Antioxidant Activity of Individual Compounds

Antioxidant activity of estrogens was compared on the basis of their activity to inhibit formation of ABTS,+. Estriol, estrone, 17/3-estradiol, and 17a-ethinyl-estradiol showed more than twofold higher activity than diethylstilbestrol, 2-hydro-xyestradiol, and 4-hydroxyestradiol. Antioxidant activity of mestranol was negligible. On a molar basis, estrone was 2.43 times more effective than Trolox (R24). 

Antioxidant activities of caffeine (1,3,7-trimethylxanthine) and its main metabolites was compared. Caffeine, 1,7-dimethylxanthine, and 3,7-dimethylxanthine did not show any peroxyl radical-scavenging capacity at concentration up to 100 fxM. Flowever, the relative antioxidant activities (with respect to Trolox) of 1-methyl-xanthine and 1-methyluric acid were 0.82 and 0.58, respectively (L9). 

Thyroid hormones and their structural analogs showed lower DPPH-scavenging activity in comparison with butylated hydroxytoluene (BHT) as a standard compound. 3,5,3, 5 -tetraiodothyroacetic acid, 3,3,5 -triiodo-L-thyronine, and thyroxine showed the highest antioxidant activity measured by DPPH reduction, 3,5,3 5 -tetraiodothyroacetic acid having over 20% of the activity of BHT (05). 

Some drugs were found to have antioxidant activity in standard assays of TAC. Aminoguanidine was found to have an antioxidant activity, although three orders of magnitude lower than Trolox on a molar basis (C36). Dimethyl sulfoxide (DMSO), used as a solvent for many compounds, also shows antioxidant activity and delays ABTS oxidation (Y5). 

Interestingly, products of chlorination of quercetin with hypochlorous acid were found to have higher antioxidant activity (estimated by inhibition of ABTS oxidation) than the parent compound, with monochloroquercetin and dichloroquercetin, showing activity 1.76 times and 1.84 times, respectively, of that of quercetin (B13). 

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Glavind and Holmer proposed a method of determination of antioxidants by TLC using the DPPH radical. They sprinkled a plate with separated substances with methanol solutionofthe radical and observed discoloration where substances able to quench radicals were present [57]. The TLC-DPPH assay allows a researcher to access the analysed substances and to assess the biological activity of individual compounds. Another advantage of the method is the possibili-... 

Fig. 1. pH dependence of total antioxidant capacity of blood plasma and antioxidant activities of individual antioxidants. After Tubaro el al. (T7). TAC was estimated by crocin bleaching the value at pH 7.4 for each was assumed as 1 for each compound and blood plasma. 

Further studies on the dynamic interactions of polyphenols with physiological compoimds endowed with antioxidant activity showed that the polyphenols may be more intricately involved with physiologically relevant antioxidant mechanisms. Using continuous-flow EPR measurement, Laranjinha and Cadenas (1999) have demonstrated that the caffeic acid-derived o-semiquinone radical formed upon regeneration of a-TOH ifom a-tocopheroxyl radical may be reduced back to caffeic by ascorbate. Therefore, a sequence of redox-coupled reactions can be envisage whereby the radical character is sequentially transferred from lipid phases to the aqueous medium through the one-electron reduction of tocopheroxyl radical by caffeic acid and, in turn, of the caffeic acid radical by ascorbate. This sequence amplifies the antioxidant effects of individual compounds in lipid structures such as LDL (Laranjinha Cadenas, 1999). 

In nature, flavonoids usually occur as mixtures of several compounds rather than as pure solutions of one single polyphenol. To simulate this, attempts have been made to deduce the antioxidant capacity in fruits and berries and their products from the antioxidant capacity of their individual polyphenols. Frankel et al. (1995) calculated the antioxidant capacity of 14 red and 6 white wines from the antioxidant activity of the individual polyphenols determined in the wines. The calculated antioxidant activity accounted for only 25% of the measured value. The difference was partly ascribed to unidentified polyphenols and polyphenolic acids and their polymers. Thus, there apparently is still no complete understanding of the factors contributing to the antioxidative capacity of a product. The best strategy at present seems to be to measure the products of interest directly. 

In products with complex combinations of various types of flavonoids and other phenolic compounds, effort has been made to ascribe the antioxidant activity of the product to different classes of polyphenols. Frankel et al. (1995) studied 20 selected California wines and related the antioxidant activity to the polyphenolic components of the wines, rather than to resveratrol. Antioxidant activity was measured by the ability of the wines to inhibit copper-catalyzed oxidation of human LDL. The correlation coefficient between antioxidant activity and total phenolic components of the wines was r = 0.94. Individual phenolic compounds (gallic acid, catechin, myricetin, quercetin, caffeic acid, rutin, epicatechin, cyanidin, malvidin-3-glucoside) contributed to the antioxidant activity. The correlation coefficient for the compounds ranged from r = 0.92 to r = 0.38 in descending order. 

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. 

Isolation of individual compounds described above produced insufficient amounts for an in vivo evaluation of their antioxidant activities [32], Instead, in vivo testing was carried out using a WPF,... 

Just as in the case of individual compounds, in mixtures of two antioxidants replacement of the active hydrogen in the functional group by an alkyl radical leads to a reduction of the effectiveness of the mixtures. 

The EO of Elionurus elegans Kunth. (African pasture grass, Poaceae) was investigated for its biological activities. GC/MS analyses showed that the oil contains a-bisabolol (1.6% in the roots and 1.2% in the aerial parts). None of the other compounds exhibited in individual test systems antioxidative activities with the exception of limonene and, which were available only in low amounts. The antioxidant activity of the EO was tested with the chemiluminescence method using a luminometer, where the chemilnminescence intensity of the reaction mixture containing the EO or a standard (a-tocopherol), AAPH, and luminol was measured. The IC50 value for the EO obtained from the aerial parts amonnted to 30% and the 50% inhibition rate of the roots EO to 46%. 

These results suggest that the antioxidant activity of some tested extracts might be attributed to the presence of non-phenohc compounds. Nevertheless, it should be taken into consideration that individual phenohcs may have distinct antioxidant activities there may be antagonistic or synergistic interactions between phenolics and other compounds like carbohydrates, proteins, etc. (Odabasoglu et al. 2005). 

Synergism is the cooperative effect of antioxidants or an antioxidant with other compounds to produce enhanced activity than the sum of activities of the individual... 

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