Metabolism of benzo [a] pyrene by guinea pig adrenal and hepatic microsomes

Debrisoquine 4-monooxygenase and bufuralol 1'-monooxygenase activities in bovine and rabbit tissues

The tissue distributions of debrisoquine 4-monooxygenase and bufuralol 1'-monooxygenase activities in microsomes from bovine and rabbit tissues were analysed. Debrisoquine 4-monooxygenase and bufuralol 1'-monooxygenase activities were found in liver, and at low levels in cerebral cortex, kidney cortex, lung, small intestine and spleen. Other tissues, such as kidney medulla, adrenocortex, adrenomedulla, blood vessels, thyroid gland, heart, ovary, uterus and testis, showed low levels of bufuralol 1'-monooxygenase activity but not detectable debrisoquine 4-monooxygenase activity. The bufuralol/debrisoquine monooxygenase activity ratios were higher in kidney and lung, and lower in cerebral cortex and spleen than in liver. Both monooxygenase activities in several bovine tissues including liver were inhibited strongly by phenylisocyanide (0.1 mM) and quinidine (0.5 mM), moderately by metyrapone (1 mM), and not at all by KCN (1 mM). NaN3 (5 mM) and sodium cholate (0.5% w/v) inhibited debrisoquine 4-monooxygenase activity strongly and moderately, but bufuralol 1'-monooxygenase activity moderately and strongly, respectively. No effect of a hydroxyl radical scavenger or of superoxide dismutase on either monooxygenase activity was observed. It was concluded from these results, as well as the NADPH dependency of the reactions, that the two monooxygenase reactions observed in these tissues were catalysed by cytochrome P450s.

Analysis of the transcripts of cytochrome P-450IID gene subfamily in bovine adrenal gland

An in situ hybridization experiment showed that the gene transcripts of P-450IID subfamily were uniformly distributed in bovine liver cells and at low levels in bovine adrenal cortex and medulla. There was no clear localization of transcripts of P-450IID genes in three adrenocortical zones, zona glomerulosa, zona fasciculata and zona reticularis. The intensity of signal obtained in the adrenal cortex and medulla showed approximately 2.5:1 ratio. The presence of gene transcripts of P-450IID subfamily in adrenocortex was confirmed by amplifying cDNA using PCR and hybridizing the PCR products with P-450IID cDNA.

Metabolism and toxicity of xenobiotics in the adrenal cortex, with particular reference to 7,12-dimethylbenz(a)anthracene

The adrenal cortex contains high amounts of detoxifying enzymes, as well as generators and protectors of reactive oxygen species. The high content of cytochrome P-450 enzymes in the adrenal cortex together with its remarkable tendency to accumulate hydrophobic substances probably contributes to the extraordinary vulnerability of the gland to a number of xenobiotics. The best studied adrenocorticolytic compounds are the potent carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) and its liver metabolite 7-hydroxymethyl-12-methylbenz(a)anthracene (7-OHM-12-MBA). Adrenocorticolysis generated by these agents in vivo as well as in vitro demonstrates high regioselective requirements and is strongly influenced by the presence of ACTH, steroids, cytochrome P-450 inhibitors and antioxidants. Furthermore, 7-OHM-12-MBA has been demonstrated to uniquely generate selective and massive oxidation of mitochondrial glutathione in cultured rat adrenal cells. The DMBA-induced adrenocorticolysis is thoroughly discussed in this review with particular emphasis on the metabolism of DMBA and the influence of various effectors. A working hypothesis involving a possible peroxidative mechanism is also presented.

Examination of differences between benzo[a]pyrene and steroid hydroxylases in guinea pig adrenal microsomes

The effects of antibodies to cytochromes P-45017 alpha,lyase and P-450C21 on benzo[a]pyrene hydroxylase activity were measured in microsomes from guinea pig adrenals. Anti-cytochrome P-45017 alpha,lyase IgG inhibited about 30% of the benzo[a]pyrene hydroxylase activity of the microsomes in the presence of excess amounts of the IgG, but anti-cytochrome P-450C21 IgG did not affect the activity. In a reconstituted system, consisting of cytochrome P-450, NADPH-cytochrome-P-450 reductase and dilauroylphosphatidylcholine, cytochrome P-45017 alpha,lyase catalyzed, in addition to steroid hydroxylation, benzo[a]pyrene hydroxylation, but cytochrome P-450C21 did not hydroxylate benzo[a]pyrene. Since anti-cytochrome P-45017 alpha,lyase IgG inhibited benzo[a]pyrene hydroxylase activity completely in the reconstituted system with cytochrome P-45017 alpha,lyase, the presence of non-inhibited benzo[a]pyrene hydroxylase in the microsomes suggests that the residual activity in the microsomes may be due to some enzyme other than cytochrome P-45017 alpha,lyase. Benzo[a]pyrene hydroxylase activity was detected in detergent-solubilized microsomes from which cytochrome P-45017 alpha,lyase was removed by using an immobilized anti-cytochrome P-45017 alpha,lyase IgG-Sepharose column. This shows the existence in microsomes of a benzo[a]pyrene hydroxylase other than cytochrome P-45017 alpha,lyase. Benzo[a]pyrene hydroxylase in the solubilized microsomes required O2, NADPH and NADPH-cytochrome-P-450 reductase for its activity and was inhibited by CO. This suggests that benzo[a]pyrene hydroxylase is a cytochrome P-450-dependent monooxygenase. This novel enzyme was also active in xenobiotic metabolism, such as 2-nitropropane denitrification and aminopyrine demethylation.

Hormonal regulation of benzo[a]pyrene metabolism in human adrenocortical cell cultures

In cultured fetal human adrenocortical cells, metabolism of the carcinogen benzo[a]pyrene was found to be unresponsive to the xenobiotic inducers 3-methylcholanthrene, benz[a]anthracene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. However, exposure of cultures to the hormone adrenocorticotropin (ACTH) for 48 hours stimulated benzo[a]pyrene metabolism 3-fold. The major metabolite was the 7,8-diol. Other compounds which stimulate the production of adrenocortical cell cyclic AMP (forskolin and cholera toxin) as well as monobutyryl cyclic AMP also increased benzo[a]pyrene metabolism. Human adrenocortical cells thus provide an unusual example of hormonal regulation of the metabolism of a carcinogen.

Metabolism of 4-chlorobiphenyl by guinea pig adrenocortical and hepatic microsomes

Studies were carried out to determine if 4-chlorobiphenyl (4-CB) was a substrate for adrenal monooxygenases and to compare its interactions with adrenal and hepatic microsomal enzymes. Addition of 4-CB to guinea pig adrenal microsomes produced a typical type I spectral change, indicative of binding to cytochrome(s) P-450 and similar to that seen in hepatic microsomal preparations. The activities of several adrenal and hepatic microsomal monooxygenases were decreased by 4-CB in vitro. High pressure liquid chromatographic analyses revealed that both adrenal and hepatic microsomes, in the presence of NADPH, converted 4-CB to a major metabolite which eluted with a retention time identical to that of 4-chloro-4'-biphenylol (4'-OH-4-CB). The identity of 4'-OH-4-CB was confirmed by mass spectrometry. The maximal rate of 4-CB metabolism was greater in adrenal, compared with liver microsomes, but 4-CB had a higher affinity for hepatic than for adrenal enzymes. The rate of adrenal 4-CB metabolism was four to five times greater in microsomes derived from the inner cortical zone (zona reticularis) than those from the outer zones (zona fasciculata and zona glomerulosa). Hepatic microsomes also converted 4-CB to a minor metabolite whose production was blocked by epoxide hydrolase inhibitors, suggesting it might be a diol. 4-CB metabolism was not demonstrable in adrenal mitochondrial preparations. The results indicate that chlorinated biphenyls can serve as substrates for adrenal microsomal monooxygenases, suggesting that local activation may contribute to their adrenocortical toxicity.

In vivo effects of 3-methylcholanthrene, phenobarbital, pyrethrum and 2,4,5-T isooctylester on liver, lung and kidney microsomal mixed-function oxidase system of guinea-pig: a comparative study

The optimum conditions (pH, microsomal protein amount and substrate concentration) of guinea-pig liver, lung and kidney microsomal aniline 4-hydroxylase, ethylmorphine N-demethylase and benzo[a]pyrene hydroxylase activities were determined. Male guinea-pigs weighing 500-700 g were administered 3-methylcholanthrene (25 mg/kg, i.p. 3 days), phenobarbital (75 mg/kg, i.p. 3 days), pyrethrum (120 mg/kg, i.p. 2 days) and 2,4,5-T isooctylester (200 mg/kg, i.p. 3 days). 3-Methylcholanthrene treatment caused significant increases in liver microsomal benzo[a]pyrene hydroxylase and kidney microsomal aniline 4-hydroxylase activities. However, with phenobarbital treatment the only significant increase was observed in liver microsomal ethylmorphine N-demethylase activity. Pyrethrum treatment decreased kidney microsomal ethylmorphine N-demethylase activity significantly. 2,4,5-T isooctylester treatment increased liver microsomal aniline 4-hydroxylase and lung microsomal ethylmorphine N-demethylase activities significantly. Liver microsomal NADPH-cytochrome c reductase activity was increased significantly by phenobarbital and pyrethrum treatment. The other treatments did not cause any significant changes in microsomal NADPH-cytochrome c reductase activities of liver, lung and kidney. Cytochrome P-450 content of guinea-pig liver microsomes were increased significantly about 2.5-fold and 2-fold by treatment with 3-methylcholanthrene and phenobarbital, respectively. 3-Methylcholanthrene also caused 1 nm spectral shift in the absorption maxima of CO difference spectrum of the dithionite-reduced liver microsomal cytochrome P-450, forming P-449.