In Vitro Studies

1. Chemical Models

The decolorization of the 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical cation is an accurate assay for screening the antioxidant activities of either lipophilic substances or food extracts (55). The total antioxidant capacity of either the DCM lipophilic extracts or the methanolic hydrophilic extracts from fruits of the yellow, red, and white Sicilian cultivars of prickly pear has been evaluated by the reaction with the ABTS radical cation, generated by reacting ABTS with potassium persulfate (9), and expressed as Trolox equivalents. Because of the low amount of lipophilic antioxidants, the organic extracts of the prickly pear fruits exhibit a modest radical-scavenging activity, when compared, for example, to extracts from tomato (55), a good source of carotenoids (Table 3). The activity of the extract from the red fruit appears higher than the activity of the yellow and white ones, possibly as a reflection of the relatively higher content of h-carotene. In contrast, the water-soluble extracts from prickly pear fruits appear very active, as compared with a number of other fruits (Table 3). The total anti-oxidant capacity is higher than that reported for pear, apple, tomato, banana, and white grape, and of the same order of pink grapefruit, red grape, and orange (56). The extract from the yellow fruit is the most effective among the three cultivars. Considering its antioxidant potential evaluated in the ABTS test (57), vitamin C, which occurs in approximately the same amount in the three cultivars of prickly pear, may account for no more than 40% of

Table 3 Total Antioxidant Activity (TAA) of Lipophilic and Water-Soluble Extracts of O. ficus indica and Other Fruits

Lipophilic extract (mmol Trolox equivalent/kg dry wt)

Water-soluble extract (Amol Trolox equivalent/g edible pulp)

Prickly pear

Lipophilic extract (mmol Trolox equivalent/kg dry wt)

Water-soluble extract (Amol Trolox equivalent/g edible pulp)

Prickly pear

Yellow

0.010

+

0.002a

5.31

+

0.49

Red

0.016

+

0.002a

4.20

+

0.51

White

0.011

+

0.001a

4.36

+

0.41

Pear

1.34

+

0.06

Tomato

5.72

+

0.21c

1.89

+

0.12

Apple

2.18

+

0.35

Banana

2.21

+

0.19

Grape

White

4.46

+

1.06

Red

7.39

+

0.48

Grapefruit

Pink

4.83

+

0.18

Orange

7.50

+

1.01

a a a b c b a Source: From Ref. 9. b Source: From Ref. 57. c Source: From Ref. 55.

a Source: From Ref. 9. b Source: From Ref. 57. c Source: From Ref. 55.

a a a b c b the evaluated antioxidant capacity of the extracts, which has suggested that other hydrophilic constituents, possibly betalain pigments, may act as efficient radical scavengers (9). This seemed to somewhat explain the activity of the extract from the yellow fruit, which contains the highest amounts of betalains (9).

In addition to the activity measured in fruits, ethanol extracts of stems of O. ficus indica var. Saboten were found to have radical-scavenging activity in a number of assays generating radicals such as 2,2-diphenyl-1-picrylhy-drazyl (DPPH), superoxide anions, and hydroxyl radicals (58). In light of the high amount of phenolics in the stems (180.3 mg/g lyophilized extract), these substances have been suggested to be the active components.

2. Biological Models

Methanolic extracts from the fruits of the yellow, red, and white cultivars of prickly pear have been found capable of preventing lipid oxidation stimulated by organic hydroperoxide in human red blood cells, and by either copper or 2,2'-azobis(2-amidinopropaane) hydrochloride (AAPH) in human low-

density lipoproteins (9). Extracts from 0.5-5 mg of fruit pulp dose-depen-dently inhibited malondialdehyde formation in red blood cells (Fig. 4). With reference to the antioxidant activity of a-tocopherol, the white cultivar has been the most effective at inhibiting lipid oxidation, the extract from 1 mg pulp being as effective as 0.2 ||M a-tocopherol. Comparable amounts of extracts from the red and yellow cultivars showed an antioxidant activity equivalent to that of 0.13 |M a-tocopherol (9).

Methanolic extracts of fruit pulp markedly elongated, in a dose-dependent fashion, the period preceding the formation of conjugated diene lipid hydroperoxides of human LDL, submitted to either metal-dependent or -independent oxidation (Fig. 5). As already observed with the red blood cell oxidation model, the extract from the white cultivar was the most effective in both LDL models, followed by the red and the yellow ones.

Incubation time (h)

Figure 4 Inibitory effect of methanolic extracts from 1 mg fresh pulp of O. ficus indica on MDA formation, in ieri-butyl hydroperoxide-treated RBCs. Human RBCs (HT 1%) were incubated with 50 aM ieri-butyl hydroperoxide at 37°C either in the absence (•) or in the presence of methanolic extract of edible pulp from white (O), yellow (▲), and red (■) cultivars. Inset. Dose-dependent inhibitory effect of methanolic extracts on MDA formation, after 4-hr incubation.

Incubation time (h)

Figure 4 Inibitory effect of methanolic extracts from 1 mg fresh pulp of O. ficus indica on MDA formation, in ieri-butyl hydroperoxide-treated RBCs. Human RBCs (HT 1%) were incubated with 50 aM ieri-butyl hydroperoxide at 37°C either in the absence (•) or in the presence of methanolic extract of edible pulp from white (O), yellow (▲), and red (■) cultivars. Inset. Dose-dependent inhibitory effect of methanolic extracts on MDA formation, after 4-hr incubation.

Methanolic extracts of prickly pear (mg of fresh pulp)

Figure 5 Elongation of the lag period during the Cu2+- (a) or AAPH- (b) induced oxidation of human LDL by methanolic extracts of prickly pear. E, yellow cultivar; ■ , red cultivar; O, white cultivar.

While taking into consideration the contribution of vitamin C, the involvement of the betalain pigments in the observed antioxidant activity of the extracts has been suggested on the basis of the measured redox potential of betanin and indicaxanthin (9). In addition, it has been pointed out that the extract from the white fruit, in which betanin is virtually absent, has the highest activity in all models of lipid oxidation. This can be an indication that, in addition to betacyanins (8), indicaxanthin may act as an antioxidant compound in biological environments.

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