In Vitro Studies

1. Chemical Models

The antiradical activity of betalains has recently been investigated in a few studies (7-9). Betacyanins such as betanin (7,9) and betanidin (7), as well as betaxanthins such as indicaxathin (9) and vulgaxanthin (7), have been found capable of reducing the cation radical from ABTS, generated either by horseradish peroxidase/hydrogen peroxide-mediated oxidation (7) or by reaction with potassium persulfate (9). When expressed as trolox equivalents, betanin has appeared very effective (9), with an antiradical activity higher than the two betaxanthins with both experimental sets (7,9) (Table 5). This appears in accordance with the redox potential of indicaxanthin and betanin. By considering the phenolic hydroxyl group, the higher scavenging capacity of betanin has been explained by the ease with which it is possible to withdraw an electron from the betacyanin, and by the stability of the resulting de-localized radical (7). In contrast, the electron abstracted from the betaxan-thins could only be from the k-orbitals, this loss being hindered by the positive charge of the N-atom. It should be mentioned that according to the ABTS assay, betanin is much more effective than a number of polyphenol com-

TABLE B Antiradical Activity of Betalains and Betalamic Acid Toward ABTS+ Radical

Trolox equivalents

Rate of ABTS + radical disappearance (Amoles ABTS + radical/first min)

Betanin

20.0 F 0.5a,b

45.5c

Indicaxanthin

1.76 F 0.1a b

Vulgaxanthin I

4.2c

Betalamic acidd

33 F 1.0a

a Each value is the mean ± SD of four determinations performed in duplicate. b Source: From Ref. 9.

c Calculated from Ref. 7. Betanin and vulgaxanthin I were assayed at 12 aM, while ABTS+ radical was 70 aM. (7). d Prepared according to Ref. 44.

a Each value is the mean ± SD of four determinations performed in duplicate. b Source: From Ref. 9.

c Calculated from Ref. 7. Betanin and vulgaxanthin I were assayed at 12 aM, while ABTS+ radical was 70 aM. (7). d Prepared according to Ref. 44.

pounds (62). Betalamic acid shows a high radical-scavenging activity (Table 5), suggesting that the products of betalain hydrolysis may have antioxidant activity.

The activity of betalains in reducing the formation of lipoperoxyl radicals has also been reported. Linoleate peroxidation by cyt c was inhibited by betanin and betanidin, with IC50 of 0.4 and 0.8 |M, respectively, and by vulgaxanthin, with IC50 around 1.0 |M (8). In comparison, vitamin E inhibited lipid oxidation with IC50 of 5 |M (8), indicating a relatively higher antioxidant potential of betalains in this system. Oxidation of linoleic acid was also inhibited by betanidin and betanin, when the lipid oxidation was stimulated by H2O2-activated metmyoglobin, or by lipoxygenase. In the latter assays the IC50 for betanidin and betanin were 0.3 and 0.6 |M, respectively (8). Interestingly, monoelectronic redox reactions between betanin and the oxoferryl catalytic forms of horseradish peroxidase have been shown (63). Such an activity could also be considered in the above-reported reactions in which activated heme proteins are involved (8), as well as to explain why either betanin or betanidin was able to inhibit the decomposition of the myo-globin heme during the oxidation of linoleate (8).

The antioxidant activity of nine betalains has been studied, and the relationship between the structure and antioxidant activity was examined, with the linetol peroxidation model (64). Betaxanthins were shown to have the highest antioxidant activity in this system.

B. Biological Models

The positive charge of betalains could favor interactions with polar head groups of lipids and/or polar sites on the protein surface. Ex vivo plasma spiking of pure either betanin or indicaxanthin has been performed to provide evidence that both betalains can bind to human LDL in a saturable fashion, with a maximum binding of 0.5 nmoles/mg LDL protein (10). The betaninas well as the indicaxanthin-enriched LDL has been shown more resistant than the homologous native LDL to copper-induced oxidation, as assessed by elongation of the lag period (Fig. 6). In addition, indicaxanthin-enriched LDL has appeared much more resistant than betanin-enriched LDL in this system, possibly as the result of synergistic interactions of indicaxanthin with the LDL vitamin E (10). Consumption of vitamin E was not varied by betanin. In contrast, indicaxanthin prevented vitamin E consumption at the beginning of LDL oxidation, and prolonged the time of its utilization (Fig. 6).

The affinity of betacyanins for microsomal membranes has been demonstrated by evaluating the rate of migration of these compounds through a dialysis tube, either in the absence or in the presence of microsomes (8).

0 50 100 150

Figure 6 Oxidation of human control-LDL, betanin- and indicaxanthin-enriched LDL, and time course of vitamin E consumption in either control- or betanin-enriched LDL (closed triangle) and in indicaxanthin-enriched LDL (open triangle).

0 50 100 150

Incubation time (min)

Figure 6 Oxidation of human control-LDL, betanin- and indicaxanthin-enriched LDL, and time course of vitamin E consumption in either control- or betanin-enriched LDL (closed triangle) and in indicaxanthin-enriched LDL (open triangle).

In addition, it has been shown that the oxidation of microsomal lipids by either FeCl2/ascorbate or H2O2-activated myoglobin was reduced by variable concentrations of betanin (8). However, because of its electron-donating capability, low amounts of betanin (<12.5 ||M) were prooxidant in the system catalyzed by iron/ascorbate, as a result of the reduction of ferric to ferrous ions. At high concentrations (25 ||M) the antioxidant works also with lipo-peroxyl radicals, thus preventing lipid oxidation (8).

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