Molecular Cloning and Expression of a Novel Cytochrome P450 from Turkey Liver with Aflatoxin B1Oxidizing Activity

Chicken CYP1A5 is able to hydroxylate aflatoxin B1 to aflatoxin M1

Aflatoxin B1 (AFB1), a naturally-occurring mycotoxin, can cause severe toxicological and carcinogenic effects in livestock and humans. Given that the chicken is one of the most important food-producing animals, knowledge regarding AFB1 metabolism and enzymes responsible for AFB1 transformation in the chicken has important implications for chicken production and food safety. Previously, we have successfully expressed chicken CYP1A5 and CYP3A37 monooxygenases in E. coli, and reconstituted them into a functional CYP system consisting of CYP1A5 or CYP3A37, CPR and cytochrome b5. In this study, we aimed to investigate the roles of CYP1A5 and CYP3A37 in the bioconversion of AFB1 to AFM1. Our results showed that chicken CYP1A5 was able to hydroxylate AFB1 to AFM1. The formation of AFM1 followed the typical Michaelis-Menten kinetics. The kinetics parameters of Vmax and Km were determined as 0.83 ± 0.039 nmol/min/nmol P450 and 26.9 ± 4.52 μM respectively. Docking simulations further revealed that AFB1 adopts a "side-on" conformation in chicken CYP1A5, facilitating the hydroxylation of the C9a atom and the production of AFM1. On the other hand, AFB1 assumes a "face-on" conformation in chicken CYP3A37, leading to the displacement of the C9a atom from the heme iron and explaining the lack of AFM1 hydroxylation activity. The results demonstrate that chicken CYP1A5 possesses efficient hydroxylase activity towards AFB1 to form AFM1.

Ancestral sequence reconstruction of a cytochrome P450 family involved in chemical defence reveals the functional evolution of a promiscuous, xenobiotic-metabolizing enzyme in vertebrates

The cytochrome P450 family 1 enzymes (CYP1s) are a diverse family of hemoprotein monooxygenases, which metabolize many xenobiotics including numerous environmental carcinogens. However, their historical function and evolution remain largely unstudied. Here we investigate CYP1 evolution via the reconstruction and characterization of the vertebrate CYP1 ancestors. Younger ancestors and extant forms generally demonstrated higher activity toward typical CYP1 xenobiotic and steroid substrates than older ancestors, suggesting significant diversification away from the original CYP1 function. Caffeine metabolism appears to be a recently evolved trait of the CYP1A subfamily, observed in the mammalian CYP1A lineage, and may parallel the recent evolution of caffeine synthesis in multiple separate plant species. Likewise, the aryl hydrocarbon receptor agonist, 6-formylindolo[3,2-b]carbazole (FICZ) was metabolized to a greater extent by certain younger ancestors and extant forms, suggesting that activity toward FICZ increased in specific CYP1 evolutionary branches, a process that may have occurred in parallel to the exploitation of land where UV-exposure was higher than in aquatic environments. As observed with previous reconstructions of P450 enzymes, thermostability correlated with evolutionary age; the oldest ancestor was up to 35 °C more thermostable than the extant forms, with a ¹⁰T₅₀ (temperature at which 50% of the hemoprotein remains intact after 10 min) of 71 °C. This robustness may have facilitated evolutionary diversification of the CYP1s by buffering the destabilizing effects of mutations that conferred novel functions, a phenomenon which may also be useful in exploiting the catalytic versatility of these ancestral enzymes for commercial application as biocatalysts.

The Mechanism Underlying the Extreme Sensitivity of Duck to Aflatoxin B1

Most metabolites of aflatoxin B1 (AFB1), especially exo-AFB1-8,9-epoxide (AFBO), can induce the production of reactive oxygen species (ROS) to vary degrees, causing oxidative stress and liver damage, and ultimately induce liver cancer in humans and animals. Duck is one of the most sensitive animals to AFB1, and severe economic losses are caused by duck AFB1 poisoning every year, but the exact mechanism of this high sensitivity is still unclear. This review highlights significant advances in our understanding of the AFB1 metabolic activation, like cytochrome P450s (CYPs), and AFB1 metabolic detoxification, like glutathione S-transferases (GSTs) in poultry. In addition, AFB1 may have other metabolic pathways in poultry, such as the mutual conversion of AFB1 and aflatoxicol (AFL) and the process of AFBO to produce AFB1-8,9-dihydrodiol (AFB1-dhd) and further metabolize it into detoxification substances. This review also summarized some exogenous regulatory substances that can alleviate AFB1-induced oxidative stress.

Bush sophora root polysaccharide could help prevent aflatoxin B1-induced hepatotoxicity in the primary chicken hepatocytes

The aim of this study was to evaluate the effects of bush sophora root polysaccharide (BSRPS) on the aflatoxin B1 (AFB1)-induced hepatotoxicity and to explore the underlying mechanisms. The primary chicken hepatocytes were used as the model in the present experiment. The results showed that AFB1 induced hepatotoxicity of chicken hepatocytes in a dose dependent manner as demonstrated by decreasing cell viability and increasing LDH activity, ALT and AST levels. AFB1 at 0.16 μM significantly increased the levels of hepatic cytochrome P450 1A5 (CYP450 1A5) mRNA and malondialdehyde (MDA) and decreased the activity and mRNA level of manganese superoxide dismutase(SOD2) and the glutathione peroxidases (GSH-Px) activity in the hepatocytes compared with the blank control. BSRPS at 8.93 μM, 17.86 μM, and 35.72 μM supplementation could significantly reverse the above-mentioned changes induced by AFB1, and 17.86 μM of BSRPS has the largest effects on protecting the AFB1-induced hepatocytes damage. Knock-down of SOD2 by SOD2-specific siRNA significantly eliminated the protective effects of BSRPS on AFB1-induced the increase of CYP450 1A5 mRNA levels and hepatotoxicity. These results suggested that the BSRPS has protective effects on AFB1-induced hepatotoxicity by down-regulating CYP450 1A5 mRNA level via up-regulating SOD2 expression in the primary chicken hepatocytes.

Comparative Response of the Hepatic Transcriptomes of Domesticated and Wild Turkey to Aflatoxin B₁

The food-borne mycotoxin aflatoxin B₁ (AFB₁) poses a significant risk to poultry, which are highly susceptible to its hepatotoxic effects. Domesticated turkeys (Meleagris gallopavo) are especially sensitive, whereas wild turkeys (M. g. silvestris) are more resistant. AFB₁ toxicity entails bioactivation by hepatic cytochrome P450s to the electrophilic exo-AFB₁-8,9-epoxide (AFBO). Domesticated turkeys lack functional hepatic GST-mediated detoxification of AFBO, and this is largely responsible for the differences in resistance between turkey types. This study was designed to characterize transcriptional changes induced in turkey livers by AFB₁, and to contrast the response of domesticated (susceptible) and wild (more resistant) birds. Gene expression responses to AFB₁ were examined using RNA-sequencing. Statistically significant differences in gene expression were observed among treatment groups and between turkey types. Expression analysis identified 4621 genes with significant differential expression (DE) in AFB₁-treated birds compared to controls. Characterization of DE transcripts revealed genes dis-regulated in response to toxic insult with significant association of Phase I and Phase II genes and others important in cellular regulation, modulation of apoptosis, and inflammatory responses. Constitutive expression of GSTA3 was significantly higher in wild birds and was significantly higher in AFB₁-treated birds when compared to controls for both genetic groups. This pattern was also observed by qRT-PCR in other wild and domesticated turkey strains. Results of this study emphasize the differential response of these genetically distinct birds, and identify genes and pathways that are differentially altered in aflatoxicosis.

Selenomethionine Alleviates AFB1-Induced Damage in Primary Chicken Hepatocytes by Inhibiting CYP450 1A5 Expression via Upregulated SelW Expression

This study aims to evaluate the protective effects of selenomethionine (SeMet) on aflatoxin B1 (AFB1)-induced hepatotoxicity in primary chicken hepatocytes. Cell viability and lactic dehydrogenase activity assays revealed the dose dependence of AFB1 toxicity to chicken hepatocytes. AFB1 concentrations of >0.05 μg/mL significantly reduced glutathione and total superoxide dismutase levels and increased the malondialdehyde concentration and cytochrome P450 enzyme 1A5 (CYP450 1A5) mRNA levels (P < 0.05). AFB1, however, did not affect CYP450 3A37 mRNA levels. Supplementation with 2 μM SeMet protected against AFB1-induced changes and significantly increased selenoprotein W (SelW) mRNA levels (P < 0.05). Additionally, SelW knockdown attenuated the protective effect of SeMet on AFB1-induced damage and significantly increased the level of CYP450 1A5 expression (P < 0.05). Therefore, SeMet alleviates AFB1-induced damage in primary chicken hepatocytes by improving SelW expression, thus inhibiting CYP450 1A5 expression.

Hepatic Transcriptome Responses of Domesticated and Wild Turkey Embryos to Aflatoxin B₁

The mycotoxin, aflatoxin B₁ (AFB₁) is a hepatotoxic, immunotoxic, and mutagenic contaminant of food and animal feeds. In poultry, AFB₁ can be maternally transferred to embryonated eggs, affecting development, viability and performance after hatch. Domesticated turkeys (Meleagris gallopavo) are especially sensitive to aflatoxicosis, while Eastern wild turkeys (M. g. silvestris) are likely more resistant. In ovo exposure provided a controlled AFB₁ challenge and comparison of domesticated and wild turkeys. Gene expression responses to AFB₁ in the embryonic hepatic transcriptome were examined using RNA-sequencing (RNA-seq). Eggs were injected with AFB₁ (1 μg) or sham control and dissected for liver tissue after 1 day or 5 days of exposure. Libraries from domesticated turkey (n = 24) and wild turkey (n = 15) produced 89.2 Gb of sequence. Approximately 670 M reads were mapped to a turkey gene set. Differential expression analysis identified 1535 significant genes with |log₂ fold change| ≥ 1.0 in at least one pair-wise comparison. AFB₁ effects were dependent on exposure time and turkey type, occurred more rapidly in domesticated turkeys, and led to notable up-regulation in cell cycle regulators, NRF2-mediated response genes and coagulation factors. Further investigation of NRF2-response genes may identify targets to improve poultry resistance.

Theoretical study on the N-demethylation mechanism of theobromine catalyzed by P450 isoenzyme 1A2

Theobromine, a widely consumed pharmacological active substance, can cause undesirable muscle stiffness, nausea and anorexia in high doses ingestion. The main N-demethylation metabolic mechanism of theobromine catalyzed by P450 isoenzyme 1A2 (CYP1A2) has been explored in this work using the unrestricted hybrid density functional method UB3LYP in conjunction with the LACVP(Fe)/6-31G (H, C, N, O, S, Cl) basis set. Single-point calculations including empirical dispersion corrections were carried out at the higher 6-311++G** basis set. Two N-demethylation pathways were characterized, i.e., 3-N and 7-N demethylations, which involve the initial N-methyl hydroxylation to form carbinolamines and the subsequent carbinolamines decomposition to yield monomethylxanthines and formaldehydes. Our results have shown that the rate-limiting N-methyl hydroxylation occurs via a hydrogen atom transfer (HAT) mechanism, which proceeds in a spin-selective mechanism (SSM) in the gas phase. The carbinolamines generated are prone to decomposition via the contiguous heteroatom-assisted proton-transfer. Strikingly, 3-N demethylation is more favorable than 7-N demethylation due to its lower free energy barrier and 7-methylxanthine therefore is the optimum product reported for the demethylation of theobromine catalyzed by CYP1A2, which are in good agreement with the experimental observation. This work has first revealed the detail N-demethylation mechanisms of theobromine at the theoretical level. It can offer more significant information for the metabolism of purine alkaloid.

Aflatoxicosis chemoprevention by probiotic Lactobacillius and lack of effect on the major histocompatibility complex

Turkeys are extremely sensitive to aflatoxin B1 (AFB1) which causes decreased growth, immunosuppression and liver necrosis. The purpose of this study was to determine whether probiotic Lactobacillus, shown to be protective in animal and clinical studies, would likewise confer protection in turkeys, which were treated for 11 days with either AFB1 (AFB; 1 ppm in diet), probiotic (PB; 1 × 10(11) CFU/ml; oral, daily), probiotic + AFB1 (PBAFB), or PBS control (CNTL). The AFB1 induced drop in body and liver weights were restored to normal in CNTL and PBAFB groups. Hepatotoxicity markers were not significantly reduced by probiotic treatment. Major histocompatibility complex (MHC) genes BG1 and BG4, which are differentially expressed in liver and spleens, were not significantly affected by treatments. These data indicate modest protection, but the relatively high dietary AFB1 treatment, and the extreme sensitivity of this species may reveal limits of probiotic-based protection strategies.

Hydroxylation of quinocetone and carbadox is mediated by CYP1As in the chicken (Gallus gallus)

Quinoxaline derivatives (quinoxalines) comprise a class of drugs that have been widely used as animal antimicrobial agents and feed additives. Although the metabolism of quinoxaline drugs has been mostly studied using chicken liver microsomes, the biochemical mechanism of biotransformation of these chemicals in the chicken has yet to be characterized. In this study, using bacteria produced enzymes, we demonstrated that both CYP1A4 and CYP1A5 participate in the oxidative metabolism of quinoxalines. For CYP1A5, three hydroxylated metabolites of quinocetone were generated. In addition, CYP1A5 is able to hydroxylate carbadox. For CYP1A4, only one hydroxylated product of quinocetone on the phenyl ring was identified. Neither CYP1A5 nor CYP1A4 showed hydroxylation activity towards mequindox and cyadox. Our results suggest that CYP1A4 and CYP1A5 have different and somewhat overlapping substrate specificity in quinoxaline metabolism, and CYP1A5 represents a crucial enzyme in hydroxylation of both quinocetone and carbadox.

Heterologous expression and functional characterization of avian mu-class glutathione S-transferases

Hepatic glutathione S-transferases (GSTs: EC2.5.1.1.8) catalyze the detoxification of reactive electrophilic compounds, many of which are toxic and carcinogenic intermediates, via conjugation with the endogenous tripeptide glutathione (GSH). Glutathione S-transferase (GST)-mediated detoxification is a critical determinant of species susceptibility to the toxic and carcinogenic mycotoxin aflatoxin B1 (AFB1), which in resistant animals efficiently detoxifies the toxic intermediate produced by hepatic cytochrome P450 bioactivation, the exo-AFB1-8,9-epoxide (AFBO). Domestic turkeys (Meleagris gallopavo) are one of the most sensitive animals known to AFB1, a condition associated with a deficiency of hepatic GST-mediated detoxification of AFBO. We have recently shown that unlike their domestic counterparts, wild turkeys (Meleagris gallopavo silvestris), which are relatively resistant, express hepatic GST-mediated detoxification activity toward AFBO. Because of the importance of GSTs in species susceptibility, and to explore possible GST classes involved in AFB1 detoxification, we amplified, cloned, expressed and functionally characterized the hepatic mu-class GSTs tGSTM3 (GenBank accession no. JF340152), tGSTM4 (JF340153) from domestic turkeys, and a GSTM4 variant (ewGSTM4, JF340154) from Eastern wild turkeys. Predicted molecular masses of tGSTM3 and two tGSTM4 variants were 25.6 and 25.8kDa, respectively. Multiple sequence comparisons revealed four GSTM motifs and the mu-loop in both proteins. tGSTM4 has 89% amino acid sequence identity to chicken GSTM2, while tGSTM3 has 73% sequence identity to human GSTM3 (hGSTM3). Specific activities of Escherichia coli-expressed tGSTM3 toward 1-chloro-2,4-dinitrobenzene (CDNB) and peroxidase activity toward cumene hydroperoxide were five-fold greater than tGSTM4 while tGSTM4 possessed more than three-fold greater activity toward 1,2-dichloro-4-nitrobenzene (DCNB). The two enzymes displayed equal activity toward ethacrynic acid (ECA). However, none of the GSTM proteins had AFBO detoxification capability, in contrast to recombinant alpha-class GSTs shown in our recent study to possess this important activity. In total, our data indicate that although turkey hepatic GSTMs may contribute to xenobiotic detoxification, they probably play no role in detoxification of AFBO in the liver.

Characterization of chicken cytochrome P450 1A4 and 1A5: inter-paralog comparisons of substrate preference and inhibitor selectivity

The chicken (Gallus gallus) is one of the most economically important domestic animals and also an avian model species. Chickens have two CYP1A genes (CYP1A4 and CYP1A5) which are orthologous to mammalian CYP1A1 and CYP1A2. Although the importance of chicken CYP1As in metabolism of endogenous compounds and xenobiotics is well recognized, their enzymatic properties, substrate preference and inhibitor selectivity remain poorly understood. In this study, functional enzymes of chicken CYP1A4 and CYP1A5 were successfully produced in Escherichia coli (E. coli). The substrate preference and inhibitor specificity of the two chicken CYP1As were compared. Kinetic results showed that the enzymatic parameters (K(m), V(max), V(max)/K(m)) for ethoxyresorufin O-deethylase (EROD) and benzyloxyresorufin O-debenzylase (BROD) differed between CYP1A4 and CYP1A5, while no significant difference was observed for methoxyresorufin O-demethylase (MROD). Lower K(m) of CYP1A4 for BROD suggests that CYP1A4 has a greater binding affinity to benzyloxyresorufin than either ethoxyresorufin or methoxyresorufin. The highest V(max)/K(m) ratio was seen in BROD activity for CYP1A4 and in MROD for CYP1A5 respectively. These results indicate that substrate preference of chicken CYP1As is more notably distinguished by BROD activity and CYP1A5 prefers shorter alkoxyresorufins resembling its mammalian ortholog CYP1A2. Differential patterns of MROD inhibition were observed between CYP1As and among the five CYP inhibitors (α-naphthoflavone, furafylline, piperonyl butoxide, erythromycin and ketoconazole). α-Naphthoflavone was determined to be a potent MROD inhibitor of both CYP1A4 and CYP1A5. In contrast, no or only a trace inhibitory effect (<15%) was observed by erythromycin at a concentration of 500 μM. Stronger inhibition of MROD activity was found in CYP1A5 than CYP1A4 by relatively small molecules α-naphthoflavone, piperonyl butoxide and furafylline. AROD kinetics and inhibition profiles between chicken CYP1A4 and CYP1A5 demonstrate that the two paralogous members of the CYP1A subfamily have distinct enzymatic properties, reflecting differences in the active site geometry between CYP1A4 and CYP1A5. These findings suggest that CYP1A4 and CYP1A5 play partially overlapping but distinctly different physiological and toxicological roles in the chicken.

Alpha-class glutathione S-transferases in wild turkeys (Meleagris gallopavo): characterization and role in resistance to the carcinogenic mycotoxin aflatoxin B1

Domestic turkeys (Meleagris gallopavo) are one of the most susceptible animals known to the toxic effects of the mycotoxin aflatoxin B₁ (AFB₁), a potent human hepatocarcinogen, and universal maize contaminant. We have demonstrated that such susceptibility is associated with the inability of hepatic glutathione S-transferases (GSTs) to detoxify the reactive electrophilic metabolite exo-AFB₁-8,9-epoxide (AFBO). Unlike their domestic counterparts, wild turkeys, which are relatively AFB₁-resistant, possess hepatic GST-mediated AFBO conjugating activity_. Here, we characterized the molecular and functional properties of hepatic alpha-class GSTs (GSTAs) from wild and domestic turkeys to shed light on the differences in resistance between these closely related strains. Six alpha-class GST genes (GSTA) amplified from wild turkeys (Eastern and Rio Grande subspecies), heritage breed turkeys (Royal Palm) and modern domestic (Nicholas strain) turkeys were sequenced, and catalytic activities of heterologously-expressed recombinant enzymes determined. Alpha-class identity was affirmed by conserved GST domains and four signature motifs. All GSTAs contained single nucleotide polymorphisms (SNPs) in their coding regions: GSTA1.1 (5 SNPs), GSTA1.2 (7), GSTA1.3 (3), GSTA2 (3), GSTA3 (1) and GSTA4 (2). E. coli-expressed GSTAs possessed varying activities toward GST substrates 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB), ethacrynic acid (ECA), cumene hydroperoxide (CHP). As predicted by their relative resistance, livers from domestic turkeys lacked detectable GST-mediated AFBO detoxification activity, whereas those from wild and heritage birds possessed this critical activity, suggesting that intensive breeding and selection resulted in loss of AFB₁-protective alleles during domestication. Our observation that recombinant tGSTAs detoxify AFBO, whereas their hepatic forms do not, implies that the hepatic forms of these enzymes are down-regulated, silenced, or otherwise modified by one or more mechanisms. These data may inform of possible molecular mechanisms of resistance to AFB₁, and may also have the benefit of identifying genetic markers which could be used to enhance AFB₁ resistance in modern domestic strains.

Functional characterization of alpha-class glutathione s-transferases from the Turkey (meleagris gallopavo)

Six Alpha-class glutathione S-transferase (GST) subunits were cloned from domestic turkey livers, which are one of the most susceptible animals known to the carcinogenic mycotoxin aflatoxin B₁. In most animals, GST dysfunction is a risk factor for susceptibility toward AFB₁, and we have shown that turkeys lack GSTs with affinity toward the carcinogenic intermediate exo-aflatoxin B(1)-8-9-epoxide (AFBO). Conversely, mice are resistant to AFB₁ carcinogenesis, due to high constitutive expression of mGSTA3 that has high affinity toward AFBO. When expressed in Escherichia coli, all six tGSTA subunits possessed conjugating activities toward substrates 1-chloro-2,4-dinitrobenzene (CDNB), 1,2-dichloro-4-nitrobenzene (DCNB), ethacrynic acid (ECA), and cumene hydroperoxide (CHP) with tGSTA1.2 appearing most active. Interestingly, tGSTA1.1, which lacks one of the four Alpha-class signature motifs, possessed enzymatic activities toward all substrates. All had comparable activities toward AFBO conjugation, an activity absent in turkey liver cytosols. E. coli-expressed mGSTA3 conjugated AFBO with more than 3-fold greater activity than that of tGSTAs and had higher activity toward GST prototype substrates. Mouse hepatic cytosols had approximately 900-fold higher catalytic activity toward AFBO compared with those from turkey. There was no apparent amino acid profile in tGSTAs that might correspond to specificity toward AFBO, although tGSTA1.2, which had slightly higher AFBO-trapping ability, shared Tyr¹⁰⁸ with mGSTA3, a residue postulated to be critical for AFBO trapping activity in mammalian systems. The observation that recombinant tGSTAs detoxify AFBO, whereas their hepatic forms do not, implies that the hepatic forms of these enzymes are silenced by one or more regulatory mechanisms.

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

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

Aflatoxin B1 in poultry: toxicology, metabolism and prevention

Aflatoxins (AF) are ubiquitous in corn-based animal feed and causes hepatotoxic and hepatocarcinogenic effects. The most important AF in terms of toxic potency and occurrence is aflatoxin B1 (AFB1). Poultry, especially turkeys, are extremely sensitive to the toxic and carcinogenic action of AFB1, resulting in millions of dollars in annual losses to producers due to reduced growth rate, increased susceptibility to disease, reduced egg production and other adverse effects. The extreme sensitivity of turkeys and other poultry to AFB1 is associated with efficient hepatic cytochrome P450-mediated bioactivation and deficient detoxification by glutathione S-transferases (GST). Discerning the biochemical and molecular mechanisms of this extreme sensitivity of poultry to AFB1, will contribute in the development of novel strategies to increase aflatoxin resistance. Since AFB1 is an unavoidable contaminant of corn-based poultry feed, chemoprevention strategies aimed at reducing AFB1 toxicity in poultry and in other animals have been the subject of numerous studies. This brief review summarizes many of the key recent findings regarding the action of aflatoxins in poultry.

Defining the turkey MHC: sequence and genes of the B locus

The MHC, the most polymorphic and gene dense region in the vertebrate genome, contains many loci essential to immunity. In mammals, this region spans approximately 4 Mb. Studies of avian species have found the MHC to be greatly reduced in size and gene content with an overall locus organization differing from that of mammals. The chicken MHC has been mapped to two distinct regions (MHC-B and -Y) of a single chromosome. MHC-B haplotypes possess tightly linked genes encoding the classical MHC molecules and few other disease resistance genes. Furthermore, chicken haplotypes possess a dominantly expressed class I and class II B locus that have a significant effect on the progression or regression of pathogenic disease. In this study, we present the MHC-B region of the turkey (Meleagris gallopavo) as a similarly constricted locus, with 34 genes identified within a 0.2-Mb region in near-perfect synteny with that of the chicken MHC-B. Notable differences between the two species are three BG and class II B loci in the turkey compared with one BG and two class II B loci in the chicken MHC-B. The relative size and high level of similarity of the turkey MHC in relation to that of the chicken suggest that similar associations with disease susceptibility and resistance may also be found in turkey.

Structure and genetic mapping of the Cytochrome P450 gene (CYP1A5) in the turkey (Meleagris gallopavo)

Cytochromes P450 (P450) are a superfamily of membrane-bound hemoproteins that oxidize a large number of endogenous and exogenous compounds. The recently cloned P450 gene (CYP1A5) encodes the primary protein responsible for epoxidation of aflatoxin B(1) (AFB(1)) in the turkey, an animal extremely sensitive to this mycotoxin. Hypersensitivity of turkeys to AFB(1) was first demonstrated by association with 'Turkey X Disease' which caused widespread deaths of turkeys and other poultry throughout Europe in the 1960s, later shown to be caused by AFB(1)-contaminated feed. In this study, comparative genomic approaches were used to selectively amplify and sequence the introns and 3' flanking region of CYP1A5. The structure of the CYP1A5 gene in the turkey is shown to be equivalent to that of the human CYP1A genes with seven exons of 38, 858, 127, 90, 124, 87 and 307 bp, respectively, and six introns. A single nucleotide polymorphism (SNP) in the 3' UTR was used to assign CYP1A5 to turkey linkage group M16 (equivalent to chicken chromosome 10). The results of this study provide the framework for identifying allelic variants of this biochemically important P450 gene in poultry.