Metabolism of aflatoxin B1 (AFB1), aflatoxin G1 (AFG1) and vitamin C intake by guinea pig liver preparation in vitro

Male and female guinea pigs weighing 150–200 g were divided into three groups, with equal number of males and females in each group. They were fed an experimental diet which varied as follows: group I, 0 mg vitamin C/g of diet; group II, 1.08 mg/g and group III, 5.4 mg/g, for 28 days. Twenty-four hours after the last feeding, liver slices and 9000 g supernatant were prepared from each group, according to sex, and used for enzyme assays. For the demethylation assay, enzyme activity expressed as amount of formaldehyde produced from AFB₁, or AFG₁/hr/g fresh liver was seen to increase with the two levels of ascorbic acid intake in females. Males showed an enhancement of activity only in group II and remained with the same production of formaldehyde as above in group III. Although in each dietary group, the activity was higher in males than in females the variation in the amount of formaldehyde produced from one group to another was higher with females than with male guinea pigs. However with both sexes, the production of formaldehyde from AFG₁, was greater than from AFB₁. For the hydroxylation assay, enzyme activity was expressed as amount of metabolites (a) and (b) produced. Compared to group II, which offered a control level of ascorbic acid, group I fed without vitamin C showed a decreased production of metabolite (a) and (b) with males and females. Moreover, high intake of ascorbic acid in group III decreased the production of metabolite (a) and (b) in males, while in female guinea pigs the reduction was observed only with (b).     

高效液相色谱-荧光检测法测定沉香中黄曲霉毒素B1、B2、G1、G2

目的建立高效液相色谱–光化学衍生–荧光检测法测定沉香药材中黄曲霉毒素B1、B2、G1、G2。方法采用高效液相色谱法,通过免疫亲和柱提取和净化,荧光检测器检测。Agilent Zorbax Ecilpse Plus C18色谱柱(250 mm×4.6 mm,5μm);流动相:甲醇–水(45∶55);体积流量:0.8 m L/min;柱温:30℃;进样盘温度:4℃;荧光激发波长为360 nm,发射波长为450 nm。结果黄曲霉毒素B1、B2、G1、G2分别在9.3~74.4、3.0~24.0、9.3~74.4、3.5~28.0 pg线性关系良好,r均大于0.998 0;检测限分别为1.86、0.60、1.86、0.70 pg,定量限分别为7.44、2.40、7.44、2.80 pg。平均回收率分别为78%、92%、82%、99%,RSD值分别为4.4%、3.0%、4.3%、2.8%。结论所建立的方法结果准确、重复性、稳定性均良好,可用于沉香药材中黄曲霉毒素的质量控制。     

高效液相色谱-光化学衍生法测定湖南产莲子中黄曲霉毒素G1、G2、B1、B2

目的对不同来源的湖南产莲子中黄曲霉毒素G_1、G_2、B_2、B_1进行高效液相色谱-光化学衍生法测定。方法采用高效液相色谱–光化学衍生法,采用岛津GL Inertsil ODS-3色谱柱(250 mm×4.6 mm,5μm),流动相为甲醇–乙腈–水(35∶13∶52),柱温35℃;光化学衍生器(254 nm)激发波长λ_(ex)=360 nm,发射波长λ_(ex)=450 nm。结果黄曲霉毒素G_1、G_2、B_2、B_1分别在6.7～33.3、11.4～56.8、10.3～51.5、4.1～20.3 pg线性关系良好;平均回收率分别为98.07%、97.72%、96.11%、99.52%,RSD值分别为1.69%、1.40%、2.72%、1.34%(n=6)。9批莲子样品中,有6批未检出黄曲霉毒素,来自于农贸市场农户自存的2批次检出黄曲霉毒素B_1,实验室塑料袋包装储存1年的1批检出黄曲霉毒素B_1、B_2,但质量分数均低于法定标准限量。结论不同来源湖南产莲子中黄曲霉毒素质量分数均低于《中国药典》2020年版规定限量。该法测定结果准确、重复性良好,可为完善莲子安全性控制和质量标准提升提供实验依据。     

Comparative effects of butylated hydroxyanisole on hepatic in vivo DNA binding and in vitro biotransformation of aflatoxin B1 in the rat and mouse

To determine the mechanisms which mediate species- and treatment-related differences in susceptibility to aflatoxin B1 (AFB), we conducted a comparative study of the effects of dietary butylated hydroxyanisole (BHA) on the hepatic in vivo DNA binding and in vitro biotransformation of AFB in the rat and mouse. Mice are resistant to the hepatocarcinogenic effects of AFB, and BHA pretreatment has been shown to inhibit the carcinogenic effects of AFB in the highly susceptible rat. Rats and mice were fed a control diet or an identical diet containing 0.75% BHA for 10 days. On the 11th day, one-half of the control and BHA animals were administered [3H]AFB (0.25 mg/kg in dimethyl sulfoxide) via intraperitoneal injection. Animals were killed 2 hr later and covalent binding of AFB to hepatic DNA was determined. The remaining animals were killed for preparation of hepatic subcellular fractions used in in vitro assays. BHA treatment resulted in a decrease in in vivo hepatic AFB-DNA adduct formation in mice to 68% of control, but, in rats, treatment decreased AFB-DNA binding to 18% of control. Furthermore, hepatic AFB-DNA binding in control mice was only 1.2% of that measured in control rats. The rate of in vitro activation of AFB to the epoxide was 3.4-fold greater in control mice relative to control rats. BHA pretreatment increased the activation of AFB in mice 3.3-fold, but had no effect on oxidative metabolism in rats. Control mice had 52 times greater glutathione S-transferase (GST) activity toward the AFB-epoxide, but only 2.6 times greater GST activity toward 1-chloro-2,4-dinitrobenzene (CDNB), compared to that of control rats. In mice, BHA did not significantly increase GST activity toward the AFB-epoxide, but increased GST activity toward CDNB 3.1-fold. In rats, BHA increased GST activity toward the AFB-epoxide and CDNB by 3.2- and 2.1-fold, respectively. Epoxide hydrolase activity toward p-nitrostyrene oxide in mice was only 52% of the activity in rats. BHA increased epoxide hydrolase activity 3.8- and 2.5-fold in mice and rats, respectively. These data indicate that mice have high levels of an AFB-epoxide-specific GST activity relative to that of the rat. The rate of formation of the AFB-epoxide and the activity of epoxide hydrolase appear to be relatively unimportant under conditions of high GST activity, whereas elevated GST activity, and thus inactivation of the AFB-epoxide, appears to be the critical component in species- and BHA-induced differences in AFB-DNA adduct formation and, presumably, AFB hepatocarcinogenicity.     

Interactions of aflatoxin B1 and blood components of various species in vitro: Interconversion of aflatoxin B1 and aflatoxicol in the blood

The fate of aflatoxin B₁ (AFB₁) in the blood of various species of animals was studied in vitro. Examination of the distribution of radioactivity in blood incubated with [¹⁴C]AFB₁ at 37°C showed that high levels of radioactivity were associated with blood cells. The radioactivity was readily removed from the blood cells by washing with fresh plasma, indicating loose binding of AFB₁ to blood cells. Most of the radioactivity in plasma was bound to protein. These results suggest that a large part of the AFB₁ in blood in vivo may be carried not only by the plasma proteins but also by the blood cells. When chloroform extracts of plasma of [¹⁴C]AFB₁-treated mouse, rat, duckling, and hamster blood were developed by thin-layer chromatography, high levels of radioactivity were found in both the AFB₁ region and the aflatoxicol (AFL) region. Incubation of blood with nonradioactive AFB₁ and AFL showed marked interconversion of AFB₁ and AFL in the blood of rats, hamsters, mice, and Mongolian gerbils, but not in the blood of guinea pigs, rhesus monkeys, squirrel monkeys, or humans. Interconversion occurred in red blood cell suspensions but not in plasma, indicating that the red blood cells are responsible for AFB₁-AFL interconversion in the blood.     

α-fetoprotein and γ-glutamyl transpeptidase response in rats fed aflatoxin B1

Weanling, male, Fischer rats were fed a semipurified diet containing 0, 0.1, 0.5, 1.0, or 2.0 ppm aflatoxin B₁ (AB₁). Plasma α-fetoprotein (AFP) concentrations were determined weekly during weeks 4 to 7. During Week 8, animals were killed and hepatic sections were histochemically stained for γ-glutamyl transpeptidase (GGT). Plasma AFP concentrations increased linearly with increasing level of dietary AB₁ (Y = 24.9 + 27.0X; r = 0.90). There was a corresponding increase in the percentage of hepatic section area stained positively for GGT (Y = 6.55 + 11.1X; r = 0.95). This study suggests that AFP and GGT may both be useful markers for dose-response relationships to hepatocarcinogens.     

Binding of aflatoxins B1 and G1 to rat liver chromatin

Direct evidence for aflatoxin-chromatin interaction was obtained using fluorescence quenching as a measure of the interaction. Interaction of aflatoxin B₁ with components of chromatin increased in the following order: nonhistone residue, DNA, histone, histone-free chromatin, and intact chromatin. Fluorescence change due to interaction of aflatoxin B₁ with various complexes of chromatin constituents was not significantly different from that obtained with DNA. Association of aflatoxin B₁ with chromatin components was very weak whereas that with reconstituted deoxyribonucleoprotein (consisting of histone, nonhistone residue, and DNA) and with native chromatin was strong as shown by increase in fluorescence polarization of aflatoxin. Such data suggest the importance of structural integrity of chromatin in the binding with aflatoxin. Interaction of aflatoxin G₁ with chromatin was less than that of aflatoxin B₁.     

高效液相色谱-串联质谱法同时测定地龙中4个黄曲霉毒素

目的:建立同时测定地龙中4个黄曲霉毒素含量的高效液相色谱串联质谱法。方法:样品经甲醇-水(80∶20,v/v)提取,通过免疫亲和柱净化后,采用高效液相色谱串联质谱法测定其中4个黄曲霉毒素的含量,以多反应监测(MRM)方式分别监测离子对m/z 313→241(黄曲霉毒素B1,CE 50 eV),m/z 315→259(黄曲霉毒素B2,CE 43 eV),m/z 329→243(黄曲霉毒素G1,CE 38 eV)和m/z 331→245(黄曲霉毒素G2,CE 40 eV)。结果:黄曲霉毒素B1、B2、G1、G2的检测限分别为0.03,0.02,0.03,0.02μg.kg-1,回收率在88.0%～100.3%范围内,RSD均低于6.1%。结论:该方法快速、灵敏,结果准确,适用于地龙中4个黄曲霉毒素的同时检测。     

HPLC法测定150种中药材中黄曲霉毒素G2、G1、B2、B1的含量

目的:建立了HPLC法测定150种中药材中的黄曲霉毒素G₂、G₁、B₂、B₁含量。方法:样品经70%甲醇提取、免疫亲和色谱柱净化后,用HPLC-柱后衍生-荧光检测器测定结果:黄曲霉毒素G₁、B₂在0.15～6.00 ng·ml^-1范围内,黄曲霉毒素C₁、B₁在0.5～20.00 ng·ml^-1范围内线性关系良好回收率为85.6%～92.0%结论:本法操作简便,结果准确、重复性好,可用于中药材中黄曲霉毒素G₂、G₁、B₂、B₁的测定     

不同粒径聚乙二醇化维生素K1脂质体的制备及体内外评价

目的 制备不同粒径聚乙二醇化维生素K₁（VK₁）脂质体,并对其进行制剂学表征,考察体内药动学和促凝药效。方法 采用薄膜分散法制备不同粒径的VK₁脂质体,采用马尔文粒径仪测定粒径、聚合物分散性指数（PDI）、Zeta电位;透射电子显微镜观察形态;以粒径为指标,考察脂质体在4℃下储存1个月、在磷酸缓冲液（PBS,pH 6.8）及pH 1.2水溶液中48 h的稳定性;采用超滤离心法测定包封率与载药量;采用大鼠在体肠吸收模型考察肠吸收特性;ig给药考察脂质体在大鼠体内药动学行为;以华法林钠诱导大鼠低凝血酶原血症,采用ELISA试剂盒检测血浆中凝血因子II、V、VII和IX含量,检测凝血酶原时间（PT）,评价不同粒径VK₁脂质体（2 mg·kg^-1）促凝效果。结果 制备了3种VK₁脂质体（Lip-180、Lip-120、Lip-60）,粒径分别为（182.40±2.17） nm、（114.38±0.60） nm和（68.42±0.73） nm,PDI分别为0.21±0.01、0.12±0.00和0.17±0.01 ,电位分别为（-27.67±1.58）、（-22.93±1.81）、（-26.63±1.37）mV,脂质体均呈球形,分布均匀,包封率均>90%,载药量均>2.90%,稳定性良好。与Lip-180相比,Lip-120和Lip-60表现出更缓慢的释放性能和更好的跨膜吸收速率。Lip-120及Lip-60的药时曲线下面积（AUC_0-∞）分别是Lip-180的1.52和1.80倍,并且Lip-120与Lip-60的AUC_0-∞分别是维生素K₁注射剂（市售对照）的1.24与1.46倍。与低凝血酶原血症模型大鼠比较,经ig给药Lip-180、Lip-120、Lip-60及维生素K₁注射剂后,PT显著降低（P<0.001）,凝血因子Ⅱ、Ⅶ、Ⅸ、Ⅹ水平均升高,其中,Lip-60作用最显著,除给药后6 h的凝血因子Ⅹ外,均差异显著（P<0.05、0.01、0.001）。结论 聚乙二醇化VK₁脂质体分布均匀,稳定性高,释放缓慢,口服生物利用度高,体内促凝效果较好,优选粒径最小的Lip-60。     

Antigenotoxic effect of Saccharomyces cerevisiae on the damage produced in mice fed with aflatoxin B1 contaminated corn

The potential of Saccharomyces cerevisiae (Sc) was evaluated for reducing the micronucleated normochromatic erythrocytes (MNNE) rate in mice fed AFB₁ contaminated corn. The study included two groups fed AFB₁ contaminated corn (0.4 and 0.8 mg/kg), a control fed uncontaminated corn, another group fed uncontaminated corn and 0.3% of Sc (1 × 10⁸ live cells/g), and two groups fed AFB₁ contaminated corn (0.4 and 0.8 mg/kg) plus 0.3% Sc. Weight and MNNE were determined weekly for six weeks. Subsequently, the same determinations were made for another three-week period, but in mice receiving only a normal diet, without AFB₁ and Sc. Results in the first period revealed the following: control and Sc fed mice had similar constant weight increase, and low MNNE rate; mice fed only AFB₁ showed weight decrease and significant MNNE increase; finally, Sc improved weight gain and reduced MNNE produced by AFB₁. In the second period, results exhibited a tendency similar to that of the previous phase in the control and Sc fed mice; the weight and MNNE values improved in the other groups. We also determined the capacity of Sc for adsorbing and modifying the mycotoxin structure. The mixture was filtered to obtain two phases, and AFB₁ content was measured. Sc revealed a potent adsorbent capacity; however, chromatographic determination suggested no structural modification.     

高效液相色谱-柱后光化学衍生法测定参苓白术散中黄曲霉毒素G2、G1、B2、B1

目的 建立中成药参苓白术散中黄曲霉毒素G2、G1、B2、B1的测定方法.方法 样品经70％甲醇提取,免疫亲和柱净化后,采用高效液相色谱-柱后光化学衍生-荧光检测法定量,以串联质谱法验证.结果 高效液相色谱法线性关系良好(r＞0.999),回收率范围76.8％～115.2％(RSD范围1.8％～12.9％),检测了57批样品,并使用串联质谱仪的多反应监测模式确证了阳性结果.结论 本法快速、简便、灵敏、准确,专属性强,可用于参苓白术散中黄曲霉毒素的检测.     

The comparative metabolism and toxic potency of aflatoxin B1 and aflatoxin M1 in primary cultures of adult-rat hepatocytes

Both aflatoxin B₁ (AFB₁) and a hydroxylated metabolite, aflatoxin M₁ (AFM₁), were potent cytotoxins and genotoxins to primary cultures of rat hepatocytes. However, AFB₁ stimulated the release of lactate dehydrogenase into the culture medium and the loss of viable cells from the monolayer at lower doses than did AFM₁. The lowest toxic doses of AFB₁ and AFM₁ were 0·05–0·1 and 0·6 μg/ culture, respectively. Genotoxicity, determined by an assay for stimulation of DNA repair, was apparent at lower doses than was cytotoxicity. AFB₁ was again more potent than AFM₁, stimulating DNA repair at 0·025 μg/culture. compared to the lowest genotoxic dose of AFM₁ of 0·05 μg/culture. At higher doses (1·2–2·4 μg/culture) the responses due to both aflatoxins in the cytotoxicity and DNA-repair assays were approximately equal. The metabolism of a low dose (c. 0·17 μg/culture) of [¹⁴C]AFB₁ and [³H]AFM₁ by cultured hepatocytes differed significantly. After 1 hr, 50% of the [¹⁴C]AFB₁ remained unchanged in the culture medium, whereas about 18 hr were required for the same amount of [³H]AFM₁ metabolism to occur. [¹⁴C]AFB₁ was metabolized to AFM₁, to polar metabolites recovered in the aqueous phase after chloroform extraction, and to metabolites covalently bound to hepatocyte macromolecules. [³H]AFM₁ was also metabolized to polar metabolites and to forms bound to macromolecules. The degree of covalent binding of the aflatoxins correlated with their cytotoxicity and genotoxicity at lower doses. After a 24-hr incubation, 12·5% of the dose of [¹⁴C]AFB₁ was covalently bound to macromolecules compared to 1·5% of [³H]AFM₁. Although AFM₁ was less potent than AFB₁ in cytotoxicity, DNA-repair and covalent-binding assays using primary cultures of hepatocytes, AFM₁ was still active at relatively low doses and therefore is probably a potent hepatotoxin in vivo.     

Changes in preneoplastic response to aflatoxin B1 in rats fed green beans,beets or squash

Weanling male Fischer rats were fed diets containing 25% freeze-dried green beans, beets or squash either with or without 1 ppm aflatoxin B₁ for 8 wk. Plasma α-foetoprotein concentrations were determined weekly from wk 4 to 7. During wk 8 the animals were killed and hepatic sections were stained for γ-glutamyl transpeptidase. All three vegetable diets, as compared to the basal semi-purified diet, enhanced both aflatoxin-B₁-induced elevation of plasma α-foetoprotein and aflatoxin-B₁-induced emergence of hepatic cell foci of γ-glutamyl transpeptidase. The data suggest that these vegetables contain natural factors that enhance aflatoxin B₁ carcinogenicity.     

Changes in hepatic polyamine levels during acute and chronic administration of aflatoxin B1 to rats

A subacute dose of aflatoxin B₁ (3 mg/kg body weight) increases liver putrescine levels within 1 hr after administration, with high levels persisting over 24 hr. Higher doses of the carcinogen elicited larger increases in liver polyamine levels. A marked elevation of putrescine, spermidine and spermine were noted in aflatoxin B₁-induced preneoplastic liver. Pretreatment of rats with phenobarbital prior to aflatoxin B₁ administration resulted in no synergistic or additive effects.     

Effects of aflatoxin B1 on the activity of drug-metabolizing enzymes in rat liver

Aflatoxin B₁ (AFB₁) is one of the most active hepatotoxic and hepatocarcinogenic compounds known for rats. In order to evaluate the mechanism of action of the toxin on the liver, the effects of aflatoxin B₁ on the enzymes involved in its transformation, such as the monooxygenase-cytochromes P-450-dependent and conjugating enzymes, were studied. At the same time, liver damage was determined by measuring the activities of plasma γ-glutamyltransferase (GGT), alkaline phosphatase (ALP), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) as well as the concentration of bilirubin.Male Sprague-Dawley rats received a single ip dose of AFB₁ (1 or 3 mg/kg). The effects of the toxic compound on the activity of drug-metabolizing enzymes were followed 20 days after this single exposure. Mortality (18%) within 7 days following administration was produced only by the higher dose. The same dose also significantly decreased the total level of hepatic proteins and impaired the activity of aminopyrine N-demethylase. AFB₁ lowered the content of cytochromes P-450 by 32 and 69% at the 1 and 3 mg/kg dose levels, respectively. Epoxide hydrase activity was increased by 121 and 170% at 1 and 3 mg/kg, respectively, whereas UDP-glucuronyltransferase activity was increased (44%) at 1 mg/kg, but also decreased at the same extent for the higher dose. The activity of GSH S-epoxide transferase was decreased by a maximum of 53% by 3 mg/kg AFB₁. Results obtained by the 3rd day following the administration of 3 mg/kg AFB₁ showed that blood levels of all the factors studied in this experiment were increased above control values, while at lower dose of AFB₁ (1 mg/kg), only the activities of AST and ALT were significantly increased. The activities of these enzymes were 27 to 42 times greater in rats treated with 3 mg/kg AFB₁ than in rats given 1 mg/kg AFB₁. Most of the biological features studied tended to return to control values between the 9th and 20th day after AFB₁ treatment. This study makes it possible to compare changes in tissue levels of drug-metabolizing enzymes at two doses of AFB₁. It can also be used to demonstrate any time lag or differing behavior of the serum enzymes, notably the sensibility of transaminases.     

The neurotoxicity of aflatoxin B1 in the rat

The effects of repeated intraperitoneal administration of aflatoxin B₁ on the peripheral and central nervous systems of rats were investigated. Biochemical markers of neurotoxicity were monitored in nervous tissues following aflatoxin B₁ dosage and after the cessation of aflatoxin B₁ administration. Aflatoxin B₁ increased the activities of β-glucoronidase and β-galactosidase in the central and peripheral nervous systems. Repeated exposure of rats to aflatoxin B₁ also activated Na⁺ K⁺-ATPase and inhibited Mg²⁺-ATPase. Nervous tissue levels of DNA and total protein increased while the concentrations of RNA and phospholipid were depressed by aflatoxin B₁. The alterations in these parameters were specific for each of the tissues examined during the recovery of the rats. The findings indicate that the repeated administration of aflatoxin B₁ to rats results in degeneration in the central and peripheral nervous systems that may be related to the overt toxicity observed following aflatoxin administration.     

Comparative histopathological effects of aflatoxin B1 and palmotoxins B0 and G0 on some organs of different strains of the newly hatched chick (Gallus domesticus)

Aflatoxin B₁ and palmotoxin B₀ are equitoxic to the developing chick-embryo (Gallus domesticus) whilst palmotoxin G₀ is relatively non-toxic. Toxic lesions are present in the heart, liver, skeletal muscle, brain and cartilage in varying severities. The liver and skeletal muscle show fatty change and toxic myositis, respectively. Lesions in the heart, brain and cartilage are relatively mild. The endocardial cushion-like plaques at the base of the atrioventricular valves are lesions peculiar to aflatoxin B₁ and palmotoxin B₀-induced cardiac damage. It appears that these mycotoxins are not selectively tissue specific in inducing organ damage in the chick-embryo. An ultrastructural study of these lesions in the chick and other species may help to elucidate the molecular mechanism of the toxicity of these mycotoxins which are suspected to be very potent human hepatocarcinogens in certain parts of the tropics. Their acute phase effects in man are, however, unknown.     

Glycine N-methyltransferase affects the metabolism of aflatoxin B1 and blocks its carcinogenic effect

Previously, we reported that glycine N-methyltransferase (GNMT) knockout mice develop chronic hepatitis and hepatocellular carcinoma (HCC) spontaneously. For this study we used a phosphoenolpyruvate carboxykinase promoter to establish a GNMT transgenic (TG) mouse model. Animals were intraperitoneally inoculated with aflatoxin B₁ (AFB₁) and monitored for 11 months, during which neither male nor female GNMT-TG mice developed HCC. In contrast, 4 of 6 (67%) male wild-type mice developed HCC. Immunofluorescent antibody test showed that GNMT was translocated into nuclei after AFB₁ treatment. Competitive enzyme immunoassays indicated that after AFB₁ treatment, the AFB₁-DNA adducts formed in stable clones expressing GNMT reduced 51.4% compared to the vector control clones. Experiments using recombinant adenoviruses carrying GNMT cDNA (Ad-GNMT) further demonstrated that the GNMT-related inhibition of AFB₁-DNA adducts formation is dose-dependent. HPLC analysis of the metabolites of AFB₁ in the cultural supernatants of cells exposed to AFB₁ showed that the AFM₁ level in the GNMT group was significantly higher than the control group, indicating the presence of GNMT can enhance the detoxification pathway of AFB₁. Cytotoxicity assay showed that the GNMT group had higher survival rate than the control group after they were treated with AFB₁. Automated docking experiments showed that AFB₁ binds to the S-adenosylmethionine binding domain of GNMT. Affinity sensor assay demonstrated that the dissociation constant for GNMT-AFB₁ interaction is 44.9 μM. Therefore, GNMT is a tumor suppressor for HCC and it exerts protective effects in hepatocytes via direct interaction with AFB₁, resulting in reduced AFB₁-DNA adducts formation and cell death.     

Trp266 determines the binding specificity of a porcine aflatoxin B1 aldehyde reductase for aflatoxin B1-dialdehyde

Aflatoxin B₁ (AFB₁) is a severe threat to human and animal health. The aflatoxin B₁ aldehyde reductase (AFAR) family specifically catalyzes AFB₁-dialdehyde, a toxic metabolic intermediate of AFB₁, producing a nontoxic dialcohol. Although several AFARs have been found and characterized, the binding specificity of the family for AFB₁-dialdehyde remains unclear. Herein, according to the published sequence, we cloned a porcine AFAR gene. Recombinant porcine AFAR was expressed and purified from Escherichia coli as hexa-histidine tagged fusion protein. Using the cloned porcine AFAR as a model, site-directed mutagenesis combined with high performance liquid chromatography studies revealed that the substitution of Trp266 with Ala resulted in almost complete loss of catalytic activity for AFB₁-dialdehyde. Interestingly, the substitution of Met86 with Ala exhibited an obviously increased activity to the dialdehyde. Based on these results and by using molecular docking simulations, this work provides a structural explanation for why the AFAR family exhibits high specificity for AFB₁-dialdehyde. The Trp266 residue in porcine AFAR plays a critical role in stabilizing the binding of AFB₁-dialdehyde in the active pocket through the hydrophobic interaction of the side-chain indole ring of Trp266 with the fused coumarin rings of the dialdehyde molecule. The enhanced activity of M86A may be attributed to the formed π–π stacking interaction between Trp266 and the dialdehyde. In addition, other hydrophobic residues (e.g. Phe and Trp) around the dialdehyde molecule also stabilize the substrate binding. The findings may contribute to understanding the substrate specificity of the AFAR family for AFB₁-dialdehyde.