Glutathione-S-transferase activity toward aflatoxin epoxide in livers of mastomys and other rodents

Detoxication and bioconversion of aflatoxin B1 by yellow mealworms (Tenebrio molitor): A sustainable approach for valuable larval protein production from contaminated grain

Yellow mealworm (Tenebrio molitor) is a supplementary protein source for food and feed and represents a promising solution to manage grain contaminated with Aflatoxin B₁ (AFB₁). In this study, AFB₁ present in different concentrations in wheat bran was treated and removed via bioconversion by yellow mealworm of different instars, with emphasis on the bioconversion performance and metabolism of AFB₁. Upon application of wheat bran spiked with 100 μg/kg AFB₁ to 5th-6th instar yellow mealworms, the conversion rate of AFB₁ was up to 87.85 %. Low level of AFB₁ (< 2 μg/kg) was accumulated in the larval bodies, and the survival rate, development and nutrition contents of yellow mealworm were not significantly affected. It was revealed that 1 kg of wheat bran contaminated with AFB₁ increased the weight of yellow mealworms from 138 g to 469 g, containing approximately 103 g of protein. The bioconversion of AFB₁ by yellow mealworms led to generation of 13 metabolites in the frass and 3 metabolites in the larvae. AFB₁ was detoxicated and removed via phase I metabolism comprising reduction, dehydrogenation, hydration, demethylation, hydroxylation, decarbonylation and ketoreduction, followed by phase II metabolism involving conjugation of amino acid, glucoside and glutathione (GSH). The toxicity of AFB₁ metabolites was deemed lower than that of AFB₁ according to their structures. This study provides a sustainable approach and theoretical foundation on using yellow mealworms for cleaner grain contamination management and valuable larval protein production via bioconversion of food and feed contaminated by AFB₁.

The metabolism and biotransformation of AFB1: Key enzymes and pathways

Aflatoxins B₁ (AFB₁) is a hepatoxic compound produced by Aspergillus flavus and Aspergillus parasiticus, seriously threatening food safety and the health of humans and animals. Understanding the metabolism of AFB₁ is important for developing detoxification and intervention strategies. In this review, we summarize the AFB₁ metabolic fates in humans and animals and the key enzymes that metabolize AFB₁, including cytochrome P450s (CYP450s) for AFB₁ bioactivation, glutathione-S-transferases (GSTs) and aflatoxin-aldehyde reductases (AFARs) in detoxification. Furthermore, AFB₁ metabolism in microbes is also summarized. Microorganisms specifically and efficiently transform AFB₁ into less or non-toxic products in an environmental-friendly approach which could be the most desirable detoxification strategy in the future. This review provides a wholistic insight into the metabolism and biotransformation of AFB₁ in various organisms, which also benefits the development of protective strategies in humans and animals.

Protective effect of glutathione S-transferase enzyme activity against aflatoxin B1 in poultry species: relationship between glutathione S-transferase enzyme kinetic parameters, and resistance to aflatoxin B1

Comparative studies designed to investigate the role of glutathione S-transferase (GST) activity on the enzyme catalyzed trapping of aflatoxin B1-8,9-epoxide (AFBO) with glutathione, and the relationship with aflatoxin B1 (AFB1) resistance have not been conducted in poultry. Hepatic cytosolic fractions of chickens, quail, turkeys and ducks were used to measure in vitro the enzymatic parameters maximal velocity (Vmax), Michaelis-Menten constant (Km) and intrinsic clearance (CLint) for GST activity. AFB1 used ranged from 2.0 to 157.5 µM and the AFB1-GSH produced was identified and quantitated by HPLC. Significant differences were found in GST Vmax values, being the highest in chickens, followed by quail, ducks and turkeys. The Km values were also significantly different, with chickens < ducks < turkeys < quail. Chickens had the higher CLint value in contrast to ducks. Differences by sex showed that duck females had a higher CLint value than the turkey and quail, whereas duck males had a CLint close to that of turkey. The ratio "AFBO production /AFB1-GSH production" follows the order duck>turkey>quail>chicken, in agreement with the known poultry sensitivity. The extremely high "AFB1 epoxidation activity/ GST activity" ratio observed in ducks might be the explanation for the development of hepatocellular carcinoma in this species.

Comparative liver metabolic enzyme activity of cytochrome P450 and glutathione-S-transferase in crocodile (Crocodylus siamensis) and livestock

This study aimed to compare the metabolism of detoxification liver enzymes activity of CYP1A2, CYP2E1, the GST enzymes activity for class pi, mu, alfa, and the universal GST between crocodile and livestock, plus the relationship of these two enzyme activities in the crocodile. Interestingly, the CYP1A2 and CYP2E1 were existent in the freshwater crocodile livers microsomal enzyme activities, and these two enzymes could be determined in both pig and chicken. Also, the universal GST, alpha, mu, and pi class of phase II were presented in the crocodile liver cytosolic fractions, and these three enzymes could also be found in pig and chicken. The kinetic activity (Vmax/Km ratio) of GST activity towards CDNB was higher in the crocodile than chicken, and pig; 80.02, 57.80 and 45.25 ml/min/mg protein, respectively. The Vmax/Km ratio GST activity towards t-PBO was highest in crocodile. In contrast, the GST activity towards EA was highest in pig, chicken and crocodile in this order. However, the Vmax/Km ratio GST activity towards CHP was very low in all species. The crocodile liver microsome fraction could metabolites AFB1 to AFM1, suggesting that the CYP1A2 activity was actively presented. The kinetic enzyme activity of crocodile liver towards CDNB revealed the highest velocity compared with other livestock species; this indicates that crocodile liver enzyme activities were very active for the detoxification function towards all xenobiotic, especially for GST activity, towards carcinogenic agent when compared with other livestock.

An unusually high production of hepatic aflatoxin B1-dihydrodiol, the possible explanation for the high susceptibility of ducks to aflatoxin B1

A study was conducted to determine the enzymatic kinetic parameters V_max, K_M, and intrinsic clearance (CL_int) for the hepatic in vitro production of aflatoxin B₁-dihydrodiol (AFB₁-dhd) from aflatoxin B₁ (AFB₁) in four commercial poultry species, ranging in sensitivity to AFB₁ from highest (ducks) to lowest (chickens). Significant but small differences were seen for V_max, while large significant differences were observed for K_M. However, the largest inter-species differences were observed for the CL_int parameter, with ducks being extraordinarily efficient in converting AFB₁ into AFB₁-dhd. Since AFB₁-dhd is considered the metabolite responsible for the acute toxic effects of AFB₁, the high hepatic production of AFB₁-dhd from AFB₁ in ducks is the possible biochemical explanation for the extraordinary high sensitivity of this poultry species to the adverse effects of AFB₁.

Toxicokinetic profile of fusarenon-X and its metabolite nivalenol in the goat (Capra hircus)

The main aim of this research was to evaluate the toxicokinetic characteristics of fusarenon-X (FX) and its metabolite, nivalenol (NIV), in goats. The amounts of FX and NIV in post-mitochondrial (S-9), microsomal and cytosolic fractions of diverse tissues of the goat were also investigated. FX was intravenously (iv) or orally (po) administered to goats at dosages of 0.25 and 1 mg/kg bw, respectively. The concentrations of FX and NIV in plasma, feces and urine were quantified by liquid chromatography tandem-mass spectrometry (LC-ESI-MS/MS). The concentrations of FX in plasma were quantified up to 8 h with both routes of administration. A large amount of NIV (metabolite) was quantifiable in plasma, urine and feces after both administrations. The C_max value of FX was 413.39 ± 206.84 ng/ml after po administration. The elimination half-life values were 1.64 ± 0.32 h and 4.69 ± 1.25 h after iv and po administration, respectively. In vitro experiments showed that the conversion FX-to-NIV mainly occurs in the liver microsomal fraction. This is the first study that evaluates the fate and metabolism of FX in ruminant species.

The effects of season and gender on the serum aflatoxins and ochratoxin A levels of healthy adult subjects from the Central Anatolia Region, Turkey

PURPOSE

This study was undertaken to determine the effects of season and gender on serum aflatoxin (AF) levels (AFG1, AFB1, AFG2 and AFB2) and ochratoxin A (OTA) concentrations of healthy adult population living in Central Anatolia Region of Turkey.

METHODS

AF levels were measured by high-performance liquid chromatography (HPLC) and OTA levels were measured by enzyme-linked immunosorbent assay (ELISA) in serum samples of healthy adults (n = 233).

RESULTS

In summer and winter, total AF levels in females were 0.98 ± 0.10 and 0.94 ± 0.12 ng/ml and in males 1.35 ± 0.17 and 0.93 ± 0.11 ng/ml, respectively. Male subjects had significantly higher serum total AF levels in summer compared with females (~38%). There was no marked seasonal change in AFG1, AFB1 and AFG2 concentrations in the whole population, except AFB2. Both of the genders had significantly higher OTA levels in winter compared with summer (~60%).

CONCLUSIONS

Overall results suggest that Central Anatolia residents are continuously exposed to AFs and OTA. Besides, season and gender can be effective in mycotoxin exposure.

Toxicokinetics and tissue depletion of Fusarenon-X and its metabolite nivalenol in piglets

Fusarenon-X (FX) is one of the trichothecene mycotoxins mainly produced by Fusarium crookwellense, which naturally occurs in agricultural commodities such as wheat and barley. To investigate the toxicokinetics of FX and its metabolite nivalenol (NIV), FX was then administered intravenously or orally to piglets at a dosage of 1mg/kg body weight. The concentrations of FX and NIV in the plasma and various tissues were measured using LC-MS/MS. The plasma concentrations of FX in the piglets were determined up to 24h and 48h after iv and po administration, respectively, and the concentration of NIV was detected up to 12h after both types of administration. The Cp(0) of FX was 580.28 ± 140.81 ng/ml after iv administration. The values of t1/2β, Vss and Foral were 1.71 ± 0.74 h, 0.009 ± 0.002 ml and 74.40 ± 18.96%, respectively. FX and NIV were detectable in the vital organs up to 24h after po administration. The peak level of FX in the liver, the kidney, and the spleen, respectively, were 165.95 ± 9.68 ng/g, 66.29 ± 8.48 and 7.35 ± 0.69 ng/g at 3h following po administration. In vitro of liver postmitochondrial fractions with FX demonstrated that the liver and kidney are capable of FX-to-NIV metabolism.

Effect of selenium in organic and inorganic form on liver, kidney, brain and muscle of Wistar rats

Selenium is a micronutrient, localized in the active sites of enzymes such as glutathione peroxidase and thioredoxin reductase, and participating together with these enzymes in an antioxidant defence system of organisms against free radicals. Administration of selenium is necessary for maintaining oxidative homeostasis. The present experiment is aimed at investigation of selenium impact on basal metabolic processes and selected antioxidants in a Wistar rat model, fed selenium in organic and inorganic forms. Liver, kidney, brain and muscle were sampled during a month-long feeding with four different doses of selenium (0.075 mg or 1.5 mg of inorganic and/or organic selenium per kg of feed). We found a significant reduction in glutathione level in liver tissue regardless of the form of the administered selenium. On the other hand, selenium caused a decreased glutathione reductase level in the liver and metallothionein level in the liver, kidney and muscle.

Cytochrome P450 enzymes involved in the metabolism of aflatoxin B1 in chickens and quail

A study was conducted to identify the cytochrome P450 (CYP, CYP450) enzyme orthologs involved in the bioactivation of aflatoxin B(1) (AFB(1)) into the highly toxic metabolite known as aflatoxin-8,9-epoxide (AFBO) in quail and chicken hepatic microsomes. The strategies used included the use of specific CYP450 inhibitors and the correlation of prototype substrate activities with AFBO production. Additionally, the presence of the enzymes was qualitatively determined using an immunoblotting technique. The results showed that both quail and chicken microsomes have CYP1A1, CYP1A2, CYP2A6, and CYP3A4 enzymatic activity. A strong relationship between CYP1A1 and CYP2A6 activities and AFB(1) bioactivation was found in both species. Inhibition studies provided more evidence for the role of CYP2A6 in the bioactivation of AFB(1). The immunoblot results showed clear bands for the CYP2A6 and CYP3A4 orthologs in both species. The results of the present study indicate that CYP2A6 and, to a lesser extent, CYP1A1 are responsible for the bioactivation of AFB(1) into AFBO in both quail and chicken hepatic microsomes.

Fate of fusarenon-X in broilers and ducks

In order to investigate the comparative fates and dispositions of fusarenon-X (FX) in broilers and ducks, FX was administered i.v. or orally (p.o.) to broilers and ducks. The FX and its metabolite (nivalenol, NIV) were determined in plasma and excreta using gas chromatography-mass spectrometry. The plasma concentrations of FX were determined up to 180 and 120 min in broilers and ducks, respectively, after i.v. and p.o. administration. The NIV was eliminated more slowly than its parent compound. The FX disposition fit an open 2-compartment pharmacokinetic model in broilers and ducks. The elimination half-life (t(1/2beta)) of FX was longer in ducks than in broilers. The elimination rate constant (kel) was higher in broilers than in ducks, whereas the oral bioavailability of FX was higher in ducks than in broilers. The gas chromatography-mass spectrometry profile in plasma showed that a large proportion of FX was recovered as NIV after administration of FX in both broilers and ducks. In vitro incubation of liver microsomal and cytosolic fractions with FX demonstrated that the liver and kidney are capable of the FX-to-NIV conversion. Thus, this study demonstrated that FX is absorbed more efficiently in ducks than in broilers, whereas it is eliminated more slowly in ducks than in broiler chickens. Consequently, the toxicity would have more serious consequences in ducks rather than broilers.

Nivalenol and the targeting of the female reproductive system as well as haematopoietic and immune systems in rats after 90-day exposure through the diet

Nivalenol (NIV) is considered to be an important trichothecene mycotoxin produced by Fusarium species because of its frequent contamination in wheat and barley worldwide. The present study examined the subchronic toxicity of NIV in male and female F344 rats fed diets containing 0, 6.25, 25 and 100 mg kg(-1) of the toxin for 90 days. During the experimental period there was a decrease in the white blood cell count at 100 mg kg(-1) in males and at > or =6.25 mg kg(-1) in females. Histopathologically, treatment-related changes were observed in the haematopoietic and immune systems in both sexes and in the female reproductive system at 100 mg kg(-1). Flow cytometric analysis of splenic cells revealed an elevation in the ratio of helper/cytotoxic T-lymphocytes at 100 mg kg(-1). In summary, NIV targets the female reproductive system as well as haematopoietic and immune systems in rats fed NIV for 90 days. Based on a significant decrease in white blood cells in female rats relative to controls, the lowest observable effect level was calculated as 0.4 mg kg(-1) body weight day(-1).

The effect of feeding piglets with the diet containing green tea extracts or coumarin on in vitro metabolism of aflatoxin B1 by their tissues

To clarify whether enzymes involved in aflatoxin B1 (AFB1) metabolism in pigs respond to antioxidant agents, the effect of feeding piglets with diets containing green tea extracts (Sunphenon) and coumarin on in vitro AFB1 metabolism by their liver and intestinal tissues was studied. The results showed that coumarin reduced AFB1-DNA adduct formation by both liver and intestinal microsomes, while Sunphenon did not have any effects. Both coumarin and Sunphenon enhanced the glutathione S-transferase (GST) activity to conjugate AFB1 to glutathione GSH in the intestine, although no effects were noted in the liver. Changes of the expression of mRNA of GSTA2 and GSTO1 were not in parallel with the observed changes of GST activity, suggesting that other GST subtypes are involved in the GST activity toward AFB1. As for lipophilic-free AFB1 metabolites, coumarin reduced the liver microsomal conversion of AFB1 to aflatoxin M1 (AFM1) and aflatoxin Q1 (AFQ1), but Sunphenon exerted no effects. Both coumarin and Sunphenon enhanced the conversion of AFB1 to aflatoxicol in the liver. All the results suggest that feeding with a diet containing coumarin affects AFB1 metabolism to enhance AFB1 detoxification through the suppression of P450 enzyme activity in the liver and the enhancement of GST activity in the intestine. Feeding with a diet containing Sunphenon enhances AFB1 detoxification, but the effects are noted mainly in the intestine.

Microsomal and cytosolic biotransformation of aflatoxin B1 in four poultry species

1. This research evaluated differences in hepatic in vitro metabolism of aflatoxin B1 (AFB1) on selected avian species. 2. Microsomal and cytosolic liver fractions were obtained from chickens, ducks, quails and turkeys; eight males and eight females of each. 3. All microsomes studied produced AFB1-8,9-exo-epoxide (AFBO), a metabolite regarded as the active product of AFB1. Turkey microsomes produced 1.8 and 3.5 times more AFBO than quails and chickens microsomes, respectively. 4. Males from evaluated birds produced more AFBO than females, but statistically-significant differences between genders were observed only in ducks and turkeys. 5. The cytosolic fraction from all four species produced aflatoxicol (AFL). Turkey and duck hepatic cytosol produced more AFL than from quail and chickens. 6. It is known that turkeys are very sensitive to AFB1, quails are intermediate and chickens are particularly resistant; the differences in AFBO production shown in our study may help to explain the difference in vivo responses among turkeys, quail and chickens. 7. Moreover, AFL may be related to AFB1 toxicity; it was produced in larger amounts by hepatic cytosol from the more susceptible species. 8. Because AFBO production by microsomes in ducks was relatively low, it is possible that other toxicity mechanisms are involved in this highly susceptible species.

Comparative activities of glutathione-S-transferase and dialdehyde reductase toward aflatoxin B1 in livers of experimental and farm animals

In order to gain a better understanding of the relative activities of glutathione-S-transferase (GST) and aldehyde reductase toward aflatoxin B1 (AFB1) in relation to the variation of species susceptibilities, we studied the in vitro cytosolic GST and reductase activities in liver tissues from male Fischer rats, ICR mice and golden hamsters, adult male rainbow trouts and female piglets. The GST activity was determined by incubating the liver cytosol with glutathione (GSH) and AFB1 in the presence of the hamster liver microsomes to metabolize AFB1 to AFB1-8, 9-epoxide. The reaction product, AFB1 and GSH conjugate (AFB1-GSH), was quantified with HPLC. The reductase activity was determined by incubating liver cytosol with AFB1-dialdehyde, followed by the quantification of the metabolic product, AFB1-dialcohol, with HPLC. All the animal species possessed the GST activities, and AFB1-GSH formed increasingly with the increase of the AFB1 concentration according to the model of first-order enzyme reaction kinetics. The V(max) and K(m) values of the GST activities in rodent species were higher and lower, respectively, than those in the trout and pig, being consistent with the relative susceptibilities to AFB1 of these animal species. However, no relationship was noted between the reductase activity and species susceptibility. Thus, the result of this study shows that GST toward AFB1, but not aldehyde reductase, is a determinant of the variation of species susceptibilities.

Polymorphism of glutathione S-transferase mu 1 and theta 1 genes and hepatocellular carcinoma in southern Guangxi, China

AIM: Glutathione S-transferase mu 1 (GSTM1) and theta 1 (GSTT1) genes are involved in the metabolism of a wide range of carcinogens, but deletions of the genes are commonly found in the population. The present study was undertaken to evaluate the association between GSTM1 and GSTT1 gene polymorphisms and hepatocellular carcinoma (HCC) risk.

METHODS: The genetic polymorphisms were studied at an aflatoxin highly contaminated region in Guangxi, China. Polymerase chain reaction (PCR) technique was used to detect the presence or absence of the GSTM1 and GSTT1 genes in blood samples. The case group was composed of 181 patients of HCC identified by the pathologists and the control group was composed of 360 adults without any tumor.

RESULTS: The frequencies of GSTM1 and GSTT1 null genotypes in the control were 47.8% and 42.7%, while those in the HCC group were 64.6% and 59.7%, respectively. The differences between HCC group and control group were very significant (P<0.01). GSTM1 and GSTT1 combined null genotypes in HCC group and control group were 38.2% and 18.5% respectively, and the difference was significant (P<0.05).

CONCLUSION: The GSTM1 and GSTT1 null genotypes are associated with an increased risk of HCC in a special geographic environment. Combination of the two null genotypes in an individual is substantially increased twice the risk of HCC.

Low hepatic glutathione S-transferase and increased hepatic DNA adduction contribute to increased tumorigenicity of aflatoxin B1 in newborn and partially hepatectomized mice

Low levels of hepatic glutathione S-transferase and increased formation of aflatoxin B1 (AFB1)-DNA adducts correlate with hepatocyte proliferation and increased hepatocarcinogenesis in both newborn mice and partially hepatectomized adult mice, as compared to normal, adult C57BL/6J mice. Newborn mice, which are highly susceptible to the hepatocarcinogenic effects of AFB1, have active proliferation of hepatocytes until 3 weeks of age, when hepatocyte proliferation abruptly ceases. At about this time, the mice become highly resistant to AFB1. In adult mice, AFB1 carcinogenicity is increased after stimulation of liver proliferation by partial hepatectomy. To become carcinogenic, AFB1 is activated in the liver by the P450 enzyme system to electrophilic intermediates, some of which form DNA adducts believed to be responsible for mutations leading to cancer. The most carcinogenic intermediate, AFB(1)-8,9-epoxide, is detoxified by glutathione S-transferase-mediated conjugation to glutathione. Glutathione levels, glutathione S-transferase levels, and AFB1-DNA adduct formation were measured at 4, 10, 30, 120, 245 and 365 days of age in C57BL/6J mice. There was a 5-fold increase in hepatic glutathione S-transferase levels and 13-fold decrease in hepatic AFB1-DNA adduct formation over these ages. Induction of hepatocyte proliferation following partial hepatectomy of 120-day-old mice lowered hepatic glutathione S-transferase levels and increased the extent of hepatic AFB1-DNA formation to levels similar to those measured in 4-day-old mice. These results indicate that increased susceptibility to AFB1 hepatocarcinogenesis in newborn mice, and in adult mice following partial hepatectomy, is due to decreased GST and increased adduct formation in proliferating liver.

Aflatoxin B1-induced changes of glutathione-S-transferase activity in the plasma and liver of the rat

BACKGROUND

The influence of low doses of aflatoxin B1(AFB1) and partial hepatectomy (PH) on glutathione-S-transferase (GST) activity was studied in the plasma and liver of the rat.

METHODS

The animals were divided into four groups. The first (I) and the second (II) group were treated with AFB1 freshly dissolved in dimethyl sulphoxide (DMSO), and administered as a single intraperitoneal dose of 50 micrograms/rat 24 hrs after the rats had undergone either sham operation or, 40% PH, respectively. The third group (III) of animals was treated with a total dose of 1 mg AFB1-5 days per week during a period of 8 weeks. The non-treated animals were used as controls (C).

RESULTS

We observed a significant increase of GST activity in the plasma of all experimental groups compared to the controls (C), (p < 0.02 to p < 0.005). In the liver, the GST activity of all experimental groups was also significantly increased, compared to the controls (from p < 0.02 to p < 0.005).

CONCLUSION

The administration of both single and multiple doses of AFB1 led to long term increase of GST activity in the rat plasma and liver, and partial hepatectomy had no significant effect on this phenomenon.