Aflatoxin B1 transport in rat blood plasma. Binding to albumin in vivo and in vitro and spectrofluorimetric studies into the nature of the interaction

Glycation reduces the binding dynamics of aflatoxin B1 to human serum albumin: a comprehensive spectroscopic and computational investigation

Aflatoxin B1 (AFB1), a potent mutagen, is synthesized by Aspergillus parasiticus and Aspergillus flavus. Human serum albumin (HSA) is a globular protein with diverse roles. As AFB1 is ingested with food and is transported in the body via blood, it becomes pertinent to comprehend the effect of the binding of this toxin on the structure and conformation of HSA, which may help to get insight into the toxic effect of the exposure of the mycotoxin. In this study, multi-spectroscopic approaches have been used to evaluate the binding efficiency of AFB1 with both the native HSA (nHSA) and the glycated HSA (gHSA). Steady-state fluorescence spectroscopy reveals the static type of fluorescence quenching in the fluorescence emission spectra of nHSA and gHSA in the presence of AFB1. The binding constant (Kb) is calculated to be 6.88 × 104 M-1 for nHSA, while a reduced Kb value of 2.95 × 104 M-1 has been obtained for gHSA. The circular dichroism study confirms the change in the secondary structure of nHSA and gHSA in the presence of AFB1, followed by alterations in the melting temperature (Tm) of nHSA and gHSA. In silico computational findings envisaged the amino acid residues and bonds involved in the binding of nHSA and gHSA with AFB1. The comprehensive study analyzes the binding effectiveness of AFB1 with nHSA and gHSA and shows reduced binding of AFB1 to gHSA.Communicated by Ramaswamy H. Sarma.

Efficient Aflatoxin B1 Sequestration by Yeast Cell Wall Extract and Hydrated Sodium Calcium Aluminosilicate Evaluated Using a Multimodal In-Vitro and Ex-Vivo Methodology

In this work, adsorption of the carcinogenic mycotoxin aflatoxin B1 (AFB1) by two sequestrants-a yeast cell wall-based adsorbent (YCW) and a hydrated sodium calcium aluminosilicate (HSCAS)-was studied across four laboratory models: (1) an in vitro model from a reference method was employed to quantify the sorption capabilities of both sequestrants under buffer conditions at two pH values using liquid chromatography with fluorescence detection (LC-FLD); (2) in a second in vitro model, the influence of the upper gastrointestinal environment on the mycotoxin sorption capacity of the same two sequestrants was studied using a chronic AFB1 level commonly encountered in the field (10 µg/L and in the presence of feed); (3) the third model used a novel ex vivo approach to measure the absorption of 3H-labelled AFB1 in the intestinal tissue and the ability of the sequestrants to offset this process; and (4) a second previously developed ex vivo model readapted to AFB1 was used to measure the transfer of 3H-labelled AFB1 through live intestinal tissue, and the influence of sequestrants on its bioavailability by means of an Ussing chamber system. Despite some sorption effects caused by the feed itself studied in the second model, both in vitro models established that the adsorption capacity of both YCW and HSCAS is promoted at a low acidic pH. Ex vivo Models 3 and 4 showed that the same tested material formed a protective barrier on the epithelial mucosa and that they significantly reduced the transfer of AFB1 through live intestinal tissue. The results indicate that, by reducing the transmembrane transfer rate and reducing over 60% of the concentration of free AFB1, both products are able to significantly limit the bioavailability of AFB1. Moreover, there were limited differences between YCW and HSCAS in their sorption capacities. The inclusion of YCW in the dietary ration could have a positive influence in reducing AFB1's physiological bioavailability.

Predicting the Acute Liver Toxicity of Aflatoxin B1 in Rats and Humans by an In Vitro-In Silico Testing Strategy

SCOPE

High-level exposure to aflatoxin B1 (AFB1) is known to cause acute liver damage and fatality in animals and humans. The intakes actually causing this acute toxicity have so far been estimated based on AFB1 levels in contaminated foods or biomarkers in serum. The aim of the present study is to predict the doses causing acute liver toxicity of AFB1 in rats and humans by an in vitro-in silico testing strategy.

METHODS AND RESULTS

Physiologically based kinetic (PBK) models for AFB1 in rats and humans are developed. The models are used to translate in vitro concentration-response curves for cytotoxicity in primary rat and human hepatocytes to in vivo dose-response curves using reverse dosimetry. From these data, the dose levels at which toxicity would be expected are obtained and compared to toxic dose levels from available rat and human case studies on AFB1 toxicity. The results show that the in vitro-in silico testing strategy can predict dose levels causing acute toxicity of AFB1 in rats and human.

CONCLUSIONS

Quantitative in vitro in vivo extrapolation (QIVIVE) using PBK modeling-based reverse dosimetry can predict AFB1 doses that cause acute liver toxicity in rats and human.

Structural dynamics studies on the binding of aflatoxin B1 to chicken egg albumin using spectroscopic techniques and molecular docking

Aflatoxin B₁, a mycotoxin produced by large number of Aspergillus species including Aspergillus flavus and Aspergillus parasiticus, has been described as the most potent carcinogenic mycotoxin. In this study, we have used a multiple spectroscopic and molecular docking approach to investigate the interaction of aflatoxin B₁ (AFB₁) with chicken egg albumin (CEA). Fluorescence spectroscopy, UV-Vis spectroscopy, and three-dimensional fluorescence spectroscopic techniques were employed to gain insight into the conformational changes in CEA in the presence of AFB₁. Fluorescence spectroscopy revealed ligand-induced quenching in the fluorescence emission spectra of CEA upon binding with AFB₁. Hyperchromic effect was observed in case of the ground state complex formation between CEA and AFB₁ by UV-Vis spectroscopy. To gain further comprehension into the site of binding of AFB₁ to CEA, competitive site marker displacement assay was performed using warfarin site marker. The magnitude of ΔG value calculated from fluorescence-based method was negative which confirmed spontaneous process. The results obtained suggest that the binding is enthalpy driven and van der Waals force and hydrogen bonds are stabilizing the AFB₁-CEA complex. Three-dimensional fluorescence studies also confirmed the quenching in the fluorescence intensity around tryptophan residues in CEA. Circular dichroism assessment revealed reduction in the alpha helical content of CEA in the presence of AFB₁. Molecular docking studies showed hydrophobic interaction, van der Waals forces, and hydrogen bonds as major forces present in interaction between CEA and AFB₁. The overall study confirms conformational and structural alteration in the protein due to binding of AFB₁.Communicated by Ramaswamy H. Sarma.

Investigation of Non-Covalent Interactions of Aflatoxins (B1, B2, G1, G2, and M1) with Serum Albumin

Aflatoxins are widely spread mycotoxins produced mainly by Aspergillus species. Consumption of aflatoxin-contaminated foods and drinks causes serious health risks for people worldwide. It is well-known that the reactive epoxide metabolite of aflatoxin B1 (AFB1) forms covalent adducts with serum albumin. However, non-covalent interactions of aflatoxins with human serum albumin (HSA) are poorly characterized. Thus, in this study the complex formation of aflatoxins was examined with HSA applying spectroscopic and molecular modelling studies. Our results demonstrate that aflatoxins form stable complexes with HSA as reflected by binding constants between 2.1 × 10⁴ and 4.5 × 10⁴ dm³/mol. A binding free energy value of −26.90 kJ mol⁻¹ suggests a spontaneous binding process between AFB1 and HSA at room-temperature, while the positive entropy change of 55.1 JK⁻¹ mol⁻¹ indicates a partial decomposition of the solvation shells of the interacting molecules. Modeling studies and investigations with site markers suggest that Sudlow’s Site I of subdomain IIA is the high affinity binding site of aflatoxins on HSA. Interaction of AFB1 with bovine, porcine, and rat serum albumins was also investigated. Similar stabilities of the examined AFB1-albumin complexes were observed suggesting the low species differences of the albumin-binding of aflatoxins.

Interactions of aflatoxin B1 with SRP components can disrupt protein targeting

Spectrofluorimetric studies have revealed that aflatoxin B1 (AFB1) interacts with signal recognition particle (SRP), which acts as an escort for polyribosomes with signal peptides to be transported and bound to the cytoplasmic face of the endoplasmic reticulum (ER). We further report that the binding of AFB1 to SRP is selective as it only binds to two (SRP9 and 14) out of its three constituent polypeptides studied. Binding of AFB1 to proteins is known to alter their conformations. Interactions of AFB1 with SRP polypeptides may generate structural and functional alterations in this particle and hinder secretory protein synthesis.

Carnitine alters binding of aflatoxin to DNA and proteins in rat hepatocytes and cell-free systems

The objective of this study was to determine effects of L-carnitine on aflatoxin B(1) (AFB(1))-DNA adduct formation in isolated rat hepatocytes, its dose response, specificity and mode of action. All experiments were conducted in either freshly isolated rat hepatocytes or cell-free systems. There was negative linear correlation between the dosage of carnitine and formation of [(3)H]AFB(1)-DNA adducts in the hepatocytes; however, the partitioning of AFB(1) into cellular compartments was not affected by carnitine. The attenuating effect of carnitine on AFB(1)-DNA adduct formation was also present in a cell-free system, but there was lack of specificity because acetylcarnitine and gamma-aminobutyric acid (GABA) were equally effective. Carnitine appears to interfere with bioactivation of AFB(1) and binding of AFB(1)-epoxide to DNA. On the contrary, carnitine enhanced the binding of AFB(1) and its epoxide to microsomal proteins, plasma proteins and bovine serum albumin. These results indicate that carnitine diverts AFB(1)-epoxide away from DNA by promoting binding to proteins. We conclude that modulation of AFB(1) binding to proteins and DNA by carnitine alters the carcinogenic and hepatotoxic potential of AFB(1) and poses concerns about the human AFB(1)-exposure data based on the AFB(1)-albumin adduct concentrations as a biomarker.

Aflatoxin B1-induced hepatic steatosis: role of carbonyl compounds and active diols on steatogenesis

Aflatoxin B1 (AFB1) exerts a chronic carcinogenic and an acute toxic effect on animals. Whereas the mechanism for carcinogenicity is known, no mechanism has been proposed for the toxic action. Among the most prominent signs of aflatoxicosis in several species, including birds and mammals, are hypolipidaemia, hypocholesterolaemia, and hypocarotenaemia, associated with severe hepatic steatosis and weight loss. We suggest that these signs of acute imbalance of lipid metabolism can be the result of the chemical modification (blocking) of key lysyl residues on the LDL protein B-100 by the activated AFB1 molecule. Modified LDLs are not recognised by their specific receptors and thus are rejected by peripheral cells. Upon return to the liver, the modified particles bind to the sinusoidal lining cells. Lipid starvation of peripheral tissues takes place while fat accumulates in the liver. This abnormal state is maintained and reinforced by further modification of nascent apoproteins, which in turn become unable to receive a lipid load for as long as aflatoxin continues to be available in the liver.

The anticalmodulin effect of aflatoxin B1 on purified erythrocyte Ca(2+)-ATPase

The genotoxic carcinogen aflatoxin B1 (AFB1) inhibited the calmodulin-stimulated membrane-bound (Ca2+Mg2+)-ATPase. Using the purified enzyme, 12 nmoles per ml of AFB1 caused maximum inhibition of 28% and 50%, of the acidic phospholipid-stimulated and calmodulin-activated Ca(2+)-ATPase activity respectively. Treatment of red cell ghosts with increasing concentrations of Triton X-100, a non-ionic detergent caused a progressive loss of both the basal and calmodulin-stimulated Ca(2+)-ATPase activity. The activity of the phospholipid-free, detergent-solubilized enzyme was almost fully restored by phosphatidyl serine (PS) and its sensitivity to calmodulin was restored in the presence of phosphatidyl choline (PC). Analysis of the results obtained using varying concentrations of ATP shows that AFB1 did not affect the Km and Vmax of the unstimulated enzyme whereas these parameters were reduced by about 75% and 50%, respectively, in the presence of calmodulin. Using the product of limited proteolysis by trypsin i.e. the 90 kDa fragment which still retains its calmodulin binding-domain and the 76 kDa fragment which has lost this domain, kinetic studies on the enzyme activity revealed that AFB1 inhibited the calmodulin-activated 90 kDa fragment by about 50% while the 76 kDa was not affected at all by the toxin and calmodulin. The toxin had no significant affect on the basal activity of the 90 kDa limited proteolysis fragment of the enzyme. These observations suggest that AFB1 inhibits the activated Ca(2+)-ATPase by binding to an important site in the calmodulin-binding domain of the enzyme. It seems likely that the toxin binds to tryptophan in the calmodulin-binding domain, thus causing a reduction in the rate at which this domain can interact with Ca(2+)-calmodulin or acidic phospholipids. The implication of these observations is that Ca(2+)-extrusion and other calmodulin-activated enzymes and processes may be slowed down during prolonged exposure to AFB1 because of its anticalmodulin effect.

Effect of aflatoxin B1 on human platelet protein kinase C

1. Aflatoxin B1 serves as an activator for protein kinase C (PKC). 2. Following activation, the enzyme translocates from the cytosol to the particulate fraction. 3. Aflatoxin B1 leads to rapid and extensive phosphorylation of the known substrate of the enzyme. 4. Aflatoxin B1 causes rapid hydrolysis of phosphatidylinositol-4,5-biphosphate to diacylglycerol and inositol-1,4,5-triphosphate.

An examination of the effect of pretreatment of rats with Sacoglottis gabonensis bark extract, a Nigerian palmwine additive, and ethanol on macromolecular binding of aflatoxin B1

In a previous study (Okoye and Neal, Food and Chemical Toxicology 1988, 26, 679) enhanced ethanol-induced reductions in albumin-bound and unbound serum aflatoxin B1 (AFB1) and increased hepatic DNA-AFB1 binding were observed in rats treated with bark extract of Sacoglottis gabonensis, a Nigerian palmwine additive. The present study was designed to examine further the mechanism of these effects. Male weanling rats were pretreated with the bark extract or ethanol, or both, in drinking-water (at three times the levels used in the previous study) for 8 days before the ip administration of a single dose of [3H]AFB1. [In the previous study the rats were fed all three compounds simultaneously.] In contrast to the results of the previous study, when both the additive and ethanol were administered, there were no significant effects on [3H]AFB1 binding to liver or serum albumin or liver DNA. The levels of DNA-bound aflatoxin were reduced in rats given the additive or ethanol alone.

Evidence for human antibodies that recognize an aflatoxin epitope in groups with high and low exposure to aflatoxins

Antibody activity against an aflatoxin epitope has been detected in serum from individuals who live in Kenya and who experience high exposure to aflatoxin B1. The activity was higher than in Danish people. The highest antibody activity was found in individuals who were recently exposed to aflatoxin B1. The ratio between IgG and IgM activities was higher in individuals with a high antibody titer. The specificity of the antibody activity differed in the serums obtained from Danish and Kenyan persons ("Danish" and "Kenyan" serum, respectively). The activity in Danish serum was inhibited by an aflatoxin-like substance isolated from human urine, whereas aflatoxin B1 did not inhibit the activity. In contrast, the activity in Kenyan serum was not inhibited by the aflatoxin-like substances. Therefore, the presence of antibodies against aflatoxin in humans indicates exposure to aflatoxin or aflatoxin-antigenic material. However, the biological consequences of these antibodies remain unknown.

Enhanced ethanol-induced changes in disposition and toxic response to dietary aflatoxin B1 due to Sacoglottis gabonensis bark extract, a Nigerian alcoholic beverage additive

The disposition of, and toxic response, to, dietary aflatoxin B1 (AFB1) was investigated in rats ingesting small doses of Sacoglottis gabonensis bark extract, ethanol, or both, in the drinking-water. Ingestion of ethanol alone or with the bark extract for 8 days resulted in a significant reduction in the level of AFB1 bound to serum albumin, but the level of unbound aflatoxin in the serum was significantly depressed only by concurrent ingestion of ethanol and the bark extract. In contrast, the bark extract alone or with ethanol significantly enhanced AFB1 binding to hepatic DNA. As with serum aflatoxin, concurrent ingestion of ethanol and the extract caused the most pronounced effect, suggesting synergism. All three treatments interfered with both the daily excretion pattern, and level, of aflatoxin in the urine. All three treatments enhanced AFB1-induction of liver gamma-glutamyltranspeptidase activity, suggesting potentiation of toxic response to AFB1. These data suggest that addition of the bark extract to alcoholic beverages may affect the biological response to dietary AFB1.

Characterization of the aflatoxin B1-binding site of rat albumin

A fluorescence-enhancement method was used to investigate the non-covalent interaction between aflatoxin B1 and rat albumin. Solvent-induced shifts in the emission spectrum of aflatoxin B1 provided evidence that the aflatoxin B1-binding site of rat albumin is a highly nonpolar environment. A dissociation constant of 20 microM was determined at 20 degrees C. The possibility that aflatoxin B1 binds one of the three major drug sites of albumin was investigated by ligand-displacement experiments. Mechanisms whereby marker ligands displace aflatoxin B1 were further investigated by comparing the experimental binding parameters with those derived theoretically, assuming competitive binding. The results indicate that: aflatoxin B1 and phenylbutazone compete for a common high-affinity site on rat albumin; high-affinity binding of aflatoxin B1 and site-II marker ligands takes place independently; aflatoxin B1 does not compete with either cholate or warfarin for the same high-affinity site, but the simultaneous binding of warfarin or cholate negatively modulates the binding of aflatoxin B1 to albumin. Fluorescence energy-transfer studies show that the lone tryptophan residue, Trp-214, is not associated with the aflatoxin B1-binding site.

Solvent effects on the spectroscopic properties of aflatoxin B1

1. Solvent-induced changes in the spectral properties of aflatoxin B1 were investigated using protic and aprotic solvents. 2. The absorption data were less sensitive to solvent effects than the fluorescence emission data. 3. Stokes shifts in protic solvents were greater than those in aprotic solvents indicating hydrogen bond formation between solvent and the excited state of aflatoxin B1. 4. From the Stokes shift data for aprotic solvents, the dipole moment of aflatoxin B1 was estimated to increase by 15.7 Debye units upon excitation to the excited singlet state.