The relationship between polarizability of polychlorinated biphenyls and their induction of mixed function oxidase activity

Assessment of hydroxylated metabolites of polychlorinated biphenyls as potential xenoestrogens: a QSAR comparative analysis∗

Alternative methods, including quantitative structure-activity relationships (QSAR), are being used increasingly when appropriate data for toxicity evaluation of chemicals are not available. Approximately 40 mono-hydroxylated polychlorinated biphenyls (OH-PCBs) have been identified in humans. They represent a health and environmental concern because some of them have been shown to have agonist or antagonist interactions with human hormone receptors. This could lead to modulation of steroid hormone receptor pathways and endocrine system disruption. We performed QSAR analyses using available estrogenic activity (human estrogen receptor ER alpha) data for 71 OH-PCBs. The modelling was performed using multiple molecular descriptors including electronic, molecular, constitutional, topological, and geometrical endpoints. Multiple linear regressions and recursive partitioning were used to best fit descriptors. The results show that the position of the hydroxyl substitution, polarizability, and meta adjacent un-substituted carbon pairs at the phenolic ring contribute towards greater estrogenic activity for these chemicals. These comparative QSAR models may be used for predictive toxicity, and identification of health consequences of PCB metabolites that lack empirical data. Such information will help prioritize such molecules for additional testing, guide future basic laboratory research studies, and help the health/risk assessment community understand the complex nature of chemical mixtures.

Molecular orbital studies on brominated diphenyl ethers. Part II--reactivity and quantitative structure-activity (property) relationships

Polybrominated diphenyl ethers (PBDEs) are widely used as flame retardants and are increasingly turning up in the environment. Their structural similarities to polychlorinated biphenyls and thyroid hormones suggest they may be a risk to human health. The present study examines the reactivity of brominated diphenyl ethers (BDEs) on the basis of the electronic structures as calculated by semiempirical AM1 self-consistent field molecular orbital (SCF-MO) method. Frontier orbital energies were used to elucidate the reactivity of BDEs in electrophilic, nucleophilic and photolytic reactions. From an examination of the frontier electron densities, the regioselectivity, or orientation, of metabolic reactions of BDEs was predicted. Furthermore, satisfactory quantitative structure-activity (property) relationship (QSAR and QSPR) models were derived to calculate gas chromatographic and ultraviolet spectral properties and luciferase induction activities from the AM1-computed electronic parameters.

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.

Identification of the Ah receptor in selected mammalian species and induction of aryl hydrocarbon hydroxylase

The Ah receptor protein, important in the mechanism of induction of aryl hydrocarbon hydroxylase activity, has been identified and partially characterized in hepatic cytosolic preparations from rat, BALB/c mouse, gerbil, hamster, rabbit, ferret and guinea-pig by means of sucrose density centrifugation analysis and hydroxyapatite binding assays. Using 2,3,7,8-tetrachloro[3H]dibenzo-p-dioxin (TCDD) as the ligand, total specific binding capacities ranged over 74-691 fmol [3H]TCDD/mg cytosolic protein and apparent dissociation constants ranged over 0.30-7.8 nM. There was no quantitative correlation between the concentration of cytosolic Ah receptors and the 3-methylcholanthrene-mediated induction of aryl hydrocarbon hydroxylase activity in the species studied. Competitive binding studies with a series of monohydroxylated benzo[a]pyrene derivatives suggested the importance of electronic character in their ability to bind to the Ah receptor and to compete with TCDD for specific binding sites on the receptor.

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.

Competitive binding to the cytosolic 2,3,7,8-tetrachlorodibenzo-p-dioxin receptor. Effects of structure on the affinities of substituted halogenated biphenyls--a QSAR analysis

The proposed mechanism of action of the toxic halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), involves the initial binding to a high-affinity, low-capacity, cytosolic receptor protein. Previous studies have shown that several 4'-halo-2,3,4,5-tetrachlorobiphenyls bind to the TCDD receptor and that a lateral substituent on both phenyl rings is required for activity. Using an extended series of eighteen 4'-substituted-2,3,4,5-tetrachlorobiphenyls as probes, the effects of a variable lateral substituent on receptor binding affinity and the induction of aryl hydrocarbon hydroxylase (AHH) in vivo and in rat hepatoma H-4-II E cells have been determined. For most substituents, there was an excellent correlation between the rank-order potency for receptor binding and the rank-order potency for AHH induction. Based on in vitro binding affinities (EC50 values) of the 4'-substituted tetrachlorobiphenyls, a multiparameter regression equation was formulated correlating the binding constants to physicochemical substituent parameters. For thirteen compounds out of the present series, multiple regression analysis of the binding data led to the following equation: log(1/EC50) = 1.53 sigma + 1.47 pi + 1.09HB + 4.08, r = 0.978. The results suggest that halogen substitution on both phenyl rings is not a requirement for binding and that hydrophobic (pi) and electronic (sigma) substituent constants and a variable for hydrogen bond (HB) formation are significant parameters describing relative binding avidities of this series of substituted biphenyls for the TCDD receptor.

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.