The results on the oxidative

The results on the oxidative imbalance after mercury intoxication corroborates with many studies showing elevated values of MDA with decreased GSH contents in kidney and other tissues [9,15]. In a significant report blood is shown to reflect tissue oxidative stress with respect to MDA and GSH [19]. We observe a protective effect of onion extract against HgCl2 induced oxidative stress evidently through marked reduction in lipid peroxidation and increased GSH content of erythrocytes and antioxidative activity of plasma. This is in agreement with previous studies which show that supplementation of onion rich diet results in increase in antioxidative ability of rat plasma [20]. The present observation may be explained due to the presence of flavonoids in red onion (especially quercetin). It is possible that the antioxidant components of the extracts might act as sacrificial antioxidants sparing the depletion of endogenous GSH during mercuric chloride-induced oxidative stress. Furthermore, dietary polyphenols have been shown to upregulate the expression of c-glutamylcysteine synthetase, the rate-limiting enzyme in the biosynthesis of GSH [21]. This may explain, in part, the enhancement of GSH in mercuric chloride-exposed rats treated with onion extracts. The high antioxidant properties of onion extract could also be attributed to the rich presence of organosulfur containing active compounds in the form of cysteine derivatives (S-methylcysteine sulfoxide) which is a rate limiting substrate in GSH biosynthesis and has also been found to be effective in preventing or ameliorating oxidative stress by scavenging free radicals [4]. The present study demonstrates that oral administration of quercetin and catechin can effectively inhibit lipid peroxidation in vivo, and increase antioxidant capacity in control as well as HgCl2-induced oxidative stressed rats. Such protection may be due to stabilization of the erythrocyte membrane owing to the incorporation of antioxidant thereby preventing physical damage of the membrane and resulting in more efficient free radical scavenging [22]. It also involves indirect activation of transcription factors (e.g., Nrf2) that regulate the expression of genes encoding for antioxidant purchase gap-26 [23]. Catechins are reported to be scavengers of superoxide radicals, peroxyl radicals and inhibitors of lipid peroxidation [8]. It chelates iron and offers superoxide scavenging and lipid peroxidation lowering properties through its structural features (catechol group in ring B and a hydroxy group in ring C). In addition, dietary flavonoids interact with phase I and phase II enzyme system, thereby modulating expression of an important enzyme glutamylcysteine synthetase, which is responsible for the synthesis of glutathione [21]. Recently, protective effect of catechin and quercetin has been highlighted on chlorpyrifos induced toxicity in rat testis tissues [24]. Eukaryotic cells including erythrocytes display a plasma membrane redox system (PMRS) that transfers electrons from intracellular substrates to extracellular electron acceptors [25]. The importance of red cell PMRS during oxidative stress has recently been highlighted [17,26]. It has been reported that PMRS is a compensatory/protective mechanism that operates to maintain the ascorbate level in plasma which is crucial for maintaining the redox balance [27]. The higher activity of red cell PMRS in HgCl2 treated rats is the result of generation of oxidative stress. Our observation of the reduction of PMRS activity upon supplementation by onion extract, catechin and quercetin suggests that such treatment enhances plasma antioxidant capacity and mitigates oxidative stress in HgCl2 treated rats. On the basis of PMRS results, it is apparent that quercetin is more powerful antioxidant compared to catechin and onion extract. Quercetin and catechin have good bioavailability. Plasma total catechin concentrations are reported to range from 0.63 to 1.8mmoL/L after ingestion of a single large dose of green tea. This plasma concentration achieved after 1.5–2.6h returned to baseline values after 24h [28]. Murota et al. [29] reported a plasma quercetin concentration of 1.031μmoL/L 1.5h after onion consumption. Significantly, it has been reported that the elimination of quercetin metabolites is very slow, with reported half-lives ranging from 17 to 28h [30]. Thus, there is strong evidence to support that repeated intake of onion and flavonoids would lead to enteric absorption which may be sufficient to reduce plasma indices of oxidant status [31].