The formation of 2,3-dihydroxy-2,3-dihydro-aflatoxin B1 by the metabolism of aflatoxin B1 by liver microsomes isolated from certain avian and mammalian species and the possible role of this metabolite in the acute toxicity of aflatoxin B1

Dealing with aflatoxin B1 dihydrodiol acute effects: Impact of aflatoxin B1-aldehyde reductase enzyme activity in poultry species tolerant to AFB1 toxic effects

Aflatoxin B1 aldehyde reductase (AFAR) enzyme activity has been associated to a higher resistance to the aflatoxin B1 (AFB1) toxicity in ethoxyquin-fed rats. However, no studies about AFAR activity and its relationship with tolerance to AFB1 have been conducted in poultry. To determine the role of AFAR in poultry tolerance, the hepatic in vitro enzymatic activity of AFAR was investigated in liver cytosol from four commercial poultry species (chicken, quail, turkey and duck). Specifically, the kinetic parameters Vmax, Km and intrinsic clearance (CLint) were determined for AFB1 dialdehyde reductase (AFB1-monoalcohol production) and AFB1 monoalcohol reductase (AFB1-dialcohol production). In all cases, AFB1 monoalcohol reductase activity saturated at the highest aflatoxin B1 dialdehyde concentration tested (66.4 μM), whereas AFB1 dialdehyde reductase did not. Both activities were highly and significantly correlated and therefore are most likely catalyzed by the same AFAR enzyme. However, it appears that production of the AFB1 monoalcohol is favored over the AFB1 dialcohol. The production of alcohols from aflatoxin dialdehyde showed the highest enzymatic efficiency (highest CLint value) in chickens, a species resistant to AFB1; however, it was also high in the turkey, a species with intermediate sensitivity; further, CLint values were lowest in another tolerant species (quail) and in the most sensitive poultry species (the duck). These results suggest that AFAR activity is related to resistance to the acute toxic effects of AFB1 only in chickens and ducks. Genetic selection of ducks for high AFAR activity could be a means to control aflatoxin sensitivity in this poultry species.

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.

Sensitivity of Arbor Acres broilers and chemoprevention of aflatoxin B1-induced liver injury by curcumin, a natural potent inducer of phase-II enzymes and Nrf2

In this study, we scrutinized the effects of curcumin and AFB₁ supplemented diet alone or in combination on phase-ӀӀ enzymes. Histopathological examination showed that after 28 days, AFB1 (5.0 mg/kg diet) induced liver injury in broilers, but curcumin supplementation partially ameliorated liver injury in a dose-dependent manner. RT-PCR data revealed that AFB₁ significantly (p < 0.01) down-regulated Nrf2 and its downstream genes mRNA expression level. Moreover, Western blot analysis showed that Nrf2, GSTM2, and GSTA3 protein expression level was markedly (p < 0.01) reduced in AFB₁-fed group. However, curcumin supplementation ameliorated AFB₁-induced liver injury via enhancing phase-ӀӀ enzymes expressions and activity. HPLC results showed that curcumin increased AFB₁-GSH conjugation in-vitro in liver cytosol. Surprisingly, similar trends were noted in mRNA, protein expression level of Nrf2 and its downstream genes at day 35, one week after the withdrawal of AFB₁ and curcumin from the diet, showing the preventive effects of curcumin.

AFM₁ in Milk: Physical, Biological, and Prophylactic Methods to Mitigate Contamination

Aflatoxins (AFs) are toxic, carcinogenic, immunosuppressive secondary metabolites produced by some Aspergillus species which colonize crops, including many dietary staple foods and feed components. AFB₁ is the prevalent and most toxic among AFs. In the liver, it is biotransformed into AFM₁, which is then excreted into the milk of lactating mammals, including dairy animals. AFM₁ has been shown to be cause of both acute and chronic toxicoses. The presence of AFM₁ in milk and dairy products represents a worldwide concern since even small amounts of this metabolite may be of importance as long-term exposure is concerned. Contamination of milk may be mitigated either directly, decreasing the AFM₁ content in contaminated milk, or indirectly, decreasing AFB₁ contamination in the feed of dairy animals. Current strategies for AFM₁ mitigation include good agricultural practices in pre-harvest and post-harvest management of feed crops (including storage) and physical or chemical decontamination of feed and milk. However, no single strategy offers a complete solution to the issue.

Sulforaphane induces glutathione S-transferase isozymes which detoxify aflatoxin B(1)-8,9-epoxide in AML 12 cells

The aflatoxin B(1)-8,9-epoxide (AFBO) is hepatocarcinogenic intermediate of aflatoxin B(1) (AFB(1)) and is detoxified by glutathione S-transferases (GSTs). In this study, we investigated whether sulforaphane (SFN) could increase the rate of conjugation between AFBO and glutathione (GSH) as well as which of the GST isozymes were involved in the conjugation reaction. The conjugation potential was inhibited dose dependently with curcumin, an inhibitor of GSTs. SFN induced the expression of GST A3, GST A4, GST M1, GST P1, and GST T1 in alpha mouse line (AML) 12 cells. The cells treated with SFN (10 microM) for 12 h showed a 35-fold increase in conjugation potential of AFBO with GSH compared with the vehicle-treated cell. The conjugation potential was blocked partially by transfection of cells with siRNAs against each of the GST isozymes. The activity of GST A3 had the strongest effect on the conjugation potential. SFN treatment also increased total GST activity detected with 1-chloro-2,4-dinitrobenzene (CDNB) up to 4.3-fold. The induction fold was much lower than that detected with AFBO. These results suggest that the chemopreventive effect of SFN on the decomposition of AFBO is related to the upregulation of several GST isozymes genes. The increase of GST activity by SFN was extremely specific toward the conjugation reaction of AFBO compared with CDNB. Therefore, this system for detecting GST activity seems to be an excellent method for screening chemopreventive compounds toward AFB(1) toxicity.

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.

Changes in hepatic cytosolic glutathione S-transferase activity and expression of its class-P during prenatal and postnatal period in rats treated with aflatoxin B1

The effect of aflatoxin B1 (AFB1) on the expression of glutathione S-transferase-P (GST-P) which is the major isoform of GST in developmental stages has been investigated in rat liver during prenatal and postnatal stages. Following administration of AFB1 (0, 0.5, 1.0, 2.0, 3.0 or 4.0 mg/kg bw) injected I.P on day 8.5 of gestation the number of dead or reabsorbed fetuses and malformed embryos were recorded. Then the fetal livers were processed for measurement of total GST and GST-P activities, using 1-chloro-2,4-dinitrobenzene (CDNB) and ethacrynic acid (ETA) as substrates respectively. RT-PCR using rat GST-P specific primers was performed on mRNA extracted from livers. Besides, the effects of AFB1 on hepatic GST and GST-P were assessed in groups of suckling rats directly injected with the toxin. The results show that a single dose of AFB1 (1.0 or 2.0 mg/kg bw) caused approximately 50-60% depletion in fetal liver GST towards CDNB. Postnatal experiments revealed that liver GST (using CDNB as substrate) was significantly induced (approximately 40%) in suckling rats injected with a single dose of AFB1 (3.0 mg AFB1/kg) 24 h before killing. Liver GST-P expression was unaffected due to AFB1 exposures of rats before and after the birth. This finding was substantiated by western blotting and RT-PCR techniques. These data suggest that AFB1-related induction in rat liver total GST after birth may be implicated in protective mechanisms against AFB1. In contrast, inhibition of this enzyme in fetal liver following placental transfer of the carcinogen may explain high susceptibility of fetal cells to trans-plancental aflatoxins. Furthermore, lack of influence of AFB1 on GST-P expression in developmental stages can role out the involvement of this class of GST in AFB1 biotransformation.

Aflatoxin residues in eggs of laying Japanese quail after long-term administration of rations containing low levels of aflatoxin B1

The aim was to evaluate the excretion of residues of aflatoxin B(1) (AFB(1)), aflatoxin M(1) (AFM(1)), aflatoxin B(2a) (AFB(2a)) and aflatoxicol (AFL) in eggs of laying Japanese quail fed rations with low levels of aflatoxin B(1) for 90 days. The quail were randomly assigned into four experimental groups and given prepared rations containing either 0 (controls), 25, 50 or 100 microg AFB(1) kg(-1) feed. Thirty-two eggs per treatment were collected on days 1-7, 10, 20, 30, 60 and 90 of the aflatoxin treatment period, and submitted to aflatoxin analysis by high-performance liquid chromatography. Average egg production and feed consumption were not affected ( p > 0.05) by AFB(1). Egg weight was significantly lower ( p<0.05) only for groups exposed to 100 microg AFB(1) kg(-1). Residues of aflatoxins were detected in eggs at levels that ranged from 0.01 to 0.08 microg kg(-1) (AFB(1)), 0.03-0.37 microg kg(-1) (AFM(1)), 0.01-1.03 microg kg(-1)(AFB(2a)) and 0.01-0.03 microg kg(-1) (AFL). Results indicate that the excretion of aflatoxin residues in quail eggs might occur at relatively low concentrations under conditions of long-term exposure of quail to low levels of AFB(1).

Mechanisms of protection against aflatoxin B(1) genotoxicity in rats treated by organosulfur compounds from garlic

Diallyl sulfide (DAS) and diallyl disulfide (DADS), two garlic constituents, were found previously to inhibit aflatoxin B(1) (AFB(1))-initiated carcinogenesis in rat liver, DADS being the most effective. In order to study the mechanisms involved in this protection, we have examined the ability of liver microsomes and cytosols from DAS- and DADS-treated rats to modulate the mutagenicity and the metabolism of AFB(1). We also examined the effects of these compounds on the expression of cytochromes P450 (CYP) and phase II enzymes known to be involved in AFB(1) metabolism. Administration of DAS (1 mmol/kg for 4 days) to rats resulted in significant inhibition of microsome-mediated mutagenicity of AFB(1), whereas DADS treatment did not alter AFB(1) mutagenicity. DAS treatment increased the metabolism of AFB(1) mainly towards the formation of AFQ(1) and AFM(1), which might account for the reduction of AFB(1) microsomal-mediated mutagenicity. DADS treatment slightly affected the oxidative metabolism of AFB(1). DAS and DADS induced CYP3A2, CYP2B1 and CYP2B2, DAS being more potent. Cytosols from DAS- and DADS-treated rats produced a significant inhibition of AFB(1)-8,9-epoxide (AFBO)-induced mutagenicity and significantly increased the cytosolic formation of AFB(1)-glutathione conjugates, DADS treatment being more effective. Western blot analysis showed that DADS is a potent inducer of glutathione S-transferase A5 (rGSTA5) and AFB(1) aldehyde reductase 1 (rAFAR1), while DAS is a weak inducer of these enzymes. Finally, we demonstrated that antibodies raised against rGSTA5 strongly reduced the antimutagenic activity of cytosols from DAS- and DADS-treated rats against AFBO. All together, these results demonstrate that DAS prevents AFB(1) mutagenicity through a dual mechanism, i.e. by modulating both the phase I and II metabolism of AFB(1), whereas DADS acts mainly by increasing the phase II metabolism of AFB(1). The induction of rGSTA5 and rAFAR1 is probably the main mechanism by which allyl sulfides give protection against AFB(1)-induced carcinogenesis.

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

In order to study the liver glutathione-S-transferase (GST) activity toward aflatoxin B1 (AFB1) epoxide in mastomys in comparison with other rodents, we performed in vitro studies of the cytosolic GST activity toward AFB1-epoxide using mastomys, rat, mouse and hamster liver. Also AFB1 metabolism by liver microsomes including formation of AFB1-DNA adducts was studied. Cytosolic GST activity toward AFB1-epoxide was highest in mastomys liver, and higher in the hamster and mouse livers than in the rat liver, correlating well with the differences of the sensitivity of these species to the toxicity of AFB1. However, no relationship was noted between the sensitivity of a given species to the toxicity of AFB1 and the microsomal activity of binding of AFB1 to DNA or metabolizing AFB1 to AFM1, AFQ1 and AFP1. These results demonstrate the importance of the GST mediated AFB1-epoxide conjugation with glutathione in determining the differing sensitivities of these species to AFB1 toxicity. The extremely high activity of GST in mastomys indicates that this species would be a good model animal for studying GST toward AFB1-epoxide.

In vitro metabolism of aflatoxin B1 by larvae of navel orangeworm, Amyelois transitella (Walker) (Insecta, Lepidoptera, Pyralidae) and codling moth, Cydia pomonella (L.) (Insecta, Lepidoptera, Tortricidae)

Larvae of the navel orangeworm (NOW), Amyelois transitella (Walker), a major pest of almonds and pistachios, and the codling moth (CM), Cydia pomonella (L.), the principal pest of walnuts and pome fruits, are commonly found in tree nut kernels that can be contaminated with aflatoxin, a potent carcinogen. The ability of larvae of these insects to metabolize aflatoxin B1 (AFB1) was examined. A field strain of NOW produced three AFB1 biotransformation products, chiefly aflatoxicol (AFL), and minor amounts of aflatoxin B2a (AFB2a) and aflatoxin M1 (AFM1). With AFL as a substrate, NOW larvae produced AFB1 and aflatoxicol M1 (AFLM1). A lab strain of CM larvae produced no detectable levels of AFB1 biotransformation products in comparison to a field strain which produced trace amounts of only AFL. Neither NOW nor CM produced AFB1-8,9-epoxide (AFBO), the principal carcinogenic metabolite of AFB1. In comparison, metabolism of AFB1 by chicken liver yielded mainly AFL, whereas mouse liver produced mostly AFM1 at a rate eightfold greater than AFL. Mouse liver also produced AFBO. The relatively high production of AFL by NOW compared to CM may reflect an adaptation to detoxify AFB1. NOW larvae frequently inhabit environments highly contaminated with fungi and, hence, aflatoxin. Only low amounts, if any, of this mycotoxin occur in the chief CM hosts, walnuts, and pome fruits. Characterizations of enzymes and co-factors involved in biotransformation of AFB1 are discussed.

Biochemical basis for the extreme sensitivity of turkeys to aflatoxin B(1)

Poultry are the most susceptible food animal species to the toxic effects of the mycotoxin aflatoxin B(1) (AFB(1)). Feed contaminated with even small amounts of AFB(1) results in significant adverse health effects in poultry. The purpose of this study was to explain the biochemical mechanism(s) for this extreme sensitivity. We measured microsomal activation of AFB(1) to the AFB(1)-8,9-epoxide (AFBO), the putative toxic intermediate, as well as cytosolic glutathione S-transferase (GST)-mediated detoxification of AFBO, in addition to other hepatic phase I and phase II enzyme activities, in 3-week-old male Oorlop strain turkeys. Liver microsomes prepared from these turkeys activated AFB(1) in vitro with an apparent K(m) of 109 microM and a V(max) of 1.25 nmol/mg/min. Preliminary evidence for the involvement of cytochromes P450 (CYP) 1A2 and, to a lesser extent, 3A4 for AFB(1) activation was assessed by the use of specific mammalian CYP inhibitors. The possible presence of avian orthologues of these CYPs was supported by activity toward ethoxyresorufin and nifedipine, as well as by Western immunoblotting using antibodies to human CYPs. Cytosol prepared from turkey livers exhibited GST-mediated conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) and 3,4-dichloronitrobenzene (DCNB), but at a much lower rate than that observed in other species. Western immunoblotting indicated the presence of alpha and sigma class GSTs and another AFB(1)-detoxifying enzyme, AFB(1)-aldehyde reductase (AFAR). Turkey liver cytosol also had quinone oxidoreductase (QOR) activity. Importantly, cytosol exhibited no measurable GST-mediated detoxification of microsomally activated AFB(1), indicating that turkeys are deficient in the most crucial AFB(1)-detoxification pathway. In total, our data indicate that the extreme sensitivity of turkeys to AFB(1) may be attributed to a combination of efficient AFB(1) activation and deficient detoxification by phase II enzymes, such as GSTs.

Metabolism and toxicity of aflatoxins M1 and B1 in human-derived in vitro systems

Aflatoxin M1 (AFM1) is the principal hydroxylated aflatoxin metabolite present in the milk of dairy cows fed a diet contaminated with aflatoxin B1, (AFB1) and the metabolite is also present in the milk of human nursing mothers consuming foodstuffs containing the toxin. AFM1 is usually considered to be a detoxification product of AFB1 and this appears warranted if the biological endpoints involved are carcinogenicity and mutagenicity. However, it may not be a valid conclusion in the case of cytotoxicity. The metabolism of AFM1 and AFB1 have been studied in vitro using human liver microsomes. Formation of primary metabolites associated with metabolic activation to the respective epoxides reflected the differences between the carcinogenic potentials of the two toxins and, similar to AFB1, the conjugation of AFM1 epoxide with reduced GSH was catalyzed by mouse, but not human liver cytosol. Although the majority of the binding of [3H]AFB1 to microsomal protein was dependent on metabolic activation, a high level of retention of [3H]AFM1 by microsomes, nonextractable in methanol and unrelated to metabolic activation, was observed. It appears possible that this property is related to the high cytotoxicity of AFM1. Experiments using human cell line cells either expressing or not expressing human cytochrome P450 enzymes in assays of acute toxicity (MTT assays) have demonstrated a directly toxic potential of AFM1 in the absence of metabolic activation, in contrast to AFB1. Caution therefore needs to be exercised in designating the formation of AFM1 as essentially detoxification when considering a biological response in which cytotoxicity may play a significant role, e.g., immunotoxicity.

The fate and acute toxicity of aflatoxin B1 in the mastomys and rat

The fate and acute toxicity of aflatoxin B1 (AFB1) were studied in the mastomys (Praomys coucha) and compared with Fischer rats. The experiment regarding the fate of [3H]AFB1 showed that the radioactivity was excreted mainly through the feces, more rapidly in the mastomys than in the rat, regardless of whether [3H]AFB1 was given orally or intravenously. The levels of radioactivity bound to the liver DNA were lower in the mastomys than in the rat, indicating that the levels of AFB1 binding to the macromolecules in the liver were lower in the mastomys. Consistent with such differences in the fate of AFB1 between the two species, the mastomys were far more resistent to the acute effects of AFB1 than were the rats. Oral administration of AFB1 at a dose of 1.0 mg/kg to rats caused marked microscopic changes in the liver, involving hepatic necrosis and proliferation of bile ducts, but at a dose of 4.0 mg/kg to mastomys caused no pathological changes in the liver or kidneys, and at a dose of 10.0 mg/kg caused only glycogen deposition in hepatic cells in a limited area. The observed differences in susceptibility to the toxic effects of AFB1 and in the fate of AFB1 between the two species are in accord with our previous finding that liver cytosol in the mastomys inhibits microsome-mediated AFB1-DNA binding in vitro more strongly than in rat liver.

Coumarin chemoprotection against aflatoxin B1-induced gene mutation in a mammalian cell system: a species difference in mutagen activation and protection with chick embryo and rat liver S9

Coumarin (1,2-benzopyrone), a natural food constituent, prevents polycyclic aromatic hydrocarbon-induced neoplasms in rats and mice, but has not been studied with other chemical carcinogens. We examined coumarin chemoprotection against aflatoxin B1 using the 6-thioguanine resistance mutation assay in two different Chinese hamster ovary cell lines (K1BH4 and AS52) with liver S9 from rats and 19-day-old chick embryos for aflatoxin B1 bioactivation. Laboratory rodents metabolize coumarin through 3-hydroxylation, whereas 7-hydroxylation predominates in chick embryos and humans. Chick embryo liver S9 was approximately 25-fold more effective in activating aflatoxin B1 to the mutagenic and cytotoxic metabolite(s) than rat liver S9. Coumarin added at 50 and 500 microM with chick embryo liver S9 reduced the mutant frequency of 1 microM aflatoxin B1 by 40 and 85%, respectively. Coumarin up to 500 microM had no effect on aflatoxin B1 mutagenicity with rat liver S9. When liver S9 from chick embryos pretreated with coumarin was used for aflatoxin B1 bioactivation, mutant frequency and cytotoxicity were decreased compared to liver S9 from vehicle-treated controls. Liver S9 from coumarin-treated rats did not significantly affect mutant frequency or cytotoxicity. HPLC analysis of chick embryo liver S9 incubated with 1 microM aflatoxin B1 showed a dose-dependent decrease by coumarin of aflatoxin B1 activation to the 8,9-epoxide ranging from 70% of controls at 5 microM coumarin to 4% of controls at 500 microM coumarin. In contrast, coumarin produced a dose-dependent increase in 20 microM aflatoxin B1 activation by rat liver S9, reaching twice the control levels at 500 microM coumarin. These findings, using a mammalian cell system as a mutagenic endpoint, demonstrate marked species differences in chemoprotection by coumarin.

Ascorbic acid protects guinea pigs from acute aflatoxin toxicity

These studies were conducted to investigate whether ascorbic acid protected guinea pigs from aflatoxin B1 (AFB1) toxicity. Young guinea pigs, fed either 0 (AA) or 25 mg (25 AA) or gavaged 300 mg ascorbic acid (300 AA) per day for 21 days, were gavaged with the LD50 dose of AFB1 on the 22nd day. Seven out of 10 animals in the AA group died within 72 hr of AFB1 administration. The livers of the animals showed regional massive necrosis and multilobular degeneration. There was no mortality in the 25 AA group. Their livers, however, showed changes similar to those seen in AA group. Serum alanine amino transferase (ALAT) and aspartate amino transferase (ASAT) levels were elevated. There was neither mortality nor pathological changes in livers in the 300 AA group. Their ALAT and ASAT levels were unaffected. In vitro production of AFM1 by liver microsomes tended to be higher than that in the other two groups. Three animals saved from the 300 AA group and continued with their supplementation were administered a second, intraperitoneal (ip) LD50 dose of AFB1 1 month after the first AFB1 dose. One animal died. Livers of the animals showed centrilobular degeneration and moderate necrosis in scattered hepatocytes. Liver microsomal cytochrome P450 and cytosolic glutathione S-transferase (GST) levels and AFM1 production were drastically reduced. ALAT and ASAT activities were raised. The results indicated that intake of 300 mg of ascorbic acid almost protected the animals from acute toxicity of AFB1 when given by gavage, but not when administered as a second dose ip.

Acute effect of aflatoxin B1 on different inbred mouse strains II

The acute effects of Aflatoxin B1(AFB1) were evaluated on C57B1/6, CBA/J, and Balb/c mice challenged with a single intraperitoneal dose of the mycotoxin (60 mg/Kg animal weight). 90 mice per strain were divided into three groups of 30 animals each: the intoxicated group and control groups I and II. Intoxicated mice were injected intraperitoneally with AFB1 dissolved in corn oil, while control I mice received corn oil only (0.01 ml/g) by the same route. Lots of 10 animals from the intoxicated and control groups were sacrificed 24, 72 and 168 hours after challenge. Control mice II remained untreated and were used as standards of normality for biochemical (hepatic and renal function) and hematological evaluation. AFB1 was detected in the liver of C57B1/6 and CBA/J mice 24 hours (1.46 and 0.75 ng/g, respectively), 72 hours (2.30 and 0.08 ng/g, respectively), and 168 hours (2.18 and 0.25 ng/g, respectively) after challenge. The mycotoxin was also observed in the liver of B10A mice (6.20 ng/g) 72 hours post-injection. The most evident histological lesions were observed 168 hours after treatment in C57B1/6 and B10A mice. Serum levels of alkaline phosphatase in intoxicated C57B1/6 and B10A mice were significantly higher than those of control I and II animals. The histopathologic lesions and biochemical changes were very discrete in Balb/c and CBA/J mice. It is included that strains C57B1/6 and B10A are more susceptible than strains CBA/J and Balb/c to the acute effects of AFB1. Such difference probably reflects each strain's ability to biotransform and eliminate AFB1 and its metabolites.

Cytochrome P450 2A of nasal epithelium: regulation and role in carcinogen metabolism

In this study, we found that rat nasal coumarin-7-hydroxylase (COH) activity was two orders of magnitude higher than rat hepatic COH activity and could be induced by adding coumarin to the rats' drinking water. In western blot analysis, an anti-cytochrome P450 (Cyp) 2a-5 (mouse liver COH) antibody recognized a sharp band in the microsomal fraction of rat nasal epithelium but not of the liver; the band comigrated with Cyp2a-5. The intensity of the band was increased by the coumarin treatment. Similarly, in northern blot analysis, a cDNA probe specific for Cyp2a-5 recognized an mRNA in the nasal epithelium having the same size as mouse liver Cyp2a-5 mRNA; however, no hybridizable mRNA was recognized in liver preparations. Unlike the protein level, the level of the mRNA was not increased by coumarin. When northern blot analyses were performed with two oligoprobes specific for rat lung CYP2A3, an mRNA of similar size to Cyp2a-5 mRNA was recognized. In immunoinhibition analysis, anti-Cyp2a-5 antibody inhibited rat nasal COH activity and aflatoxin B1 (AFB1) metabolism completely. It inhibited N-nitrosodiethylamine (NDEA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism by 80-90%. In contrast, the hepatic metabolism of the four compounds was not affected by the antibody. When coumarin instead of anti-Cyp2a-5 antibody was used, a strong but variable inhibition of the nasal metabolism of AFB1, NDEA, and NNK was seen. The results suggest that an enzyme or enzymes similar to mouse liver Cyp2a-5, one of which may be CYP2A3, is expressed at high levels in rat nasal epithelium but not in the liver and that its expression is increased by coumarin, an odorant and a substrate of Cyp2a-5. The increase probably occurs by protein stabilization or stimulation of translation. The results also show that the enzyme has a key role in the nasal metabolism of three well-known carcinogens, AFB1, NDEA, and NNK and may therefore be an important contributing factor in nasal carcinogenesis.

Metabolism of nitrosamines and aflatoxin B1 by hamster liver CYP2A enzymes

Using a specific antibody against mouse CYP2A5 and coumarin as inhibitors, and pretreatments known to affect the activities of CYP2A-related enzymes, we studied the contribution of hamster liver CYP2As in the dealkylation of N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA) and hydroxylation of aflatoxin B1 (AFB1). CYP2A5 antibody inhibited AFB1 and NDEA metabolism by 40-60% in untreated hamsters and after treatment with phenobarbital (PB) or 3-methylcholanthrene (MC). After pyrazole (PYR) treatment, there was no inhibition, although PYR increased the metabolism above the controls. NDMA metabolism was inhibited only weakly in all groups. This suggests that CYP2As may contribute significantly to NDEA and AFB1 metabolism and less to NDMA metabolism in untreated hamster liver and after MC or PB treatments, but that PYR treatment may change the expression pattern of CYPs so that the metabolism is mainly catalysed by CYPs not recognized by the antibody. Coumarin inhibited NDEA and NDMA metabolism in all groups of hamsters, including pyrazole-pretreated animals, suggesting that it interacts not only with CYP2As but also with other CYPs that are important in nitrosamine metabolism. On the other hand, coumarin inhibited AFB1 metabolism significantly only in control and PB-treated animals but not at all after PYR or MC treatment. This suggests that enzymes involved in AFB1 metabolism may be different depending on the pretreatment. It is concluded that CYP2As may contribute significantly to nitrosamine and aflatoxin metabolism in hamster liver. However, in view of previous results showing essential differences in their regulation between mouse and hamster, interspecies comparisons may be difficult.

Aflatoxin B1-binding proteins in primary cultured hepatocytes of chicken embryo: studies in vivo and in vitro

Among several aflatoxin B1 (AFB1)-binding proteins in primary cultured hepatocytes of chicken embryo, a cytosolic 25 kDa protein proved to be an AFB1-specific binding protein. Binding of AFB1 to a cytosolic protein of the same molecular mass as that seen in vivo was also detectable in a cell-free system and was dependent on the microsomal components. The binding was observed in the in vitro systems derived from AFB1-sensitive animals such as chicken, rabbit, and rat. These findings suggest that the 25 kDa AFB1-binding protein(s) is involved in the intracellular process of AFB1-mediated toxicity.

A novel aldehyde reductase with activity towards a metabolite of aflatoxin B1 is expressed in rat liver during carcinogenesis and following the administration of an anti-oxidant

In contrast with fractions from control animals, an aldehyde reductase, which catalyses the reduction of aflatoxin B1-dihydrodiol, in the dialdehyde form at physiological pH values, to aflatoxin B1-dialcohol, is expressed in cytosolic fractions prepared from rat livers bearing pre-neoplastic lesions, or following treatment with the anti-oxidant ethoxyquin. This expression parallels the development of resistance to the toxin. Unlike the aflatoxin B1-dihydrodiol, the dialcohol does not undergo binding to protein. This enzyme activity could play a mechanistic role in hepatocarcinogenesis and chemoprotection in the rat. Correlated n.m.r. and m.s. spectra are provided in Supplementary Publication SUP 50171 (3 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1993) 289, 9.

Investigation of the assay of AFB1-albumin adducts using proteolysis products in ELISA

Epidemiological evidence of the involvement of aflatoxins in the aetiology of human liver cancer has led to an increasing interest in the development of appropriate techniques for monitoring human exposure. The assay for aflatoxin adducts in albumin has a better potential for assessing long-term exposure than analyses of urine samples, and several protocols for ELISA of these adducts, following proteolysis of albumin, have been examined. However, there is usually an incomplete release of a major adduct, aflatoxin-lysine, even after prolonged hydrolysis, and the adduct is very unstable under some conditions of proteolysis for unknown reasons. Therefore, before such techniques can be recommended for general application, the significance of such factors in the quantitive estimation of aflatoxin adducts needs to be evaluated. This study has detected the presence of a considerable fraction of aflatoxin-modified material, produced by proteolysis of in vivo aflatoxin-modified rat albumin or in vitro modified bovine albumin, and which is not recognized in ELISA by an anti-aflatoxin polyclonal antibody having a wide spectrum of aflatoxin metabolite detection. This fraction increases in parallel with the proteolysis protocols.

The isolation and culture of DHBV-infected embryo and duckling hepatocytes and the effect of aflatoxin B1 or irradiation on these cells

The preparation of primary cultures of control and DHBV-infected duck hepatocytes from embryos and young ducklings is described. Cultures of both embryo and duckling hepatocytes secreted duck serum proteins. Cultures of hepatocytes established from ducklings maintained initial morphology for up to 3 weeks in culture and also exhibited high levels of metabolism of aflatoxin B1. Embryonic cell cultures rapidly lost ability to metabolise AFB1 and became overgrown by spindle-shaped cells. Both embryo and duckling cell cultures secreted infective DHBV, and had intracellular replicative forms of the virus. No integration of the virus into the duck genome was observed, and attempts to induce viral integration in the duckling hepatocytes using irradiation and aflatoxin B1 toxicity were unsuccessful. The results of the study lend further support to the suggestion that the rarity of liver cancer in DHBV-infected experimental ducks is related to an innate resistance of the hepatocytes to develop DHBV-DNA integration. Another possibility may be related to the lower oncogenic potential of the DHBV strain used for the study. However DHBV infected duckling hepatocytes would appear to offer a suitable material for studying viral replication and mechanisms of aflatoxin B1 toxicity during prolonged cell culture.

Effect of the leafy vegetable Solanum nigrum on the activities of some liver drug-metabolizing enzymes after aflatoxin B1 treatment in female rats

Wistar albino female rats were maintained for 10 d on diets containing various levels of the vegetable Solanum nigrum. Simultaneously, they received daily intraperitoneal injections of aflatoxin B1 (AFB1) (either 0.2 or 0.4 mg/kg body-weight) diluted in propylene glycol. At the end of the experiment, all animals were killed and their serum and hepatic microsomes were prepared for assay of enzymes. Results showed that aminopyrine N-demethylase activity increased 2.5-fold with 200 (S200) and 600 (S600) g S. nigrum/kg diets. Activity of uridine diphosphate glucuronyltransferase (UDPGT) (EC 2.4.1.17) also increased twofold. Similar results were obtained with glutathione S-transferase (EC 2.5.1.18) activity which increased by 60% with diet S600. After AFB1 treatment, a general increase in the activities of the above enzymes was found, except for UDPGT in the group fed on diet S600. When rats were fed on the diet without S. nigrum, AFB1 induced an increase in alkaline phosphatase (ALP) (EC 3.1.3.1), aspartate aminotransferase (AST) (EC 2.6.1.1) and gamma-glutamyltransferase (gamma-GT) (EC 2.3.2.2) levels in the serum. AFB1 also induced increases in serum ALP and gamma-GT levels when rats were fed on diet S600.

Modulatory effect of crocetin on aflatoxin B1 cytotoxicity and DNA adduct formation in C3H10T1/2 fibroblast cell

Crocetin is a carotenoid isolated from the seeds of Cape jasmine (Gardenia jasminoides). The cytotoxicity and DNA-adduct formation of rat microsome-activated aflatoxin B1 (AFB1) in the C3H10T1/2 cells were significantly inhibited by pretreatment of crocetin. Most significant inhibition was found at the time of 9 h after crocetin pretreatment. Under these experimental conditions, consistent elevation in the cytosolic glutathione (GSH) levels and the activities of GSH S-transferase (GST) and GSH-peroxidase (GSH-Px) were observed. Crocetin treatment also resulted in a decrease in AFB1-DNA adduct formation in vitro, while no effect of crocetin on the formation of AFB1-8,9-oxide in vitro system was detected as measured by the Trisdiol method. From these results, we suggested that the protective effect of crocetin on the AFB1-cytotoxicity in C3H10T1/2 cells might be due to the cellular defense mechanisms that elevated the cytosol GSH and the activities of GST and GSH-Px.

Effects of phenobarbital and beta-naphthoflavone on the in vivo toxicity and in vitro metabolism of aflatoxin in an aflatoxin-resistant and control line of chickens

The in vivo toxicity of aflatoxin and the in vitro microsomal metabolism of aflatoxin B1 (AFB1) were investigated in a population of chickens previously selected for resistance to aflatoxin (AR line) and a corresponding control population (NS line) after in vivo pretreatment with saline, sodium phenobarbital (PB), or beta-naphthoflavone (BNF) solutions. PB pretreatment increased survival and BNF pretreatment increased mortality in both the NS and AR lines when a single oral dose of aflatoxin was administered. The rate of in vitro metabolism of AFB1 was greater with microsomes from saline pretreated AR chicks than with microsomes from similarly treated NS chicks. In vivo pretreatment with PB increased AFB1 metabolism by NS and AR microsomes. After BNF pretreatment of vivo, AR microsomes metabolized more AFB1 than NS microsomes, and there was a dramatic decrease in AFB1 metabolism in NS microsomes. AFB1-dihydrodiol was the major metabolite produced by both lines, with aflatoxin M1 and aflatoxin Q1 recovered in small quantities from BNF-pretreated AR microsomal incubations only. These data indicate that increased in vivo resistance of the AR line to acute aflatoxicosis may be related to increased hepatic AFB1 metabolism and that genetic selection has resulted in altered in vitro quantitative and qualitative metabolism of AFB1 in the AR line.

Evidence for involvement of multiple forms of cytochrome P-450 in aflatoxin B1 metabolism in human liver

Liver cancer is a major cause of premature death in many areas of Africa and Asia and its incidence is strongly correlated with exposure to aflatoxin B1 (AFB1). Because AFB1 requires metabolic activation to achieve a biological response, there is a need for detailed knowledge of the mechanism of activation to assess individual risk. We have carried out an extensive study using a total of 19 human liver samples to determine the individual variability in the metabolism of the toxin to mutagenic or detoxification products and to identify the specific cytochrome P-450 forms involved in these processes. Metabolism to the toxic 8,9-epoxide or to products mutagenic in the Ames test was found to exhibit very large individual variation. The rates of metabolic activation were highly correlated with both the level of proteins of the P450IIIA gene family and with the total cytochrome P-450 content of the microsomes. In agreement with this, antibodies reacting with P450IIIA proteins were strong inhibitors of both the metabolism and mutagenicity in the majority of the samples. However, the inhibition varied between 50% and 100%. The expression of a protein in the P450IIC gene family also correlated with AFB1 metabolism and mutagenicity. This result therefore indicated the involvement of cytochromes other than P450IIIA in the activation of AFB1 by human liver microsomes. This hypothesis was strongly supported by the finding that antibodies to P450IA2 and P450IIA1 were also effective inhibitors of metabolism in many of the samples. These data demonstrate that, although P450IIIA probably plays an important role in AFB1 activation, several other cytochrome P-450 forms have the capacity to activate the toxin. Similar considerations apply to detoxifying metabolism to aflatoxin Q1 and aflatoxin M1. The levels of expression of many of the forms of cytochrome P-450 involved in AFB1 metabolism are known to be highly sensitive to environmental factors. This indicates that such factors will be an important determinant in individual susceptibility to the tumorigenic action of AFB1.

Cytotoxic effects of aflatoxin B1 and its association with cellular components in chicken embryo primary cultured cells

We have examined the cytotoxicity and cellular incorporation of aflatoxin B1 (AFB1) in several types of established and primary cultured cells. The inhibition of DNA synthesis by AFB1 at 1 microgram/ml was about 0-30% in the established cell lines, including human hepatic cells. In chicken primary hepatocytes, however, DNA synthesis as well as RNA and protein syntheses were strongly inhibited by much lower concentrations of AFB1, e.g., 0.1 microgram/ml. In contrast, chicken primary fibroblasts showed almost no significant response to the toxin. Microsomal cytochrome P-450 activities in hepatic tissues were 10-20-fold higher than those in fibroblastic tissues. The amount of [3H]AFB1 incorporated into acid-insoluble materials in the primary hepatocytes was also 10-100-fold more than that in the primary fibroblasts. However, a significant amount of AFB1, which was enough to induce cytotoxic effects on the primary hepatocytes, could be incorporated into the primary fibroblasts when the concentrations of AFB1 were increased. Characterization of the AFB1-associated cellular components showed that most of them were DNA, RNA, and proteins in the primary hepatocytes, while in the primary fibroblasts a large portion of the incorporated AFB1 was recovered from lipid fractions. In addition, the selective binding of [3H]AFB1 to several proteins was observed only in the primary hepatocytes. The possible role of the AFB1-binding proteins are also discussed.

Long-term administration of low doses of mycotoxins to poultry. 1. Residues of aflatoxin B1 and its metabolites in broilers and laying hens

A study was performed to determine aflatoxin residues in tissues and organs of male broilers and hens that had been fed a diet contaminated with 50 micrograms/kg aflatoxin B1 (AFB1). Residue levels of AFB1, aflatoxicol (Ro), aflatoxin M1 (AFM1) and aflatoxin B2a (AFB2a) were determined by an HPLC method and, with the exception of AFB2a, were detected in the liver, kidney and thigh of both male broilers and hens. The highest levels found were for Ro in liver (1.10 and 0.60 micrograms/kg for male broilers and hens, respectively). On the other hand no detectable amounts of aflatoxins were found in any tissue after withdrawal periods of 14 and 33 days for male broilers and laying hens respectively.

Effects of modulation of hepatic glutathione on biotransformation and covalent binding of aflatoxin B1 to DNA in the mouse

Relative to the rat and most other species tested, the mouse is resistant to the carcinogenic effects of aflatoxin B1 (AFB). Previous investigations in our laboratory demonstrated that mouse liver cytosol has 52 times greater hepatic glutathione S-transferase (GST) activity toward the AFB-epoxide, compared with rat liver cytosol. To determine the importance of GST-mediated detoxification of the AFB-epoxide in the mouse in vivo, we examined the effects of glutathione (GSH) depletion on the covalent binding of AFB to hepatic DNA in control and 2(3)-butyl-4-hydroxyanisole (BHA)-treated mice. Male Swiss-Webster mice were fed control or 0.75% BHA diet for 14 days. Depletion of hepatic GSH was accomplished with D,L-buthionine-S-sulfoximine (BSO, 0.6 g/kg in saline) and diethyl maleate (DEM, 0.75 ml/kg), administered by ip injection at 2 and 1.5 hr, respectively, prior to administration of 3H-AFB (0.25 mg/kg, ip in DMSO). The combined BSO-DEM treatment depleted hepatic GSH by 97 and 70% in control and BHA-treated mice, respectively. In mice receiving the control diet, GSH depletion was associated with a 30-fold increase in the covalent binding of AFB to hepatic DNA. AFB-DNA binding in mice treated with dietary BHA alone was reduced to 54% of control. In BHA-treated mice, BSO-DEM treatment increased AFB-DNA binding by 62%. Dietary BHA increased hepatic S-9 mediated activation of AFB to the AFB-epoxide by eightfold in both control and BSO-DEM mice. BHA also increased GST activity toward the AFB-epoxide by 52 and 68% in control and BSO-DEM mice, respectively. The BSO-DEM treatment alone had no significant effect on the in vitro biotransformation of AFB. These results support the hypothesis that GST is the principal determinant of species differences in susceptibility to AFB-induced hepatocarcinogenicity. The results also support the hypothesis that BHA may protect against the toxic and carcinogenic effects of xenobiotics in part by preventing the depletion of hepatic GSH.

Conjugation of model substrates or microsomally-activated aflatoxin B1 with reduced glutathione, catalysed by cytosolic glutathione-S-transferases in livers of rats, mice and guinea pigs

Glutathione-S-transferase (GST) activity has been examined in liver cytosol fractions from guinea pigs, mice, control fed rats or rats with pre-neoplastic nodular liver lesions. The levels of activity in unfractionated cytosols have been assayed using the model substrates 1-chloro-2,4-dinitrobenzene (CDNB), 3,4-dichloronitrobenzene (DCNB) and monobromobimane (mBrB) with reduced glutathione (GSH). The order of activities in the various liver fractions using CDNB as substrate were: mouse greater than pre-neoplastic nodular rat greater than guinea pig greater than control rat and paralleled the capacities of the cytosols to catalyse the formation of aflatoxin B1-GSH from microsomally-activated aflatoxin B1 (AFB1) and GSH. Quantitative differences between the activities of the cytosols using the three model substrates were observed. In the mouse fractionation of GST activity by isoelectric focusing (I.E.F.) on preparative granular gels showed that the most basic component (isoelectric point pH 9.4) with the highest conjugating activity with respect to microsomally-activated AFB1 did not correspond with the peak of most activity for conjugating CDNB. In the pre-neoplastic nodular rat liver the CDNB conjugating activities of all fractions separated on granular I.E.F. gels, were higher than the corresponding fractions isolated from control rat liver, with particular enhancement of the peak containing the 3:3 isoenzyme. In contrast to control rat liver the 7:7 isoenzyme was detected in pre-neoplastic nodular liver preparations. These isoenzymes (3:3 and 7:7) did not contribute significantly to the enhanced level of AFB1-GSH formation catalysed by cytosol fractions prepared from pre-neoplastic nodular rat liver. The microsomally-activated AFB1-conjugating activity of unfractionated rat liver cytosols was increased to a relatively greater extent than CDNB conjugating activity during the induction of pre-neoplastic nodular liver lesions, and the elevated level of the activated AFB1-conjugating activity was found to be associated with the most basic fraction (isoelectric point pH 9.0). Analytical isoelectric focusing gels using mBrB as substrate demonstrated the presence of a basic GST isoenzyme in the pre-neoplastic nodular rat liver, not detected in preparations from the livers of control rats. The low level of activated AFB1-conjugating activity present in unfractionated guinea-pig cytosol was found to correspond with the fraction containing the peak of CDNB conjugating activity on preparative isoelectric focusing (isoelectric point pH 7.5). The lack of correlation between the conjugation of model substrates and the conjugation of xenobiotics could be of import

HPLC determination of the total content of aflatoxins in naturally contaminated eggs in free and conjugate forms

A study was undertaken to evaluate the total aflatoxin content in naturally contaminated eggs. Two pools of eggs from laying hens were collected after 2 and 7 days of treatment with aflatoxin B1 (AFB1). An HPLC method has been developed for the determination of AFB1, aflatoxin M1 (AFM1), aflatoxin B2a (AFB2a) and aflatoxicol (Ro) both in free form and after release from water-soluble conjugates. The bound form was cleaned up after acid hydrolysis of the aqueous phase. Eggs collected after 2 days of treatment revealed residues of AFB1, AFB2a and Ro in the organic phase, but none in the aqueous portion. After 7 days of treatment both AFB1 and its hydroxy derivative were found in the organic phase but the aqueous portion showed only hydroxylated metabolites accounting for 35% of the total aflatoxin content.

Influence of avian aflatoxicosis on the synthesis of polyamines

Effects were examined of inanition, dietary aflatoxin (2.5 mg/kg), and dietary supplements of threonine, lysine, and arginine on the activities of renal arginase and hepatic ornithine decarboxylase and on the accumulation of polyamines in liver and brain of 24 or 26-day-old broiler cockerels. Aflatoxicosis and inanition lowered the activity of renal arginase by 58 and 37%, respectively. Supplemental dietary threonine (.4%) did not suppress the activity of renal arginase, while fortification of diets of controls with lysine (.53%), but not diets containing aflatoxin, elevated the activity of renal arginase. Supplements of dietary lysine and/or arginine did not influence the hepatic content of putrescine but lowered the concentrations of spermidine and spermine. Aflatoxicosis, but not inanition, increased the activity of hepatic ornithine decarboxylase (ODC; 22-fold), increased hepatic concentrations of putrescine and spermidine, but decreased spermine concentrations. The elevation of hepatic ODC, putrescine, and the ratio of spermidine to spermine parallels the enlargement of the liver caused by aflatoxicosis. Cadaverine and putrescine were not detected in avian brain, while cerebral concentrations of spermidine and spermine were not altered by aflatoxin, inanition, or by supplements of dietary lysine, arginine, or both lysine and arginine.

Modulation by certain factors of metabolic activation of aflatoxin B1 as detected in vitro in a simple fluorimetric assay

Forty compounds belonging to various chemical groups have been tested for their ability to suppress metabolic activation of aflatoxin B1 (AFB1) mediated by rat liver microsome. Microsomal activation has been carried out in an in vitro system containing Tris-buffer at pH 7.2. Production of the reactive metabolite, AFB1 8,9-epoxide, has been measured by separation and detection of its hydrolysis product AFB1 8,9-dihydrodiol as the Tris-diol complex. The complex is separated using simple procedures of biphasic extraction and deproteinization, and detected by its characteristic fluorescence. Quantitation is made by direct comparison of its fluorescence with that of an authentic Tris-diol prepared synthetically. The method is rapid and proved to be highly sensitive and reproducible. A large number of compounds have been observed to modulate at varying degrees the activation of AFB1 in this in vitro system. Many compounds have been tested at several concentration ranges and inhibition curve is constructed in each case from which ID50 values, i.e., the dose needed to bring about 50% inhibition can be obtained. These values expressed as nmol afford a direct and realistic comparison of the inhibitory potential of various modulators. Factors having great inhibitory potential have been identified as retinoids (retinol, retinal, retinoic acid, retinyl acetate), beta-carotene, riboflavin, ascorbic acid, copper, zinc, linoleic acid, p-hydroxy benzoic acid, butylated hydroxyanisole, butylated hydroxytoluene, disulfiram, and phenothiazine. Several other compounds have shown moderate inhibitory potential. The strong inhibition on Tris-diol formation by several vitamins, antioxidants and trace metals shows similarity with their effect on AFB1-DNA adduct formation. It is suggested that these agents may have potential anticarcinogenic activity against AFB1.

The in vitro metabolism of aflatoxin B1 catalyzed by hepatic microsomes isolated from control or 3-methylcholanthrene-stimulated rats and quail

The rate of microsomal metabolism of aflatoxin B1 (AFB1) by the male quail hepatic microsomal polysubstrate monooxygenase P-450 in vitro was eight times greater than that catalyzed by male rat cytochrome. In the quail almost all the metabolism proceeded via 8,9-aflatoxin B1 epoxidation (assessed by Tris-AFB1-8,9-dihydrodiol formation), whereas in the rat only 36% of soluble metabolites were by this pathway. Pretreatment with 3-methylcholanthrene in the quail resulted in a 9-fold induction of cytochrome P-450 per unit liver weight, but only a 1.7-fold increase in the rate of aflatoxin B1 metabolism. In the rat the corresponding inductions were 1.7 and 1.3, respectively. The aflatoxin B1 metabolism catalyzed by control and 3-methylcholanthrene-stimulated quail microsomes differed qualitatively from that observed with the corresponding fractions from the rat. It was concluded that not only do the basal aflatoxin B1 microsomal metabolism of rat and quail differ, but also that 3-methylcholanthrene induces cytochromes with very different properties in the two species. Assays of ethoxyresorufin O-deethylase activities and polyacrylamide gel electrophoresis analyses further demonstrated these differences between the species. These studies provide further insight into the metabolic differences underlying individual species sensitivities to aflatoxin B1.

Effect of ethanol on the in vivo covalent binding and in vitro metabolism of aflatoxin B1 in rats

The binding of [3H]aflatoxin B1 (AFB1) to the DNA, RNA and protein of liver after i.p. administration to rats with and without ethanol pretreatment was studied. The quantities of AFB1 binding to DNA and RNA were significantly increased by ethanol pretreatment but the formation of protein adducts was not affected. AFB1 metabolism by hepatic microsomes from ethanol-treated rats to aflatoxins M1 (AFM1) and Q1 (AFQ1) was increased when compared to those of control microsomes. These results suggest that an increase in AFB1 binding to liver nucleic acids and AFB1 metabolism after pretreatment of ethanol resulted from an increase in hepatic mixed-function oxidases and a possible decrease in hepatic glutathione (GSH) content which subsequently lead to an increase in hepatotoxicity of AFB1.

Aflatoxin carryover and clearance from tissues of laying hens

A feeding trial was conducted to determine the levels of aflatoxins deposited in the tissues of hens fed a diet contaminated with 3310 micrograms AFB1/kg and 1680 micrograms AFB2/kg for 4 wk. At the end of aflatoxin feeding, the livers were pale, enlarged and haemorrhagic and the ovaries were significantly smaller than those from control hens and contained only small ova. Only a small fraction of the aflatoxins consumed was deposited in the tissues, either as the original compounds or as their metabolites, which were widely distributed in all tissues. The highest levels of aflatoxins were detected in the gizzard, kidneys and liver, with average total concentrations of less than 3 micrograms/kg. The lowest residue levels were detected in the breast, blood serum and leg, with breast muscle having a total concentration of less than 0.1 microgram/kg. Two days after removal of the contaminated feed, aflatoxin residues in all tissues had decreased markedly, with no aflatoxins being detected in the heart or spleen. No aflatoxin residues were detected in the breast, leg, gizzard and ovaries of hens killed 8 days after withdrawal, or in the kidneys and blood at 16 days. However, one of seven hens had measurable amounts of AFB2 in the liver 32 days after withdrawal. Although few residues were detected in most tissues after 8 days on the aflatoxin-free diet, variation existed between tissues and between individual hens in the amount of time required to achieve tissue clearance.

Metabolism of aflatoxin B1 by rat hepatic microsomes induced by polyhalogenated biphenyl congeners

The metabolism of aflatoxin B1 to aflatoxins M1 and Q1 by rat liver microsomes from animals pretreated with polychlorinated or polybrominated biphenyl congeners depended on the structure of the halogenated biphenyl inducers. Microsomes from rats treated with phenobarbital (PB) or halogenated biphenyls that exhibit PB-type activity preferentially enhanced the conversion of aflatoxin B1 to aflatoxin Q1. In contrast, microsomes from rats treated with 3-methylcholanthrene (MC) or halogenated biphenyls that exhibit MC-type induction activity increased the metabolism of aflatoxin B1 to aflatoxin M1. The coadministration of PB and MC produced microsomes that exhibited both types of induction activity (mixed type) in catalyzing the oxidative metabolism of diverse xenobiotic agents. However, PB-plus-MC-induced hepatic microsomes from immature male Wistar rats preferentially increased the metabolism of aflatoxin B1 to aflatoxin M1 but did not enhance the conversion of aflatoxin B1 to aflatoxin Q1. Comparable results were observed with microsomes from rats pretreated with halogenated biphenyls classified as mixed-type inducers; moreover, in some cases there was a significant decrease in the conversion of aflatoxin B1 to aflatoxin Q1 (compared with that of controls treated with corn oil).

Tissue deposition and clearance of aflatoxins from broiler chickens fed a contaminated diet

A study was conducted to determine aflatoxin levels in the tissues of broiler chickens that had been fed a diet containing 2057 micrograms aflatoxin B1 and 1323 micrograms aflatoxin B2/kg for 35 days. Results showed that aflatoxins were deposited in all tissues. The highest levels of aflatoxins were present in the gizzards, livers and kidneys. There was evidence that the high levels of aflatoxins B1 and B2 in the gizzards might have been caused by contamination by the gizzard contents during the slaughtering process. After feeding the aflatoxin-contaminated diet for 35 days, mean values for the combined aflatoxins were less than 3 micrograms/kg of tissue. Four days after withdrawal of the aflatoxin-contaminated ration, there were no detectable amounts of aflatoxins in any of the tissues. The results indicate that broiler chickens rapidly clear aflatoxins from their tissues once they are transferred to an aflatoxin-free diet.

Aflatoxin B1 dihydrodiol antibody: production and specificity

A specific antibody for 2,3-dihydro-2,3-dihydroxyaflatoxin B1 (AFB1-diol) was prepared, and its reactivity was characterized for the major aflatoxin (AF) B1 (AFB1) metabolites. Reductive alkylation was used to conjugate AFB1-diol to ethylenediamine-modified bovine serum albumin (EDA-BSA) and horseradish peroxidase for use as an immunogen and an enzyme-linked immunosorbent assay (ELISA) marker, respectively. High reactant ratios, 1:5 and 1:10, for AFB1-diol-EDA-BSA (wt/wt) resulted in precipitated conjugates which were poorly immunogenic. However, a soluble conjugate obtained by using a 1:25 ratio of AFB1-diol to EDA-BSA could be used for obtaining high-titer AFB1-diol rabbit antibody within 10 weeks. Competitive ELISAs revealed that the AFB1-diol antibody detected as little as 1 pmol of AFB1-diol per assay. Cross-reactivity of AFB1-diol antibody in the competitive ELISA with AF analogs was as follows: AFB1-diol, 100%; AFB1, 200%; AFM1, 130%; AFB2a, 100%; AFG1, 6%; AFG2, 4%; aflatoxicol, 20%; AFQ1, 2%; AFB1-modified DNA, 32%; and 2,3-dihydro-2-(N7-guanyl)-3-hydroxy AFB1, 0.6%. These data indicated that the cyclopentanone and methoxy moieties of the AF molecule were the primary epitopes for the AFB1-diol antibody. The AFB1-diol competitive ELISA was subject to substantial interference by human, rat, and mouse serum albumins but not by BSA, Tris, human immunoglobulin G, or lysozyme. By using a noncompetitive, indirect ELISA with an AFB1-modified DNA solid phase, a modification level of one AFB1 residue for 200,000 nucleotides could be determined.

Metabolic basis of the species difference to aflatoxin B1 induced hepatotoxicity

Primary metabolism of aflatoxin B1 by the liver microsomal enzymes from a range of animal species showed both quantitative and qualitative differences. Quail was shown to have the most rapid metabolism of aflatoxin B1. The major product of metabolism in this case was found to be aflatoxin B1-8,9-dihydrodiol suggesting that the quail microsomes produced high levels of the proposed reactive intermediate aflatoxin B1-8,9-epoxide. Using this system to generate the epoxide, the ability of the cytosol prepared from each species to conjugate epoxide with reduced glutathione was investigated. Large differences in ability to conjugate were observed ranging from 0 to 72% for quail and mouse respectively. Differences in both primary and secondary metabolism of AFB1 were noted between male and female Fischer 344 rats.

The requirement for glutathione S-transferase in the conjugation of activated aflatoxin B1 during aflatoxin hepatocarcinogenesis in the rat

The formation of an aflatoxin B1-reduced glutathione (AFB1-GSH) conjugate in in vitro systems has been examined. AFB1 was activated by a chicken liver microsomal system and factors affecting the subsequent conversion to the AFB1-dihydrodiol or conjugation with GSH were investigated by HPLC. A requirement for glutathione S-transferase in the formation of the AFB1-GSH conjugate was observed. Studies using CM-cellulose columns showed the fractions containing glutathione S-transferase B activity were the most effective in catalysing the formation of the AFB1-GSH conjugate. The possibility of changes in the level of AFB1-GSH conjugate production in the liver during carcinogenesis by AFB1 has been examined. It has been found, using freshly isolated rat hepatocytes, that low level feeding with AFB1 in vivo increases the production of the conjugate in vitro. Further increases in the production of the conjugate by hepatocytes in vitro, accompanying increases in the preneoplastic lesions, are achieved by partially hepatectomising the AFB1-fed animals. Partial hepatectomy of control-fed animals yielded no similar changes. The AFB1/partial hepatectomy treatment resulted in increased levels of all the glutathione S-transferase activities fractionated on CM-cellulose. Macromolecular binding of AFB1 and/or of its metabolites was detected in the fractions containing glutathione S-transferase activity, but there was no evidence for a greater binding in the glutathione S-transferase B/ligandin containing fractions. Furthermore fractionation on Sephadex G-75 indicated a predominance of binding of AFB1 to proteins of a higher molecular weight than the glutathione S-transferases, although some binding in the molecular weight range of the latter was observed.