3,4,5,3',4'-Pentachlorobiphenyl as a useful inducer for purification of rat liver microsomal cytochrome P448

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.

Inducing effect of oxfendazole on cytochrome P450IA2 in rabbit liver. Consequences on cytochrome P450 dependent monooxygenases

Male New Zealand rabbits were dosed with either 0.9, 4.5 or 22.5 mg/kg/day of oxfendazole by gastric intubation for 10 days. Oxfendazole administered at the therapeutic dose (4.5 mg/kg) and at the highest dose (22.5 mg/kg) increased 1.54- and 2.36-fold the total liver microsomal cytochrome P450 and more particularly the isoenzyme P450IA2 (95 and 184% increases) as demonstrated by western blotting. Increases in ethoxyresorufin O-deethylation and hydroxylations of benzopyrene and acetanilide occurred in livers of the same animals without any change in N-demethylation of aminopyrine, benzphetamine or erythromycin. Because of the unchanged level of mRNA specific to cytochrome P450IA2, as shown by northern blot analysis of poly mRNA, an enzyme stabilization rather than a transcriptional activation of IA2 genes should be involved in the P450IA2 regulation mechanisms. Oxfendazole bound strongly to cytochrome P450, giving rise to a type II spectrum, and inhibited noncompetitively the ethoxyresorufin O-deethylase and acetanilide hydroxylase activities, this confirmed that oxfendazole interacts only with the P450IA2 family. On the basis of a comparison of the enzymatic activities induced by various imidazole drugs, it was concluded that oxfendazole, like omeprazole and albendazole, behaved as a 3-methylcholanthrene-type inducer. These three benzimidazoles did not all belong to the same category of cytochrome P450 inducers as the antifungal drugs miconazole, clotrimazole and ketoconazole.

Differential induction of cytochromes P450 and cytochrome P450-dependent arachidonic acid metabolism by 3,4,5,3',4'-pentachlorobiphenyl in the rat and the guinea pig

Differential induction of hepatic cytochromes P450 by 3,4,5,3',4'-pentachlorobiphenyl (PENCB) has been observed in the rat and the guinea pig: (1) in rat and guinea pig, treatment with the chosen dose levels resulted in significant induction of total, carbon monoxide-discernible cytochrome P450 content; the absorption maximum of the CO-adduct of the dithionite-reduced microsomes from PENCB-induced rat liver was shifted from 450 to 448 nm, whereas its counterpart in the guinea pig did not; (2) PENCB treatment significantly increased EROD activity in rat liver microsomes (up to 60-fold), but the increase in the guinea pig was less than fivefold; (3) PENCB-induced rat liver microsomes significantly induced the omega-1 hydroxylation of arachidonic acid (AA); however, omega-1 hydroxylation of AA was hardly affected by PENCB treatment in the guinea pig. Instead, omega-hydroxylation was significantly increased in this latter species. In addition to omega-1 hydroxylation in the rat or omega-hydroxylation in the guinea pig, an additional AA metabolite (designated peak III) was significantly induced by PENCB in both rat and guinea pig; (4) Western blot and ELISA analyses with polyclonal anti-P450 IA1/IA2 and IVA1 antibodies demonstrated that P450 IA1 was significantly induced in the rat but only slightly induced in the guinea pig, whereas P450 IVA1 was significantly suppressed in the rat but significantly induced in the guinea pig by PENCB treatment. The induction of the third arachidonic acid metabolite peak, Peak III, in both rat and guinea pig, particularly in the guinea pig, is obviously neither mediated by P450 IA1 nor by P450 IV A1. At present, it is still unclear which form(s) of cytochrome P450 isoenzymes is responsible for this latter hydroxylation of arachidonic acid.

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.

Inducers of cytochrome P-450d: influence on microsomal catalytic activities and differential regulation by enzyme stabilization

The interaction of isosafrole, 3,4,5,3',4',5'-hexabromobiphenyl (HBB) and hexachlorobiphenyl (HCB) with cytochrome P-450d was evaluated by characterization of estradiol 2-hydroxylase activity. Displacement of the isosafrole metabolite from microsomal cytochrome P-450d derived from isosafrole-treated rats resulted in a 160% increase in estradiol 2-hydroxylase. The increase was fully reversed by incubation with 1 microM HBB. Although isosafrole is capable of forming a complex with many different cytochrome P-450 isozymes, it appears to bind largely to cytochrome P-450d in vivo as was demonstrated by measuring the enzymatic activity of microsomal cytochromes P-450b, P-450c, and P-450d from isosafrole-treated rats. When estradiol 2-hydroxylase was measured in rats treated with increasing doses of HCB, there was a gradual decrease in microsomal enzyme activity despite a 20-fold increase in cytochrome P-450d. The ability of cytochrome P-450d ligands to stabilize the enzyme was investigated in two ways. First, cytochromes P-450c and P-450d were quantitated immunochemically in microsomes from rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), at a dose which maximally induced total cytochrome P-450, followed by a single dose of a second inducer. The specific content of cytochrome P-450d was significantly increased when isosafrole or HCB was the second inducer but not when 3-methylcholanthrene was the second inducer. Second, the relative turnover of cytochrome P-450d was measured by the dual label technique. Following TCDD treatment, microsomal protein was labeled in vivo with [3H]leucine, the second inducer was given and protein was again labeled 3 days later with [14C]leucine. A higher ratio of 3H/14C in the cytochrome P-450d from isosafrole + TCDD- and HCB + TCDD-treated rats relative to TCDD (control)-treated rats suggested that isosafrole and HCB were able to retard the degradation of cytochrome P-450d, presumably by virtue of being tightly bound to the enzyme.

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.

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.