No abstract

The oxidation of 2',7'-dichlorofluorescin (DCFH) to a fluorescent product is currently used to evaluate oxidant stress in cells. However, there is considerable uncertainty as to the enzymatic and nonenzymatic pathways that may result in DCFH oxidation. Iron/hydrogen peroxide-induced DCFH oxidation was inhibited by catalase or by the hydroxyl radical scavenger dimethylsulfoxide; however, superoxide dismutase (SOD) had no effect on DCFH oxidation. The formation of hydroxyl radical (indicated by the oxidation of salicylic acid to 2,3-dihydroxybenzoic acid) was proportional to DCFH oxidation, suggesting that the hydroxyl radical is responsible for the iron/peroxide-mediated oxidation of DCFH. Utilizing a superoxide generating system consisting of hypoxanthine/xanthine oxidase, oxidation of DCFH was unaffected by SOD, catalase or desferoxamine, and stimulated by removing hypoxanthine from the reaction mixture. In contrast, SOD or elimination of hypoxanthine abolished superoxide formation. In addition, potassium superoxide did not support the oxidation of DCFH. Thus, superoxide is not involved in DCFH oxidation. Boiling xanthine oxidase eliminated its concentration-dependent oxidation of 1 microM DCFH, indicating that xanthine oxidase can enzymatically utilize DCFH as a high affinity substrate. Kinetic studies of the oxidation of DCFH by xanthine oxidase indicated a Km(app) of 0.62 microM. Hypoxanthine competed with DCFH with a Ki(app) of 1.03 mM. These studies suggest that DCFH oxidation may be a useful indicator of oxidative stress. However, other types of cellular damage may produce DCFH oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

N-acetylcysteine (NAC) is a thiol-containing compound which nonenzymatically interacts and detoxifies reactive electrophiles and free radicals. NAC was shown to effectively protect human bronchial fibroblasts against the toxic effects of tobacco smoke condensates and the isolated perfused lung against the glutathione (GSH)-depleting effect of tobacco smoke. NAC was also shown to reduce the reactive oxygen intermediate hydrogen peroxide (H₂O₂) and protect against the toxic effects of H₂O₂. In vivo studies, however, demonstrated that NAC when administered orally has very low bioavaibility due to rapid metabolism to GSH among other metabolites. Thus, even though NAC is very effective in protecting cells of different origins from the toxicity of reactive components in tobacco smoke and reactive oxygen species, a direct scavenging effect by NAC in vivo, particularly when administered orally, does not seem likely. The bioavailability of NAC itself is very low when given this route. A more relevant mechanism in vivo for any protective effect NAC may exert against toxic species may be due to NAC acting as a precursor of GSH and facilitating its biosynthesis. GSH will then serve as the protective agent and detoxify reactive species both enzymatically and nonenzymatically

No abstract

The EFSA ANS Panel was asked to provide a scientific opinion on the safety of green tea catechins from dietary sources including preparations such as food supplements and infusions. Green tea is produced from the leaves of Camellia sinensis (L.) Kuntze, without fermentation, which prevents the oxidation of polyphenolic components. Most of the polyphenols in green tea are catechins. The Panel considered the possible association between the consumption of (-)-epigallocatechin-3-gallate (EGCG), the most relevant catechin in green tea, and hepatotoxicity. This scientific opinion is based on published scientific literature, including interventional studies, monographs and reports by national and international authorities and data received following a public 'Call for data'. The mean daily intake of EGCG resulting from the consumption of green tea infusions ranges from 90 to 300 mg/day while exposure by high-level consumers is estimated to be up to 866 mg EGCG/day, in the adult population in the EU. Food supplements containing green tea catechins provide a daily dose of EGCG in the range of 5-1,000 mg/day, for adult population. The Panel concluded that catechins from green tea infusion, prepared in a traditional way, and reconstituted drinks with an equivalent composition to traditional green tea infusions, are in general considered to be safe according to the presumption of safety approach provided the intake corresponds to reported intakes in European Member States. However, rare cases of liver injury have been reported after consumption of green tea infusions, most probably due to an idiosyncratic reaction. Based on the available data on the potential adverse effects of green tea catechins on the liver, the Panel concluded that there is evidence from interventional clinical trials that intake of doses equal or above 800 mg EGCG/day taken as a food supplement has been shown to induce a statistically significant increase of serum transaminases in treated subjects compared to control.

Exposure of human umbilical vein endothelial cells to oxidants such as hydrogen peroxide, tertbutyl hydroperoxide and diamide has been shown to induce oxidant-specific S-thiolation of cellular proteins. In this study one of the main S-thiolated proteins in hydrogen-peroxide-treated cells was identified as the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase. Additionally, we have shown that the post-translational modification of the cysteinyl thiols of glyceraldehyde-3-phosphate dehydrogenase accompanies an inhibition of the enzyme and that both events are simultaneously and rapidly reversed upon the removal of the oxidative stimulus.

No abstract

The EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS) provides a scientific opinion re-evaluating the safety of silicon dioxide (E 551) when used as a food additive. The forms of synthetic amorphous silica (SAS) used as E 551 include fumed silica and hydrated silica (precipitated silica, silica gel and hydrous silica). The Scientific Committee on Food (SCF) established a group acceptable daily intake (ADI) 'not specified' for silicon dioxide and silicates. SAS materials used in the available biological and toxicological studies were different in their physicochemical properties; their characteristics were not always described in sufficient detail. Silicon dioxide appears to be poorly absorbed. However, silicon-containing material (in some cases presumed to be silicon dioxide) was found in some tissues. Despite the limitations in the subchronic, reproductive and developmental toxicological studies, including studies with nano silicon dioxide, there was no indication of adverse effects. E 551 does not raise a concern with respect to genotoxicity. In the absence of a long-term study with nano silicon dioxide, the Panel could not extrapolate the results from the available chronic study with a material, which does not cover the full-size range of the nanoparticles that could be present in the food additive E 551, to a material complying with the current specifications for E 551. These specifications do not exclude the presence of nanoparticles. The highest exposure estimates were at least one order of magnitude lower than the no observed adverse effect levels (NOAELs) identified (the highest doses tested). The Panel concluded that the EU specifications are insufficient to adequately characterise the food additive E 551. Clear characterisation of particle size distribution is required. Based on the available database, there was no indication for toxicity of E 551 at the reported uses and use levels. Because of the limitations in the available database, the Panel was unable to confirm the current ADI 'not specified'. The Panel recommended some modifications of the EU specifications for E 551.