Stimulation of hepatic drug hydroxylation and conjugation by a cutaneously applied PCB-mixture (clophen A 50)

Differential regulation of hepatic glutathione transferase and glutathione peroxidase activities in the rat

The effects of the xenobiotics, i.e. butylated hydroxytoluene, beta-naphthoflavone, isosafrole, pregnenolone-16 alpha-carbonitrile, trans-stilbene oxide, 3-methylcholanthrene, phenobarbital, 3,3',4,4'-tetrachlorobiphenyl, 2,2',4,4',5,5'-hexachlorobiphenyl, on rat liver cytosolic glutathione transferase and glutathione peroxidase activities have been investigated. Although the glutathione transferase isozymes (measured by the specific substrates ethacrynic acid and delta 5-androstene-3,17-dione) which have been shown to possess peroxidase activity were significantly increased, little or no increase in peroxidase activity (toward cumene hydroperoxide, tert-butyl hydroperoxide or hydrogen peroxide) was observed. Likewise during a 16-day time course following the administration of Aroclor 1254 or fireMaster BP-6 (each 500 mg/kg, i.p.), potent induction of glutathione transferase activities was seen without any significant increases in peroxidase activities. In fact during the second week of the time course, there were significant decreases in selenium-dependent glutathione peroxidase activity (toward hydrogen peroxide). The inverse regulation of these activities, i.e. the depression of selenium-dependent glutathione peroxidase activity following sustained induction of glutathione transferases, may have direct implications for the toxicity of the polyhalogenated aromatic hydrocarbons.

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.

Metabolic activation of promutagens, detectable in Ames' Salmonella assay, by 5000 X g supernatant of rat ventral prostate

Induction of aryl hydrocarbon hydroxylase (AHH) and 7-ethoxyresorufin-O-deethylase (7-EOD) activities as well as of benzo[a]pyrene (BP) metabolite formation in rat prostatic microsomes has been demonstrated after treatment with beta-naphthoflavone (BNF). The capacity to convert promutagenic compounds to ultimate mutagenic metabolites in the Ames' Salmonella assay by 5000 X g supernatant of rat ventral prostate was investigated. Male rats were treated with BNF, polychlorinated biphenyls (PCB; Arochlor 1254), phenobarbital (PB) and the vehicle, corn oil. PCB or BNF pretreatment increased the AHH- and 7-EOD activities 100-200-fold in the rat prostate 5000 X g supernatant (S-5 fraction). Epoxide hydrolase (EH) and glutathione-S-transferase (GST) activities were not affected while UDP-glucuronosyltransferase (UDP-GT) was increased 2.2- and 2.5-fold by PCB and BNF, respectively. PB did not significantly affect any of the enzyme activities measured. A dose-dependent increase in mutagenic response versus amount of 5000 X g supernatant and promutagen (aflatoxin B1 (AFB), 2-aminofluorene (2-AF), BP) was observed. The most pronounced activation was obtained with S-5 fraction from BNF- or PCB-treated rats. The great sensitivity of prostatic AHH to certain inducers and the capacity of the prostate to produce mutagenic metabolites might be of importance for initiation of prostatic cancer by environmental factors.

Effects of short-term dietary exposure to polychlorinated biphenyls on pharmacokinetics of intravenous pentobarbital in rats

The intravenous pharmacokinetics of pentobarbital (30 mg/kg as pentobarbital sodium) in rats were studied at 0, 1, 2, 3, and 10 days after pretreatment with 0, 1, 5, 25, and 125 ppm of polychlorinated biphenyls in food. The polychlorinated biphenyls then were removed from the food, and the residual effects of the exposure on pentobarbital pharmacokinetics were studied at 15, 25, 45, and 70 days after initiation of the polychlorinated biphenyl exposure. The pharmacokinetics of pentobarbital were approximated to a one-compartment model. After pretreatment at 2 and 5 ppm for up to 10 days, all pentobarbital pharmacokinetic parameters obtained were comparable to control values. Pretreatment at 125 ppm significantly reduced the biological half-life and raised the total body elimination rate constant, total body clearance, and intrinsic clearance of pentobarbital after a 1-day exposure; all parameters apparently reached a new steady-state value by Days 5--10,. Enhanced pentobarbital elimination at 25 ppm was observed after a 3-day exposure, but, again, the elimination parameters appeared to have reached a steady state after 5--10 days of pretreatment. Upon removal of the polychlorinated biphenyls, the various pharmacokinetic parameters showed a lag phase prior to a gradual return to control values. The study shows that intrinsic clearances rather than total body clearances or half-lives are more appropriate in assessing enzymatic induction in agents undergoing facile liver metabolic clearance that borders on blood flow rate dependency.

Effect of short-term exposure to polychlorinated biphenyls on first-pass metabolism of pentobarbital in rats

The enhancement of the first-pass metabolism of orally administered pentobarbital in rats was examined after a 10-day exposure to food contaminated with polychlorinated biphenyls at 25 and 125 ppm. The degree of the first-pass effect and the influence of the polychlorinated biphenyl exposure were quantitated by comparing the areas under the plasma concentration--time curves after oral and intravenous dosing in control and treated animals. By using the clearance model and assuming that pentobarbital was eliminated totally by liver metabolism, the experimentally determined oral availability was predicted adequately from both the oral and intravenous data. The enhanced first-pass effect was principally in the intrinsic clearance term, although liver blood flow rates also appeared to be enhanced in animals treated with polychlorinated diphenyls at 125 ppm.

Senescent changes in rodent hepatic epoxide metabolism

Age-related alterations in epoxide metabolism were examined in subcellular fractions of liver from 3-, 12- and over 24-month-old male rats and mice. Using styrene oxide as the substrate, glutathione-S-transferase activity remained unchanged while the activity of epoxide hydrase increased with age in both species. The microsomally-mediated binding of benzo[a]pyrene to DNA was also increased in the old animals. Thus, senescent rodents retain or increase their ability to metabolize epoxides. The effect on epoxide metabolism of pretreatment of the senescent rodents with polychlorinated biphenyls was also examined. Glutathione-S-transferase activity was induced only in old animals. However, epoxide hydrase activity, while inducible in all age groups of rats, increased only in young mice. Therefore, there is an age-related difference in response of epoxide metabolizing enzymes to polychlorinated biphenyl treatments between rats and mice.