The antioxidant and antiinflammatory effects of flavonols have been suggested to be structure-related. Results revealed that selected flavonols, including fisetin (F), kaempferol (K), morin (MO), myricetin (MY), and quercetin (Q), exhibited distinctive free radical scavenging properties against different kinds of free radicals. The H donation (DPPH bleaching) potential was Q > F approximately equals MY > MO > K, indicating that the presence of a 3',4'-catechol moiety in the B ring correlated with high activity. The 4'-OH in the B ring was suggested to be important for reducing xanthing/xanthine oxidase-generated superoxide; while an additional OH moiety on the ortho sites (3' or 5') attenuated the effect as the observed inhibitory potency was K approximately equals MO > Q > F > MY. The relative inhibitory effect for Fenton-mediated hydroxyl radical was K approximately equals MO approximately equals Q > F > MY. This result implies the involvement of 4-keto, 5-OH region in Fe++ chelating and the negative effect of pyrogallol moiety in the B ring. Similar to the inhibitory activity against a N-formyl-methionyl-leucyl-phenylalanine (f-MLP)-stimulated oxidative burst in human polymorphonuclear neutrophils (PMN), our result showed that the structural peculiarity of the di-OH in the B ring obviously rendered F, Q, and MO more potent as ROS inhibitors than MY and K, which have tri- and mono-OH in the B ring, respectively. All of the previous data indicated that the structure prerequisite to reinforce the free radical scavenging activity varies with the type of free radical. We further analyzed the effects of flavonols on nitric oxide (NO) production in endotoxin-stimulated murine macrophages, RAW264.7 cells. Results showed that all flavonols (up to 10 microM) inhibited NO production without exerting detectable cytotoxicity. F, K, and Q dose-dependently repressed iNOS mRNA expression and prostaglandin E2 (PGE2) production, in part through an attenuating NF-kappaB signaling pathway. This result indicates that flavonols, despite structural similarity, have different antioxidant and antiinflammatory effects.

Atherosclerosis is a chronic inflammatory disease of the arterial wall. Kaempferol and rhamnocitrin (kaempferol 7-O-methyl ether) are two anti-inflammatory flavonoids commonly found in plants. The aim of this study is to investigate the function of kaempferol and rhamnocitrin on prevention of atherosclerosis. Chemical analyses demonstrated that kaempferol and rhamnocitrin were scavengers of DPPH (1,1-diphenyl-2-picrylhydrazyl) with IC50 of 26.10 +/- 1.33 and 28.38 +/- 3.07 microM, respectively. Copper-induced low-density lipoprotein (LDL) oxidation was inhibited by kaempferol and rhamnocitrin, with similar potency, as measured by decreased formation of malondialdehyde and relative electrophoretic mobility (REM) on agarose gel, while rhamnocitrin reduced delayed formation of conjugated dienes better than kaempferol. Cholesterol-laden macrophages are the hallmark of atherogenesis. The class B scavenger receptor, CD36, binds oxidized low-density lipoprotein (oxLDL), is found in atherosclerotic lesions, and is up-regulated by oxLDL. Addition of kaempferol and rhamnocitrin (20 microM) caused significant reductions in cell surface CD36 protein expression in THP-1-derived macrophages (p < 0.05). Reverse transcription quantitative PCR (RT-Q-PCR) showed that kaempferol and rhamnocitrin (20 microM) decreased oxLDL-induced CD36 mRNA expression (p < 0.01 and p < 0.05, respectively). Kaempferol- and rhamnocitrin-treated macrophages also showed reduction in 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanide perchlorate (DiI)-labeled oxLDL uptake. Current evidences indicate that kaempferol and rhamnocitrin not only protect LDL from oxidation but also prevent atherogenesis through suppressing macrophage uptake of oxLDL.

Several antioxidant compounds have been previously identified from sword brake fern (Pteris ensiformis Burm.) by DPPH bleaching and Trolox equivalent antioxidant capacity (TEAC) analyses. Among the isolates, 7-O-caffeoylhydroxymaltol 3-O-beta-D-glucopyranoside and hispidin 4-O-beta- D-glucopyranoside [6-(3,4-dihydroxystyryl)-4-O-beta-D-glucopyranoside-2-pyrone] were two new compounds. The aim of this study is to elucidate the possible effect of the aqueous extract of sword brake fern (SBF) and these two compounds in preventing atherosclerosis. The results demonstrated that SBF and these two compounds strongly inhibited Cu2+-mediated low-density lipoprotein (LDL) oxidation measured by thiobarbituric acid-reactive substances assay (TBARS), conjugated diene production, and relative electrophoretic mobility. The commercial antioxidant dl-alpha-tocopherol showed lower antioxidant activity than these two compounds at the same molecular concentration. SBF and these two compounds also suppressed N-formylmethionyl-leucylphenylalanine (fMLP)-stimulated reactive oxygen species (ROS) production in human polymorphonuclear neutrophils (PMN). These findings indicate that sword brake fern may prevent atherosclerosis via inhibition of both LDL oxidation and ROS production.