Antioxidant activities of carotenoids

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. 

Many reviews have been written about the antioxidant activities of carotenoids. Some also describe prooxidant activities. - In consequence, only selected points about this very broad subject will be presented in the first part of this chapter. Linked to these properties and important for food nutritional value is the stability of caro-... 

Experimental evidence in humans is based upon intervention studies with diets enriched in carotenoids or carotenoid-contaiifing foods. Oxidative stress biomarkers are measured in plasma or urine. The inhibition of low density lipoprotein (LDL) oxidation has been posmlated as one mechanism by which antioxidants may prevent the development of atherosclerosis. Since carotenoids are transported mainly via LDL in blood, testing the susceptibility of carotenoid-loaded LDL to oxidation is a common method of evaluating the antioxidant activities of carotenoids in vivo. This type of smdy is more precisely of the ex vivo type because LDLs are extracted from plasma in order to be tested in vitro for oxidative sensitivity after the subjects are given a special diet. 

The antioxidant activity of carotenoids depends on the number of conjugated double bonds and possibly the presence of oxygenated functions in the molecule (Schmidt 2004). The high antioxidant activity of lycopene has been identified against singlet... 

Stahl W and Sies H. 2003. Antioxidant activity of carotenoids. Mol Aspects Med 24 345-351. 

Some years later, Miller and others (1996) described a modified TEAC assay that is able to determine the antioxidant activity of carotenoids. In the improved version, ABTS,+, the oxidant, is generated by oxidation of 2,2 -azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS2 ) with manganese dioxide. A similar approach was described by Re and others (1999) in which ABTS was oxidized with potassium persulfate (Fig. 10.2), this version of the TEAC assay is applicable to both water soluble and lipophilic antioxidants (Re and others 1999 Pellegrini and others 1999). 

Muller L, Frohlich K, Bohm V. Comparative antioxidant activities of carotenoids measured by ferric reducing antioxidant power (FRAP), ABTS bleaching assay (aTEAC), DPPH assay and peroxyl radical scavenging assay. Food Chemistry. 2011 129 139-148... 

Soffers, A. E., Van Haandel, M. J., Boersma, M. G., Tyrakowska, B., Laane, C., and Rietjens, I., Antioxidant activities of carotenoids Quantitative relationships between theoretical calculations and experimental literature data. Free Radic. Res. 30,233—240 (1999). 

Woodall AA, Britton G and Jackson MJ (1994) Antioxidant activity of carotenoids in phosphatidylcholine vesicles Chemical and stmctural considerations. Biochem Soc Trans 22 133S... 

To understand the mechanism of antioxidant activity of carotenoids it is important to analyze the oxidation products that are formed during their action as antioxidants. " The most important part of the molecule involved in those reactions is the polyene chain. This is a highly reactive electron rich system, susceptible to attack by electrophilic reagents such as peroxyl radicals, and thus responsible for the instability of the carotenoids toward oxidation. 

A large body of scientific evidence suggests that carotenoids scavenge and deactivate free radicals both in vitro and in vivo. It has been reported that their antioxidant action is determined by (1) electron transfer reactions and the stability of the antioxidant free radical (2) the interplay with other antioxidants and (3) their structure and the oxygen pressure of the microenvironment. Moreover, the antioxidant activity of carotenoids is characterized by literature data for (1) their relative rate of oxidation by a range of free radicals, or (2) their capacity to inhibit lipid peroxidation in multilamellar liposomes. ... 

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. 

In all groups of terpenes, it is possible to find compounds with bioactive functions, and for this reasmi, they have been used since ancient times for their pharmacological properties. Terpenes also give plants their characteristic tastes and flavors and have been used as food additives or cosmeceuticals, and more recently, properties such as the antioxidant activity of carotenoids have given these... 

The relative antioxidant activities of carotenoids in multilamellar liposomes, as assayed by inhibition of the formation of thiobarbituric acid-reactive substances (TBARs), were lycopene>a-carotene>p-cryptoxanthin >zeaxanthin= P-carotene>lutein (Stahl et al, 1998). Mixtures of carotenoids were more effective than the single compounds, and this synergistic effect was most pronounced when lycopene or lutein was present. The superior antioxidant activity of mixtures of carotenoids may be related to the specific positioning of different carotenoids within cell membranes. 

Epidemiological evidence has suggested that dietary carotenoids may inhibit certain types of cancer. This may be due to their provitamin A activity and/or because carotenoids have the ability to act as antioxidants (Peto et al., 1981 Stahl and Sies, 1996). The antioxidant activity of carotenoids is conferred by the hydrophobic chain of polyene units that can quench singlet oxygen, neutralize thiyl radicals, and combine with and stabilize peroxyl structures (Palace et al., 1999). 

Electrochemical and Electron paramagnetic resonance (EPR) studies have been focused on neutral carotenoid radicals. These species are derived from carotenoid radical cations deprotonation (Focsan et al., 2015). At lower radical concentration this behaves like a scavenger for reactive species (single oxygen, hydroxyl or peroxyl radicals) (Foot, 1976). Although the antioxidant activity of carotenoids is higher than that of a-tocopherol, a modest contribution (less than 5%, due to modest concentration levels of carotenoids in oil) to the total activity is expected (Muller et al., 2011). 

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