Kinetic models for association of 2,3,7,8-tetrachlorodibenzo-p-dioxin with the Ah receptor

Persistent binding of ligands to the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the biological and toxic effects of halogenated aromatic hydrocarbons (HAHs), polycyclic aromatic hydrocarbons (PAHs), and other structurally diverse ligands. While HAHs are several orders of magnitude more potent in producing AhR-dependent biochemical effects than PAHs or other AhR agonists, only the HAHs have been observed to produce AhR-dependent toxicity in vivo. Here we have characterized the dissociation of a prototypical HAH ligand ([(3)H] 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) and PAH-like ligand ([(3)H] beta-naphthoflavone [betaNF]) from the guinea pig, hamster, mouse, and rat hepatic cytosolic AhR in order to elucidate the relationship between the apparent ligand-binding affinities and the divergent potency of these chemicals. Both compounds dissociated very slowly from the AhR with the amount of specific binding remaining at 96 h ranging from 53% to 70% for [(3)H]TCDD and 26% to 85% for [(3)H] betaNF, depending upon the species examined. The rate of ligand dissociation was unaffected by protein concentration or incubation temperature. Preincubation of cytosol with 2,3,7,8-tetrachlorodibenzofuran, carbaryl, or primaquine, prior to the addition of [(3)H]TCDD, shifted the apparent IC(50) of these compounds as competitive AhR ligands by approximately 10- to 50-fold. Our results support the need for reassessment of previous AhR ligand-binding affinity calculations and competitive binding analysis since these measurements are not carried out at equilibrium binding conditions. Our studies suggest that AhR binding affinity/occupancy has little effect on the observed differences in the persistence of gene expression by HAHs and PAHs.

Application of the ethoxyresorufin-O-deethylase (EROD) assay to mixtures of halogenated aromatic compounds

The ethoxyresorufin-O-deethylase (EROD) assay monitors the induction of the xenobiotic-metabolizing enzyme cytochrome P-450 (CYP) 1A1 and is a widely used biomarker for exposure of wildlife to substances that bind the aryl hydrocarbon (Ah) receptor. In this work the induction of EROD activity by single compounds and binary mixtures in primary rat hepatocytes was compared with the predictions of a kinetic model involving mixtures of inducers. The inducing agents were also examined for their ability to activate the Ah receptor to its DNA-binding form and for their ability to act as competitive inhibitors for CYP 1A1. Xenobiotics that failed to activate the Ah receptor did not induce EROD activity. Competitive inhibition for CYP 1A1 between the xenobiotic and 7-ethoxyresorufin caused EROD activity to fall at high xenobiotic concentrations. Competition for a limited number of Ah receptor sites depressed the EROD activity of a strong inducer such as 2,3,7,8-tetrachlorodibenzo-p-dioxin at high concentrations of a weak inducer. Application of the kinetic model to the example of a mixture of low concentrations of dibenzo-p-dioxins and much higher concentrations of polychlorinated biphenyls indicated that EROD assays often seriously underestimate the true potency of an environmental sample. Hence the EROD assay cannot be used for determining dioxin equivalent concentrations using the toxic equivalence factor approach.

Competitive behavior in the interactive toxicology of halogenated aromatic compounds

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that binds and mediates responses to many halogenated aromatic compounds (HACs). Exposure to mixtures of HACs frequently results in nonadditive behavior in both in vivo and in vitro assays. One cause is antagonism, which results when two or more ligands compete for a limited supply of the AhR; one interacts agonistically to induce a strong response, and the other interacts unproductively, eliciting little or no response. This study involves the mechanism by which HACs induce CYP 1A1. Agonistic (e.g., TCDD) and unproductive (e.g., PCB 153) HACs behaved similarly through the stages of initial AhR binding and conversion of the initial AhR-ligand complex to the form that possesses increased affinity for the bound ligand. They diverged in the ability of the AhR-HAC complex to bind to a synthetic oligonucleotide containing the consensus dioxin response enhancer sequence, as studied by the gel retardation assay. Competition for the Ah receptor was used to explain antagonistic behavior between TCDD and other HACs in both the gel retardation assay and the downstream response of CYP 1A1 induction in primary rat hepatocytes.

Competitive inhibition by inducer as a confounding factor in the use of the ethoxyresorufin-O-deethylase (EROD) assay to estimate exposure to dioxin-like compounds

The ethoxyresorufin-O-deethylase (EROD) assay has been extensively used in whole animals and in cell culture as a biomarker of exposure to environmental contaminants such as dioxin-like compounds (DLCs). This paper addresses two controversial phenomena that arise when DLCs are examined by the EROD assay. Firstly, the maximum level of induced EROD activity varies with the identity of the inducing compound; secondly, the induced EROD activity reaches a concentration-dependent maximum level that is followed by an apparent reduction in activity when the concentration of inducer is further increased. These phenomena are completely explained by competitive inhibition of the EROD enzyme-substrate reaction by the dioxin-like compound. A kinetic model explains the biphasic appearance of EROD induction curves as a function of a compound's binding affinity with the Ah receptor (Kd) and its binding affinity to CYP 1A1 (Ki) which results in inhibition of the EROD enzyme-substrate reaction. These results limit the reliability of the information obtained from calibration curves of EROD activity versus concentration of a standard DLC such as 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Characterization of the Ah receptor from human placental tissue

The rate of thermal inactivation of the unliganded human Ah receptor, studied by sucrose density gradient centrifugation, with respect to loss of ligand binding ability, was found to be greater than those of most rodents at 20 degrees C, but the temperature coefficient of the rate constant was much smaller than for the rodent species. This implies that the unliganded human Ah receptor would be thermally more stable than the rodent analogs at physiological temperatures. The liganded form of the human Ah receptor was found to be less stable with respect to ligand release than the rodent receptors. These differences in behavior between human and rodent Ah receptors underline the difficulties in using rodent data in the development of receptor-based models of dioxin toxicity. Attempts to develop an alternative to sucrose density gradient centrifugation, comparable with the hydroxylapatite adsorption method used to assay rodent hepatic Ah receptor, were unsuccessful.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.

Cellular and molecular biology of aryl hydrocarbon (Ah) receptor-mediated gene expression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds elicit diverse toxic and biochemical responses in laboratory animals and mammalian cells in culture. TCDD induces CYP1A1 gene expression and results of extensive research have delineated the molecular mechanism of this response. In target cells, TCDD initially binds to the aryl hydrocarbon (Ah) receptor which accumulates in the nucleus as an Ah-receptor:aryl hydrocarbon nuclear translocator (Arnt) protein heterodimeric complex. The nuclear Ah receptor complex acts as a ligand-induced transcription factor which binds to transacting genomic dioxin/xenobiotic responsive elements (DREs/XREs) located in the 5'-regulatory region upstream from the initiation start site and this interaction results in transactivation of gene transcription. DREs have been identified in several other genes which are induced by TCDD, including CYP1A2, aldehyde-3-dehydrogenase, NAD(P)H quinone oxidoreductase, and glutathione S transferase Ya and similar induction response pathways have been observed or proposed. However, TCDD and other Ah receptor agonists also inhibit expression of several genes and research in this laboratory has investigated inhibition of estrogen (E2)-induced genes including uterine epidermal growth factor, c-fos protooncogene, and the progesterone receptor, estrogen receptor (ER) and cathepsin D genes in human breast cancer cell lines. In MCF-7 human breast cancer cells, E2 induces cathepsin D gene expression and this is associated with formation of an ER/Sp1 complex at the sequence in the promoter region (-199/-165) of this gene. Within 30 min TCDD causes a rapid inhibition of E2-induced cathepsin D gene expression in MCF-7 cells. Moreover, using a series of synthetic oligonucleotides which include the wild-type ER/Sp1 and various mutants, it was shown by gel electromobility shift and transient transfection assays that the nuclear Ah receptor complex binds to an imperfect DRE located between the ER and Sp1 binding sequences. This interaction results in disruption of the ER/Sp1 complex and inhibition of E2-induced gene expression. These results illustrate that the nuclear Ah receptor complex also exhibits activity as a negative transcription factor via a mechanism which is similar to that reported for Ah receptor-mediated induction of gene expression.

Comparative kinetic study of the binding between 2,3,7,8-tetrachlorodibenzo-p-dioxin and related ligands with the hepatic Ah receptors from several rodent species

Kinetic analysis of the time course of association of [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin with hepatic cytosol from five rodent species gave additional evidence for differences in the properties of the Ah receptor ligand binding subunit between species. A parallel study of the association of six tritiated polychlorinated dibenzo-p-dioxins and dibenzofurans with hepatic Ah receptor from Wistar rat and C57BL/6 mouse showed that their rank order for kinetic affinity did not correlate with the rank ordering of their toxic potency and may vary according to the source of the Ah receptor.

Transformation of the aryl hydrocarbon receptor to a DNA-binding form is accompanied by release of the 90 kDa heat-shock protein and increased affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the aryl hydrocarbon receptor (AhR) elicits a sequence of poorly defined molecular events that ultimately yield a heteromeric transformed AhR that is active as a transcription factor. We have previously developed a model of the ligand-initiated transformation of the AhR to the DNA-binding state based on characterization of several forms of the AhR with respect to their physicochemical properties and DNA-binding affinities. The present studies were designed to determine whether, and at what stage, this process of transformation alters the receptor's affinity for TCDD. In rat hepatic cytosol, approx. 10% of the TCDD specifically bound to the AhR rapidly dissociated (t1/2 approximately 1 h), while the remainder was only slowly dissociable (t1/2 approximately 70 h). The isolated DNA-binding forms of the receptor (monomeric and transformed) bound TCDD very tightly (t1/2 > 100 h), whereas TCDD was dissociable from the non-DNA-binding receptor form(s). A lower incubation temperature (0-4 degrees C) and the presence of molybdate partially stabilized the non-DNA-binding fraction of the TCDD.receptor complex and also enhanced TCDD dissociation in crude cytosol. Immunoprecipitation of the different AhR forms with an anti-AhR antibody and immunoblotting with antibody to the 90 kDa heat-shock protein (hsp90) demonstrated that hsp90 was associated with the unoccupied receptor complex as well as with a fraction of the non-DNA-binding TCDD.receptor complex; isolated DNA-binding forms did not contain detectable hsp90. We conclude that while hsp90 remains associated with the AhR, TCDD is readily dissociable; following release of hsp90, however, TCDD becomes very tightly bound, and remains so upon completion of transformation.

Effects of ligand structure on the in vitro transformation of the rat cytosolic aryl hydrocarbon receptor

Incubation of radiolabeled, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF),1,2,3,7,8-pentachlorodibenzo-p-dioxin(PeCDD), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF), 1,2,7,8-TCDF, and 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) with rat hepatic cytosol for 2 h at 0 degrees C gave liganded aryl hydrocarbon (Ah) receptor complexes which were indistinguishable as determined by velocity sedimentation analysis and DNA-Sepharose column chromatography. Incubation of the cytosol plus the different radioligands for 2 h at 20 degrees C resulted in the formation of Ah receptor complexes which exhibited increased retention times on DNA-Sepharose columns. It was apparent that the amount of specifically bound Ah receptor complex or the levels of the transformed Ah receptor complex which eluted from the column with 0.2-0.3 M salt were dependent on the structure of the radioligand. For example, after incubation for 2 h at 20 degrees C the overall yields of the specifically bound transformed Ah receptor complex were 3.4, 2.0, 1.2, 1.9, 0.3, and 0.1%, respectively, using 2,3,7,8-TCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDD, 1,2,3,7,8-PeCDF, 1,2,7,8-TCDF, and 2,3,7,8-TrCDD as radioligands. A more quantitative measure of the structure-dependent transformation of the liganded cytosolic Ah receptor complex was determined using a gel retardation assay with a consensus synthetic dioxin-responsive element (DRE) (26-mer, duplex). The EC50 values obtained for the concentration-dependent formation of the retarded DRE-Ah receptor complex using 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDF, 2,3,7-TrCDD, and 1,2,7,8-TCDF as ligands were 0.26, 0.35, 0.78, 1.75, 27.0, and 220 nM, respectively. The structure-dependent differences in these values were similar to their different potencies as Ah receptor agonists and these data suggest that the structure-dependent transformation of the liganded cytosolic Ah receptor may significantly contribute to the structure-activity relationships observed for 2,3,7,8-TCDD and related compounds.

Factors affecting the toxicity of dioxin-like toxicants: a molecular approach to risk assessment of dioxins

The numerous toxic responses of dioxin-like compounds are mediated by the intracellular Ah (aryl hydrocarbon) receptor. It has been suggested that the regulation of dioxins and similar substances could be placed on a molecular foundation by considering the proportion of Ah-receptor sites occupied by toxicant molecules. The present work has shown that the following formation not yet available would be needed in order to develop this approach: correlation between dioxin exposure and human tissue levels; accurate determination of the association constants for human Ah-receptor with toxicant, and for human receptor-ligand complex with DNA; and knowledge of the intracellular concentrations of both receptor binding sites and DNA binding sites. Furthermore, since not all dioxin-like substances behave identically, this information would need to be gathered for a wide variety of substances.

Characterization of the aryl hydrocarbon receptor in the human C-4II cervical squamous carcinoma cell line

Treatment of C-4II human cervical squamous carcinoma cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) gave a concentration-dependent increase in ethoxyresorufin O-deethylase (EROD) activity. The EC50 for this response was approximately 1 nM and the maximum induced activity was 27 pmol/min/mg protein. The molecular properties of the cytosolic and nuclear aryl hydrocarbon (Ah) receptor complex were determined by velocity sedimentation analysis, photoaffinity labeling, gel retardation using a consensus dioxin responsive element (DRE), and DNA-Sepharose, DRE-Sepharose and Sephacryl S-300 gel permeation column chromatography. The apparent molecular masses of the cytosolic and nuclear photoaffinity-labeled Ah receptor complexes were 110 kDa and differed from the corresponding values obtained for the Ah receptor from other animal species. In contrast, most of the other molecular properties of the Ah receptor were not significantly different from those previously reported for other species. The relative Ah-responsiveness of the C-4II cells was assessed by determining the ratio of the induced EROD activity/nuclear Ah receptor levels for a submaximal inducing dose of [3H]TCDD. The induced activity/binding ratio for the human C-4II cells was 0.77 and was at least one order of magnitude lower than the corresponding value for the Ah-responsive rat hepatoma H-4-II E cells.

The Ah receptor and the mechanism of dioxin toxicity

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Mechanism of action of 2,3,7,8-tetrachlorodibenzo-p-dioxin antagonists: characterization of 6-[125I]methyl-8-iodo-1,3-dichlorodibenzofuran-Ah receptor complexes

6-Methyl-8-iodo-1,3,-dichlorodibenzofuran (I-MCDF) and its radiolabeled analog [125I]MCDF have been synthesized and used to investigate the mechanism of action of 1,3,6,8-substituted dibenzofurans as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) antagonists. Like 6-methyl-1,3,8-trichlorodibenzofuran (MCDF), I-MCDF partially antagonized the induction by TCDD of microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activities in rat hepatoma H-4-II E cells and male Long-Evans rat liver. Incubation of rat liver cytosol with [125I]MCDF followed by velocity sedimentation analysis on sucrose gradients gave a specifically bound peak which sedimented at 9.6 S. This radioactive peak was displaced by coincubation with a 200-fold excess of unlabeled I-MCDF, 6-methyl-1,3,8-trichlorodibenzofuran (MCDF), 2,3,7,8-tetrachlorodibenzofuran (TCDF), and benzo [a]pyrene. Based on the velocity sedimentation results and the elution profile from a Sephacryl S-300 gel permeation column, the Stokes radius and apparent molecular weights of the cytosolic [125I]MCDF-Ah receptor complex were 6.5 nm and 259,200, respectively. In addition, the nuclear [125I]MCDF-receptor complex eluted at a salt concentration of 0.29 M KCl from a DNA-Sepharose column. Velocity sediment analysis of the nuclear [125I]MCDF-Ah receptor complex from rat hepatoma H-4-II E cells gave a specifically bound peak at 5.6 +/- 0.8 S. All of these properties were similar to those observed using [3H]TCDD as the radioligand. In addition, there were several ligand-dependent differences observed in the properties of the I-MCDF and TCDD receptor complexes; for example, the [125I]MCDF rat cytosolic receptor complex was unstable in high salt buffer and was poorly transformed into a form with increased binding affinity on DNA-Sepharose columns; Scatchard plot analysis of the saturation binding of [3H]TCDD and [125I]MCDF with rat hepatic cytosol gave KD values of 1.07 and 0.13 nM and Bmax values of 137 and 2.05 fmol/mg protein, respectively. The nuclear extract from rat hepatoma H-4-II E cells treated with I-MCDF or TCDD interacted with a dioxin-responsive element in a gel retardation assay. These results suggest that the mechanism of antagonism may be associated with competition of the antagonist receptor complex for nuclear binding sites.

Characterization of the interaction of transformed rat hepatic cytosolic Ah receptor with a dioxin responsive transcriptional enhancer

Many of the biological and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin), a highly toxic environmental contaminant, are mediated by a soluble intracellular protein (the Ah receptor (AhR)). Following a poorly defined process of transformation, during which the TCDD:AhR complex acquires the ability to bind to DNA with high affinity, TCDD:AhR complexes activate gene transcription by binding to dioxin responsive enhancers (DREs) adjacent to the responsive gene. Here we have utilized gel retardation analysis to study the interaction of rat hepatic cytosolic TCDD:AhR complexes, transformed in vitro, with dioxin responsive enhancer DNA. Cytosol contains a protein(s) that binds to the DRE in a TCDD-inducible, sequence-specific, time- and temperature-dependent manner and exhibits AhR ligand binding specificity. These results imply that this inducible protein-DNA complex represents the binding of liganded:AhR complex to the DRE. The TCDD:AhR complex bound to the DRE with an equilibrium dissociation constant of 1.2 +/- 0.1 nM, an affinity at least 3800-fold stronger than that for binding to nonspecific DNA. Assuming one DNA binding site per AhR molecule, the total concentration of transformed AhR in these studies was approximately 56.1 +/- 6.6 fmol/mg protein (representing transformation of 45% of the total amount of AhR present in the same cytosolic preparations). Inhibition of AhR transformation, but not ligand or DNA binding, by EDTA and EGTA suggests that a chelatable divalent cation(s) may play a critical role in the transformation process.

Competitive binding of 7-substituted-2,3-dichlorodibenzo-p-dioxins with human placental ah receptor--a QSAR analysis

The competitive binding affinities of thirteen 7-substituted-2,3-dichlorodibenzo-p-dioxins to the human placental cytosolic aryl hydrocarbon (Ah) receptor were determined using [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin as the radioligand. Multiple parameter linear regression analysis of the competitive binding C50 values for these compounds gave the following equation: pEC50 (M) = 6.246 + 1.632 pi - 1.764 sigma 0m + 1.282 HB where pi, sigma m and HB are the physiochemical parameters for substituent lipophilicity, meta-directing electronegativity, and hydrogen bonding capacity respectively. The 7-t-butyl- and 7-phenyl-2,3-dichlorodibenzo-p-dioxins were treated as outliers for the derivation of this equation, and these results suggest that only substituents with van der Waals' volumes less than 40 cm3/mol were accommodated in the receptor binding site. The equations previously derived from the binding of the 7-substituted-2,3-dichlorodibenzo-p-dioxins to the rat, mouse, guinea pig, and hamster hepatic cytosolic receptor were different than the correlation obtained using human placental receptor and provide further evidence for the interspecies differences in the molecular and binding properties of the Ah receptor protein.

Chemically induced hepatic cytosol from the Sprague-Dawley rat: evidence for specific binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to components kinetically distinct from the Ah receptor

A series of exogenous chemicals was used as potential inducers for the hepatic Ah receptor in the Sprague-Dawley rat. 2,3,7,8-Tetrachlorodibenzo-p-dioxin, 2,2',4,4',5,5'-hexachlorobiphenyl and phenobarbital all induced an elevated level of 3H-2,3,7,8-tetrachlorodibenzo-p-dioxin specific binding, while 3,3',4,4'-tetrachloroazobenzene and trans-3,3',4,4'-tetrachlorostilbene caused a depression. Mixtures of these chemicals caused additive effects. Elevated levels of specific binding appeared to be heterologous, comprising a binding species having the normal high stability of the Ah receptor in its liganded form, and another less stable substance having a half-life of approximately 2 h at 37 degrees C.

Hepatic Ah receptor from the Wistar rat: role of solvation in receptor structure and inactivation

Repeated freezing and thawing, the addition of salts, and elevated temperatures all promote the inactivation of the rat hepatic Ah receptor. The reduced availability of bulk water to solvate the protein is proposed to be the factor linking all these routes for inactivation. Prospective protocols for purification of unliganded Ah receptor should therefore minimize the number of freeze/thaw cycles; long-term freezing of cytosolic samples at -20 degrees C is inadequate to maintain long-term viability of the unliganded receptor. The stability of rat hepatic receptor is greatly increased upon binding the ligand, and the extent of ligand-induced stabilization is much greater than what is observed with steroid hormone receptors. Concentrations of NaCl and K2HPO4 up to 0.5 M inactivate the unbound Ah receptor irreversibly, with the loss of approximately 50% of the specific binding. At 2.0 M NaCl, a further reversible reduction in ligand binding activity is observed. The results at lower salt concentrations are interpreted in terms of the irreversible dissociation of a single binding unit from the trimeric cytosolic Ah receptor (which consists of two ligand-binding units and a 90-kDa heat shock protein), with the release of bound ligand from that subunit.

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.

Dioxins and the Ah receptor

Despite continuing controversies related to public policy, information on the molecular biology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has advanced significantly over the past decade. Current understanding of the biological mechanisms of TCDD action is based upon the interactions of TCDD with a genetically expressed cytosolic macromolecule that functions as a receptor in many cells across many species. The Ah receptor recognizes TCDD and structurally similar molecules and serves as the transducing step whereby TCDD alters gene expression through the association of the TCDD:receptor complex with specific TCDD-responsive elements on the genome. Understanding these molecular events and their relevance to the organ-level manifestations of TCDD toxicity may be critical to formulating scientifically based assessments of the risk of TCDD exposure.