Carotenoids autoxidation

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... 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

In conclusion, oxidation of carotenoids by molecular oxygen, the so-called autoxidation process, is a complex phenomenon that is probably initiated by an external factor (radical, metal, etc.) and for which different mechanisms have been proposed. The autoxidation of a carotenoid is important to take into account when studying antioxidant activity because it can lower the apparent antioxidant activity of a carotenoid. ... 

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. 

Studies on the autoxidation of carotenoids in liposomal suspensions have also been performed since liposomes can mimic the environment of carotenoids in vivo. Kim et al. have studied the autoxidation of lycopene (Kim et al. 2001), -carotene (Kim 2004), and phytofluene (Kim et al. 2005) in liposomal suspensions and identified oxygenated cleavage compounds. The stability to oxidation at room temperature of various carotenoids has also been studied when incorporated in pig liver microsomes (Socaciu et al. 2000), and taking into account membrane dynamics. After 3 h of reaction, P-carotene and lycopene had completely degraded, whereas the xanthophylls tested were shown to be more stable. 

The autoxidation of carotenoids in cell medium is highly probable when experiments are conducted over periods of a few hours. The autoxidation of canthaxanthin in a cell culture medium was shown to give all-( )- and 13-(Z)-4-oxoretinoic acid, both of which were shown to induce gap junction communication (Hanusch et al. 1995). 

The interaction of carotenoids with cigarette smoke has become a subject of interest since the results of the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study Group 1994 (ATBC) and CARET (Omenn et al. 1996) studies were released. P-Carotene has been hypothesized to promote lung carcinogenesis by acting as a prooxidant in the smoke-exposed lung. Thus, the autoxidation of P-carotene in the presence of cigarette smoke was studied in model systems (toluene) (Baker et al. 1999). The major product was identified as 4-nitro-P-carotene, but apocarotenals and P-carotene epoxides were also encountered. 

Carotenoids are present in edible oils at different levels. These are powerful antioxidants against both autoxidation and photo-oxidation. Therefore, attempts have been made to retain them or recover them, as in the case of palm oil. However, carotenoids may be degraded to colorless products at high temperatures exceeding 150°C. 

Antiphotooxidative effect of carotenoids on the induction lime of soybean oil as measured by the Rancimat method. The effect of 100 ppm or 200 ppm capsanthin, P-carotene, or lutein on the photooxidative stability of soybean oil containing 200 ppm chlorophyll was measured by the Rancimat method after 4 hours light exposure at 2S°C. Preliminary studies showed that the induction lime of purified soybean oil containing 100 or 200 ppm carotenoids without chlorophyll was slightly lower than that of the blank sample which contained no carotenoids (data not shown. Therefore, there is no anti-autoxidative effect of carotenoids on the soybean oil- Table I shows that as the concentration of the carotenoids increased from 100 to 200 ppm, the induction time as well as the anti-photooxidation index (API) increased. The induction time of soybean oil containing the carotenoids was longer than that of (he control sample which contained no carotenoid however, it was still shorter than that of the blank sample which contained no carotenoid and no chlorophyll. 

During the refining, the sensory value of the oil is substantially improved but the nutritional value is impaired by partial removal of carotenoids, tocopherols, and phytosterols, and by moderate isomerization of polyunsaturated acyls into their cis,trans isomers linolenic acid is especially sensitive (Cmolik et al., 2000). The amount of trans isomers should not exceed 1%. Refined oils are nearly flavorless, tasteless, and colorless, and possess good stability against autoxidation. 

Farombi and Burton, examined the effects of several carotenoids on autoxidized triglycerides and concluded that potential prooxidant effects could occur at high concentrations. Moreover, Henry et al., suggested that at concentrations >500 ppm both P-carotene and lycopene acted as prooxidants by decreasing the induction time significantly, during the heat catalyzed oxidation of safflower seed oil. Several studies have shown that the presence of primary antioxidants and especially tocopherols stabilize carotenoids so they exhibit synergistic antioxidant character, instead of the prooxidant action that carotenoids would present individually. 

The carotenoid activity during oxidation is strongly influenced by the oxygen pressure (PO2) of the experimental conditions. Kiokias and Oreopoulou have shown that certain natural carotenoid mixtures (paprika, bixin and tomato, and palm-oil preparations) inhibited the azo-initiated oxidation of sunflower oil-in-water emulsions (operated rapidly under low pOj) in terms of both primary and secondary oxidation products. However, other studies " concluded that carotenoids not only did not inhibit aerial lipid autoxidation (high PO2) but even exerted a prooxidant character, a phenomenon also observed at high carotenoid concentrations that could be due mainly to a more increased formation of carotene-peroxyl radicals, promoting the propagation of autoxidation. 

In the same oxidation system, an enhanced antioxidant activity of carotenoid mixtures (lutein, lycopene, paprika, bixin, etc.) have been reported as compared to each separate compound. Moreover, Kiokias and Gordon found that mixtures of olive oil phenolics with various carotenoids exhibited a strong activity against the autoxidation of bulk and emulsified olive oil, whereas individual carotenoids presented no inhibitory effect. 

Isol. from extracts of the spider mite Oligonychus bessardi synthesized by autoxidation of 4-oxo-carotenoids. Prisms. Mp 173-174°. 486 nm (Py) 487 nm... 

The main formation pathway of C9 apocarotenoids with cyclohexanone and cyclohexenone structures is conversion of hydroperoxides derived from P-damascol (Figure 9.29). Hydroperoxides generated by autoxidation of carotenoids are further oxidised, reduced and hydrated to form a variety of different structures. The most important compound of this apocarotenoid... 

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