The scavenging abilities of compound and fractions on hydrog

The scavenging abilities of compound 1 and fractions 2–6 on hydrogen peroxide need to be evaluated (Table 2) because even though it is not very reactive in human cells, hydrogen peroxide sometimes is toxic because it gives rise to the hydroxyl radical in cells [41]. Thus, antioxidants that remove hydrogen peroxide are important in biological and food systems. The reducing power assay measures the ability of antioxidants to reduce the ferric (Fe3+) ion to ferrous (Fe2+) ion through the donation of an electron, that can be monitored at 700nm. The ability of an antioxidant to reduce the ferric ion to ferrous ion is an indication of its ability to act as a pro-oxidant in a biological or food system. In the present study, we used gallic npy receptor and ascorbic acid equivalents to determine reducing power abilities. The higher the reducing power value, the greater the reducing power of the tested constituents and, thus, the higher the antioxidant activity. The reducing powers of the isolated constituents ranged between 19.34 and 54.35μg/mL in gallic acid equivalents and between 56.16 and 91.72μg/mL in ascorbic acid equivalents (Fig. 5a). These potential reducing powers may have been due to the presence of the dihydroxy type of benzene derivatives, catechin and epicatechin, which are integral parts of condensed tannins. The reducing powers of bioactive constituents including the high molecular weight tannins extracted from the peanut hulls and stem bark of Indian laburnum have been closely linked to strong antioxidant activities [39]. The β-carotene-linoleate bleaching assay measures the ability of an antioxidant to inhibit lipid peroxidation. The β-carotene-linoleate bleaching assay was conducted because food generally consists of a lipid and water system with some emulsifier. Therefore, the aqueous emulsion system of β-carotene and linoleic acid was used to evaluate the antioxidant activities of the isolated constituents. The free peroxy radical in this system was formed when oxidized linoleic acid attacked β-carotene molecules, which consequently underwent rapid decolorization. The rate of bleaching of the β-carotene solution was measured by the difference between the initial reading in spectral absorbance at 470nm at time 0min and after 120min. Antioxidant activity was expressed as percent inhibition relative to the control. The results obtained showed that most of the investigated constituents efficiently inhibited the oxidation of emulsified linoleic acid and, as a result, inhibited β-carotene bleaching. The antioxidant activities of compound 1 and fractions 2–6 from Q. phillyraeoides at 40μg/mL in the β-carotene linoleate model system resulted in fraction 6 having the strongest ability to protect against β-carotene bleaching, followed by fractions 3, 5, 4, 2, and compound 1, which still retained antioxidant activities of 20.01%, 25.78%, 22.10%, 18.42%, 14.73%, and 1.05%, respectively, after 120min of the assay. The results obtained for fractions 2–6 were higher than that of gallic acid as the standard, which had an antioxidant ability of 6.32%. A comparison of antioxidant activities measured with the four methods, i.e., DPPH radical scavenging, hydrogen peroxide radical scavenging, reducing power, and β-carotene-linoleate bleaching assays, demonstrated that all the isolated constituents showed similar results among the four methods. Therefore, we concluded that all these methods were consistent with each other for evaluating the antioxidant activities of isolated constituents from the leaves of Q. phillyraeoides. Antioxidants have been reported to provide synergistic benefits in the treatment of DM because of their insulin-enhancing potentials [42]. To the best of our knowledge, this study is the first reported work on the bioassay-guided isolation of active constituents from the leaves of Q. phillyraeoides through evaluation of both their α-glucosidase inhibitor and antioxidant activities.