Inhibition of rat, mouse, and human glutathione S-transferase by eugenol and its oxidation products

A weight of evidence evaluation of the mode of action of isoeugenol

Isoeugenol is one of several phenylpropenoid compounds that is used as a fragrance, food flavoring agent and in aquaculture as a fish anesthetic. Carcinogenicity testing in rats and mice by NTP resulted in clear evidence of carcinogenicity (hepatic adenomas/carcinomas) in male mice only. A nongenotoxic threshold mode of action (MOA) is postulated for isoeugenol and is discussed considering the IPCS MOA and Human Relevance Framework. The weight of evidence indicates that isoeugenol is not genotoxic and that the carcinogenic outcome in male mice relates directly to the metabolism of individual compounds. Benchmark Dose (BMD) modeling was conducted to determine a Point of Departure (POD) and potential threshold of carcinogenicity. The results of the BMD evaluation for isoeugenol resulted in an estimated POD for carcinogenicity in the male mouse of 8 mg/kg with a lower limit of 4 mg/kg, representing a POD for the determination of an acceptable daily intake. With application of uncertainty factors, an ADI of 40 μg/kg is calculated. This daily dose in humans would be protective of human health, including carcinogenicity. A corresponding maximum residual level (MRL) of 3200 μg/kg fish is also estimated based on this POD that considers the threshold MOA.

Persistent susceptibility of Aedes aegypti to eugenol

Botanical insecticides are preferred for their environment and user-friendly nature. Eugenol is a plant-based monoterpene having multifarious biocidal activities. To understand whether eugenol would persistently work against Aedes aegypti, we performed larvicidal bioassays on thirty successive generations and determined median lethal concentration (LC50) on each generation. Results showed no apparent differences between LC50 at F0 (63.48 ppm) and F30 (64.50 ppm) indicating no alteration of susceptibility toward eugenol. To analyze, if eugenol has any effect on metabolic detoxification-associated enzymes, we measured esterases (alpha and beta), cytochrome P450, and GST activities from the survived larvae exposed to LC50 concentration from F0–F30. Results revealed a decrease of esterases, GST, and cytochrome P450 activities at the initial 4–8 generations and then a gradual increase as the generations progressed. GST activity remained significantly below the control groups. Synergists (TPP, DEM, and PBO) were applied along with eugenol at F30 and LC50 concentration, and the said enzyme activities were recorded. Results showed a noticeable decrease in LC50 and enzyme activities indicating effective inhibitions of the respective enzymes. Overall, present results inferred that eugenol would effectively work as a larvicide for a longer period in successive generations without initiating rapid resistance and therefore could be advocated for controlling A. aegypti.

The role of cinnamon as a modulator of the expression of genes related to antioxidant activity and lipid metabolism of laying quails

Since cinnamon has vitamins and minerals in addition to antioxidants compounds in its chemical composition studies have shown the potential of cinnamon supplementation on some important characteristics in the performance of birds. Thus, this study was conducted under the hypothesis that the inclusion of cinnamon in the laying quail diet could influence the performance of the birds through the expression of genes related to antioxidant activity and lipid metabolism. To test this hypothesis, 144 Japanese quail (Coturnix japonica) with an initial age of 18 weeks and average weight of 133g were distributed in a completely randomized design with two treatments: no cinnamon supplementation (NCS—control group) and with supplementation of 9g/kg of cinnamon powder (CPS). The experiment lasted for 84 days. At the end of the experimental period, six animals from each treatment were euthanized by cervical dislocation, blood was collected and organs weighed. Liver tissue was collected for gene expression and biochemical analyses. We observed a significant effect of cinnamon inclusion on the weight of the pancreas (P = 0.0418), intestine (P = 0.0209) and ovary (P = 0.0389). Lower weights of the pancreas and intestine, and a higher ovary weight was observed in birds receiving the CPS diet. Quails fed with cinnamon supplementation also had better feed conversion per egg mass (2.426 g /g, P = 0.0126), and higher triglyceride (1516.60 mg/dL, P = 0.0207), uric acid (7.40 mg/dL, P = 0.0003) and VLDL (300.40 mg/dL, P = 0.0252) contents. A decreased content of thiobarbituric acid reactive substances (TBARS) and lower catalase activity was observed in the liver of quails from the CPS diet (0.086 nmoles/mg PTN, and 2.304 H₂O₂/min/mg PTN, respectively). Quails from the CPS group presented significantly greater expression of FAS (fatty acid synthase, 36,03 AU), ACC (Acetyl-CoA Carboxylase, 31.33 AU), APOAI (apolipoprotein A-I, 803,9 AU), ESR2 (estrogen receptor 2, 0.73 AU) SOD (superoxide dismutase, 4,933.9 AU) and GPx7 (glutathione peroxidase 7, 9.756 AU) than quails from the control group. These results allow us to suggest that cinnamon powder supplementation in the diet of laying quails can promote balance in the metabolism and better performance through the modulation of antioxidant activity and the expression of genes related to lipid metabolism.

Increased sensitivity of Hep G2 cells toward the cytotoxicity of cisplatin by the treatment of piper betel leaf extract

Piper betel leaves (PBL) are used in Chinese folk medicine for the treatment of various disorders. PBL has the biological capabilities of de-toxication, anti-oxidation and anti-mutation. In this study we first examined the effect of PBL extract on the activity of Glutathione S-transferase (GST) isoforms, and found that it inhibited total GST and the alpha class of GST (GSTA), but not the pi class of GST (GSTP), and the mu class of GST (GSTM), activity in Hep G2 cells. RT-PCR results verified a reduction in the expression of GSTA1. Next, we examined whether PBL extract could increase the sensitivity of Hep G2 cells to anti-cancer drugs. The data showed that the cytotoxicity of cisplatin was significantly enhanced by the presence of PBL extract, accompanied by a reduction in the expression of multidrug resistance protein 2 (MRP2). These effects of PBL extract were compared to its major constitute, eugenol. Although eugenol decreased MRP2 level more effectively than PBL extract, it exhibited less sensitizing effect. In conclusion, we demonstrated that PBL extract was able to increase the sensitivity of Hep G2 cells to cisplatin via at least two mechanisms, reducing the expression of MRP2 and inhibiting the activity of total GST and the expression of GSTA. The data of this study support an application of PBL as an additive to reduce drug resistance.

Safety assessment of allylalkoxybenzene derivatives used as flavouring substances - methyl eugenol and estragole

This publication is the seventh in a series of safety evaluations performed by the Expert Panel of the Flavor and Extract Manufacturers' Association (FEMA). In 1993, the Panel initiated a comprehensive program to re-evaluate the safety of more than 1700 GRAS flavouring substances under conditions of intended use. In this review, scientific data relevant to the safety evaluation of the allylalkoxybenzene derivatives methyl eugenol and estragole is critically evaluated by the FEMA Expert Panel. The hazard determination uses a mechanism-based approach in which production of the hepatotoxic sulfate conjugate of the 1'-hydroxy metabolite is used to interpret the pathological changes observed in different species of laboratory rodents in chronic and subchronic studies. In the risk evaluation, the effect of dose and metabolic activation on the production of the 1'-hydroxy metabolite in humans and laboratory animals is compared to assess the risk to humans from use of methyl eugenol and estragole as naturally occurring components of a traditional diet and as added flavouring substances. Both the qualitative and quantitative aspects of the molecular disposition of methyl eugenol and estragole and their associated toxicological sequelae have been relatively well defined from mammalian studies. Several studies have clearly established that the profiles of metabolism, metabolic activation, and covalent binding are dose dependent and that the relative importance diminishes markedly at low levels of exposure (i.e. these events are not linear with respect to dose). In particular, rodent studies show that these events are minimal probably in the dose range of 1-10 mg/kg body weight, which is approximately 100-1000 times the anticipated human exposure to these substances. For these reasons it is concluded that present exposure to methyl eugenol and estragole resulting from consumption of food, mainly spices and added as such, does not pose a significant cancer risk. Nevertheless, further studies are needed to define both the nature and implications of the dose-response curve in rats at low levels of exposure to methyl eugenol and estragole.

The role of biotransformation in dietary (anti)carcinogenesis

The fact that dietary compounds influence the susceptibility of human beings to cancer, is widely accepted. One of the possible mechanisms that is responsible for these (anti)carcinogenic effects is that dietary constituents may modulate biotransformation enzymes, thereby affecting the (anti)carcinogenic potential of other compounds. This ambiguous theme is the basis for the present paper. The possible effects of enzymatic bioactivation and detoxification of dietary constituents are discussed using two representative examples of phase I and phase II biotransformation enzymes i.e., cytochrome P450 and glutathione S-transferase. Furthermore, the impact of genetic polymorphisms of these two enzyme systems is considered. Although it is very difficult on the basis of the enzyme inducing or inhibiting properties of dietary compounds, especially to characterize them as anticarcinogenic, for certain constituents it is acknowledged that they have anticarcinogenic properties. As such, this provides for an important mechanistic substantiation of the established cancer chemopreventive effect of a diet rich in fruits and vegetables.

Influence of non-nutrient plant components on biotransformation enzymes

Plant foods contain numerous non-nutritive substances which exert biological activity. Most attention has been focused on the anticarcinogenic effects of these compounds. Many of the mechanisms involved include induction or inhibition of biotransformation enzymes. Each individual has its own isoenzyme pattern for the various drug-metabolizing enzymes. The multiplicity of these enzymes results in differential responses to dietary constituents. A substance may increase the level of a certain P450, and decrease the level of another. Although this complicates matters considerably, it also offers the possibility of specifically influencing biotransformation directed at a particular compound, e.g., a cytostatic agent. Using the important class of the glutathione S-transferase (GST) as an example, the various phenotypic and genetic origins of interindividual variation are described. Genetic variation is especially important for the mu and thetra class enzymes. The induction of individual isoenzymes of the GST has been studied in man rat. It was shown that the changes in the GST isoenzyme pattern induced by Brussels sprouts in rat liver and intestine were very similar to that caused by administration of ally isothiocyanate, and not to that resulting from goitrin. In man Brussels sprouts led to induction of GST alpha only. A number of naturally occurring catechols, or more likely the quinones derived from them, are effective irreversible inhibitors of GST. Eugenol, for instance, lowers GST activity in man. A second class of compounds which shows promise are alpha, beta-unsaturated aldehydes and ketones. A number of naturally occurring representatives of this class inhibit GST pi irreversibly, and ethacrynic acid, a drug with a similar reactive moiety in its structure, has already been shown to be quite useful to inhibit GST activity in cellular systems. Several approaches for future studies on the effects of dietary constituents are indicated: 1) further studies on the mechanisms of induction and inhibition of biotransformation enzymes: 2) careful studies using human volunteers, where the effects can be studied in isolation as much as possible; 3) studies of the disposition and kinetics of the dietary constituents themselves, to assess the relevance of inducing agents in food for the day-to-day human situation.

Modulation of glutathione conjugation in vivo: how to decrease glutathione conjugation in vivo or in intact cellular systems in vitro

Glutathione conjugation is involved in detoxification and toxification of a variety of electrophilic substrates. Thus it plays a major role in protection against reactive intermediates. At the same time this conjugation may cause resistance of tumor cells against certain cytostatics. In this review the methods available to decrease glutathione conjugation in vivo are discussed. So far the only in vivo active inhibitors of glutathione S-transferases are ethacrynic acid and a number of glutathione-derived structures; the latter seem very promising for further development. For (chronic) glutathione-depletion, buthionine sulfoximine is most effective, and surprisingly safe in clinical studies. Diethylmaleate can be used for acute depletion. Inhibition of glutathione transferases offers advantages over glutathione depletion as a method of decreasing glutathione conjugation since inhibition may be accomplished without changing the activities of other glutathione-dependent reactions in the cell. However, clinically safe, in vivo effective and isoenzyme-selective glutathione S-transferase inhibitors have not yet been developed.

Oxidation of 4-alkylphenols and catechols by tyrosinase: ortho-substituents alter the mechanism of quinoid formation

Numerous phenols and catechols are known to be substrates for tyrosinase. While the catalytic mechanism of phenol oxidation by tyrosinase has been well studied, little work has been done to determine the influence of substituents on the reaction. In the present investigation, we explored the effects of changing substituents at the 2 and 6 position on the mechanism of tyrosinase-catalyzed oxidation of 4-allyl and 4-propylphenols and catechols. We have previously demonstrated that tyrosinase initially oxidizes hydroxychavicol (4-allyl-catechol) to an o-quinone (3,5-cyclohexadien-1,2-dione) which because of the relatively acidic protons in the benzyl position, readily isomerizes to the tautomeric p-quinone methide (4-allylidene-2,5-cyclohexadien-1-one, QM) (Bolton et al., 1994). We have confirmed through GSH trapping studies that oxidation of 4-allylphenol by tyrosinase yields the same o-quinone GSH conjugates as hydroxychavicol. In contrast, the presence of additional ortho substituents dramatically alters the mechanism of tyrosinase-catalyzed oxidation of 4-alkylphenols. For example, eugenol (4-allyl-2-methoxyphenol), which possesses 1 ortho-methoxy substituents, is not oxidized to a o-quinone or a QM. However, when both ortho o-quinones or QMs which may be selectively toxic to the malignant melanocyte. Although mammalian tyrosinase is much more substrate specific compared to the mushroom tyrosinase used in this study [42], it should be possible to identify compounds which are substrates for the mammalian form but are otherwise oxidatively stable. In order to develop such target compounds an improved understanding of substituent effects on tyrosinase-catalyzed oxidation of catechols and phenols is necessary. This should for the development of strategies for therapeutic compounds that are selectively toxic toward melanoma.