Flavonoid antioxidants: chemistry, metabolism and structure-activity relationships

Individual and Interactive Effects of Apigenin Analogs on G2/M Cell-Cycle Arrest in Human Colon Carcinoma Cell Lines

Apigenin has been previously shown to induce G2/M cell-cycle arrest in human colon cancer cell lines. The present study assessed the individual and interactive influence of seven apigenin analogs on cell cycle, cell number, and cell viability in human SW480 and Caco-2 colonic carcinoma cells. Cellular concentration of selected apigenin analogs was further assessed by high-performance liquid chromatography to assess cellular availability. The apigenin analogs studied were acacetin, chrysin, kampherol, luteolin, myricetin, naringenin, and quercetin. DNA flow cytometric analysis indicated that treatment with either chrysin or acacetin at 0 to 80 microM for 48 h resulted in cell-cycle arrest at the G2/M phase in a dose-dependent manner in the SW480 cells but not in the Caco-2 cells. The percentage of SW480 cells at G2/M also increased when cells were treated with kampherol, luteolin, or quercetin between 5 and 30 microM, but the percentage of cells in G2/M decreased at doses greater than 40 microM. Cell number was significantly decreased in a time- and dose-dependent manner following the treatments with each analog except for naringenin and myricetin. The interactive effects of these analogs with apigenin were further assessed by combining each analog at doses from 0 to 80 microM with apigenin at 20 microM, a dose at which apigenin was found to double the proportion of SW480 cells in G2/M. When either acacetin, chrysin, luteolin, kampherol, or quercetin at doses between 5 and 30 microM were combined with apigenin at 20 microM, there was an increase of 22% in the proportion of G2/M cells over that observed with 20 microM apigenin alone (P < 0.05). At doses higher than 40 microM, however, the interaction became antagonistic, and the proportion of cells in G2/M decreased below that observed with apigenin alone. Cell viability, as assessed by Trypan blue exclusion assay, significantly decreased by treatments with high doses of each agent or each agent combined with apigenin. Cellular concentration of apigenin, chrysin, or naringenin in SW480 cells significantly increased at doses of 40 microM or greater, but it was not correlated with their impact on G2/M cell-cycle arrest. The induction of cell-cycle arrest by five of seven tested apigenin analogs and the additive induction by the combination of flavonoids at low doses suggest that apigenin-related flavonoids may cooperatively protect against colorectal cancer through conjoint blocking of cell-cycle progression.

Effects of Flavonoid-Rich Beverages on Prostacyclin Synthesis in Humans and Human Aortic Endothelial Cells: Association withEx VivoPlatelet Function

Diets rich in flavonoids have been associated with reduced risk for cardiovascular disease. This may be due, in part, to flavonoid-induced alterations in eicosanoid synthesis. Our objective was to identify plant-derived beverages that alter synthesis of prostacyclin in cultured human aortic endothelial cells (HAEC), and to determine if these beverages could alter in vivo 6-keto-prostaglandin F(1alpha) (a stable metabolite of prostacyclin) synthesis and platelet function. HAEC were treated with nine commonly consumed beverages to determine their effects on prostacyclin synthesis under acute and chronic treatment regimens. Orange, purple grape, and pomegranate juices and coffee (6-9 mL/kg) were then provided to 28 fasted, healthy adult subjects (eight men and 20 women) on five separate days. Plasma samples were collected immediately following juice consumption (baseline), and at 2 and 6 hours post-consumption. On an acute basis, administration of HAEC with pomegranate juice increased media prostacyclin. Chronic exposure to purple grape and pomegranate juice increased aortic endothelial cell prostacyclin synthesis (38% and 61%, respectively; P <.05). The consumption of purple grape, pomegranate, and orange juice prolonged epinephrine/collagen-induced clotting time (P <.05). Purple grape juice increased plasma 6-keto-prostaglandin F(1alpha) (20%; P <.05) at 2 hours; pomegranate and orange juice did not significantly influence plasma prostacyclin concentrations. Consistent with the in vitro data, coffee consumption did not influence clotting time or plasma prostacyclin concentrations. These results indicate that the HAEC model system can provide a qualitative means to screen food and food-derived products for biologic activity related to cardiovascular health.

Role of phenolic O-H and methylene hydrogen on the free radical reactions and antioxidant activity of curcumin

To understand the relative importance of phenolic O-H and the CH-H hydrogen on the antioxidant activity and the free radical reactions of Curcumin, (1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione), biochemical, physicochemical, and density functional theory (DFT) studies were carried out with curcumin and dimethoxy curcumin (1,7-bis[3, 4-dimethoxy phenyl]-1,6-heptadiene-3,5-dione). The antioxidant activity of these compounds was tested by following radiation-induced lipid peroxidation in rat liver microsomes, and the results suggested that at equal concentration, the efficiency to inhibit lipid peroxidation is changed from 82% with curcumin to 24% with dimethoxy curcumin. Kinetics of reaction of (2,2'-diphenyl-1-picrylhydrazyl) DPPH, a stable hydrogen abstracting free radical was tested with these two compounds using stopped-flow spectrometer and steady state spectrophotometer. The bimolecular rate constant for curcumin was found to be approximately 1800 times greater than that for the dimethoxy derivative. Cyclic voltammetry studies of these two systems indicated two closely lying oxidation peaks at 0.84 and 1.0 V vs. SCE for curcumin, while only one peak at 1.0 V vs. SCE was observed for dimethoxy curcumin. Pulse radiolysis induced one-electron oxidation of curcumin and dimethoxy curcumin was studied at neutral pH using (*)N(3) radicals. This reaction with curcumin produced phenoxyl radicals absorbing at 500 nm, while in the case of dimethoxy curcumin a very weak signal in the UV region was observed. These results suggest that, although the energetics to remove hydrogen from both phenolic OH and the CH(2) group of the beta-diketo structure are very close, the phenolic OH is essential for both antioxidant activity and free radical kinetics. This is further confirmed by DFT calculations where it is shown that the -OH hydrogen is more labile for abstraction compared to the -CH(2) hydrogen in curcumin. Based on various experimental and theoretical results it is definitely concluded that the phenolic OH plays a major role in the activity of curcumin.

Effect of O-glycosilation on the antioxidant activity and free radical reactions of a plant flavonoid, chrysoeriol

Chrysoeriol and its glycoside (chrysoeriol-6-O-acetyl-4'-beta-D-glucoside) are two natural flavonoids extracted from the tropical plant Coronopus didymus. The aqueous solutions of both the flavonoids were tested for their ability to inhibit lipid peroxidation induced by gamma-radiation, Fe (III) and Fe (II). In all these assays chrysoeriol showed better protecting effect than the glycoside. The compounds were also found to inhibit enzymatically produced superoxide anion by xanthine/xanthine oxidase system; here the glycoside is more effective than the aglycone. The rate constants for the reaction of the compounds with superoxide anion determined by using stopped-flow spectrometer were found to be nearly same. Chrysoeriol glycoside reacts with DPPH radicals at millimolar concentration, but the aglycone showed no reaction. Using nanosecond pulse radiolysis technique, reactions of these compounds with hydroxyl, azide, haloperoxyl radicals and hydrated electron were studied. The bimolecular rate constants for these reactions and the transient spectra of the one-electron oxidized species indicated that the site of oxidation for the two compounds is different. Reaction of hydrated electron with the two compounds was carried out at pH 7, where similar reactivity was observed with both the compounds. Based on all these studies it is concluded that chrysoeriol exhibits potent antioxidant activity. O-glycosylation of chrysoeriol decreases its ability to inhibit lipid peroxidation and reaction with peroxyl radicals. However the glycoside is a more efficient scavenger of DPPH radicals and a better inhibitor of xanthine/xanthine oxidase than the aglycone.