Marked differences in the inductive effects of two symmetrical hexachlorobiphenyls and the corresponding unsymmetrical isomer on hepatic monooxygenases

Assessing the role of ortho-substitution on polychlorinated biphenyl binding to transthyretin, a thyroxine transport protein

ortho-Substituted polychlorinated biphenyls (PCBs) make up a large part of the PCB residue found in the environment and human tissues. Our laboratory as well as others have demonstrated that ortho-substituted congeners exhibit important biological activities by aryl hydrocarbon (Ah) receptor-independent mechanisms, including changes in second messenger systems necessary for normal cell function and growth. Previous structure-activity relationship (SAR) studies on second messengers and transthyretin (TTR; prealbumin) binding focused little attention on the ortho-substituted PCBs. Disruption of thyroid hormone (TH) transport is one potentially important mechanism by which PCBs can alter TH homeostasis. A more systematic study of PCB binding to TTR, a major TH transport protein, was undertaken, in which the role of ortho-substitution was more thoroughly investigated. Results from this study indicated that the ortho-only substituted series showed significant binding activity and the relative affinities were 2,2',6 > 2,2' = 2,6 >> 2 = 2,2',6, 6'. As anticipated on the basis of steric considerations, bromine was shown to be more active as an ortho-substituent where the relative affinity of 2,2'-Br was equivalent to 2,2',6-Cl. The congener patterns (di-meta-substitution in one or both rings) most closely resembling the diiodophenolic ring of thyroxine (T(4)) showed the highest binding activity. Multiple ortho-substituents were shown to decrease binding activity in such patterns. Congener patterns (single meta-substitution in one or both rings) more closely resembling the monoiodophenolic ring of T(3) showed significantly lower binding activity, consistent with the relatively low binding activity of T(3) and smaller size of chlorine compared to iodine. The addition of ortho-substitution to such patterns gave variable results depending on the substituent relationship (adjacency or nonadjacency) to the pattern. Some patterns such as 2, 2',4,4',5,5' showed good binding activity and represent common congeners in the commercial Aroclor mixtures and in the environment. The binding potencies of ortho-PCBs to TTR may represent a signature SAR that predicts specific biologic/toxic effects. In this regard, the binding potencies were consistent with measured biological activities of these PCBs, including effects on cell dopamine content, Ca(2+) homeostasis, and protein kinase C translocation in neuronal cells and brain homogenate preparations.

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.

Dose requirements, assay procedures and tissue specificity for PCB inductation of P-450 dependent mono-oxygenase activity in the rat: implications for design of studies measuring in vivo induction of human placental mono-oxygenases

Pregnant Sprague-Dawley and Fisher 344 rats were treated on day 15 of gestation with Aroclor 1254 in a single dose ranging from 0 to 500 mg kg-1 body weight and killed on day 18 of gestation. In the small groups of animals used for this study, no effect was observed on mean maternal liver or placental weight, or the number of fetal resorptions at any of the doses tested. Measurement of aryl hydrocarbon hydroxylase (AHH) and 7-ethoxycoumarin O-deethylase (7ECD) activity in tissue homogenates, however, showed that administration of Aroclor 1254 (15 mg kg-1 body weight or greater) induced mono-oxygenase activity in fetal liver. Both the AHH and 7ECD assay detected effects of PCBs with similar sensitivity, and the findings were comparable when homogenates were assayed instead of microsomes. These data were used to suggest technical approaches to the detection of mono-oxygenase induction in placental tissue human populations exposed to PCBs.

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.