Polychlorinated biphenyls as inducers of hepatic microsomal enzymes: effects of di-ortho substitution

The disposition of polychlorinated biphenyls (PCBs) differs between germ-free and conventional mice

The disposition of toxicants, such as polychlorinated biphenyls (PCBs), in germ-free (GF) vs. conventional (CV) mice has received little attention to date. Here, we investigate PCB levels in three-month-old female CV and GF mice exposed orally daily for 3 days to 0, 6, or 30 mg/kg body weight of the Fox River Mixture (FRM), an environmental PCB mixture. We euthanized animals 24 h after the final dose. PCB profiles in tissues differed from the FRM profile but were similar in tissues across all 4 PCB exposure groups. PCB levels in CV but not GF mice followed the difference in PCB dose. Importantly, PCB levels were higher in CV than GF mice exposed to the same dose. Hepatic cytochrome P450 enzyme or lipid levels did not explain these trends in PCB tissue levels. Thus, toxicity studies with CV and GF animals need to assess the toxicokinetics of the toxicant investigated. CAPSULE: PCB levels are typically higher in conventional than germ-free mice exposed to the same dose of PCBs.

Pure non-dioxin-like PCB congeners suppress induction of AhR-dependent endpoints in rat liver cells

The relative potencies of non-ortho-substituted coplanar polychlorinated biphenyl (PCB) congeners to activate the aryl hydrocarbon receptor (AhR) and to cause the AhR-dependent toxic events are essential for their risk assessment. Since some studies suggested that abundant non-dioxin-like PCB congeners (NDL-PCBs) may alter the AhR activation by PCB mixtures and possibly cause non-additive effects, we evaluated potential suppressive effects of NDL-PCBs on AhR activation, using a series of 24 highly purified NDL-PCBs. We investigated their impact on the model AhR agonist-induced luciferase reporter gene expression in rat hepatoma cells and on induction of CYP1A1/1B1 mRNAs and deregulation of AhR-dependent cell proliferation in rat liver epithelial cells. PCBs 128, 138, and 170 significantly suppressed AhR activation (with IC50 values from 1.4 to 5.6 μM), followed by PCBs 28, 47, 52, and 180; additionally, PCBs 122, 153, and 168 showed low but still significant potency to reduce luciferase activity. Detection of CYP1A1 mRNA levels in liver epithelial cells largely confirmed these results for the most abundant NDL-PCBs, whereas the other AhR-dependent events (CYP1B1 mRNA expression, induction of cell proliferation in confluent cells) were less sensitive to NDL-PCBs, thus indicating a more complex regulation of these endpoints. The present data suggest that some NDL-PCBs could modulate overall dioxin-like effects in complex mixtures.

Human receptor activation by aroclor 1260, a polychlorinated biphenyl mixture

Polychlorinated biphenyls (PCBs) are persistent environmental toxicants, present in 100% of U.S. adults and dose-dependently associated with obesity and non-alcoholic fatty liver disease (NAFLD). PCBs are predicted to interact with receptors previously implicated in xenobiotic/energy metabolism and NAFLD. These receptors include the aryl hydrocarbon receptor (AhR), pregnane xenobiotic receptor (PXR), constitutive androstane receptor (CAR), peroxisome proliferator-activated receptors (PPARs), liver-X-receptor (LXRα), and farnesoid-X-receptor (FXR). This study evaluates Aroclor 1260, a PCB mixture with congener composition mimicking that of human adipose tissue, and selected congeners, as potential ligands for these receptors utilizing human hepatoma-derived (HepG2) and primate-derived (COS-1) cell lines, and primary human hepatocytes. Aroclor 1260 (20 μg/ml) activated AhR, and PCB 126, a minor component, was a potent inducer. Aroclor 1260 activated PXR in a simple concentration-dependent manner at concentrations ≥10 μg/ml. Among the congeners tested, PCBs 138, 149, 151, 174, 183, 187, and 196 activated PXR. Aroclor 1260 activated CAR2 and CAR3 variants at lower concentrations and antagonize CAR2 activation by the CAR agonist, CITCO, at higher concentrations (≥20 μg/ml). Additionally, Aroclor 1260 induced CYP2B6 in primary hepatocytes. At subtoxic doses, Aroclor 1260 did not activate LXR or FXR and had no effect on LXR- or FXR-dependent induction by the agonists T0901317 or GW4064, respectively. Aroclor 1260 (20 μg/ml) suppressed PPARα activation by the agonist nafenopin, although none of the congeners tested demonstrated significant inhibition. The results suggest that Aroclor 1260 is a human AhR, PXR and CAR3 agonist, a mixed agonist/antagonist for CAR2, and an antagonist for human PPARα.

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

PCB exposure and in vivo CYP1A2 activity among Native Americans

Cytochrome P-450 1A2 (CYP1A2) is an enzyme involved in the metabolic activation of some carcinogens and is believed to be induced by xenobiotics. Very few studies, however, have investigated the association between environmental exposures and in vivo CYP1A2 activity in humans. To address this issue, a study was conducted of CYP1A2 activity among Native Americans exposed to polychlorinated biphenyls (PCBs) from the consumption of fish from the St. Lawrence River. At the Mohawk Nation at Akwesasne (in New York and in Ontario and Quebec, Canada), 103 adults were interviewed, and they donated blood for serum PCB analysis and underwent the caffeine breath test (CBT), a safe and noninvasive procedure that uses caffeine as a probe for CYP1A2 activity in vivo. The results supported the findings of other studies that CBT values are higher among smokers and men and lower among women who use oral contraceptives. Despite a relatively low average total PCB body burden in this population, the sum of serum levels for nine mono- or di-ortho-substituted PCB congeners showed positive associations with CBT values (p = 0.052 wet weight and p = 0.029 lipid adjusted), as did toxic equivalent quantities (TEQs; p = 0.091 for wet weight and 0.048 for lipid adjusted). Regarding individual congeners, serum levels of PCB-153, PCB-170, and PCB-180 were significantly correlated with CBT values. The results support the notion that CYP1A2 activity may be a marker of an early biological effect of exposure to PCBs in humans and that the CBT may be a useful tool to monitor such effects.

Polychlorinated biphenyl-induced effects on metabolic enzymes, AP-1 binding, vitamin E, and oxidative stress in the rat liver

Environmental pollutants, such as polychlorinated biphenyls (PCBs), may induce drug metabolism and may be substrates for the induced metabolic enzymes. Both processes may lead to oxidative stress. The goal of this study was to determine the influence of polychlorinated biphenyls, selected as inducers and substrates of drug metabolism, on oxidative events within the liver over a 3-week time course. Male and female Sprague-Dawley rats received two ip injections per week of 4-chlorobiphenyl, 2,4,4'-trichlorobiphenyl, 3,4,5-trichlorobiphenyl, 3,3',4,4'-tetrachlorobiphenyl (PCB 77), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), or both PCB 77 and 153 (100 micromol/kg/injection) and were euthanized at the end of 1, 2, or 3 weeks. Hepatic cytochrome P450 1A1 (EROD) activity, DT-diaphorase activity, AP-1 DNA-binding activity, conjugated dienes, and alpha-tocopherol (vitamin E) as well as alpha-tocopheryl quinone (oxidized vitamin E) were determined. While the lower chlorinated biphenyls (at these doses and times) showed little or no effect on these oxidative stress parameters, both CYP 1A1 and DT-diaphorase activities were significantly increased in both male and female rats receiving PCB 77, a ligand for the aryl hydrocarbon receptor. In addition, the DNA-binding activity of the transcription factor AP-1 was increased in rats treated with PCB 77 or PCB 153. Within the lipid fraction there was no significant increase observed in conjugated diene concentrations, but there was a significant increase in alpha-tocopheryl quinone upon treatment with all PCBs tested. These data indicate that alpha-tocopheryl quinone may be a sensitive marker for PCB exposure and is possibly increased by a wide range of PCBs.

Structure-dependent induction of CYP2B by polychlorinated biphenyl congeners in female Sprague-Dawley rats

The dose-response induction of hepatic microsomal pentoxyresorufin O-dealkylase (PROD) activity by phenobarbital (PB) and several polychlorinated biphenyl (PCB) mixtures and congeners was determined in the immature female Sprague-Dawley rat. At a dose of 75 mg/kg/day of PB for 3 days, the microsomal PROD activity was 2154 pmol/min/mg protein. Aroclors 1260, 1254, 1242, and 1016 did not induce maximal PROD activity at doses up to 500 mg/kg, and only Aroclor 1016 induced > a half-maximal response at the 500 mg/kg dose. The relative potencies of eighteen different PCB congeners were also determined, and the structures of these compounds differed with respect to the degree of chlorination (tri- to octochloro) and substitution patterns. The relative potencies of these compounds were estimated by comparing their induced activities at the high dose (150 or 100 mg/kg) with that of PB. The most potent inducers were 2,3,3',4',5,6-hexaCB and 2,2',3,4',5,5',6-heptaCB; at a dose of 150 mg/kg, the PROD activity induced by 2,2'3,4',5,5',6-heptaCB was comparable to that observed for PB. 2,3,3',4',5,6-HexaCB was the most potent inducer, and hepatic PROD activity in rats treated with 150 mg/kg was 4202 pmol/min/mg; this value was higher than that observed for PB at a dose of 75 mg/kg. A second group of congeners including 2,2',3,4,4',5,5'-heptaCB, 2,2',4,4',5,5'-hexaCB, 2,2',3,3',4,4',5,5'-octaCB 2,2',4,4'-tetraCB, 2,2',4,5,5'-pentaCB, 2,2',3,4,4',5',6-heptaCB, 2,2',4,4',5-pentaCB and 2,2',3,3',4',5,5',6-octaCB induced PROD activity > or = 1090 pmol/min/mg at the 150 mg/kg dose, and this value was > 50% of the maximal response observed for PB. The remaining compounds, namely 2,4,4'-triCB, 2,2',3,4'-tetraCB, 2,2',5,5'-tetraCB, 2,3',4,4',5-pentaCB, 2,3,3',4,4'-pentaCB, 2,2',4,4',5,6'-hexaCB, 2,3,3',4,4',5,5'-heptaCB and 2,2',3,3',4,4,5-heptaCB were all relatively weak inducers of hepatic microsomal PROD activity ( < 450 pmol/min/mg). In parallel experiments, western blot analysis of immunoreactive CYP2B1 and CYP2B2 protein showed that PB, the PCB mixtures, and congeners induced both proteins. Previous studies have identified a cis-acting DNA element that plays a role in regulating CYP2B1/B2 gene expression and binds nuclear trans-acting factor(s) induced by PB. The results of gel electrophoretic mobility shift assays with nuclear extracts showed that both PB and 2,2',3,4',5,5',6-heptaCB induce formation of a common retarded band using a 32P-labeled oligonucleotide corresponding the the cis-acting DNA promoter sequence. Both PB and PCBs appear to induce CYP2B1/B2 via a common mechanism. Although the results of this study do not define structure-induction (CYP2B1/B2) relationships for PCBs, two compounds, namely 2,3,3',4',5,6-hexaCB and 2,2',3,4',5,5',6-heptaCB, were identified as highly potent inducers.

Induction of cytochrome P-450 in the Norway rat, Rattus norvegicus, following exposure to potential environmental contaminants

Cytochrome P-450 (CYP) induction (consisting of increases in cellular RNA and protein content and associated catalytic activities) occurs predominantly in the liver, but also in small intestine, lung, kidney, and placenta, of Norway rats (Rattus norvegicus) exposed to certain types of potential environmental contaminants. The specific isoform(s) induced in the rat and the magnitudes of the increases observed depend upon the chemical nature of the xenobiotic. For instance, the predominant isoforms induced by nonhalogenated polycyclic aromatic hydrocarbons, such as petroleum derivatives and coal-tar constituents such as the benzopyrenes and the anthracenes, are those of the CYP1A subfamily. Polyhalogenated aromatic hydrocarbons, such as the halogenated dibenzodioxins, dibenzofurans, and biphenyls, may cause the induction of predominantly the CYP1A subfamily, predominantly the CYP2B subfamily, or mixed CYP1A- and CYP2B-type induction, depending upon the halogen substitution pattern. In contrast, the chlorinated hydrocarbon pesticides, such as DDT, dieldrin, chlordane, and mirex, cause almost exclusively the induction of isoforms of the CYP2B (and to a lesser extent the CYP3A) subfamilies. The commonly employed plasticizing agent di-(2-ethylhexyl)phthalate elicits predominantly induction of the CYP4A subfamily. Those xenobiotics that would be expected to be the most pervasive environmental contaminants are typically those that have also been found to cause the most profound CYP induction responses. Such chemicals are extremely lipophilic and tend to accumulate in animal tissues, especially fatty tissues such as the liver. The hepatic CYP induction response to such potential environmental contaminants is typical of the animals' response to lipophilic xenobiotics in general, and serves as a mechanism by which the excretion of such compounds from the body is facilitated.

Induction of cytochrome P-450 in Sigmodon hispidus (cotton rats) exposed to dietary Aroclor 1254

The induction of immunoreactive cytochrome P-450 protein and associated catalytic activities in 10-wk-old male and female Sigmodon hispidus (cotton rats) exposed for 2 wk to low dietary levels of Aroclor 1254 (0.33, 1.0, 3.3, 10, and 33 ppm), or the prototype P-450 inducers phenobarbital, DDT, clotrimazole, and beta-naphthoflavone was examined. Ethoxy-(ETR), methoxy- (MTR), pentoxy- (PTR), and benzyloxyresorufin (BZR) O-dealkylation activities were significantly increased at 0.33 ppm Aroclor for males and 1.0 ppm for females, when compared to control levels. O-Dealkylation activities peaked at 3.3 ppm for males and 10 ppm for females. ETR and MTR O-dealkylation activities were increased four- to eightfold while PTR and BZR O-dealkylation activities increased only two- to threefold. Liver/body weight ratios also increased, with the maximum ratios observed at the highest Aroclor dose, and were associated with histopathologic hepatocyte lesions. While increases in liver/body weight ratio, immunoreactive CYP2B protein, and BZR O-dealkylation were detected following phenobarbital treatment, no increase in PTR O-dealkylation activity was observed. These results demonstrate that S. hispidus (both males and females) are extremely sensitive to low dietary levels of Aroclor 1254, responding with increases in liver/body weight ratio, immunoreactive P-450 protein, and O-dealkylation activities. The cotton rat would appear to be a sensitive feral target species for detecting exposure to certain environmental contaminants.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.

Relative tumour promoting activity of three polychlorinated biphenyls in rat liver

The relative tumour promoting activity of three structurally and toxicologically diverse polychlorinated biphenyls (3,4,5,3',4'-penta- 2,3,4,3',4'-penta- and 2,4,5,2',4',5'-hexachlorobiphenyl) was measured in an initiation/promotion assay in nitrosodiethylamine-initiated female Sprague-Dawley rats. The congeners under study were administered by once-weekly subcutaneous injections for 20 weeks. Evaluation of the development of gamma-glutamyl transpeptidase (GGT)- and glutation transferase P (GST-P)-positive hepatic foci showed that all congeners promoted altered hepatic foci, although 3,4,5,3',4'-pentachlorobiphenyl was far more potent. The volume fraction of the liver occupied by GGT-positive tissue in the 3,4,5,3',4'-pentachlorobiphenyl-treated animals (100 micrograms/kg per week) was 23%, while the volume fractions of altered liver tissue in the rats treated with 2,3,4,3',4'-pentachlorobiphenyl (5000 micrograms/kg per week) and 2,4,5,2',4',5'-hexaCB (20,000 micrograms/kg per week) were 1.2 and 2.3, respectively. The enhancement of GGT- and GST-P-positive foci was accompanied by an increased incidence of histological changes in the livers.

Distribution and effects on cytochrome P450 system of two hexachlorobiphenyl isomers in the rat

Tissue distribution and effects induced by 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB) on cytocrome P450 isozymes were compared with those of 2,2',3,3',6,6'-hexacholorobiphenyl (236-HCB). Male Wistar rats were given a single intragastric dose (23 mg/kg body wt) of either isomer, and killed after 72 h. At termination the tissue concentrations of 245-HCB were considerably higher than those of 236-HCB, suggesting a more effective metabolism of the latter. The binding affinity of 236-HCB to cytochrome P450 was higher and the magnitude of binding greater than of 245-HCB. 245-HCB-treatment elevated the hepatic concentration of cytochrome P450 and also the activities of 7-pentoxyresorufin O-depentylase (50-fold), aniline p-hydroxylase (2-fold) and 7-ethoxycoumarin O-deethylase (2-fold), a response typical of phenobarbital-type inducers. In the Western immunoblot of liver microsomes from 245-HCB treated rats, an increased amount of P450IIB 1/2 was detected by a monoclonal antibody 2-66-3, which specifically detects phenobarbital inducible isoenzymes. The minimum molecular mass of the P450 isozyme induced was 52 kDa. After 236-HCB administration, a weak inducing effect was observed.

Induction of cytochrome P450 and other drug metabolizing enzymes in rat liver following dietary exposure to Aroclor 1254

Selected drug metabolizing activities were measured in female F344/NCr rats exposed to graded dietary concentrations of Aroclor 1254 (1 to 1000 ppm) for 7 days or to lower concentrations of Aroclor (1 to 10 ppm) for up to 28 days. Following the 7-day exposure, the hepatic O-dealkylation of ethoxyresorufin (ETR), mediated primarily by cytochrome P450IA, was increased 60-, 10-, and 4-fold by 33, 10, and 3 ppm Aroclor, respectively. In rats exposed to 10 and 3 ppm Aroclor for 28 days, this activity was increased approximately 30- and 10-fold, respectively. Hepatic ETR O-dealkylase activities correlated with Aroclor concentrations in the livers of exposed rats (r = 0.99, p less than 0.01). Although the O-dealkylation of benzyloxyresorufin was highly increased by 7-days dietary exposure to 1000 ppm Aroclor, the levels of Aroclor necessary for detection of induction were substantially higher than those required for detection of ETR O-dealkylase induction. Examination of the non-P450-mediated drug metabolizing activities, epoxide hydrolase and DT-diaphorase, similarly showed limited (approximately 10-fold) increases. In contrast, aldehyde dehydrogenase (benzaldehyde, NADP+) activity was highly increased (greater than 40-fold) at 1000 ppm, however this activity was increased to only a limited extent at lower Aroclor concentrations (e.g. approximately 3-fold at 33 ppm). These results support the potential use of cytochrome P450 activities as potential biomarkers for environmental exposure to PCBs and related compounds.

Differential potency of atropisomers of polychlorinated biphenyls on cytochrome P450 induction and uroporphyrin accumulation in the chick embryo hepatocyte culture

The atropisomers of 2,2',3,4,6-pentachlorobiphenyl (PeCB), 2,2',3,4,4',6-hexachlorobiphenyl (HeCB), and 2,2',3,3',4,4',6,6'-octachlorobiphenyl (OCB) were studied in the chick embryo hepatocyte culture to determine if chirality plays a role in the recognition events associated with the induction of cytochromes P450 and the accumulation of uroporphyrin (URO). Concentration-related induction of cytochrome P450 content, ethoxyresorufin-O-deethylase (EROD) and benzphetamine N-demethylase (BPDM) activities were measured. The rank order of potency for total cytochrome P450 induction was HeCB greater than OCB greater than or equal to PeCB. The (+)- and (-)-enantiomers of PeCB and OCB were of equal potencies as inducers of cytochromes P450, whereas the (+)-HeCB was greater than the (-)-HeCB. HeCB was a much more potent inducer of EROD activity than was either PeCB or OCB. EROD activity was induced to a much greater extent by the (+)-enantiomers of all compounds, with the (-)-enantiomers of PeCB and OCB being inactive. BPDM activity was induced by all three compounds in the order of OCB greater than or equal to HeCB greater than PeCB. The (-)-enantiomers were more potent inducers of BPDM activities than were the (+)-enantiomers, except for HeCB, in which the (+)- was more potent than the (-)-enantiomer. Analysis of porphyrin accumulation in cultures treated with delta-aminolevulinic acid revealed that (+)-HeCB caused the greatest percent URO accumulation, which also correlated with the greatest increase in EROD activity. All other enantiomers caused up to 47% URO accumulation, which did not correlate with an increase in EROD activity.

Immunosuppressive activities of polychlorinated biphenyls in C57BL/6N mice: structure-activity relationships as Ah receptor agonists and partial antagonists

The immunosuppressive activity of polychlorinated biphenyl (PCB) congeners is structure-dependent and 2 classes of compounds, namely the coplanar (class I) and monoortho coplanar (class II) congeners exhibit immunotoxicity. This study extends the structure-immunotoxicity relationships for PCBs by investigating representative congeners from the following structural classes of PCBs: monoortho coplanar (2,3,3',4,4',5-hexachlorobiphenyl, class II); monoortho coplanar minus a single parachloro group (2,3,3',4,5,5'-hexachlorobiphenyl and 2,3,3',4,5'-pentachlorobiphenyl, class III); diortho coplanar (2,3',4,4',5',6-hexachlorobiphenyl, class IV); triortho coplanar (2,2',4,4',5,6'-hexachlorobiphenyl, class V) and a tetraortho-substituted PCB (2,2',4,4',6,6'-hexachlorobiphenyl, class VI). The effects of these compounds on the splenic plaque forming cell response to sheep red blood cells was determined in 7-8 week old male C57BL/6N mice. The results showed that the class II-IV congeners were immunotoxic and with only one exception these compounds also induced hepatic microsomal aryl hydrocarbon hydroxylase and ethoxyresorufin O-deethylase activities and displaced [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from the cytosolic aryl hydrocarbon (Ah) receptor in competitive binding assays. These results thus extend the structure-activity relationships for PCBs as Ah receptor agonists. The interaction of these PCB congeners with an ED70-90 dose of TCDD (3.7 nmol/kg) showed that only one structural class of compounds, namely class III, partially antagonized TCDD-mediated immunotoxicity.

Induction of drug metabolism enzymes by dihalogenated biphenyls

The effects of pretreatment with symmetrically dihalogenated biphenyls (DXBs, X-F, Cl(C), Br(B) and I) on rat liver drug metabolism enzymes were investigated. 4,4'-DFB, -DCB, and -DBB as well as 2,2'-DFB appeared to be inducers of microsomal cytochrome P-450-linked monoxygenases (N-demethylases of aminopyrine and ethylmorphine). However, no structure-induction relationship was found. 4,4'-DXBs also induced a cytochrome P-448-linked mono-oxygenase (ethoxyresorufin O-deethylase), and their order of induction potential seemed to parallel the increase of the size of the halogen substituent. Therefore, 4,4'-DXB's may be categorized as mixed-type inducers, the cytochrome P-450 component being the more pronounced. Data on the cytochrome P-448 induction by dihalogenated biphenyls with only para substituents may be considered as a refinement of the previously described structure-activity relationship in this respect. All of the DXBs except 3,3'-DCB and 4,4'-DIB, enhanced, like phenobarbital, the activity of UDP-glucuronyltransferase toward 4-hydroxybiphenyl. Only 4,4'-DFB was able to induce the activity of glutathione S-transferase toward 1,2-epoxy-3-(p-nitrophenoxy)propane. Studies after 4,4'-DBB-treatment revealed, like phenobarbital, a preferential induction of ethylmorphine N-demethylase on rough endoplasmic reticulum-derived microsomes, whereas UDP-glucuronyltransferase activity toward 4-hydroxybiphenyl was induced to a larger extent on smooth endoplasmic reticulum microsomes, suggesting a dissimilar enzyme induction in microsomal subfractions.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.

Effects of polychlorinated biphenyls on cytochrome P450 induction in the chick embryo hepatocyte culture

The effects of pure synthetic polychlorinated biphenyl (PCB) congeners on the induction of cytochrome P450 and associated activities were examined in cultured chick embryo hepatocytes. Dose-response effects for the induction of total cytochrome P450 ethoxyresorufin-O-deethylase (EROD) activity, and benzphetamine demethylase (BPDM) activity were studied using 10 selected tetra- to hexachlorinated PCB congeners. These studies revealed that PCBs caused effects in the chick hepatocyte culture different from previously observed effects in rat liver. Based on their effects in chick hepatocytes, the PCBs could be categorized into two groups. The first group (consisting of 3,3',4,4'-PCB, 3,3',4,4',5-PCB, 3,3',4,4',5,5'-PCB, 2',3,3',4,5-PCB, 2,3,3',4,4',5'-PCB, and 2,3,4,4',5-PCB) induced total cytochrome P450 2.4- to 2.9-fold and EROD activity from 1-2 pmol/min/mg protein to 162-247. There was marked variation in potency, but all these congeners had a maximal inducing dose above which cytochrome P450 concentrations and EROD activities declined. BPDM activities were increased only slightly (1.2- to 1.6-fold) at the maximal cytochrome P450 inducing dose. The second group of congeners (consisting of 2,2',4,5,5'-PCB. 2,2',4,4',5,5'-PCB, and 2,2',3,4,4',6-PCB) induced total cytochrome P450 concentrations 4.0-fold and BPDM activities 2.2- to 2.6-fold with greatest activity occurring at the highest doses which could be added (10-50 microM). However, EROD activities were also increased by these congeners to 60-112 pmol/min/mg protein with declining activities seen at the highest PCB doses (i.e., resembling EROD induction patterns of the first group). The EROD induction patterns with these latter PCB congeners are noteworthy since these PCBs do not induce EROD activity in the rat. For both groups of PCB congeners, EROD induction was associated with increased accumulation of uroporphyrin in cultures exposed to exogenous 5-aminolevulinate. Studies investigating the reason for the depression of cytochrome P450 concentrations and/or EROD activities by high doses of the PCBs revealed that with the first group there was slightly decreased total protein synthesis, decreased total cell heme concentrations, and decreased accumulation of radiolabeled heme synthesized from 5-[14C]aminolevulinate. These changes might represent nonspecific toxic effects of the first group of PCBs. However, since these changes were not seen with the second group of PCBs, it is unlikely that either inhibition of heme synthesis or toxicity cause the depression of EROD activity with high PCB doses.

Synthesis, biologic and toxic effects of the major 2,3,7,8-tetrachlorodibenzo-p-dioxin metabolites in the rat

The two major mammalian metabolites of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), namely 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin and 2-hydroxy-1,3,7,8-tetrachlorodibenzo-p-dioxin, have been synthesized. The compounds were individually administered to immature male Wistar rats and their effects on body weight loss, thymic atrophy, liver and spleen weights and their activities as inducers of hepatic microsomal benzo[a]pyrene hydroxylase, 4-chlorobiphenyl hydroxylase and ethoxyresorufin O-deethylase were determined using dose levels of 100, 1000 and 5000 micrograms/kg. The 2 metabolites did not affect organ or body weights after 14 days of exposure and only 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin was active as an inducer of the microsomal monooxygenases at dose levels of 1000 and 5000 micrograms/kg. A comparison of the relative enzyme induction activities of 2,3,7,8-TCDD and 2-hydroxy-3,7,8-trichlorodibenzo-p-dioxin indicates that the former compound was greater than 3 orders of magnitude more active than the metabolite.

The relative induction of mixed-function oxidase specific activity to C-H and C-C1 bond strengths in polychlorinated derivatives of dibenzo-p-dioxin (PCDDs)

The structure-activity relationships between the mixed-function oxidase (MFO) system and six polychlorinated derivatives of dibenzo-p-dioxin (PCDDs) were studied using Sprague Dawley rats. The study was set up in accordance with previous work in these laboratories involving monocyclic hydrocarbons. Three possible mechanisms at the MFO active site are proposed. C-H bond hydroxylation in low chlorine substituted PCDDs, and probable expoxidation in the intermediate chlorinated species including 2378 tetrachlorodibenzodioxin. Benzo[a]pyrene activity induced by some PCDDs appears irrelevant to the desired metabolic result.

PCBs and PBBs: biologic and toxic effects on C57BL/6J and DBA/2J inbred mice

Treatment of genetically inbred "responsive" C57BL/6J and "non-responsive" DBA/2J mice with Aroclor 1254 or fireMaster BP-6 resulted in the induction of hepatic microsomal benzo[a]pyrene hydroxylase only in the former mouse strain and aminopyrine N-demethylase in both strains of mice. In contrast, 3,3',4,4',5-pentachlorobiphenyl and 3,3',4,4'-tetrabromobiphenyl, induced benzo[a]pyrene hydroxylase in both C57BL/6J and DBA/2J but did not enhance aminopyrine N-demethylase in either strain of mouse. Both these coplanar halogenated biphenyls also caused thymic atrophy in the responsive and non-responsive mice and their effects resembled those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Treatment of the inbred mice with several mono-ortho substituted analogs of the coplanar halogenated biphenyls, including 2,3,3',4,4'-pentachloro-, 2,3',4,4',5-pentabromo-, 2,3,3',4,4',5-hexachloro- and 3',4'-dibromo-2,3,4,5-tetrachlorobiphenyl, gave hepatic enzyme-induction results similar to those observed for the commercial halogenated biphenyls. At dose levels of 1500 mumol/kg, most of these compounds caused thymic atrophy in C57BL/6J mice but not in DBA/2J mice. The structure-activity correlations in the mice complement similar studies with the halogenated biphenyls in rats and support the proposed receptor-mediated mechanism for the toxic halogenated aromatics.

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action

Polychlorinated and polybrominated biphenyls are industrial chemical mixtures which have been implicated in numerous human poisonings in Taiwan and Japan (PCBs) and Michigan (PBBs). Moreover, these polyhalogenated biphenyls have been widely detected in the environment including the air, water, fish, wildlife, human adipose tissue, and blood and breast milk. A major problem associated with the analysis and toxicology of this group of chemicals is their chemical complexity (e.g., there are 209 possible PCB isomers and congeners) and the remarkable effects of structure on activity. This article will discuss the effects of structure on the biologic and toxic effects of individual PCB and PBB congeners as well as reconstituted mixtures. The results clearly show that like "dioxin" (or 2,3,7,8-TCDD), the PCBs and PBBs elicit their effects through a cytosolic receptor protein which preferentially binds with the toxins which are approximate isostereomers of 2,3,7,8-TCDD. The evidence for this mechanism of action will be discussed in detail.

Polychlorinated biphenyls as phenobarbitone-type inducers of microsomal enzymes. Structure-activity relationships for a series of 2,4-dichloro-substituted congeners

Several polychlorinated biphenyl (PCB) isomers and congeners resemble phenobarbitone (PB) in their mode of induction of the hepatic drug-metabolizing enzymes; however, unlike PCBs which induce aryl hydrocarbon hydroxylase, no apparent structure-activity correlations have been reported. This study examines the effects of structure on the activity of a series of 2,4-dichloro-substituted biphenyls as inducers of several microsomal enzyme activities including dimethylaminoantipyrine N-demethylase, benzo[a]pyrene hydroxylase, aldrin epoxidase, and ethoxyresorufin O-deethylase. The results clearly illustrate a marked effect of structure on activity: all of the 2,4-dichloro-substituted PCBs resembled PB in their mode of induction. However, the potency of the induction response was dependent on the substitution pattern of the second phenyl ring (i.e. 2,3,4,5-tetrachloro greater than or equal to 2,3,4,5,6-pentachloro greater than 2,3,4,6-tetrachloro greater than 2,3,5,6-tetrachloro greater than 2,4,6-trichloro); the structure of the lower chlorinated ring also determined induction potency since the 2,4-dichloro-substituted PCBs were generally more active than their 4-chloro-substituted analogs, whereas the 2-substituted PCB homologs were inactive. The structural factors which typify the most active PB-type inducer, 2,2',3,4,4',5-hexachlorobiphenyl, include the presence of two para-, at least two meta- and two ortho-chloro substituents. In addition to the structure-activity correlations noted for PCBs, the 2,2',3,4,4',5-hexachlorobiphenyl congener also elicited a dose-response induction of two PB-inducible enzymes, aldrin epoxidase and dimethylaminoantipyrine N-demethylase.

The comparative biologic and toxic potencies of polychlorinated biphenyls and polybrominated biphenyls

Aroclor 1254 and fireMaster BP-6, two commercial polychlorinated biphenyl (PCB) and polybrominated biphenyl (PBB) preparations, exhibit comparable biologic and toxic effects. In the present study the commercial PBB was more active than Aroclor 1254 in causing thymic atrophy in male Wistar rats. However, a direct comparison of the relative effects of bromine vs chlorine substituents is not possible with the commercial PBB and PCB mixtures due to their complex congeneric composition. This study reports the synthesis and biologic and toxic effects of a series of laterally substituted 3,3',4,4'-tetrahalobiphenyls which contain the following variable molecular Cl/Br ratios; Br4, Br3Cl, Br2Cl2 (two isomers), BrCl3, and Cl4. 3,3',4,4'-Tetrabromobiphenyl and 3,4,4'-tribromo-3'-chlorobiphenyl (150 mumol/kg)-pretreated animals significantly inhibited the growth rate of and caused thymic atrophy in immature male Wistar rats whereas those isostereomers with reduced Br (and increased Cl) content were either less active or inactive. Pretreatment of male Wistar rats with 10 mumol/kg of the 3,3',4,4'-tetrahalobiphenyls and determination of their effects as inducers of the hepatic microsomal drug-metabolizing enzymes also illustrated the effects of the relative Cl/Br ratios on induction potencies. Both 3,3',4,4'-tetrabromo- and 3,4,4'-tribromo-3'-chlorobiphenyl maximally induced the cytochrome P-448-dependent monooxygenases, benzo[a]pyrene and 4-chlorobiphenyl hydroxylase; the order of potency of the other isostereomers was 4,4'-dibromo-3,3'-dichloro- congruent to 3,4-dibromo-3',4'-dichlorobiphenyl greater than 4-bromo-3,3',4'-trichloro- greater than 3,3',4,4'-tetrachlorobiphenyl. With few exceptions this order of potency was observed for the induction of benzo[a]pyrene hydroxylase and ethoxyresorufin O-deethylase in rat hepatoma cells in culture and for their relative binding affinities to the rat cytosolic receptor protein. The data clearly demonstrate that the biologic activities of this group of isosteric halogenated biphenyls are enhanced with increasing bromine substitution and also support the hypothesis that the activities of this class of chemicals are mediated through the receptor.

Differential time course of induction of rat liver microsomal cytochrome P-450 isozymes and epoxide hydrolase by Aroclor 1254

The time course of induction of rat liver microsomal cytochromes P-450a, P-450b + P-450e, P-450c, and P-450d and epoxide hydrolase has been determined in immature male rats administered a single large dose [1500 mumol (500 mg)/kg body wt] of the polychlorinated biphenyl mixture Aroclor 1254. Differential regulation of these xenobiotic-metabolizing enzymes was indicated by their characteristic patterns of induction. The rate of induction of cytochrome P-450a and epoxide hydrolase was relatively slow, and steady-state levels of these enzymes were maintained from approximately Days 9 to 15 after Aroclor 1254 treatment. In contrast, cytochrome P-450c was maximally induced 2 days after Aroclor 1254 treatment and remained at a constant level through Day 15. Steady-state levels of cytochrome P-450d, beginning 1 week after Aroclor 1254 treatment, were preceded by a fairly rapid rate of induction and possibly by a small decline from maximal levels observed around Days 4 to 5. Like those of the other cytochrome P-450 isozymes and epoxide hydrolase, the levels of cytochromes P-450b + P-450e were constant from Day 9 to 15 after Aroclor 1254 treatment. However, an unexpected but reproducible decline (approximately 25%) in total cytochrome P-450 content observed between Days 4 and 9 after Aroclor 1254 treatment principally reflected a dramatic and totally unanticipated decrease (approximately 45%) in the level of cytochromes P-450b + P-450e. This transient decline in the level of cytochromes P-450b + P-450e was not due to an unusual effect of a mixture of polychlorinated biphenyls, since identical results were obtained with two individual congeners, namely 2,3,4,5,4'-penta- and 2,3,4,5,3',4'-hexachlorobiphenyl, that induced the same isozymes as Aroclor 1254. In contrast, when rats were treated with 2,4,5,2',4',5'-hexachlorobiphenyl, which induces cytochromes P-450a and P-450b + P-450e and epoxide hydrolase but not cytochromes P-450c or P-450d, maximal levels of cytochromes P-450b + P-450e were attained on Day 4 and no decrease was observed over the next 11 days. These results suggest that there may be an interaction in the regulation of induction of certain individual cytochrome P-450 isozymes.

Induction of rat hepatic microsomal cytochrome P-450 by 2,3',4,4',5,5'-hexachlorobiphenyl

The following evidence suggests that 2,3',4,4',5,5'-hexachlorobiphenyl resembles isosafrole as an inducer of hepatic microsomal cytochrome P-450d in the immature male Wistar rat. First, the major hepatic microsomal polypeptide (Mr = 52,000), intensified after treatment of rats with 2,3',4,4',5,5'-hexachlorobiphenyl, comigrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with cytochrome P-450d (i.e. the major isosafrole-inducible polypeptide) but had an electrophoretic mobility intermediate between cytochrome P-450b (Mr approximately equal to 51,500) and cytochrome P-450c (Mr = 56,000) (i.e. the major phenobarbital- and 3-methylcholanthrene-inducible polypeptides respectively). Second, when pairs of various xenobiotics were coadministered to rats at doses effecting maximal induction of hepatic microsomal cytochrome P-450, the inductive effects of 2,3',4,4',5,5'-hexachlorobiphenyl were additive with those of phenobarbital, 3-methylcholanthrene and pregnenolone-16 alpha-carbonitrile but not with those of isosafrole. The inductive effects of phenobarbital, 3-methylcholanthrene, pregnenolone-16 alpha-carbonitrile and isosafrole were all expressed additively with each other. Third, in contrast to phenobarbital and pregnenolone-16 alpha-carbonitrile treatment, treatment of rats with 2,3',4,4',5,5'-hexachlorobiphenyl, isosafrole or 3-methylcholanthrene failed to increase markedly the proportion of total cytochrome P-450 capable of forming a 446 nm-absorbing complex with metyrapone. Fourth, the in vitro metabolism of isosafrole, catalyzed by hepatic microsomes from rats treated with 2,3',4,4',5,5'-hexachlorobiphenyl, isosafrole or 3-methylcholanthrene, produced complexes between ferrous cytochrome P-450 and a methylenedioxyphenyl metabolite, the spectra of which were between 400 and 500 nm and were similar to each other but which were readily distinguishable from the spectra of the product adducts formed during the metabolism of isosafrole by hepatic microsomes from rats treated with corn oil (control), phenobarbital, or pregnenolone-16 alpha-carbonitrile.

Polybrominated biphenyls as aryl hydrocarbon hydroxylase inducers: structure-activity correlations

The synthesis of all possible laterally-substituted polybrominated biphenyl (PBB) congeners containing two para bromines is described. Using enzymic, electrophoretic and ligand-binding assays that distinguish between phenobarbitone(PB)- and 3-methylcholanthrene(MC)-type inducers, the synthetic PBBs were evaluated as inducers of liver microsomal drug-metabolizing enzymes in the immature male Wistar rat. 4,4'-Dibromobiphenyl resembled PB in its mode of induction whereas all the meta-brominated derivatives of 4,4'-dibromobiphenyl, namely 3,4,4'-tri, 3,4,4',5-tetra-, 3,3', 4,4'-tetra-, 3,3',4,4',5-penta- and 3,3',4,4',5,5'-hexabromobiphenyl, resembled MC in their mode of induction. The results obtained with 3,4,4'-tribromobiphenyl demonstrate that, in contrast to the polychlorinated biphenyls (PCBs), a single meta halogen substituent is sufficient to abolish the PB-type characteristics of 4,4'-dibromobiphenyl and convert it to a strictly MC-type inducer. PBBs which induce AHH activity must be substituted at both para positions and at one, two, three or four meta positions. Ortho-substitution of PBBs which contain only lateral bromine groups may also give compounds which are aryl hydrocarbon hydroxylase (AHH) inducers. One of the MC-type PBBs, namely 3,3',4,4'-tetrabromobiphenyl, which has been tentatively identified in the commercial PBB mixture, fireMaster BP-6, was at least 50 times more potent as an inducer of AHH activity than the commercial PBB mixture. The induction of AHH by 3,3',4,4'-tetrabromobiphenyl was accompanied by a dose-dependent decrease in both thymus and spleen weights. The thymus and/or spleen weights were decreased in rats treated with the other MC-type PBBs which further supports the correlation between the toxicity of the PBBs and their ability to induce AHH.

PCB isomers and congeners: induction of aryl hydrocarbon hydroxylase and ethoxyresorufin O-deethylase enzyme activities in rat hepatoma cells

The effects of structure on the activity of 15 polychlorinated biphenyl (PCB) isomers and congeners as inducers of cytochrome P-448-dependent monooxygenases in rat hepatoma cell cultures was investigated. All the PCBs which have previously been classified as aryl hydrocarbon hydroxylase (AHH) inducers in the immature male Wistar rat also induced benzo[a]pyrene hydroxylase and ethoxyresorufin (ER) O-deethylase activity in the rat hepatoma H-4-II-E cells. The relative activities of the compounds were evaluated by comparing the doses required to half-maximally induce the two enzyme activities. The most potent PCB inducer, 3,3',4,4',5-pentachlorobiphenyl, exhibited molar EC50 values of 2.48 X 10(-10) and 2.40 X 10(-10) for the induction of ER O-deethylase and benzo[a]pyrene hydroxylase enzyme activities, respectively. These values were only 3- to 4-fold lower than the data obtained for the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The relative activities of the PCB congeners as inducers of cytochrome P-448-dependent monooxygenase in vitro were similar to their potencies as microsomal enzyme inducers in the immature male rat.

Halogenated biphenyls as AHH inducers: effects of different halogen substituents

4'-Iodo-, 4'-bromo-, 4'-chloro- and 4'-fluoro-2,3,4,5-tetrachlorobiphenyl were administered to immature male Wistar rats and the effects of this homologous series of 4'-halo-2,3,4,5-tetrachlorobiphenyls on the microsomal drug-metabolizing enzymes were determined. All the halogenated biphenyls increased microsomal benzo[a]pyrene hydroxylase (or aryl hydrocarbon hydroxylase, AHH), ethoxyresorufin (ER) O-deethylase and dimethylaminoantipyrine (DMAP) N-demethylase. The effects of the 4'-halo-2,3,4,5-tetrachlorobiphenyls on the microsomal enzyme activities and on the relative peak intensities and spectral shifts of the reduced cytochrome P-450:CO and ethylisocyanide (EIC) binding difference spectra were similar to those observed after coadministration of phenobarbitone (PB) and 3-methylcholanthrene (MC). The relative activities of the halogenated biphenyls were determined using two in vitro assays; namely cytochrome P-448 associated induction in rat hepatoma H-4-II E cells in culture and competitive binding to the hepatic cytosolic Ah receptor protein from male Wistar rats. Dose-response experiments for the iodo, bromo, chloro and fluoro analogs gave EC50(M) values of 8.5 x 10(-9), 6.6 x 10(-8), 5.7 x 10(-7), and 3.3 x 10(-5), and 1.5 x 10(-6), 2.5 x 10(-6), 4.1 x 10(-6) and 2.5 x 10(-5) for the Er O-deethylase induction and receptor binding assays respectively. The relative potencies of the 4'-halo-2,3,4,5-tetrachlorobiphenyls followed the order I greater than Br greater than Cl greater than F for both assays and differences in the EC50 values for the iodo and fluoro analogs were greater than three orders of magnitude for ER O-deethylase induction in rat hepatoma cells in culture. One possible explanation for these effects may be associated with differences in the polarizability of the laterally substituted halogen groups. However, other differences in the physico-chemical properties of the halogen atoms may also be important.

Binding of polychlorinated biphenyls classified as either phenobarbitone-, 3-methylcholanthrene- or mixed-type inducers to cytosolic Ah receptor

It has been postulated that reversible, high-affinity binding of 3-methylcholanthrene(MC)-type inducers to a receptor protein (the Ah receptor) in hepatic cytosol is essential for induction of aryl hydrocarbon hydroxylase (AHH) enzymic activity. To test this postulate, the binding affinities of 16 highly-purified, synthetic chlorinated biphenyl (PCB) congeners, which have been categorized either as phenobarbitone(PB)-, MC- or mixed (PB + MC)-type inducers of cytochrome P-450-dependent monooxygenase have been examined. The affinity of individual biphenyl congeners for the receptor was determined by their competition with 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin ([3H]TCDD) for specific cytosolic binding sites as measured by sucrose density gradient analysis following dextran-charcoal treatment. This assay demonstrates (a) that the receptor binds with highest affinity to 3,3', 4,4',5-pentachlorobiphenyl and 3,3',4,4'-tetrachlorobiphenyl ('pure MC-like' inducers); (b) mixed-type PCB inducers also bind to the receptor but with an affinity (average EC50-value of 8.6 microM) lower than that for 3,3',4,4',5-pentachlorobiphenyl; this corresponds with their relatively lower potencies as AHH inducers; (c) the receptor binds 2,2',4,4'-tetra-, 2,3,4,5-tetra-, 2,2',4,4',5,5'-hexa- and 2,3',4,4',5',6-hexachlorobiphenyl at high concentrations (0.1 mM), whereas PB fails to bind, even at a concentration of 10 mM. All PCBs tested competed with [3H]TCDD for Ah receptor, but there was a great variation in their relative binding affinities. The fact that two chlorinated biphenyls classed as 'PB-like' inducers and two chlorinated biphenyls which are neither 'PB- nor MC-type' inducers competed, coupled with the fact that PB did not bind to the receptor suggest that chemicals other than "pure MC-type' inducers can bind to the cytosolic receptor. Affinity of the binding dictates the relative potency of given PCB congeners as inducers of cytochrome P-448.