Differential inhibition of hepatic microsomal alkoxyresorufin O-dealkylation activities by tetrachlorobiphenyls

Visible Light-Driven, Gold(I)-Catalyzed Preparation of Symmetrical (Hetero)biaryls by Homocoupling of Arylazo Sulfones

The preparation of symmetrical (hetero)biaryls via arylazo sulfones has been successfully carried out upon visible light irradiation in the presence of PPh₃AuCl as the catalyst. The present protocol led to the efficient synthesis of a wide range of target compounds in an organic-aqueous solvent under photocatalyst-free conditions.

Interspecies comparison of selected pollution biomarkers in dreissenid spp. inhabiting pristine and moderately polluted sites

Stress biomarkers, which can outline impacts of contaminants in aquatic biota at the biochemical level, are increasingly used as early warning tools in environmental monitoring. Reliable biomarker based assessment schemes, however, request appropriate knowledge of baseline levels of selected endpoints, and the potential influence of a range of natural influencing factors (both abiotic and biotic) as well. In this study, we examined the interspecies variability of various biomarkers (metallothioneins (MT), ethoxyresorufin-O-deethylase activity (EROD), lipid peroxidation (LPO), DNA strand breaks (DNA_sb), vitellogenin-like proteins (Vtg)) in Dreissena polymorpha and Dreissena bugensis inhabiting either pristine- or moderately impacted sites of Lake Balaton (Hungary). Levels of all biomarkers considered revealed low interspecies variability in the two dreissenid species at all sampling sites, with consistently higher (but statistically insignificant) values in Dreissena polymorpha. Levels of all biomarkers varied within the two investigated seasons, with significant influence of the reproduction cycle particularly on the levels of metallothioneins and vitellogenin-like proteins. Each biomarker considered was elevated by October, with significantly higher values in the mussels inhabiting harbours. Insignificant spatial and temporal variability in the general health indicators (condition index, total protein content) of dreissenids was observed, which, in parallel with evident rise in biomarker levels, apparently suggest that the anthropogenic impacts in harbours affect mussel fitness yet at sub organismal level. Our data might serve useful basis for future environmental monitoring surveys, especially in habitats where the progressive replacement of Dreissena polymorpha by Dreissena bugensis is taking place, as the interspecies variability in susceptibility to chemical stress of the two species is well comparable.

Monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 enzymes

This review examines the monooxygenase, peroxidase and peroxygenase properties and reaction mechanisms of cytochrome P450 (CYP) enzymes in bacterial, archaeal and mammalian systems. CYP enzymes catalyze monooxygenation reactions by inserting one oxygen atom from O2 into an enormous number and variety of substrates. The catalytic versatility of CYP stems from its ability to functionalize unactivated carbon-hydrogen (C-H) bonds of substrates through monooxygenation. The oxidative prowess of CYP in catalyzing monooxygenation reactions is attributed primarily to a porphyrin π radical ferryl intermediate known as Compound I (CpdI) (Por•+FeIV=O), or its ferryl radical resonance form (FeIV-O•). CYP-mediated hydroxylations occur via a consensus H atom abstraction/oxygen rebound mechanism involving an initial abstraction by CpdI of a H atom from the substrate, generating a highly-reactive protonated Compound II (CpdII) intermediate (FeIV-OH) and a carbon-centered alkyl radical that rebounds onto the ferryl hydroxyl moiety to yield the hydroxylated substrate. CYP enzymes utilize hydroperoxides, peracids, perborate, percarbonate, periodate, chlorite, iodosobenzene and N-oxides as surrogate oxygen atom donors to oxygenate substrates via the shunt pathway in the absence of NAD(P)H/O2 and reduction-oxidation (redox) auxiliary proteins. It has been difficult to isolate the historically elusive CpdI intermediate in the native NAD(P)H/O2-supported monooxygenase pathway and to determine its precise electronic structure and kinetic and physicochemical properties because of its high reactivity, unstable nature (t½~2 ms) and short life cycle, prompting suggestions for participation in monooxygenation reactions of alternative CYP iron-oxygen intermediates such as the ferric-peroxo anion species (FeIII-OO-), ferric-hydroperoxo species (FeIII-OOH) and FeIII-(H2O2) complex.

Applicability of the TCDD-TEQ approach to predict sublethal embryotoxicity in Fundulus heteroclitus

The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxic equivalent quantity (TCDD-TEQ) approach was used successfully to predict lethal embryotoxicity in salmonids, but its applicability to sublethal effects of mixtures of organohalogenated compounds in other fish species is poorly known. The sublethal toxicity of two dioxin-like compounds (DLCs), 3,3',4,4'-tetrachlorobiphenyl (PCB77) and 2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PnCDF), two non-dioxin-like (NDL) polychlorinated biphenyls (PCBs), 2,2',5,5'-tetrachlorobiphenyl (PCB52) and 2,3,3',4',6-pentachlorobiphenyl (PCB110), and of Aroclor 1254, a complex commercial mixture of PCBs, was assessed in Fundulus heteroclitus embryos exposed by intravitelline injection. At 16 days post-fertilization, the two DLCs and Aroclor 1254 altered prey capture ability in addition to inducing classical aryl hydrocarbon receptor-mediated responses: ethoxyresorufin-O-deethylase (EROD) induction, craniofacial deformities and reduction in body length. None of these responses was induced by the two NDL PCBs, at doses up to 5400 ng g(-1)wet weight. Dose-response curves for prey capture ability for the 2 DLCs tested were not parallel to that of TCDD, violating a fundamental assumption for relative potency (ReP) estimation. Dose-response curves for EROD induction were parallel for 2,3,4,7,8-PnCDF and TCDD, but the ReP of 2,3,4,7,8-PnCDF for F. heteroclitus was 5-fold higher than the World Health Organization (WHO) fish toxic equivalent factor (TEF) based on embryolethality in salmonids. The chemically derived TCDD-TEQs of Aroclor 1254, calculated using 3,3',4,4',5-pentachlorobiphenyl (PCB126) concentrations and it ReP for F. heteroclitus, overestimated its potency to induce EROD activity possibly due to antagonistic interactions among PCBs. This study highlights the limitations of using TEFs based on salmonid toxicity data alone for risk assessment to other fish species. There is a need to assess the variability of RePs of DLCs in different species for a variety of endpoints and to better understand interactions between DLCs and other toxic chemicals.

Factors affecting phase I stereoselective biotransformation of chiral polychlorinated biphenyls by rat cytochrome P-450 2B1 isozyme

In vitro incubations of rat cytochrome P-450 (CYP) 2B1 isozyme with three chiral polychlorinated biphenyl (PCB) congeners (PCBs 45, 95, and 132) were performed to investigate factors affecting phase I stereoselective biotransformation. Rat CYP2B1 preferentially biotransformed the second-eluting atropisomers of PCBs 45 and 95 at low substrate concentration ranges (≤15 μM). Biotransformation competition by different congeners was also observed, with increasing competition at higher chlorination. Competition decreased the biotransformation rates of each congener stereoselectively, affecting atropisomeric composition. No atropisomeric enrichment was observed for PCB 132 upon incubation of the racemate. However, under the same conditions, significant differences in biotransformation kinetics were observed in individual atropisomer incubations, indicating that (+)-PCB 132 and (-)-PCB 132 were competitively biotransformed. Homology modeling and docking studies suggested that each atropisomer had different interactions with rat CYP2B1 and could dock with the isozyme at different locations. This is one possible explanation for stereoselective biotransformation and competition of chiral PCBs at the molecular level. Our results suggest that the lack of predictive capability for stereoselectivity of PCBs and other chiral pollutants in biota may be due to competitive and/or inhibitory activities of different substrates, including individual enantiomers of the same compound.

Comparative oxidative metabolism of BDE-47 and BDE-99 by rat hepatic microsomes

Polybrominated diphenyl ethers (PBDEs) are flame-retardant chemicals that have become ubiquitous environmental pollutants. 2,2',4,4'-Tetrabromodiphenyl ether (BDE-47) and 2,2',4,4',5-pentabromodiphenyl ether (BDE-99) are among the most prevalent PBDEs detected in humans, wildlife, and abiotic environmental matrices. The purpose of this study was to investigate the oxidative metabolism of BDE-47 and BDE-99 in rat hepatic microsomes by comparing metabolite formation rates, kinetic parameters associated with metabolite formation, and the effects of prototypical cytochrome P450 (CYP) inducers. The CYP enzymes involved were also identified. Incubation of BDE-47 with hepatic microsomes from phenobarbital-treated rats generated a total of five hydroxylated (OH-BDE) metabolites, among which 4'-hydroxy-2,2',4,5'-tetrabromodiphenyl ether (4'-OH-BDE-49) and 3-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (3-OH-BDE-47) were the major metabolites, as identified using authentic standards and quantified by liquid chromatography/mass spectrometry. Incubations of BDE-99 with hepatic microsomes from dexamethasone-treated rats produced a total of seven hydroxylated metabolites, among which 4-hydroxy-2,2',3,4',5-pentabromodiphenyl ether (4-OH-BDE-90) and 6'-hydroxy-2,2',4,4',5-pentabromodiphenyl ether (6'-OH-BDE-99) were the major metabolites. Although the overall rate of oxidative metabolism of BDE-99 by hepatic microsomes was greater than that of BDE-47, para-hydroxylation involving a National Institutes of Health shift mechanism represented a major metabolic pathway for both PBDE congeners. Among the rat recombinant CYP enzymes tested, CYP2A2 and CYP3A1 were the most active in BDE-47 and BDE-99 metabolism, respectively. However, CYP1A1 exhibited the highest activity for 4'-OH-BDE-49 and 6'-OH-BDE-99 formation, and CYP3A1 exhibited the highest activity for 3-OH-BDE-47 and 4-OH-BDE-90 formation. Collectively, the results demonstrate that oxidative metabolism of BDE-47 and BDE-99 is mediated by distinct but overlapping sets of CYP enzymes and represents a key process that determines the bioaccumulation of BDE-47 and BDE-99 in mammals.

Contaminant accumulation and multi-biomarker responses in field collected zebra mussels (Dreissena polymorpha) and crayfish (Procambarus clarkii), to evaluate toxicological effects of industrial hazardous dumps in the Ebro river (NE Spain)

Large amounts of industrial waste containing high concentrations of mercury, cadmium and organochlorine residues were dumped in a reservoir adjacent to a chlorine-alkali plant in the village of Flix(Catalonia, Spain), situated at the shore of the lower Ebro river. Effects of these contaminants to aquatic river invertebrates were assessed by integrating analyses of metals and organochlorine residues in field collected zebra mussels and crayfish with a wide range of biomarkers. Biological responses included levels of metallothioneins, activities of ethoxyresorufin-O-deethylase, oxidative stress biomarkers (glutathione content, enzymatic activities of superoxide dismutase, catalase, glutathione s-transferase, glutathione peroxidise and glutathione reductase), levels of lipid peroxidation and of DNA strand breaks. The results obtained evidenced similar response patterns in mussels and crayfish with increasing toxic stress levels from upper parts of the river towards the meander located immediately downstream from the most polluted site, close to the waste dumps. The aforementioned stress levels could be related with concentrations of mercury, cadmium, hexachlorobenzene, polychlorobiphenyls and dichlorodiphenyltrichloroethanes from 4- to 195-fold greater than local background levels. The response of biomarkers to these pollutant concentrations differences was reflected in high activities and levels of antioxidant enzymes, metallothioneins, lipid peroxidation and DNA strand breaks and decreased levels of glutathione.

Hyper- and hypo-induction of cytochrome P450 activities with Aroclor 1254 and 3-methylcholanthrene in Cyp1a2(-/-) mice

The response of hepatic mono-oxygenase activities to Aroclor 1254 or 3-methylcholanthrene was investigated in wild-type and Cyp1a2(-/-) mice. Cytochrome P450 concentrations were similar in naïve Cyp1a2(-/-) and wild-type mice. There was no difference between naïve wild-type and Cyp1a2(-/-) animals in 7-ethoxyresorufin and 7-ethoxy-4-trifluoromethylcoumarin dealkylase activities, nor was the induction response after 3-methylcholanthrene any different between the two genotypes. However, both activities were induced to a higher extent in Cyp1a2(-/-) mice after Aroclor 1254. In contrast, 7-pentoxyresorufin dealkylation activity was lower in Cyp1a2(-/-) mice and this differential was maintained during induction by both agents. 7-Methoxy- and 7-benzoxyresorufin dealkylation activities were also lower than wild-type in naïve Cyp1a2(-/-) animals and during 3-methylcholanthrene induction, but showed accelerated induction in Cyp1a2(-/-) mice with Aroclor 1254. Bufuralol 1'- and testosterone 6beta-hydroxylation activities, and P450 characteristics were evaluated 48h after inducer administration. Bufuralol 1'-hydroxylation, a sexual dimorphic activity (female>male) showed no genotype differences in naïve animals. Activity changes varied across gender and genotype, with 3-methylcholanthrene and Aroclor 1254 inducing in male Cyp1a2(-/-), and Aroclor 1254 inducing in female wild-type. Testosterone 6beta-hydroxylation activity was 16% higher in Cyp1a2(-/-) mice and neither 3-methylcholanthrene nor Aroclor 1254 elicited induction. After Aroclor 1254, a 24% increase in P450 concentration with a hypsochromic shift in the ferrous-CO maximum characteristic of CYP1A enzymes occurred in wild-type, compared to no change in either parameter in Cyp1a2(-/-) mice. Induction changes with 3-methylcholanthrene were greater in wild-type mice, a 60% increase in concentration and approximately 2 nm hypsochromic shift versus a 10% increase and approximately 1nm hypsochromic shift in Cyp1a2(-/-) mice. The study demonstrates that deletion of a single P450 can profoundly affect the induction response, as monitored with activities of other P450s, in a manner unrelated to the contribution of the deleted P450 to the activity.

Western mosquitofish as a bioindicator of exposure to organochlorine compounds

The evaluation of the alochthonous and cosmopolitan mosquitofish species Gambusia affinis suitability as a bioindicator species and the induction of its liver cytochrome P450-dependent mixed function oxygenase (MFO), measured as the 7-ethoxyresorfin-O-deethylase (EROD) activity, as well as changes in DNA integrity, measured by the Fast Micromethod, for the monitoring of organochlorine fresh water pesticide contamination, were the main aims of the study. The test mosquitofish were exposed under laboratory conditions to several doses (0.1, 10 and 100 microg l(-1)) of lindane in experimental basins for up to 7 days, and a subsequent field study was carried out at five natural ponds in the south-western Istrian peninsula, Croatia, where up to 10 fish were collected from each pond. Results obtained during the studies showed positive correlations between the measured biomarkers in G. affinis liver (EROD activity and DNA integrity status) and lindane (laboratory experiment) or persistent organochlorine pollutant amounts in natural pond sediments (field study). The clear dose-responses of EROD activity and DNA integrity deterioration in G. affinis were recorded after exposure to 0.1-10 microg/l lindane and 96 h exposure to lindane, respectively. The results indicate that the mosquitofish G. affinis, due to its biological-ecological characteristics and the biomarker dose-response, is suitable for the monitoring of fresh water organochlorine pesticide contamination in general and lindane in particular.