Cloning, tissue distribution, and functional studies of a new cytochrome P450 3A subfamily member, CYP3A45, from rainbow trout (Oncorhynchus mykiss) intestinal ceca

Organ-specific biotransformation in salmonids: Insight into intrinsic enzyme activity and biotransformation of three micropollutants

Aquatic ecosystems continue to be threatened by chemical pollution. To what extent organisms are able to cope with chemical exposure depends on their ability to display mechanisms of defense across different organs. Among these mechanisms, biotransformation processes represent key physiological responses that facilitate detoxification and reduce the bioaccumulation potential of chemicals. Biotransformation does not only depend on the ability of different organs to display biotransformation enzymes but also on the affinity of chemicals towards these enzymes. In the present study, we explored the ability of different organs and of two freshwater fish to support biotransformation processes through the determination of in vitro phase I and II biotransformation enzyme activity, and their role in supporting intrinsic clearance and the formation of biotransformation products. Three environmentally relevant pollutants were evaluated: the polycyclic aromatic hydrocarbon (PAH) pyrene (as recommended by the OECD 319b test guideline), the fungicide azoxystrobin, and the pharmaceutical propranolol. Comparative studies using S9 sub-cellular fractions derived from the liver, intestine, gills, and brain of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) revealed significant phase I and II enzyme activity in all organs. However, organ- and species-specific differences were found. In brown trout, significant extrahepatic biotransformation was observed for pyrene but not for azoxystrobin and propranolol. In rainbow trout, the brain appeared to biotransform azoxystrobin. In this same species, propranolol appeared to be biotransformed by the intestine and gills. Biotransformation products could be detected only from hepatic biotransformation, and their profiles and formation rates displayed species-specific patterns and occurred at different magnitudes. Altogether, our findings further contribute to the current understanding of organ-specific biotransformation capacity, beyond the expression and activity of enzymes, and its dependence on specific enzyme-chemical interactions to support mechanisms of defense against exposure.

Tissue-specific expression and activity of cytochrome P450 1A and 3A in rainbow trout (Oncorhynchus mykiss)

Piscine cytochrome P450 (CYP) enzymes play an important role in the metabolism of xenobiotics. Xenobiotics often act as inducers of CYP1A1 and CYP3A expression and activity in fish. We compared constitutive mRNA expression of CYP1A1, CYP3A27, and CYP3A45 and catalytic activity of CYP1A (7-ethoxyresorufin-O-deethylation, EROD) and CYP3A-like (benzyloxy-4-trifluoromethylcoumarin-O-debenzyloxylation, BFCOD) enzymes in the following six rainbow trout tissues: liver, gill, heart, brain, intestine, and gonad. mRNA expression and activity were present in all investigated tissues. The CYP1A1 mRNA expression was higher in the liver, gill, heart, and brain compared to gonad and intestine. The intestine was the main site of CYP3A27 and CYP3A45 expression. The highest EROD and BFCOD activity was observed in liver tissue followed in descending order by heart, brain, gill, intestine, and gonad. Such differences might be related to the role of CYP physiological functions in the specific tissue. Rainbow trout exposure to 50 mg/kg of β-naphthoflavone for 48 h resulted in a 7.5- and 5.9-fold increase in liver EROD and BFCOD activity, respectively. In vitro EROD activity inhibition with ellipticine showed tissue-specific inhibition, while ketoconazole decreased BFCOD activity by 50-98 % in all tissues. Further studies are needed to identify all CYP isoforms that are responsible for these activities and modes of regulation.

In vitro-in vivo extrapolation of hepatic and gastrointestinal biotransformation rates of hydrophobic chemicals in rainbow trout

Hepatic in vitro biotransformation assays, in combination with in vitro-in vivo extrapolation (IVIVE) and bioaccumulation modeling, can be used to support regulatory bioaccumulation assessments. In most applications, however, these methods ignore the possibility of extrahepatic metabolism. Here we evaluated intestinal biotransformation in rainbow trout using S9 fractions prepared from the upper intestinal (GIT) epithelium. Measured levels of activity determined using standard substrates for phase I and phase II biotransformation enzymes were within 2-fold of activities measured in hepatic S9 fractions. In vitro intrinsic clearance rates for 2-ethylhexyl-4-methoxycinnamate (EHMC; an organic sunscreen agent) and two polycyclic aromatic hydrocarbons (pyrene [PYR] and benzo(a)pyrene [BAP]) were significantly higher in liver S9 fractions than in GIT S9 fractions. For octocrylene (OCT; a second sunscreen agent), however, in vitro intrinsic clearance rates were higher in GIT S9 fractions compared to liver S9 fractions. An existing 'liver only' IVIVE model was expanded to consider biotransformation in both the liver and GIT. Relevant IVIVE scaling factors were developed by morphological, histological, and biochemical evaluation of trout intestines. For chemicals biotransformed at higher rates by hepatic S9 fractions (i.e., BAP, PYR, EHMC), the 'liver & GIT' model yielded whole-body biotransformation rate constants (k_MET) that were within 1.2 to 1.4-fold of those estimated using the 'liver only' model. In contrast to these findings, the mean k_MET for OCT obtained using the 'liver & GIT' model was 3.3 times higher than the mean k_MET derived using the 'liver only' model and was in good agreement with empirical k_MET estimates determined previously for trout (<20 % difference). The results of this study suggest that current 'liver only' IVIVE approaches may underestimate in vivo biotransformation rates for chemicals that undergo substantial biotransformation in the GIT.

Independent losses of a xenobiotic receptor across teleost evolution

Sensitivity to environmental stressors largely depend on the genetic complement of the organism. Recent sequencing and assembly of teleost fish genomes enable us to trace the evolution of defense genes in the largest and most diverse group of vertebrates. Through genomic searches and in-depth analysis of gene loci in 76 teleost genomes, we show here that the xenosensor pregnane X receptor (Pxr, Nr1i2) is absent in more than half of these species. Notably, out of the 27 genome assemblies that belong to the Gadiformes order, the pxr gene was only retained in the Merluccidae family (hakes) and Pelagic cod (Melanonus zugmayeri). As an important receptor for a wide range of drugs and environmental pollutants, vertebrate PXR regulate the transcription of a number of genes involved in the biotransformation of xenobiotics, including cytochrome P450 enzymes (CYP). In the absence of Pxr, we suggest that the aryl hydrocarbon receptor (Ahr) have evolved an extended regulatory role by governing the expression of certain Pxr target genes, such as cyp3a, in Atlantic cod (Gadus morhua). However, as several independent losses of pxr have occurred during teleost evolution, other lineages and species may have adapted alternative compensating mechanisms for controlling crucial cellular defense mechanisms.

Effects of phenanthrene on early development of the Pacific oyster Crassostrea gigas (Thunberg, 1789)

Phenanthnere (PHE) is a polycyclic aromatic hydrocarbon continuously discarded in the marine environment and bioavailable to many aquatic species. Although studies about PHE toxicity have been documented for adult oysters, the effects on early developmental stages are poorly characterized in bivalves. In this study, the effects of PHE (0.02 and 2.0μg.L^-1) were evaluated on the embryogenesis and larval development of Crassostrea gigas. Toxicity bioassays, growth and deformities assessment, analysis of shell calcium abundance and transcript levels of genes related to xenobiotic biotransformation (CYP2AU2, CYP30C1), immune system (Cg-Tal) and tissue growth and shell formation (Ferritin, Insulin-like, Cg-Try, Calmodulin and Nacrein) were assayed in D-shape larvae after 24h of PHE exposure. At the highest concentration (2.0μg.L^-1), PHE decreased the frequency of normal development (19.7±2.9%) and shell size (53.5±2.8mm). Developmental deformities were mostly related to abnormal mantle and shell formation. Lower calcium levels in oyster shells exposed to PHE 2.0μg.L^-1 were observed, suggesting effects on shell structure. At this same PHE concentration, CYP30C1, Cg-Tal, Cg-Tyr, Calmodulin were upregulated and CYP2AU2, Ferritin, Nacrein, and Insulin-Like were downregulated compared to control larvae. At the lowest PHE concentration (0.02μg.L^-1), it was observed a minor decrease in normal larval development (89,6±6%) and the remaining parameters were not affected. This is the first study to provide evidences that exposure to PHE can affect early oyster development at the molecular and morphological levels, possibly threatening this bivalve species.

Toxicological hazard induced by sucralose to environmentally relevant concentrations in common carp (Cyprinus carpio)

Sucralose (SUC) is an artificial sweetener that is now widely used in North American and Europe; it has been detected in a wide variety of aquatic environments. It is considered safe for human consumption but its effects in the ecosystem have not yet been studied in depth, since limited ecotoxicological data are available in the peer-reviewed literature. This study aimed to evaluate potential SUC-induced toxicological hazard in the blood, brain, gill, liver and muscle of Cyprinus carpio using oxidative stress biomarkers. Carps were exposed to two different environmentally relevant concentrations (0.05 and 155μgL^-1) for different exposure times (12, 24, 48, 72 and 96h). The following biomarkers were evaluated: lipid peroxidation (LPX), hydroperoxide content (HPC) and protein carbonyl content (PCC), as well as the activity of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT). SUC was determined by high pressure liquid chromatography tandem mass spectrometry techniques (HPLC)-MS/MS. Results show a statically significant increase in LPX, HPC, PCC (P<0.05) especially in gill, brain and muscle, as well as significant changes in the activity of antioxidant enzymes in gill and muscle. Furthermore, the biomarkers employed in this study are useful in the assessment of the environmental impact of this agent on aquatic species.

The cytochrome P450 genes of channel catfish: their involvement in disease defense responses as revealed by meta-analysis of RNA-Seq data sets

BACKGROUND

Cytochrome P450s (CYPs) encode one of the most diverse enzyme superfamily in nature. They catalyze oxidative reactions of endogenous molecules and exogenous chemicals.

METHODS

We identified CYPs genes through in silico analysis using EST, RNA-Seq and genome databases of channel catfish. Phylogenetic analyses and conserved syntenic analyses were conducted to determine their identities and orthologies. Meta-analysis of RNA-Seq databases was conducted to analyze expression profile of CYP genes following bacterial infection.

RESULTS

A full set of 61 CYP genes was identified and characterized in channel catfish. Phylogenetic tree and conserved synteny provided strong evidence of their identities and orthorlogy. Lineage-specific gene duplication was evident in a number of clans in channel catfish. CYP46A1 is missing in the catfish genome as observed with syntenic analysis and RT-PCR analysis. Thirty CYPs were found up- or down-regulated in liver, while seven and eight CYPs were observed regulated in intestine and gill following bacterial infection.

CONCLUSION

We systematically identified and characterized a full set of 61 CYP genes in channel catfish and studied their expression profiles after bacterial infection. While bacterial challenge altered the expression of large numbers of CYP genes, the mechanisms and significance of these changes are not known.

GENERAL SIGNIFICANCE

This work provides an example to systematically study CYP genes in non-model species. Moreover, it provides a basis for further toxicological and physiological studies in channel catfish.

Molecular cloning and expression analysis of cytochrome P450 3A gene in the turbot Scophthalmus maximus

In this study, the cytochrome P450 3A (CYP3A) gene was cloned from the turbot Scophthalmus maximus for the first time using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends approaches. The amino acid sequences were analyzed with corresponding software programs. The cDNA of CYP3A was 1,969 bp in length, which contained a 5'-untranslated region (UTR) of 34 bp, a 3'-UTR of 404 bp and an open reading frame of 1,530 bp encoding a predicted protein of 509 amino acids (GenBank accession No. JN216889). The deduced protein had a molecular weight of 58.09 kDa and an isoelectric point of 5.75. Amino acid sequence alignment indicated that turbot CYP3A shared 60-67% homology with other fish species. It consists of a signal peptide, six conservative substrate recognition sites (SRS 1-6) and the conserved heme-binding motif FXXGXXXCXG in all CYP3As. Quantitative real-time RT-PCR analysis indicated that turbot CYP3A mRNA was widely expressed in liver, kidney, gill, muscle, stomach, intestine, gallbladder and spleen, with the highest level in liver and the lowest in muscle. After oral administration of sulfamethazine, CYP3A expression in all experimental groups enhanced compared with control, and the expression varied with administration time. It suggested that CYP3A expression could be induced by sulfamethazine. Our findings provided molecular characterization and expression profile of turbot CYP3A, and revealed the important role that turbot CYP3A played in drug metabolisms.

Cytochrome P450 (CYP) in fish

Cytochrome P450 (CYP) enzymes are members of the hemoprotein superfamily, and are involved in the mono-oxygenation reactions of a wide range of endogenous and exogenous compounds in mammals and plants. Characterization of CYP genes in fish has been carried out intensively over the last 20 years. In Japanese pufferfish (Takifugu rubripes), 54 genes encoding P450s have been identified. Across all species of fish, 137 genes encoding P450s have been identified. These genes are classified into 18 CYP families: namely, CYP1, CYP2, CYP3, CYP4, CYP5, CYP7, CYP8, CYP11, CYP17, CYP19, CYP20, CYP21, CYP24, CYP26, CYP27, CYP39, CYP46 and CYP51.We pinpointed eight CYP families: namely, CYP1, CYP2, CYP3, CYP4, CYP11, CYP17, CYP19 and CYP26 in this review because these CYP families are studied in detail. Studies of fish P450s have provided insights into the regulation of P450 genes by environmental stresses including water pollution. In this review, we present an overview of the CYP families in fish.

Assessment of typical pollutants in waterborne by combining active biomonitoring and integrated biomarkers response

Organic pollutants, heavy metals and pharmaceuticals are continuously dispersed into the environment and have become a relevant environmental emerging concern. In this study, a situ assay to assess ecotoxicity of mixed pollutants was carried out in three typical sites with different priority contaminations in Guangzhou, China. Chemical analysis of organic pollutants, metals and quinolones in three exposure sites were determined by GC-ECD/MS, ICP-AES and HPLC, as well as, a combination of biomarkers including: ethoxyresorufin O-deethylase (EROD); aminopyrine N-demethylase (APND); erythromycin N-demethylase (ERND); glutathione S-transferase (GST); malondialdehyde (MDA); CYP1A; and P-glycoprotein (P-gp) mRNA expressions were evaluated in Mugilogobius abei. Results of chemical analysis in sediment samples revealed that the dominant chemicals were organic pollutants and heavy metals in Huadi River while quinolones in the pond. Bioassays indicated that differences among sites were in relation to some specific biomarkers. EROD and GST activities significantly increased after 72 h in situ exposure, but no difference was observed among the exposure sites. APND, ERND and MDA exhibited dissimilar change patterns for different priority pollutants. CYP1A and P-gp mRNA expressions were significantly induced at all exposure sites, whilst P-gp activity was typical for S2 with the highest levels of quinolones. The molecular biomarkers seemed to be more susceptible than enzyme activities. These assays confirmed the usefulness of applying a large array of various combined biomarkers at different levels, in assessing the toxic effects of mixed pollutants in a natural aquatic environment.

Identification of a CYP3A form (CYP3A126) in fathead minnow (Pimephales promelas) and characterisation of putative CYP3A enzyme activity

Cytochrome P450-dependent monooxygenases (CYPs) are involved in the metabolic defence against xenobiotics. Human CYP3A enzymes metabolise about 50% of all pharmaceuticals in use today. Induction of CYPs and associated xenobiotic metabolism occurs also in fish and may serve as a useful tool for biomonitoring of environmental contamination. In this study we report on the cloning of a CYP3A family gene from fathead minnows (Pimephales promelas), which has been designated as CYP3A126 by the P450 nomenclature committee (GenBank no. EU332792). The cDNA was isolated, identified and characterised by extended inverse polymerase chain reaction (PCR), an alternative to the commonly used method of rapid amplification of cDNA ends. In a fathead minnow cell line we identified a full-length cDNA sequence (1,863 base pairs (bp)) consisting of a 1,536 bp open reading frame encoding a 512 amino acid protein. Genomic analysis of the identified CYP3A isoenzyme revealed a DNA sequence consisting of 13 exons and 12 introns. CYP3A126 is also expressed in fathead minnow liver as demonstrated by reverse transcription PCR. Exposure of fathead minnow (FHM) cells with the CYP3A inducer rifampicin leads to dose-dependent increase in putative CYP3A enzyme activity. In contrast, inhibitory effects of diazepam treatment were observed on putative CYP3A enzyme activity and additionally on CYP3A126 mRNA expression. This indicates that CYP3A is active in FHM cells and that CYP3A126 is at least in part responsible for this CYP3A activity. Further investigations will show whether CYP3A126 is involved in the metabolism of environmental chemicals.

Effects of atrazine on cytochrome P450 enzymes of zebrafish (Danio rerio)

In this study, the effects of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) in males and females of adult zebrafish (Danio rerio) were studied. The liver microsomal cytochrome P450 content, NADPH-P450 reductase, aminopyrine N-demethylase (APND), and erythromycin N-demethylase (ERND) activity were measured. Zebrafish were exposed to control and 3 treatments (0.01, 0.1, and 1 mg L(-1)) of atrazine for 5, 10, 15, 20, and 25 days. The results indicated that, within the range of test atrazine concentrations, either P450 content or P450 isozyme activities could be induced by atrazine. Compared to controls, P450 content was significantly increased at all atrazine concentrations at days 10, 15, and 20; thereafter, at day 25, all concentrations decreased to approximately the control levels, both in males and females. In addition, the strongest induction of P450 content was observed on day 15 in males and day 10 in females at treatment concentrations of 1 mg L(-1). NADPH-P450 reductase activities showed mild increase in males; however, the females exhibited significant induction on days 15, 20, and 25; especially, at concentrations of 0.01 mg L(-1), the induction level was consistently increased during the experiment. The inducements of APND and ERND in males were mainly observed on the days 5, 10, and 15, which showed less distinct induction, while significant induction was observed in cases of treatments during all days in females. In conclusion, atrazine induces P450 enzymes in zebrafish, and the effects may function as significant toxicity mechanisms in zebrafish. Additionally, it also confirms the importance of using a combined multi-time and multi-index diagnostic method to enhance the sensitivity and effectiveness of the indices adopted.

Effects of synthetic polycyclic musks on estrogen receptor, vitellogenin, pregnane X receptor, and cytochrome P450 3A gene expression in the livers of male medaka (Oryzias latipes)

This study demonstrates the effects of synthetic polycyclic musks such as Galaxolide (HHCB), Tonalide (AHTN), Traseolide (ATII), Celestolide (ADBI), Phantolide (AHMI) and Cashmeran (DPMI), both on the early life stage and on gene expression in the livers of male medaka (Oryzias latipes). The toxicity ranking (the 96-h median lethal concentration) of the chemicals tested on 24-h-old medaka larvae descended in the order HHCB (0.95 mg/L)=ATII (0.95 mg/L)>AHTN (1.0 mg/L)>AHMI (1.2 mg/L)>ADBI (2.0 mg/L)>>DPMI (12 mg/L), indicating high acute toxicity of these compounds on the early life stages of medaka. Expression analysis of hepatic vitellogenin (VTG) protein showed potential estrogenic effects upon the addition of AHTN and HHCB, indicative of the induction of VTG synthesis in the livers of male medaka. We also investigated mRNA expression levels of two estrogen receptor (ER) subtypes (ERalpha and beta), two VTGs (VTG I and II), pregnane X receptor (PXR), and two cytochromes P450 (CYP) 3As (CYP3A38 and 3A40) in the livers of male medaka treated with AHTN and HHCB at 5, 50 and 500 microg/L. Quantitative real-time PCR analyses revealed that hepatic ERalpha, VTG I, VTG II, and CYP3A40 mRNA responded to 500 microg/L of AHTN and/or HHCB after 3 days exposure, whereas no effects of these compounds on ERbeta, PXR, and CYP3A38 mRNA transcription were observed. These results suggest that certain polycyclic musks, including AHTN and HHCB, induce the expression levels of hepatic ERalpha and VTG mRNA/protein and modulate expression levels of CYP3A40 mRNA in the livers of male medaka.

Effects of mammalian CYP3A inducers on CYP3A-related enzyme activities in grass carp (Ctenopharyngodon idellus): Possible implications for the establishment of a fish CYP3A induction model

Unexpected drug-drug interactions in fish are generally associated with the induction of CYP3A activity and may lead to the formation of drug residues and thus threaten the safety of fishery products. However, little information is available about CYP3A induction in fish. In the present study, we determined the in vivo and in vitro effects of typical mammalian CYP3A inducers (rifampicin, phenobarbital and dexamethasone) on CYP3A-related enzyme activities in a freshwater teleost, the grass carp (Ctenopharyngodon idellus). Our results showed that the response to rifampicin was similar for grass carp liver cell line (GCL), liver microsomes and the primary hepatocytes of grass carp, as indicated by the activity of aminopyrine N-demethylase (APND). When erythromycin N-demethylase (ERND) and 6beta-testosterone hydroxylase (6beta-TOH) were taken into consideration, the GCL displayed a greater capacity for conducting CYP3A metabolism and induction than the C. idellus kidney cell line (CIK). Using erythromycin and testosterone as substrates, we demonstrated that CYP3A catalysis exhibited non-Michaelis-Menten kinetics in GCL cells, and that V(max)/K(m) values were significantly increased due to rifampicin-treatment. Overall, this study may have implications for the use of GCL as a CYP3A induction model to identify physiological changes in fish as well as the similarities or differences between fish and mammals.

Cloning, tissue expression, and inducibility of CYP 3A79 from sea bass (Dicentrarchus labrax)

Multiple members of the CYP3A subfamily have been identified and intensively studied in mammals as they represent prominent CYP enzymes involved in drug metabolism. Also in fish, some CYP3A genes have been identified by cDNA cloning and immunological techniques, but relatively little is known about their function, distribution, and inducibility. In this study, a novel CYP3A, designated as CYP3A79 was isolated from adult male sea bass, an economically valuable species in fisheries. The sea bass CYP3A79 that was cloned contained an open-reading frame of 1512 bp that encoded a 504 amino acid protein and shared a high-sequence identity with medaka, killifish, and trout CYP3As. Interestingly, CYP3A79 also shares five of six substrate recognition sites (SRS) with the SRS of other fish CYP3As, suggesting an evolutionary conservation of the function of these enzymes. In this fish, we also investigated the expression of CYP3A79 and its susceptibility to induction by various compounds including clotrimazole and dehydroepiandrosterone, two strong ligands of zebrafish PXR. The expression of CYP3A79 mRNA was detected by RT-PCR only in the intestine and liver. The immunoblot analysis by antitrout CYP3A27 confirmed the presence of a CYP3A-like protein in the microsomes of these tissues, but, in addition, a immunoreactive protein with this antibody was also observed in the heart microsomes, suggesting the presence of other CYP3A isoforms in this fish. Accordingly, the southern blot analysis of genomic DNA indicated that multiple CYP 3As may be present in sea bass. All attempts to induce 6beta-testosterone hydroxylase, as a marker of CYP3A79, by dexametasone, 17beta-estradiol, pregnenolone 16alpha-carbonitrile, corticosterone, clotrimazole, and dehydroepiandrosterone failed. On the contrary, the administration of 17beta-estradiol, pregnenolone 16alpha-carbonitrile, and corticosterone strongly inhibited this activity and, in parallel, reduced the expression of CYP3A79 transcript. Thus, the sea bass CYP3A79 appears to be resistant to induction, suggesting that this enzyme and likely other CYP3As are regulated differently compared to those of mammals.

The physiology and toxicology of salmonid eggs and larvae in relation to water quality criteria

The purpose of this review is to collate physiological knowledge on salmonid eggs and larvae in relation to water quality criteria. Salmonid genera reviewed include Coregonus, Thymallus, Salvelinus, Salmo, and Oncorhynchus spp. When physiological data for salmonids are lacking, the zebrafish and medaka models are included. The primary focus is on the underlying mechanisms involved in the hydro-mineral, thermal, and respiratory biology with an extended section on the xenobiotic toxicology of the early stages. Past and present data reveal that the eggs of salmonids are among the largest shed by any broadcast spawning teleost. Once ovulated, the physicochemical properties of the ovarian fluid provide temporary protection from external perturbations and maintain the eggs in good physiological condition until spawning. Following fertilisation and during early development the major structures protecting the embryo from poor water quality are the vitelline membrane, the enveloping layer and the chorion. The vitelline membrane is one of the least permeable membranes known, while the semi-permeable chorion provides both physical and chemical defense against metals, pathogens, and xenobiotic chemicals. In part these structures explain the lower sensitivity of the eggs to chemical imbalance compared to the larvae, however the lower metabolic rate and the chronology of gene expression and translational control suggest that developmental competence also plays a decisive role. In addition, maternal effect genes provide a defense potential until the mid-blastula transition. The transition between maternal effect genes and zygotic genes is a critical period for the embryo. The perivitelline fluids are an important trap for cations, but are also the major barrier to diffusion of gases and solutes. Acidic environmental pH interferes with acid-base and hydromineral balance but also increases the risk of aluminium and heavy metal intoxication. These risks are ameliorated somewhat by the presence of ambient humic acid. High temperatures during development may be teratogenic, cause sexual bias, or long-term effects on muscle cellularity. Xenobiotics cause inhibition of neural acetylcholine esterase and carboxylases and disrupt the normal signalling pathways of hormones by binding to relevant receptors and mimicking their actions. A complex suite of genes is activated in response to environmental or parentally transmitted xenobiotics. The primary defense mechanism in embryos involves resistance to uptake but later biotransformation via the aryl hydrocarbon receptor (AHR)-mediated activation of members of the cytochrome mixed-function mono-oxygenase superfamily (CYP1A, CYP2B, and CYP3A) and subsequent glucuronidation or glutathionation. Due to the number of duplicate or triplicate genes coding for intermediates in the signalling pathways, and cross-talk between nuclear orphan receptors and steroid hormone receptors, a large number of complications arise in response to xenobiotic intoxicaton. One such syndrome, known as blue-sac disease causes an anaphylactoid response in hatched larvae due to increased permeability in the vascular endothelium that coincides with AHR-mediated CYP induction. Early embryos also respond to such xenobiotic insults, but apparently have an immature translational control for expression of CYP proteins, which coincides with a lack of excretory organs necessary for the end-point of biotransformation. Other syndromes (M74 and Cayuga) are now associated with thiamine deficiency. Where possible guidelines for water quality criteria are suggested.

Expression and characterization of cytochrome P450 2X1 in channel catfish (Ictalurus punctatus)

Previous studies in channel catfish identified a novel cDNA encoding the cytochrome P450 isoform, CYP2X1. To characterize the substrate specificity of CYP2X1, the 57 kDa protein was expressed in Sf9 cells. Microsomes from Sf9 cells transfected with CYP2X1 demonstrated a maximum carbon monoxide-reduced difference spectrum at 450 nm and catalyzed aminopyrine and benzphetamine demethylase activity with catalytic efficiency (Vmax/Km) values of 0.82 pmol/nmol P450/min and 4.39 pmol/nmol P450/min, respectively. However, enzymatic activity was not observed following incubation with p-nitrophenol, benzyloxyresorufin or pentoxyresorufin. Expression of CYP2X1 transcription was significantly elevated in the gills and liver relative to that detected in brain, kidney and heart. In the brain, liver and heart, intraperitoneal injections with clofibric acid, ethanol, pyridine and rifampin failed to alter expression of CYP2X1 mRNA. In kidney, pyridine significantly suppressed the expression of CYP2X1 transcription (p < or = 0.05). These results indicate CYP2X1 displays minimal catalytic activities consistent with other piscine CYP2 isoforms, and unique tissue expression and regulation patterns in juvenile channel catfish.

Functional characterization of medaka CYP3A38 and CYP3A40: kinetics and catalysis by expression in a recombinant baculovirus system

Phylogenic analysis of the teleost genomic lineages has demonstrated the precedent for multiple genome duplications. Among many of the genes duplicated, cytochrome P450 genes have undergone independent diversification, which can be traced to a single ancestral gene. In teleosts, cytochrome P450s, from all major families, have been identified. Among these, the CYP3A family has been cloned in several teleost species and demonstrated to contain multiple paralogs differing in gene expression patterns and tissue distribution. Herein we characterized the catalytic and kinetic activities of two medaka CYP3A paralogs (CYP3A38 and CYP3A40) with benzyloxyresorufin (BFC), a fluorescent 3A-selective substrate, and testosterone, a known metabolic substrate for CYP3A enzymes. Recombinant CYP3A was produced using the baculovirus expression vector system in Spodoptera frugiperda (Sf9) and Trichoplusia ni (Tn5) insect cells and accounted for up to 24% of total cellular protein. Following addition of a heme-albumin conjugate to log phase cells, spectral P450 content reached a maximum of 560 and 2350 pmol/mg microsomal protein for CYP3A38 and CYP3A40, respectively. Incubations containing recombinant CYP3A, human NADPH-cytochrome P-450 oxidoreductase reductase, human cytochrome b5, and a NADPH generation system catalyzed the dealkylation of BFC and hydroxylation of testosterone with a high degree of stereoselectivity. However, efficiencies and specificities were significantly different between the two isoforms. Km and Vmax activities based on BFC-catalysis were 0.116 and 0.363 muM, and 7.95 and 7.77 nmol/min/nmol P450 for CYP3A38 and CYP3A40, respectively. CYP3A38 preferentially catalyzed testosterone hydroxylation at the 6beta-, 2beta- and 16beta-positions with minor hydroxylation at other positions within the steroid nucleus. Testosterone catalysis with CYP3A40 was limited predominantly to the 6beta- and 2beta-positions. Putative identification of CYP3A substrate recognition sites (SRS) 1-6 indicates that 12 of the 49 amino acid differences between CYP3A38 and CYP3A40 OFRs occur in SRS regions previously known to be associated with steroid hydroxylation. We suggest that differences in kinetics and catalytic activities are a result of amino acid substitutions in SRS regions 1, 3 and 5 within the CYP3A38 and CYP3A40 protein sequence.

CYP2K6 from zebrafish (Danio rerio): cloning, mapping, developmental/tissue expression, and aflatoxin B1 activation by baculovirus expressed enzyme

A full-length zebrafish (Danio rerio) cytochrome P450 (CYP) 2K6 cDNA, was obtained (GenBank accession No. AF283813) through polymerase chain reaction cloning using degenerated primers based on a consensus CYP2 sequence and the heme-binding domain. This first CYP2K family member cloned from zebrafish had 1861 bp which contained 27 bp of 5'-untranslated region (5'-UTR), an open reading frame (ORF) of 1518 bp, and a 300 bp 3'-UTR with a poly A tail. The deduced 506 amino acid sequence of CYP2K6 had 63%, 62% and 59% identity with rainbow trout CYP2K1, CYP2K4 and CYP2K3, respectively; and 45%, 42%, and 42% identity with rabbit CYP2C1, human CYP2C19 and mouse CYP2C39, respectively. CYP2K6 mapped to 107.49cR on LG3 using the LN54 radiation hybrid panel. Its mRNA was detected at 5 days post-fertilization and in the adult liver and ovary among nine tissues examined. The ORF, including the 27 bp of the 5'-UTR, was cloned into pFastBac donor vector and then transferred into the baculovirus genome (bacmid DNA) in DH10Bac competent cells. The recombinant bacmid DNA was used to infect Spodoptera frugiperda insect cells to express the CYP2K6 protein (Bv-2K6). As its ortholog, rainbow trout Bv-2K1 [Yang, Y.H., Miranda, C.L., Henderson, M.C., Wang-Buhler, J.-L., Buhler, D.R., 2000. Heterologous expression of CYP2K1 and identification of the expressed protein (Bv-2K1) as lauric acid (omega-1)-hydroxylase and aflatoxin B1 exo-epoxidase. Drug Metab. Disp. 28,1279-83.], Bv-2K6 also catalyzed the conversion of aflatoxin B1 (AFB1) to its exo-8,9-epoxide as assessed by the trapping of a glutathione (GSH) adduct in the presence of a specific mouse alpha class glutathione S-transferase. The identity of the AFB1-GSH adduct was verified by liquid chromatography-mass spectrometry (LC-MS) and mass spectrometry-mass spectrometry (MS-MS) analysis. Although rainbow trout Bv-2K1 was capable of oxidizing lauric acid, zebrafish Bv-2K6 protein showed no activity against this substrate.