Metabolism of benzo(a)pyrene by microsomes from tissues of pregnant and fetal hamsters

In vitro and in vivo studies on oxygen free radical and DNA adduct formation in rat lung and liver during benzo[a]pyrene metabolism

Reactive oxygen species (ROS), possibly produced during the metabolic conversion of benzo(a)pyrene (B[a]P), could be involved in B[a]P-induced genotoxicity and, eventually, carcinogenicity. Therefore, ROS formation by rat lung and liver microsomes was studied in vitro by electron spin resonance (ESR/EPR) spectrometry. B[a]P-mediated generation of ROS was detected in incubations with rat lung, but not with liver microsomes. Inhibition of cytochrome P450 (CYP450) by the non isoform-specific inhibitor SKF-525A resulted in a complete inhibition of B[a]P-dependent ROS formation, whereas ROS formation was not affected by inhibition of prostaglandin H synthase by indomethacin. Subsequently, bulky DNA adduct formation and 8-oxo-dG levels after a single oral dose of B[a]P were examined in vivo in rat lung and liver, in combination with urinary excretion of 8-oxodG. B[a]P exposure resulted in increased urinary 8-oxo-dG levels. On the contrary, 8-oxo-dG levels decreased in liver and lung after B[a]P exposure. Bulky DNA adducts reached higher levels and were more persistent in rat lung than in liver. These results indicate that ROS are generated during the CYP450 dependent metabolism of B[a]P, particularly in the rat lung, but this does not necessarily result in increased levels of oxidative DNA damage in vivo, possibly by induction of DNA repair mechanisms.

Qualitative and quantitative differences in the induction and inhibition of hepatic benzo[a]pyrene metabolism in the rat and hamster

The present study compared the induction and inhibition of the metabolism of the prototype polycyclic aromatic hydrocarbon, benzo[a]pyrene (BaP), in rat and hamster liver microsomes. The production of total polar metabolites was quantitated by separating 3H-metabolites from [3H]-BaP using reverse-phase thin-layer chromatography. The rate of hepatic microsomal BaP metabolism was similar in the rat and hamster (0.81 vs 0.72 nmol/min/nmol cytochrome P-450 respectively). In the rat, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 5 micrograms/kg, i.p.) and 3-methylcholanthrene (3-MC; 50 mg/kg, i.p., X 3 days) pretreatments doubled the rate of BaP metabolism, whereas phenobarbital pretreatment (PB; 80 mg/kg, i.p., X 3 days) had no effect. In contrast, hamster hepatic microsomal BaP metabolism was elevated 2.3-fold by PB pretreatment, whereas TCDD and 3-MC pretreatments had no effect. Isosafrole pretreatment (ISO; 150 mg/kg, i.p., X 3 days) elevated the rate by almost 2-fold in each species. Another cytochrome P-448-mediated activity, 7-ethoxyresorufin O-deethylase (EROD), was induced by the same compounds that induced BaP metabolism in the rat. In hamster liver microsomes, in contrast to BaP metabolism, EROD was induced by TCDD and 3-MC but not PB or ISO pretreatments. The results suggest differences in the substrate specificity of the cytochromes P-448-450 induced by TCDD, 3-MC and PB in these species. This was supported by the different selectivity of the in vitro inhibitors, metyrapone and 7,8-benzoflavone, towards BaP metabolism and EROD in hepatic microsomes from TCDD- or PB-pretreated rats and hamsters. Reverse-phase HPLC analysis indicated that, while 3-hydroxy-BaP was the major metabolite formed by the untreated rat, untreated hamster liver microsomes formed predominantly BaP-4,5-diol. Microsomes from TCDD-treated rats generated elevated levels of all BaP-diols, diones and 3-hydroxy-BaP, with the major metabolites being BaP-9,10- and BaP-7,8-diols. In contrast, the metabolite profile from TCDD-pretreated hamsters was unchanged from the control. PB-treated hamster microsomes produced elevated levels of BaP-diones and 3-hydroxy-BaP. However, the major hepatic metabolite formed by PB-pretreated hamsters was BaP-4,5-diol, while BaP-9,10- and BaP-7,8-diols were not detected. The results of the study indicate differences in the induced cytochrome P-450s and the generation of toxic BaP metabolites in the liver of the rat and hamster.

Somatic mutation, DNA damage and cytotoxicity induced by benzo[a]pyrenedione/benzo[a]pyrenediol redox couples in cultured mammalian cells

BP-3,6-dione was found to be mutagenic, cytotoxic and to induce DNA damage in a transformed line of Syrian hamster fibroblasts at low concentrations, 2 micrograms/ml and less. Inhibition of sulfate and glucuronic acid conjugating enzymes with salicylamide potentiated the above effects of BP-3,6-dione. Diminishing cellular capacity to scavenge superoxide anion radicals also potentiated the mutagenic and cytotoxic action of the dione. The presence of dicumarol, a specific inhibitor of the two-electron reduction of quinones by DT-diaphorase, afforded some protection against cytotoxicity. The results indicate that BP-3,6-dione undergoes two-electron reduction to an unstable hydroquinone, BP-3,6-diol, or one-electron reduction to a semiquinone radical intermediate and that both of these reduced forms undergo rapid univalent oxidation to generate active reduced oxygen species. The data are consistent with the hypothesis that active oxygen species generated by BP-dione/BP-diol redox cycling are responsible, at least in part, for the mutagenic and cytotoxic effects observed with BP-3,6-dione.

Benzo[a]pyrene dione-benzo[a]pyrene diol oxidation-reduction couples; involvement in DNA damage, cellular toxicity, and carcinogenesis

Three isomeric quinone metabolites of the environmental carcinogen benzo[a]pyrene undergo reversible, univalent oxidation-reduction cycles involving the corresponding benzo[a]pyrene diols and intermediate semiquinone radicals. Under anaerobic conditions, benzo[a]pyrene 1,6-dione, benzo[a]pyrene 3,6-dione, and benzo[a]pyrene 6,12-dione are readily reduced by mild biological agents such as NADH and glutathione. The benzo[a]pyrene diols, in turn, are very rapidly autooxidized to diones when exposed to air. Substantial amounts of hydrogen peroxide are produced during these autooxidations. The benzo[a]pyrene diol/benzo[a]pyrene dione interconversions proceed by one-electron steps; the corresponding semiquinone radicals were detected as intermediates when the reactions were carried out at high pH. Benzo[a]pyrene diones are electron-acceptor substrates for NADH dehydrogenase. Catalytic amounts of these metabolites, together with this respiratory enzyme, function as cyclic oxidation-reduction couples to link NADH and molecular oxygen in the continuous production of hydrogen peroxide. Benzo[a]pyrene diones induce strand scissions when incubated with T7 DNA. The damage is modified by conditions that indicate that reduced oxygen species propagate the reactions responsible for strand scission. Benzo[a]pyrene diones are cytotoxic at low concentrations to cultured hamster cells. The cytotoxic effect can be substantially reduced by depletion of oxygen from the growth medium and the atmosphere in which the cells are incubated. The results support the hypothesis that the biological activity of benzo[a]pyrene diones is due to the regenerative oxidation-reduction cycles involving quinone and hydroquinone forms; activated oxygen species and semiquinone radicals formed during these cycles are most likely responsible for the observed cytotoxic action. The role of activated oxygen species in carcinogenesis is discussed.

Identification and partial purification of hamster microsomal cytochrome P-450 isoenzymes

We have identified and partially purified three forms of cytochrome P-450 from hamster liver microsomes. Phenobarbital (PB) treatment induced three major polypeptides with relative mobilities (Mr) of 47,000, 50,000 and 51,500. The 47,000 polypeptide was assigned as epoxide hydrolase, since it was also enhanced by trans-stilbene oxide (TSO) treatment. Two polypeptides (Mr = 48,500 and 53,500) were induced by both 3-methylcholanthrene (3-MC) and beta-naphthoflavone (BNF) treatments. Treatment with Aroclor 1254 induced three polypeptides (Mr = 48,500, 50,000 and 53,500), indicating the induction of both drug- and carcinogen-inducible cytochrome P-450s. Liver microsomal benzo[a]pyrene hydroxylase activity was not affected significantly by any of these inducers. In contrast, it was induced 2- to 3-fold in lung microsomes by 3-MC, BNF or Aroclor 1254 treatment. Benzphetamine N-demethylase and 7-ethoxycoumarin O-deethylase activities, expressed as nmoles of product formed per min per mg of liver microsomal protein, were increased 3- to 4-fold by either PB or Aroclor treatment. The activity of 7-ethoxycoumarin O-deethylase was the only one enhanced significantly by 3-methylcholanthrene or beta-naphthoflavone treatment in liver microsomes. Pregnenolone-16-alpha-carbonitrile (PCN) and TSO did not alter any of these activities. The major polypeptides induced by PB (Mr = 50,000) and 3-MC (Mr = 48,500 and 53,500 respectively) were partially purified, to a specific content of 6-10 nmoles P-450/mg of protein and were active in catalyzing N-demethylation of benzphetamine, hydroxylation of benzo[a]pyrene, and O-deethylation of 7-ethoxycoumarin with different substrate specificity. None of these isoenzymes immuno-cross-reacted with antibodies prepared against rabbit cytochrome P-450LM2 or P-450LM4.

Developmental onset of mixed-function oxidase activity in preimplantation mouse embryos

Two-cell embryos, obtained from the C57BL/6N and DBA/2N strains, were cultured in media that supported in vitro differentiation and that contained [3H]benzo[a]pyrene. High-pressure liquid chromatography of the activated intermediates formed during in vitro early embryonic development indicated that the onset of polynuclear aromatic hydrocarbon activation coincided with blastocyst formation. Comparison of individual oxygenated intermediates metabolically formed from embryos genetically "responsive" or "nonresponsive" to aromatic hydrocarbons revealed significant quantitative differences in the production of dihydrodiol, quinone, and phenolic derivatives. In addition to exhibiting basal mixed-function oxidase activity, blastocysts were also responsive to enzymatic induction when exposed to 2,-3,7,8-tetrachlorodibenzo-p-dioxin. The presence of operative metabolite-detoxifying pathways was also assayed. Enzymatic treatment of water-soluble metabolites with beta-glucuronidase or arylsulfatase revealed that neither glucuronic acid conjugates nor sulfate ester derivatives were present. These data, therefore, provide direct evidence that late preimplantation mouse embryos (day 3 1/2 of gestation) are similar to later developmental stages in having the enzymatic capability for xenobiotic activation and enzyme induction but are dissimilar with respect to their detoxification mechanism(s). Moreover, the ability of preimplantation embryos to activate directly polynuclear aromatic hydrocarbon to bioreactive intermediates may be of importance in assessing the ontological susceptibility of the developing embryo to carcinogenic or teratogenic chemicals.

Environmental influences on human foetal and placental xenobiotic metabolism

The human foetus is more capable of metabolizing xenobiotics than foetuses of common laboratory animal species. However, xenobiotic metabolism in animal foetuses is inducible by the exposure of the mother to various inducers during late pregnancy. Xenobiotic metabolism in neonates is more easily inducible than in foetal animals. With respect to the human foetus at mid-pregnancy, the hepatic enzyme systems do not seem to be readily inducible by exoaenous inducers, whereas the placental monooxygenase system is almost totaly dependent on maternal cigarette smoking. In the human newborn, indirect evidence points to the possibility of induction by potential inducers. The ontogenetic development of xenobiotic metabolism is probably regulated by endogenous hormones. It is possible that environmental factors may effect these normal regulatory and "imprinting" phenomena and thus lead to permanent disturbances in xenobiotic metabolism.

Benzo[a]yrenedione/benzo[a]pyrenediol oxidation-reduction couples and the generation of reactive reduced molecular oxygen

The ability of the isomeric quinone metabolites of benzo[a]pyrene, benzo[a]pyrene-6,12-dione, benzo[a]pyrene-1,6-dione, and benzo[a]pyrene-3,6-dione to undergo reversible, univalent oxidation-reduction cycles involving the corresponding benzo[a]pyrenediols and intermediate semiquinone radicals has been characterized. Under anaerobic conditions, all three benzo[a]pyrenediones are easily reduced to benzo[a]pyrenediols, even by mild biological agents such as NAD(P)H, cysteamine, and glutathione. The benzo[a]pyrenediols, in turn, are very rapidly autoxidized to the benzo[a]pyrenediones when exposed to air. Substantial amounts of hydrogen peroxide are produced during these autoxidations, and other reactive reduced oxygen species, such as the superoxide and hydroxyl radicals, are probably formed transiently as well. The benzo[a]pyrenediol-benzo[a]pyrenedione interconversions proceed by one-electron steps; the corresponsing semiquinone radicals can be monitored by electron spin resonance spectroscopy as inter mediates during these reactions carried out at high pH. Benzo[a]pyrenediones induce DNA strand scission when incubated with bacteriophage T7 DNA. This damage is modified by conditions which indicate that reduced oxygen species propagate the free-radical reactions responsible for the strand scission. Benzo[a]pyrenediones are electron-acceptor substrates for NADH dehydrogenase from Clostridium kluyveri. Catalytic amounds of these benzo[a]pyrene metabolites, together with this respiratory enzyme function as cyclic oxidation-reduction couples which link NADH and molecular oxygen in the continuous production of hydrogen peroxide. These data, together with preliminary results with cells in culture, indicate that benzo[a]pyrenediones are potentially harmful metabolites of benzo[a]pyrene, acting by processes which lead to their regeneration rather than depletion; nucleic acid and call damage is probably produced by the reactive reduced oxygen species resulting from such regenerative oxidation-reduction cycles.