Effect of superoxide dismutase on the autoxidation of hydroquinones formed during DT-diaphorase catalysis and glutathione nucleophilic addition

Cellular uptake and metabolism of flavonoids and their metabolites: implications for their bioactivity

Flavonoids have been proposed to act as beneficial agents in a multitude of disease states, including cancer, cardiovascular disease, and neurodegenerative disorders. The biological effect of these polyphenols and their in vivo circulating metabolites will ultimately depend on the extent to which they associate with cells, either by interactions at the membrane or more importantly their uptake. This review summarises the current knowledge on the cellular uptake of flavonoids and their metabolites with particular relevance to further intracellular metabolism and the generation of potential new bioactive forms. Uptake and metabolism of the circulating forms of flavanols, flavonols, and flavanones into cells of the skin, the brain, and cancer cells is reviewed and potential biological relevance to intracellular formed metabolites is discussed.

Intracellular metabolism and bioactivity of quercetin and its in vivo metabolites

Understanding the cellular effects of flavonoid metabolites is important for predicting which dietary flavonoids might be most beneficial in vivo. Here we investigate the bioactivity in dermal fibroblasts of the major reported in vivo metabolites of quercetin, i.e. 3'-O-methyl quercetin, 4'-O-methyl quercetin and quercetin 7-O-beta-D-glucuronide, relative to that of quercetin, in terms of their further metabolism and their resulting cytotoxic and/or cytoprotective effects in the absence and presence of oxidative stress. Uptake experiments indicate that exposure to quercetin led to the generation of two novel cellular metabolites, one characterized as a 2'-glutathionyl quercetin conjugate and another product with similar spectral characteristics but 1 mass unit lower, putatively a quinone/quinone methide. A similar product was identified in cells exposed to 3'-O-methyl quercetin, but not in the lysates of those exposed to its 4'-O-methyl counterpart, suggesting that its formation is related to oxidative metabolism. There was no uptake or metabolism of quercetin 7-O-beta-D-glucuronide by fibroblasts. Formation of oxidative metabolites may explain the observed concentration-dependent toxicity of quercetin and 3'-O-methyl quercetin, whereas the formation of a 2'-glutathionyl quercetin conjugate is interpreted as a detoxification step. Both O -methylated metabolites conferred less protection than quercetin against peroxide-induced damage, and quercetin glucuronide was ineffective. The ability to modulate cellular toxicity paralleled the ability of the compounds to decrease the level of peroxide-induced caspase-3 activation. Our data suggest that the actions of quercetin and its metabolites in vivo are mediated by intracellular metabolites.

The autoxidation of the reduced forms of EO9

The properties of the semiquinone radical from [3-hydroxy-5-aziridinyl-1-methyl-2-(1H-indole-4,7-indi one)-prop-beta-en-alpha-ol], EO9, have been studied using pulse-radiolysis techniques. The reduction potential of the semiquinone of EO9 at pH7.4, E(EO9/EO9-), is -253 +/- 6 mV and hence this quinone can be readily reduced by one-electron reducing enzymes such as cytochrome P450 reductase and xanthine oxidase. However, the radical is unstable in the presence of oxygen (k = 1.3 +/- 0.15 x 10(8) M-1 s-1). The semiquinone radicals and the hydroquinone are in equilibrium although the formation of the hydroquinone is favoured t physiologically relevant pH. The hydroquinone of EO9 is also unstable in the presence of oxygen and it is predicted that in fully aerated solutions, its half life is 1.5 +/- 0.3 seconds. These results are discussed in view of the selective cytotoxicity of EO9 and its ability to undergo bioreductive activation by one-electron reducing enzymes and DT-diaphorase.

The antifungal activities of some 6-[N-(halophenyl)amino]-7-chloro-5,8-quinolinediones against Candida species

A series of 6-[N-(halophenyl)amino]-7-chloro-5,8-quinolinedione derivatives 1-10 were tested for antifungal susceptibilities, in vitro, against pathogenic Candida species such as C. albicans, C. glabrata, C. krusei, C. parapsilosis and C. tropicalis. The MICs were determined by the standard macrodilution techniques, according to the NCCLS 1992 guidelines. The 6-[N-(halophenyl)amino]-7-chloro-5,8-quinolinedione derivatives showed generally potent antifungal activities against pathogenic Candida species. Among them, derivative 1, 2, 5 and 7 showed more potent antifungal activities than ketoconazole. All derivatives 1-10 had specially potent activities against C. tropicalis. Derivative 1 and 2 containing (N-3,4-dihalo-phenyl)amino moiety exhibited the potent antifungal activities. Derivative 2 with (3,4-dichlorophenyl)amino substituent was the most effective in preventing the growth of Candida species at MICs 4 micrograms/ml respectively.

One- and two-electron reduction of menadione in guinea-pig and rat cardiac tissue

1. In both guinea-pig and rat heart, mitochondrial NADH-ubiquinone-reductase and soluble DT-diaphorase accounted for 49-50% and 48-50% of menadione metabolism, respectively. Microsomal NADPH-cytochrome P450-reductase was responsible for less than 1% of menadione reduction. 2. Menadione was a high-affinity substrate for all reductases (Km values from 1 to 10 microM). 3. Marked amounts of O2-. (superoxide anion) were generated as a consequence of cardiac metabolism of menadione. 4. Menadione-induced O2-. generation was about 3-fold higher in guinea-pig than in rat heart. 5. All results were compared with data obtained on guinea-pig and rat liver.