Multiple forms of solubilized and partially resolved cytochrome P-450 from rats induced by 2,3,5,2',3',5'- and 3,4,5,3',4',5'-hexachlorobiphenyls

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.

Induction of pulmonary and hepatic cytochrome P-450 species by coal fly ash inhalation in rats

The effect of fly ash inhalation on xenobiotic metabolizing enzymes and heme metabolism in lung and liver has been studied in rats. Fly ash inhalation induced pulmonary and hepatic cytochrome P-450 content, aryl hydrocarbon hydroxylase and glutathione S-transferase activity. Induction of cytochrome P-450 was accompanied by induction of delta-amino levulinic acid synthetase in lung and inhibition of heme oxygenase in both lung and liver. Fly ash inhalation induced those species of cytochrome P-450 which closely resembled cytochrome P-448 in spectral properties and electrophoretic mobility.

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.

Establishment of two rat hepatoma cell strains produced by a carcinogen initiation, phenobarbital promotion protocol

Two Fischer 344 rat hepatoma cell strains, JM1 and JM2, have been isolated from a primary hepatocellular carcinoma. Primary tumor formation was induced in a two-thirds partially hepatectomized rat by a single low dose (70 mg/kg of diethylnitrosamine followed by chronic phenobarbital administration (0.1 g/100 ml drinking water). The primary tumors were passed three times by subcutaneous implantation of tumor fragments into the inguinal region of syngeneic recipients. The fourth pass was by injection of tumor cells directly into the livers of recipient rats. Several weeks later, the tumor-containing rat livers were subjected to collagenase perfusion. Two cell lines emerged from tissue culture of the cells isolated by perfusion. Each cell line was cloned by serial dilution. Cells JM1 and JM2 were tumorigenic when injected into syngeneic rats. The tumors, which arose from injected cell strains, exhibited several characteristics of hepatocellular carcinoma. Morphology was examined by light and electron microscopy. Histochemical studies of JM1 and JM2 cells grown in vitro and in vivo were done. The levels of tyrosine aminotransferase and three microsomal enzymes of importance to drug and carcinogen metabolism were investigated. To our knowledge, this is the first report of cell strains derived from an initiation promotion protocol in rats.

Structure-activity relationships of chlorinated benzenes as inducers of different forms of cytochrome P-450 in rat liver

Hexachlorobenzene (HCB) produced increases in ethoxyresorufin (ERR) O-deethylase, aryl hydrocarbon hydroxylase (AHH) and aminopyrine N-demethylase activities in rat liver phenobarbital which were intermediate between those produced by phenobarbital and 3,4-benzpyrene (BP), alpha-Naphthoflavone (ANF) selectivity inhibited ERR activity in BP and HCB-induced microsomes (94% and 88%). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of liver microsomes indicated that HCB did not produce a detectable increase in a polypeptide with electrophoretic properties similar to those of purified cytochrome P-448 (Mr = 56,000). However, HCB did induce a polypeptide with Mr = 53,000 corresponding to one of two polypeptide bands induced by BP. This polypeptide may represent a second form of cytochrome P-448. Purification of HCB to remove possible dibenzo-p-dioxin impurities did not alter the 'mixed-type' induction produced by HCB. In contrast to HCB, all other chlorinated benzenes tested resembled phenobarbital as inducers.