Antioxidant action of carotenoids in vivo

In vivo experiments devoted to examining the antioxidant properties of carotenoids have been previously described (Palozza and Krinsky, Reference PALOZZA and KRINSKY1992; Moller et al., Reference MOLLER, BIARD, BLOUNT, HOUSTON, NINNI, SAINO and SURAI2000; Surai, Reference SURAI2002). In other trials, various carotenoids were included in the diet of chickens, mice, rats, guinea pigs, fish, and humans. In some cases, diets were deficient in antioxidants (vitamin E and Se), whilst in others, diets were sufficient in major antioxidants. In many situations various stress factors were applied. This included injection with CCl_4, tumour development or inoculation with tumour cells. In addition, exposure to UV or X-ray radiation, dietary inclusion of oxidised oils, a zinc deficient diet, a diet with iron overload or challenged with Aeromonas salmonicida were used (Surai, Reference SURAI2002).

When a Se and vitamin E deficient chicken diet was supplemented with CX (0.5 g CX/kg diet), liver homogenates exhibited significantly (P=0.02) decreased formation of thiobarbituric acid-reactive substances over time in ferrous ion-induced peroxidation conditions (Mayne and Parker, Reference MAYNE and PARKER1989). It is worth mentioning the protective effect of CX observed in an experiment where chickens were fed diets containing full-fat flaxseed supplemented with mixed tocopherols and CX (Ajuyah et al., Reference AJUYAH, AHN, HARDIN and SIM1993). Cooked meat from both the tocopherol and CX-supplemented chickens was more stable during refrigerated storage. The best stability of meat lipids was obtained with the combination of the two compounds. Tocopherol is known to act as a hydrogen donor; however, сanthaxanthin can stop peroxy free radical chain propagation by trapping the radical in its conjugated polyene system (Ruiz et al., Reference RUIZ, PEREZ-VENDRELL and ESTEVE-GACIA1999). When vitamin E and CX were both included in the diet of fish, a strong antioxidant effect was shown. This was attributed to the concept of antioxidant interaction. Indeed, these two compounds use different mechanisms to control lipid oxidation. α-Tocopherol is a hydrogen donor and can donate the hydrogen from its C-6 carbon while CX captures the peroxyl free radical in its conjugated polyene system. It is interesting to mention that in the liver of chickens fed on CX-enriched diet vitamin E concentrations significantly increased (Mayne and Parker, Reference MAYNE and PARKER1989). Furthermore, antioxidant interactions play an important role in antioxidant defences. For example, SOD activity was increased in plasma newly hatched chicks obtained from CX-enriched eggs (Zhang et al., Reference ZHANG, ZHANG, DING, BAI, HERNANDEZ, YAO and Q. ZHU2011)

When carotenoid antioxidant activities were measured, a range of state of the art techniques of assessing oxidative stress and lipid peroxidation were employed. In most of the cases the accumulation of TBARS was used as a test for lipid peroxidation. In other cases phospholipid hydroperoxide accumulation, lysis of erythrocytes, pentane production, levels of antioxidant vitamins in tissues, plasma antioxidant capacity, activities of antioxidant enzymes, resistance of LDL to oxidative stress, oxidative damage to lipids, lipoproteins and DNA, the frequency of micronuclei of polychromatic erythrocytes and the mitotic index of bone marrow cells, photosensitivity, survival rate after irradiation or survival of lymphoma-bearing animals were used as the end points in experiments (Surai, Reference SURAI2002). In most of in vivo experiments β-carotene was tested. But other carotenoids showed positive effects as well including lutein, CX, citranaxanthin and carotenoic acid, zeaxanthin and CX, ethyl-β-apo-8^I-carotenoate, CX, astaxanthin, lutein and lycopene (Surai, Reference SURAI2002).

The protective effect of carotenoid dietary supplementation varied but was sufficient to conclude that the carotenoids express their antioxidant properties in vivo. It is necessary to underline that the efficiency of antioxidant protection afforded by carotenoids depends on their accumulation in the experimental tissues. On the other hand, an interaction of carotenoids with other antioxidants could be considered as an additional factor regulating the efficiency of antioxidant defence in the tissue.

Taken together, the results clearly show that carotenoids express their antioxidant properties not only in vitro but in vivo as well. The efficiency of the antioxidant defence provided by carotenoids depends on many factors including stress conditions, method of oxidative stress detection, concentrations of carotenoid used, model system employed, oxygen tension and interaction with other antioxidants (Rock, Reference ROCK1997; Rice-Evans et al., Reference RICE-EVANS, SAMSON, BRAMLEY and HOLLOWAY1997; Edge et al., Reference EDGE, MCGARVEY and TRUSCOTT1997). It has been suggested that depending on the redox potential of the carotenoid molecules and oxygen tension, carotenoid concentration and interactions with other antioxidants these pigments could show antioxidant or pro-oxidant properties (Palozza, Reference PALOZZA1998). However, it is necessary to underline that in physiological conditions all those factors are usually favourable for antioxidant activity of carotenoids. Therefore, carotenoids, such as CX, are efficient quenchers of singlet oxygen and are also effective scavengers of free radicals. Indeed, in biological systems carotenoids could be considered as an integral part of the antioxidant systems operating inside the membranes. Furthermore, recent results demonstrate a very strong modifying effect of CX with respect to the dynamic and structural properties of lipid membranes (Sujak et al., Reference SUJAK, GABRIELSKA, MILANOWSKA, MAZUREK, STRZAŁKA and GRUSZECKI2005). However, the precise details of the processes associated with the quenching of radical species by carotenoids, as well as inhibition of the propagation of chain reactions, remain to be elucidated.

Сonclusions

Among more than 750 known carotenoids, CX has a special place as a carotenoid with proven antioxidant and other biologically-relevant functions. A great body of evidence indicates that CX possesses high antioxidant activity that was shown in various in vitro model systems as well as in animal experiments in vivo. It seems likely that the highest protective effects of CX would be seen under various stress conditions and this compound could be considered as an important element of the integrated antioxidant system of various tissues in the body, including chicken embryo development. A possibility of the recycling of vitamin E by various carotenoids, including CX, is of great importance for further investigation. Taking together, the aforementioned results clearly indicate that CX could provide a great deal of benefits for animals, including chicken eggs, embryos and chickens during early postnatal development.