Identification of Six Flavonoids as Novel Cellular Antioxidants and Their Structure-Activity Relationship

Abstract

This study is aimed at determining the relationship of flavonoid structures to their chemical and intracellular antioxidant activities. The antioxidant activities of 60 flavonoids were investigated by three different antioxidant assays, including 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, oxygen radical absorption capacity (ORAC), and cellular antioxidant activity (CAA) assays. The result showed 6 flavonoids as good cellular antioxidants evaluated for the first time. The cellular antioxidant activities of compounds 7-methoxy-quercetin, 3-O-methylquercetin, 8-hydroxy-kaempferol, quercetin-3-O-α-arabinofuranose, kaempferol-7-O-glucopyranoside, and luteolin6-C-glucoside were linked with the upregulation of antioxidant enzyme activities (superoxide dismutase, catalase, and glutathione peroxidase). A structure-activity relationship suggested that 2,3-double bond, 4-keto groups, 3',4'-catechol structure, and 3-hydroxyl in the flavonoid skeleton played important roles in the antioxidant behavior. Furthermore, the cell proliferative assay revealed a low cytotoxicity for 3-O-methylquercetin. The present results provide valuable information for the dietary application of flavonoids with different structures for high antioxidant.

Figure 1

The antioxidant activities of flavonoids determined by ORAC (a, b), DPPH (c), and the cellular antioxidant (d) assays. The IC₅₀ and EC₅₀ of compounds that were not in the Figure were >200 μM. The data are presented as the mean with standard deviation (SD) bar of three replicates. The values having no letters in common are significantly different (P < 0.05). The data was listed in Table S1.

Figure 2

Peroxyl radical-induced oxidation of DCFH to DCF in HepG2 cells and the inhibition of oxidation by compounds 2 (a), 5 (b), 8 (c), 10 (d), 13 (e), and 32 (f) over time, using the protocol having no PBS wash.

Figure 3

The rate and structures of the compounds 2 (a, g, p), 5 (b, h, q), 8 (c, i, r), 10 (d, j, s), 13 (e, k, t), and 32 (f, o, u) of PC value on the activities of antioxidant enzymes. The activities of CAT, SOD, and GSH-Px of the PC were 106.82 ± 5.32, 4.86 ± 0.84, and 33.3746 ± 2.25 U/mg protein, respectively. The activities of CAT, SOD, and GSH-Px of the NC were 45.11 ± 2.21, 2.12 ± 0.21, and 14.4231 ± 1.25 U/mg protein, respectively. The data are presented as the mean with standard deviation (SD) bar of three replicates. The values having no letters in common are significantly different (P < 0.05).

Figure 4

The antiproliferative activities, cytotoxicities, and structures of compounds 2 (a), 5 (b), 8 (c), 10 (d), 13 (e), and 32 (f) against HepG2 cells. The data are presented as the mean with standard deviation (SD) bar of three replicates. Bars with no letters in common were significantly different (P < 0.05).