The metabolism of benzo(a)pyrene by English sole (Parophrys vetulus): comparison between isolated hepatocytes in vitro and liver in vivo

Exposure of Polycyclic Aromatic Hydrocarbons (PAHs) and Crude Oil to Atlantic Haddock (Melanogrammus aeglefinus): A Unique Snapshot of the Mercapturic Acid Pathway

Fish exposed to xenobiotics like petroleum-derived polycyclic aromatic hydrocarbons (PAHs) will immediately initiate detoxification systems through effective biotransformation reactions. Yet, there is a discrepancy between recognized metabolic pathways and the actual metabolites detected in fish following PAH exposure like oil pollution. To deepen our understanding of PAH detoxification, we conducted experiments exposing Atlantic haddock (Melanogrammus aeglefinus) to individual PAHs or complex oil mixtures. Bile extracts, analyzed by using an ion mobility quadrupole time-of-flight mass spectrometer, revealed novel metabolites associated with the mercapturic acid pathway. A dominant spectral feature recognized as PAH thiols set the basis for a screening strategy targeting (i) glutathione-, (ii) cysteinylglycine-, (iii) cysteine-, and (iv) mercapturic acid S-conjugates. Based on controlled single-exposure experiments, we constructed an interactive library of 33 metabolites originating from 8 PAHs (anthracene, phenanthrene, 1-methylphenanthrene, 1,4-dimethylphenanthrene, chrysene, benz[a]anthracene, benzo[a]pyrene, and dibenz[a,h]anthracene). By incorporation of the library in the analysis of samples from crude oil exposed fish, PAHs conjugated with glutathione and cysteinylglycine were uncovered. This qualitative study offers an exclusive glimpse into the rarely acknowledged mercapturic acid detoxification pathway in fish. Furthermore, this furnishes evidence that this metabolic pathway also succeeds for PAHs in complex pollution sources, a notable discovery not previously reported.

Responses of juvenile fathead minnow (Pimephales promelas) gut microbiome to a chronic dietary exposure of benzo[a]pyrene

The microbiome has been described as an additional host "organ" with well-established beneficial roles. However, the effects of exposures to chemicals on both structure and function of the gut microbiome of fishes are understudied. To determine effects of benzo[a]pyrene (BaP), a model persistent organic pollutant, on structural shifts of gut microbiome in juvenile fathead minnows (Pimephales promelas), fish were exposed ad libitum in the diet to concentrations of 1, 10, 100, or 1000 μg BaP g^-1 food, in addition to a vehicle control, for two weeks. To determine the link between exposure to BaP and changes in the microbial community, concentrations of metabolites of BaP were measured in fish bile and 16S rRNA amplicon sequencing was used to evaluate the microbiome. Exposure to BaP only reduced alpha-diversity at the greatest exposure concentrations. However, it did alter community composition assessed as differential abundance of taxa and reduced network complexity of the microbial community in all exposure groups. Results presented here illustrate that environmentally-relevant concentrations of BaP can alter the diversity of the gut microbiome and community network connectivity.

In vitro metabolism of pesticides and industrial chemicals in fish

Metabolism is one of the most important factors in controlling the toxicity and bioaccumulation of pesticides in fish. In vitro systems using subcellular fractions, cell lines, hepatocytes and tissues of a specific organ, each of which is characterized by usability, enzyme activity and chemical transport via membrane, have been applied to investigate the metabolic profiles of pesticides. Not only species and organs but also the fishkeeping conditions are known to greatly affect the in vitro metabolism of pesticides. A comparison of the metabolic profiles of pesticides and industrial chemicals taken under similar conditions has shown that in vitro systems using a subcellular S9 fraction and hepatocytes qualitatively reproduce many in vivo metabolic reactions. More investigation of these in vitro systems for pesticides is necessary to verify their applicability to the estimation of pesticide metabolism in fish.

Applicability of in vitro methods in evaluating the biotransformation of polycyclic aromatic hydrocarbons (PAHs) in fish: Advances and challenges

The biotransformation of polycyclic aromatic hydrocarbons (PAHs) and the biochemical mechanisms involved in such process continue to be intensively studied in the fields of environmental science and toxicology. The investigation of PAH biotransformation in fish is fundamental to understand how piscine species cope with PAH exposure, as these compounds are ubiquitous in aquatic ecosystems and impact different levels of biological organization. New approaches are continuously developed in the field of ecotoxicology, allowing live animal testing to be combined with and, in some cases, replaced with novel in vitro systems. Many in vitro techniques have been developed and effectively applied in the investigation of the biochemical pathways driving the biotransformation of PAH in fish. In vitro experimentation has been fundamental in the advancement of not only understanding PAH-mediated toxicity, but also in highlighting suitable cell-based models for such investigations. Therefore, the present review highlights the value and applicability of in vitro systems for PAH biotransformation studies, and provides up-to-date information on the use of in vitro fish models in the evaluation of PAH biotransformation, common biomarkers, and challenges encountered when developing and applying such systems.

Estimating the Bioconcentration Factors of Hydrophobic Organic Compounds from Biotransformation Rates Using Rainbow Trout Hepatocytes

Determining the biotransformation potential of commercial chemicals is critical for estimating their persistence in the aquatic environment. In vitro systems are becoming increasingly important as screening methods for assessing the potential for chemical metabolism. Depletion rate constants (k_d) for several organic chemicals with high octanol-water partition coefficient (K_ow) values (9-methylanthracene, benzo(a)pyrene, chrysene, and PCB-153) in rainbow trout hepatocytes were determined to estimate biotransformation rate constants (k_MET) that were used in fish bioconcentration factor (BCF) models. Benzo[a]pyrene was rapidly biotransformed when incubated singly; however, its depletion rate constant (k_d) declined 79% in a mixture of all four chemicals. Chrysene also exhibited significant biotransformation and its depletion rate constant declined by 50% in the mixture incubation. These data indicate that biotransformation rates determined using single chemicals may overestimate metabolism in environments containing chemical mixtures. Incubations with varying cell concentrations were used to determine whether cell concentration affected k_d estimates. No statistically significant change in depletion rate constants were seen, possibly due to an increase in nonspecific binding of hydrophobic chemicals as cell density increased, decreasing overall biotransformation. A new model was used to estimate BCFs from k_MET values calculated from empirically derived k_d values. The inclusion of k_MET in models resulted in significantly lower BCF values (compared k_MET = 0). Modelled BCF values were consistent with empirically derived BCF values from the literature.

Hepatocytes as in vitro test system to investigate metabolite patterns of pesticides in farmed rainbow trout and common carp: Comparison between in vivo and in vitro and across species

In vitro tools using isolated primary fish hepatocytes have been proposed as a useful model to study the hepatic metabolism of xenobiotics in fish. In order to evaluate the potential of in vitro fish hepatocyte assays to provide information on in vivo metabolite patterns of pesticides in farmed fish, the present study addressed the following questions: Are in vitro and in vivo metabolite patterns comparable? Are species specific differences of metabolite patterns in vivo reflected in vitro? Are metabolite patterns obtained from cryopreserved hepatocytes comparable to those from freshly isolated cells? Rainbow trout and common carp were dosed orally with feed containing the pesticide methoxychlor (MXC) for 14days. In parallel, in vitro incubations using suspensions of freshly isolated or cryopreserved primary hepatocytes obtained from both species were performed. In vivo and in vitro samples were analyzed by thin-layer chromatography with authentic standards supported by HPLC-MS. Comparable metabolite patterns from a qualitative perspective were observed in liver in vivo and in hepatocyte suspensions in vitro. Species specific differences of MXC metabolite patterns observed between rainbow trout and common carp in vivo were well reflected by experiments with hepatocytes in vitro. Finally, cryopreserved hepatocytes produced comparable metabolite patterns to freshly isolated cells. The results of this study indicate that the in vitro hepatocyte assay could be used to identify metabolite patterns of pesticides in farmed fish and could thus serve as a valuable tool to support in vivo studies as required for pesticides approval according to the EU regulation 1107.

Concentration dependence of biotransformation in fish liver S9: Optimizing substrate concentrations to estimate hepatic clearance for bioaccumulation assessment

In vitro bioassays to estimate biotransformation rate constants of contaminants in fish are currently being investigated to improve bioaccumulation assessments of hydrophobic contaminants. The present study investigates the relationship between chemical substrate concentration and in vitro biotransformation rate of 4 environmental contaminants (9-methylanthracene, pyrene, chrysene, and benzo[a]pyrene) in rainbow trout (Oncorhynchus mykiss) liver S9 fractions and methods to determine maximum first-order biotransformation rate constants. Substrate depletion experiments using a series of initial substrate concentrations showed that in vitro biotransformation rates exhibit strong concentration dependence, consistent with a Michaelis-Menten kinetic model. The results indicate that depletion rate constants measured at initial substrate concentrations of 1 μM (a current convention) could underestimate the in vitro biotransformation potential and may cause bioconcentration factors to be overestimated if in vitro biotransformation rates are used to assess bioconcentration factors in fish. Depletion rate constants measured using thin-film sorbent dosing experiments were not statistically different from the maximum depletion rate constants derived using a series of solvent delivery-based depletion experiments for 3 of the 4 test chemicals. Multiple solvent delivery-based depletion experiments at a range of initial concentrations are recommended for determining the concentration dependence of in vitro biotransformation rates in fish liver fractions, whereas a single sorbent phase dosing experiment may be able to provide reasonable approximations of maximum depletion rates of very hydrophobic substances.

Potency of polycyclic aromatic hydrocarbons (PAHs) for induction of ethoxyresorufin-O-deethylase (EROD) activity in hepatocyte cultures from chicken, Pekin duck, and greater scaup

The potency of tetrachlorodibenzo-p-dioxin (TCDD) and 18 polycyclic aromatic hydrocarbons (PAHs) for induction of ethoxyresorufin-O-deethylase (EROD) activity was assessed in primary hepatocyte cultures prepared from chicken (Gallus domesticus), Pekin duck (Anas platyrhynchos domesticus), and greater scaup (Aythya marila). TCDD and 8 of the PAHs induced EROD activity in a concentration-dependent manner. Seven of these were previously shown to be acutely toxic to avian embryos, while the 10 congeners that did not produce an EROD response caused limited mortality. The rank order potency of the EROD-active congeners in all three species was as follows: TCDD>dibenz[ah]anthracene>benzo[k]fluoranthene>indeno[1,2,3-cd]pyrene>benzo[a]pyrene>chrysene≈benz[a]anthracene≈benz[ghi]perylene>benzo[b]naphtho[2,3-d]thiophene. Chicken hepatoctyes were more sensitive than duck hepatocytes to EROD induction by all test compounds, but the gap in species sensitivity was 100-fold for TCDD, and generally ≤10-fold for PAHs. This study is the first to use in vitro methods to rank the AHR-mediated potency of PAHs in birds. These data may be useful for assessing risks associated with exposure to PAHs in the environment.

Gill cell culture systems as models for aquatic environmental monitoring

A vast number of chemicals require environmental safety assessments for market authorisation. To ensure acceptable water quality, effluents and natural waters are monitored for their potential harmful effects. Tests for market authorisation and environmental monitoring usually involve the use of large numbers of organisms and, for ethical, cost and logistic reasons, there is a drive to develop alternative methods that can predict toxicity to fish without the need to expose any animals. There is therefore a great interest in the potential to use cultured fish cells in chemical toxicity testing. This review summarises the advances made in the area and focuses in particular on a system of cultured fish gill cells grown into an epithelium that permits direct treatment with water samples.

Dynamic passive dosing for studying the biotransformation of hydrophobic organic chemicals: microbial degradation as an example

Biotransformation plays a key role in hydrophobic organic compound (HOC) fate, and understanding kinetics as a function of (bio)availability is critical for elucidating persistence, accumulation, and toxicity. Biotransformation mainly occurs in an aqueous environment, posing technical challenges for producing kinetic data because of low HOC solubilities and sorptive losses. To overcome these, a new experimental approach based on passive dosing is presented. This avoids using cosolvent for introducing the HOC substrate, buffers substrate depletion so biotransformation is measured within a narrow and defined dissolved concentration range, and enables high compound turnover even at low concentrations to simplify end point measurement. As a case study, the biodegradation kinetics of two model HOCs by the bacterium Sphingomonas paucimobilis EPA505 were measured at defined dissolved concentrations ranging over 4 orders of magnitude, from 0.017 to 658 μg L(-1) for phenanthrene and from 0.006 to 90.0 μg L(-1) for fluoranthene. Both compounds had similar mineralization fluxes, and these increased by 2 orders of magnitude with increasing dissolved concentrations. First-order mineralization rate constants were also similar for both PAHs, but decreased by around 2 orders of magnitude with increasing dissolved concentrations. Dynamic passive dosing is a useful tool for measuring biotransformation kinetics at realistically low and defined dissolved HOC concentrations.

Energy intake affects the biotransformation rate, scope for induction, and metabolite profile of benzo[a]pyrene in rainbow trout

The metabolic conversion of benzo[a]pyrene (B[a]P) by rainbow trout (Oncorhynchus mykiss) hepatocytes was not significantly different between any group of fed fish (fed one of three isoenergetic diets that varied in protein and lipid content at full satiation levels or half rations), however at 12 weeks, fasted fish exhibited significantly reduced B[a]P biotransformation rates (by 58%). Alterations in metabolite profiles were also seen: fasted fish produced significantly more Phase I metabolites, higher levels of both glucuronide and sulphate conjugates, and lower levels of presumptive glutathione conjugates, compared to fed fish. When fish were fasted, higher proportions of phenols were produced, with lower proportions of quinones, triols and tetrols. Inducing metabolism (using beta-naphthoflavone) increased metabolic scope for B[a]P by 2-fold, regardless of each diet's baseline metabolic rate. However, the balance between Phase I and II reactions was altered with induction and fasting: higher proportions of Phase I metabolites were found, with lower glutathione conjugates and higher proportions of triols/tetrols. Fasting-mediated reductions in glutathione conjugation, and increased induction of oxidation vs. conjugating enzymes, can explain altered metabolite profiles. These results suggest that in contaminated habitats, where pollution-induced reductions in food quantity or quality are combined with the presence of toxic compounds and inducers, detoxification rates can be diminished.

A compilation of in vitro rate and affinity values for xenobiotic biotransformation in fish, measured under physiological conditions

Scientific literature from the past 25 years was searched to obtain in vitro biotransformation rate and affinity data for fish. To maximize the environmental relevance of this dataset, we focused on studies conducted at multiple substrate concentrations, and established acceptance criteria with respect to assay temperature and pH. Altogether, enzyme rate and affinity parameters are provided for 43 species and 77 compounds. In all but three instances, the reported reactions exhibited saturation at high substrate concentrations and could be used to calculate Michaelis-Menten rate (Vmax) and affinity (Km) constants. Most of this information was obtained using in vitro systems derived from liver tissue. Information from non-hepatic tissues was included, however, to provide a basis for comparisons among tissues. Where possible, in vitro enzyme parameters were examined to compare: (1) hepatic metabolism of a common substrate within a species, (2) hepatic metabolism of common substrates by different species, and (3) metabolism of a common substrate by different tissues of one species. Comparisons within species highlight a number of factors that may substantially influence xenobiotic metabolism in fish including gender, life stage, and acclimation temperature. Limited data suggest that Vmax and Km for the same reaction may vary by up to three orders of magnitude among species.

Establishment and validation of primary hepatocytes of the African sharptooth catfish (Clarias gariepinus)

In vitro systems such as primary cells and continuous cell lines are gaining momentum in ecotoxicological studies. Cytotoxicity tests with fish cells as well as tests using specific endpoints such as CYP1A induction are valuable in the toxicity assessment of environmental samples. The main objective of this study was to establish and validate the use of primary hepatocytes from the African sharptooth catfish (Clarias gariepinus) as an in vitro toxicity monitoring system. The successful isolation of primary hepatocytes from the sharptooth catfish was achieved using an in situ perfusion method. The primary hepatocytes responded to CYP1A induction, while a continuous Chinese hamster ovary (CHO-K1) cell line showed no activity when exposed to various concentrations of benzo[a]pyrene (B[a]P) (p<0.0001). Cytotoxicity, as measured by the methyl thiazol tetrazolium (MTT) assay, was not observed following a 72 h exposure of the primary hepatocytes and the CHO-K1 cell line to different B[a]P concentrations. However, the hepatocytes were damaged at higher B[a]P concentrations (>10(-6)M) as shown by transmission electron microscopy. This cytotoxicity effect was also confirmed by the trypan blue exclusion assay (TD(50) of 10(-6)M). Differences in the results between the MTT and trypan blue exclusion assays are probably due to mitochondria that are still metabolically active, causing the tetrazolium salt to be dehydrogenated. The internal architecture of normal primary hepatocytes included large quantities of rough endoplasmic reticulum (often in close proximity to the nucleus), mitochondria, aggregates and scattered glycogen, a few lipid droplets and spherical nuclei with distinct nucleoli. The primary catfish hepatocyte cell culture system, expressing CYP1A when exposed to B[a]P, could be used as a biomarker for aromatic hydrocarbon pollutants in aquatic ecosystems of southern and East Africa.

Effects of dietary phytoestrogens in vivo and in vitro in rainbow trout and Siberian sturgeon: interests and limits of the in vitro studies of interspecies differences

A study of the effects of dietary genistein on trout and sturgeon in vivo showed that sturgeon was sensitive to 20 ppm of genistein, whereas trout was not. To analyze the origin of this interspecies difference in sensitivity, a cell culture technique was developed with hepatocytes from sturgeon and compared to results obtained with hepatocytes from trout in the same system. The hepatocyte culture proved to be useful as bioassay for estrogenicity. Vitellogenin (VTG), assayed by a specific enzyme-linked immunosorbent assay, was used as a biomarker of the estrogenic activity. 17 beta-Estradiol, its glucuronide and sulfate derivatives, and estradiol analogues (ethynylestradiol and diethylstilbestrol) were tested. Nonestrogenic compounds such as androgens, progesterone, and cortisol were tested as negative controls. VTG production was monitored at doses ranging from 1 nM to 10 microM estradiol. Phytoestrogens, from the isoflavone family, were tested individually at increasing doses exhibiting dose response curves for concentrations from 500 nM to 10 microM. With tamoxifen, an antagonist of estrogen receptors, the estrogenic effect was partially reduced. The effect was the same with ICI182,780 in sturgeon, whereas the effect was the opposite in trout. The estrogenic potency of the isoflavones ranged differently between the two species in the following order: biochanin A < daidzein = formononetin < genistein < equol in trout and biochanin A < genistein < daidzein < formononetin < equol in sturgeon. Further, in sturgeon, formononetin was the most potent phytoestrogen in vitro, whereas its activity was weakest in vivo. These data suggest that one must reconsider the relevance of heterologous estrogenic tests and of homologous in vitro tests for estrogenic potency of chemicals.

Metabolic activity in primary cultures of fish hepatocytes

In aquatic toxicology, isolated liver cells from fish can be used as a tool to generate initial information on the hepatic metabolism of xenobiotics, and on the mechanisms of xenobiotic activation or deactivation. This isolation of teleost liver cells is achieved by enzymic dissociation, and monolayer cultures of fish hepatocytes in serum-free medium maintain good viability for 3-8 days. During in vitro culture, fish liver cells express stable levels of phase I and phase II enzymes, such as cytochrome P4501A or glutathione S-transferase, and the cells show an induction of biotransformation enzymes after exposure to xenobiotics. The xenobiotic metabolite pattern produced by fish hepatocytes in vitro is generally similar to that observed in vivo. Limitations to more-intensive application of cultured fish hepatocytes as a screen in aquatic hazard assessment are partly due to the rather limited scope of existing studies, i.e. the focus on one particular species (rainbow trout), and on one particular biotransformation enzyme (cytochrome P4501A), as well as a lack of comparative in vitro/in vivo studies.

Sex-related responses to oxidative stress in primary cultured hepatocytes of European flounder (Platichthys flesus L.)

Effects of oxidative stress induced by xenobiotic compounds were studied in primary cultures of isolated hepatocytes of immature European flounder (Platichthys flesus L.) of both sexes caught in a relatively unpolluted area of the German Bight (North Sea). Cells were exposed to oxidative stressors such as 100 microM hydrogen peroxide (H2O2), 100 microM benzo[a]pyrene (B[a]p) and 50 microM nitrofurantoin (N-(5-nitro-2-furfurylidene)-1-aminohydantoin; NF) for 2 and 24 h. Cell mortality was determined with the use of the fluorescent ethidium homodimer-1 and calcein. Oxidative stress response was assessed by quantitative analysis of (1) intracellular reactive oxygen species (ROS) formation with dihydrorhodamine 123, (2) lipid peroxidation on the basis of concentrations of lipid hydroperoxides and the lipid peroxidation products malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) and (3) cellular total oxidant-scavenging capacity (TOSC) using the TOSC assay (Winston et al., 1998). An increase in ROS formation was detected as early as 2 h after exposure to H2O2, B[a]p and NF. After 24 h, stress responses were lower, except following exposure to NF. The pattern of responses differed with the different oxidative stressors. Lipid peroxidation and the capacity to scavenge ROS were increased significantly in both sexes only after exposure to H2O2, whereas B[a]p and NF provoked sex-dependent responses. B[a]p-induced lipid peroxidation and increase in scavenging capacity were observed only in hepatocytes of females, whereas NF initiated these responses only in cells of males. Sex differences in oxidative stress response only after exposure to pro-oxidants that require enzymatic activation infer the importance of biotransformation pathways in stress responses. Because of their sensitivity to oxidative stress, flounder hepatocytes provide a useful model for early risk assessment of xenobiotics.

DNA adducts in liver and leukocytes of flounder (Platichthys flesus) experimentally exposed to benzo

In the present study the levels of hydrophobic DNA adducts detected by 32P-postlabelling were followed in liver and leukocytes of flounder (Platichthys flesus) over 10 days following single i.p. injections of two doses of BaP (10 and 50 mg kg(-1) fish weight, respectively). DNA adducts were detected in both tissues of exposed fish 2 days post injection and continued to rise on day 5 and day 10. In flounder exposed to the lower dose of BaP, the levels of hepatic DNA adducts reached higher values on the fifth day compared with flounder exposed to the higher dose. However, at the end of the experiment, the DNA adduct level was again higher in fish from the high dose group compared with the low dose group. There was no substantial increase of DNA adducts in liver of flounder from the low dose group after day 5, while the adduct levels in flounder liver from the high dose group increased throughout the experiment. Earlier studies detecting DNA adducts in BaP-exposed flatfish with the 32P-postlabelling technique have reported declining adduct levels from about 2 days after the exposure, regardless of exposure route. In contrast, the results from our study did not confirm a rapid increase and successive decline of hydrophobic adducts in liver of BaP-exposed flounder.

In vitro vitellogenin production by carp (Cyprinus carpio) hepatocytes as a screening method for determining (anti)estrogenic activity of xenobiotics

The yolk protein precursor vitellogenin (Vtg) is secreted by the liver of female as well as male fish, in response to estrogenic compounds. In this study, an in vitro assay was developed for measuring Vtg induction, using cultured primary hepatocytes from genetically uniform strains of carp (Cyprinus carpio). Vtg production was measured by indirect competitive ELISA, using a polyclonal antiserum against goldfish Vtg that cross-reacts with carp Vtg. Vtg was dose-dependently induced by 17beta-estradiol (E2) in hepatocytes of both sexes. E2 had a lowest observed effect concentration (LOEC) for Vtg induction of 2 nM, an EC50 between 50 and 150 nM, and a maximum response at 2 microM. The plasticizer and xenoestrogen bisphenol-A induced Vtg secretion by hepatocytes of both sexes at 50 and 100 microM. This carp hepatocyte (CARP-HEP) assay can also be used to detect antiestrogenic activity, which was measured as the reduction of E2-stimulated Vtg synthesis. Two well-known antiestrogenic compounds, tamoxifen and 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), were tested. TCDD caused a reduction in Vtg synthesis in female hepatocytes at concentrations <0.1 nM, making it approximately 10,000-fold more potent than tamoxifen. Carp hepatocytes were also sensitive to induction of cytochrome P4501A (CYP1A) activity, measured as ethoxyresorufin O-deethylase (EROD). Depending on the exposure time, 18 or 96 h, EROD EC50 values for TCDD were 27 or 6 pM, respectively. The CARP-HEP assay, using the 96-well plate format, offers good possibilities to screen large numbers of compounds for (anti)estrogenic properties. In addition, it can simultaneously determine aryl hydrocarbon receptor agonist properties, measured as CYP1A induction.

Biotransformation of pentachlorophenol, aniline and biphenyl in isolated rainbow trout (Oncorhynchus mykiss) hepatocytes: comparison with in vivo metabolism

1. The biotransformation of pentachlorophenol (PCP), aniline and biphenyl in rainbow trout (Oncorhynchus mykiss) isolated liver cells was investigated to examine if fish hepatocytes represent a suitable alternative to the in vivo approach for studying the biotransformation of chemicals. Each compound was incubated at two concentrations (10 and 60 microM) for 2 h. For comparison, the metabolic profile of these xenobiotics was also studied in urine and bile of trout orally exposed to 1.8-4.0 mg/kg wet wt of each compound. 2. In vitro as in vivo, PCP glucuronide and to a lesser extent PCP sulphate were the metabolites formed by trout from PCP. 3. Aniline was mainly metabolized to acetanilide and to a lesser extent to 2-aminophenol by isolated hepatocytes, but neither hydroxylated acetanilide nor conjugates were found in vitro whereas they were present in bile and urine of trout treated with this chemical. 4. Trout hepatocytes metabolized biphenyl to hydroxylated and dihydroxylated products and the corresponding glucuronides. These results correlated well with the metabolic profile obtained from the bile of trout exposed to this pesticide. 5. It is concluded that although hepatocytes are well suited for several types of biotransformation studies, the fact that this system may in some cases produce a different metabolic pattern than in vivo should be considered when attempting to extrapolate in vitro to in vivo data.

Benzo(a)pyrene toxicokinetics in the cricket following injection into the haemolymph

The metabolic disposition of (14)C-labelled benzo(a)pyrene (BP) in the cricket (Acheta domesticus) was investigated after injection into the haemolymph. (14)C-BP was taken up rapidly by the nerve cord, malpighian tubules, reproductive organs, gut, and muscle:cuticle of the cricket. The elimination half-lives of (14)C-BP in these tissues ranged from 8.9 to 17.8 h. The haemolymph (14)C-BP concentration-time curve could be described by a one-compartment open pharmacokinetic model. (14)C-BP was metabolized by the cricket mainly to unconjugated and conjugated BP metabolites since very little unchanged (14)C-BP was found in the excreta at 48 h post-dosing. GLPC-MSD and HPLC/ES-MS analyses showed the presence of at least two BP metabolites in the excreta. The BP metabolites were identified tenatively as the diol derivatives of benzo(a)pyrene and benzo(a)pyrene quinone.

Xenobiotics and xenoestrogens in fish: modulation of cytochrome P450 and carcinogenesis

As is the case with mammals, an ever-increasing number of cytochromes P450 (CYPs) are being characterized from fish. The focus of work on fish CYPs has been primarily on environmental induction of CYP1A by pollutants such as the polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins and dibenzofurans. This response has been the basis for a sensitive biomonitoring tool of ecosystem health for a number of years. Studies have documented a correlation between CYP1A induction, pollutant levels and tumor incidence, especially in bottom-dwelling species. The rainbow trout has been utilized as a tumor model to document the role of CYP1A modulation in the inhibition or promotion of cancer. Fish are also very responsive to the class of chemicals known as xenoestrogens. Recent evidence is presented documenting the modulation of CYPs by xenoestrogens and their potential role as modulators of the tumor response. In this paper, we summarize the current knowledge concerning the occurrence of CYPs in fish and focus on the role of CYP1A induction in environmental monitoring of various genotoxic carcinogens and in the modulation of cancer in the trout model. Finally, the important class of aquatic pollutants known as xenoestrogens have now been shown to modulate CYP levels perhaps leading to alterations in tumor response or other adverse effects.

32P-postlabeling analysis of DNA adduct formation and persistence in English sole (Pleuronectes vetulus) exposed to benzo[a]pyrene and 7H-dibenzo[c,g]carbazole

The formation and persistence of benzo[a]pyrene (BaP)- and 7H-dibenzo[c,g]-carbazole (DBC)-DNA adducts in liver of English sole (Pleuronectes vetulus) were investigated. BaP is a putative hepatocarcinogen in English sole based on its ability to induce formation of preneoplastic foci, while DBC is a hepatocarcinogen in mammals but whose carcinogenicity in fish is not known. English sole liver was sampled from 2 h through 84 days after a single intermuscular injection of a BaP and DBC mixture (100 mumol of each/kg body wt.), and DNA adduct levels were measured by the nuclease P1 version of the 32P-postlabeling assay. The major BaP adducts detected were from binding of BaP-7,8-diol-9,10-epoxide to DNA, whereas multiple uncharacterized DBC-DNA adducts were detected. Total adduct levels for both BaP and DBC reached a maximum at 2 days post exposure. The levels of DBC-DNA adducts were greater than the levels of BaP adducts at all time points and increased more rapidly than did the levels of BaP-DNA adducts. The DBC to BaP adduct ratio was 33 +/- 8.8 at 2 h and declined to 4.2 +/- 0.48 by 12 h post exposure. From 2 to 28 days, the levels of both BaP and DBC adducts declined with apparent half-lives of 11 and 13 days, respectively. There was no apparent decline from 28 to 84 days in the levels of the remaining BaP or DBC adducts; these persistent adducts represented 32 and 36% of maximum levels, respectively. These results provide the first data on the kinetics of adduct formation and removal of a carcinogenic nitrogen-containing polycyclic aromatic compound in fish. The results showing greater binding and similar persistence of DBC-DNA adducts compared to BaP-DNA adducts suggest that DBC may be hepatotoxic and potentially carcinogenic in English sole. In a separate experiment, the effect of multiple doses of BaP (30 mumol/kg body wt.) on the levels of hepatic BaP-DNA adducts showed that adduct levels increased linearly (r = 0.815, P = 0.0007) with 5 successive doses administered at 2 day-intervals and sampled 2 days after the last dose. The persistence of both BaP-DNA and DBC-DNA adducts in liver, together with the increase in BaP-DNA adducts in English sole exposed to successive doses of BaP, suggest that hepatic xenobiotic-DNA adducts in English sole are molecular dosimeters of relatively longterm environmental exposure to genotoxic polycyclic aromatic compounds.