Characteristics of benzo(a)pyrene metabolism and cytochrome P-450 heterogeneity in rat liver nuclear envelope and comparison to microsomal membrane

Life and Times in Biochemical Toxicology

The biochemical facets of toxicology have always had a major role in providing insight into mechanisms. Some of the history of the development of this area is summarized, including metabolism, enzymology, and the chemistry of reactive intermediates. Knowledge in these fields has had a major impact in the areas of drug metabolism and safety assessment, which are both critical steps in the development of pharmaceuticals and the rational use of commodity chemicals. The science of toxicology has developed considerably with input from other disciplines and today is poised to emerge as a predictive science with even more dramatic impact. The challenges ahead are considerable but there is renewed excitement in the potential of the field. As in the past, further advances in the field of toxicology will require the input of knowledge from many disciplines.

The bioavailability of polycyclic aromatic hydrocarbons from different dose media after single and sub-chronic exposure in juvenile swine

Humans are constantly exposed to contaminants in the environment, which may lead to changes in physiological processes by altering enzyme activities that could affect bioavailability. However, bioavailability estimates are typically made from a single exposure to an animal model, which may lead to overestimating bioavailability. This study uses juvenile swine to model human exposure to benzo[a]pyrene (BaP) and anthracene in certified reference material (CRM), spiked soil, spiked food, or spiked corn oil after one and seven days of dosing. Area under the curve (AUC) was calculated after one and seven days of exposure for both BaP and anthracene for each exposure media. Whereas there were significant differences in AUC between different media, there were no significant changes in AUC after sub-chronic exposure to BaP or anthracene. Average BaP bioavailability for CRM, spiked soil, spiked food and corn oil was 71%, 0.72%, 0.03% and 0.97% respectively. Average anthracene bioavailability was 1.7% and 43% for corn oil and CRM respectively. Anthracene was not detected above background in swine exposed to spiked food and spiked soil. Thus, this study indicates that exposure media impacts bioavailability, but there is no statistical evidence that sub-chronic exposure affects systemic exposure.

Exposure of primary porcine urothelial cells to benzo(a)pyrene: in vitro uptake, intracellular concentration, and biological response

More than 90 % of all bladder cancers are transitional cell carcinomas arising from the cells lining the inside of the hollow organ (uroepithelium). Cell cultures from primary urinary bladder epithelial cells (PUBEC) of pigs were established to assess the uptake, intracellular concentration, and subcellular distribution of the environmental pollutant benzo(a)pyrene (BaP). During treatment of the cells with 0.5 μM BaP for up to 24 h, intracellular concentration of BaP increased without saturation but with marked differences between various PUBEC pools. Analysis of BaP uptake by laser scanning microscopy indicated that BaP is rapidly partitioned into the cell membrane, while only a slight but significant increase in BaP fluorescence intensity was observed in the cytosol and nucleus. Spectrofluorometric quantification of BaP in PUBEC using ex situ calibration revealed a strong accumulation of BaP, leading to intracellular concentrations ranging from 7.28 to 35.70 μM in cells exposed to 0.5 μM BaP and from 29.9 to 406.64 μM in cells exposed to 10 μM BaP. These results were confirmed by gas chromatographic mass spectrometric analysis. Apoptotic cell nuclei were assessed by TUNEL analysis to see whether BaP exposure at the given concentrations results in a toxic effect. While apoptotic cells were barely detectable in control epithelial cells, there was a marked elevation in apoptosis in the BaP-exposed cells. In conclusion, a comprehensive study on uptake and quantification of BaP in epithelial cells from pig bladder is reported for the first time. The study may be helpful in understanding the pattern of BaP uptake and distribution in bladder and its possible implication in bladder cancer development.

Hepatic binding proteins translocating azo dye carcinogen metabolites from cytoplasm into nucleus in rats

When liver cytosol prepared from rats administered [(14)C]-3'-Methyl-N,N-dimethyl-4-aminoazobenzene was subjected to Sephadex gel chromatography, four peaks of radioactivity containing proteins (Peak-I-IV) and one peak devoid of protein (Peak-V) were obtained. Translocation of azo dye metabolites from these various cytosolic fractions into nucleus was studied in an in vitro system and a maximum of about 10% of the radioactivity associated with a particular cytosolic fraction (Peak-II) could translocate into the nuclei. Radioactivity (%) translocated did not increase upon addition of excess nuclei. Passage of this protein fraction through an immobilized protease column reduced the azo dye metabolite translocation by 65%, concomitant with the degradation of proteins. Translocation was not observed with protein-free metabolites extracted from this cytosolic fraction; addition of proteins corresponding to peak-II from normal rat liver cytosol significantly restored the metabolite translocation. This observation suggests that specific cytosolic proteins are involved in the translocation of azo dye carcinogen metabolites from liver cytoplasm into the nucleus. When the liver cytosolic proteins corresponding to this fraction (Peak-II) were iodinated with (125)I-iodine and incubated with purified nuclei, translocation of three specific proteins into nucleus was observed as seen by SDS-PAGE and fluorography of nuclear proteins. Covalent binding of azo dye metabolites to DNA was not observed when cytosolic peak-II fraction containing azo dye metabolites was incubated with isolated liver DNA instead of liver nuclei. This suggests that the interaction of azo dye metabolites with nuclear macromolecules necessitate further prior processing which actually may occur in the nucleus.

Impact of cellular metabolism on the biological effects of benzo[a]pyrene and related hydrocarbons

Polycyclic aromatic hydrocarbons are ubiquitous contaminants in the environment. Benzo[a]pyrene (BaP), a prototypical member of this class of chemicals, has been extensively studied for its toxic effects in laboratory animals and human populations. BaP toxicity is often mediated by oxidative metabolism to reactive intermediates that interact with macromolecules leading to alterations in target cell structure and function. More recent evidence suggests that disruption of cellular signaling pathways involved in the regulation of growth and differentiation contribute significantly to the toxicity of BaP and its metabolites. This review summarizes recent advances in our understanding of biological mechanisms of BaP toxicity at the molecular level, and the role of metabolic intermediates in carcinogenesis, atherogenesis, and teratogenesis.

Nuclear calcium and the regulation of the nuclear pore complex

In eukaryotic cells the nucleus and its contents are separated from the cytoplasm by the nuclear envelope. Macromolecules, as well as smaller molecules and ions, can cross the nuclear envelope through the nuclear pore complex. Molecules greater than approx. 60 kDa and containing a nuclear localization signal are actively transported across the nuclear membranes, but there has been little evidence for regulatory mechanisms for smaller molecules and ions. Recently, diffusion across the nuclear envelope has been observed to be regulated by nuclear cisternal Ca2+ concentrations. Following depletion of Ca2+ from the nuclear store by inositol 1,4,5-trisphosphate or Ca2+ chelators, a fluorescent 10 kDa marker molecule was no longer able to enter the nucleus. Distinct conformational states of the nuclear pore complexes depended on the Ca2+ filling state of the nuclear envelope, supporting the assumption that a switch in the conformation of the nuclear pore complex may control the transport of intermediate-sized molecules across the nuclear envelope. Thus nuclear Ca2+ stores may regulate the conformational state of the nuclear pore complex, and thereby passive diffusion of molecules between the cytosol and the nucleoplasm. The physiological significance of this finding is currently unknown.

Metabolism of (+)-trans-benzo[a]pyrene-7,8-dihydrodiol by 3-methylcholanthrene-induced rat liver homogenates

Using a new sensitive reverse-phase HPLC assay with on-line radioactivity detector, metabolism of (+)-trans-benzo[a]pyrene-7,8-dihydrodiol (B[a]P diol) to the ultimate carcinogen benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) was studied using 3-methylcholanthrene-induced rat liver homogenates. The results demonstrate that the stereoselectivity of B[a]PDE formation is a function of the concentration of the cellular constituents in the incubation media. At more dilute concentrations of the homogenate, the ratio of anti- to syn-B[a]PDE was the highest and decreased as the homogenate protein was increased in the incubation medium. However, there was a marked and parallel decrease of free B[a]PDE and DNA-bound radioactivity with increasing concentrations of cellular constituents in the incubation medium. The decreased DNA-bound radioactivity appears to be due to the preferential binding of B[a]PDE to glutathione and to proteins as the homogenate concentration was increased in the incubation media. These results indicate that liver homogenates, while apparently preserving the function of microsomes, present additional opportunities to study the interrelationship among cytochrome P450 monooxygenase activity, water-soluble conjugates, and binding of B[a]P diol metabolites to macromolecules in the study of benzo[a]pyrene-induced carcinogenesis.

Effect of environmental tobacco smoke on the metabolism of (-)-trans-benzo[a]pyrene-7,8-dihydrodiol in juvenile ferret lung and liver

To evaluate the effects of "environmental tobacco smoke" (ETS) on developing lungs, juvenile ferrets were exposed to ETS at an average total particulate concentration of 381 +/- 97 mg/m3 for 2 h at the breathing zone. Twenty-four hours after the exposure, the ferrets were sacrificed and the metabolism of (-)-trans-benzo[a]pyrene-7,8-dihydrodiol was studied in the lung and liver homogenates. The rate of conversion of (-)-trans-benzo[a]pyrene-7,8-dihydrodiol to the ultimate carcinogen (+)-anti-benzo[a]pyrene-7,8-dihydrodiol-9,10- epoxide was twofold higher in the liver than that observed in the lung of control ferrets. After ETS exposure, the formation of free benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide was increased by 62% in the lung (p < .01). The DNA-bound metabolites were significantly increased only in the lung, while protein-bound metabolites were significantly increased in the liver after ETS exposure. Although glutathione conjugates tended to be increased both in the lung and liver, sulfate conjugates were significantly decreased in the lung after ETS exposure (p < .05). (+)-trans-Benzo[a]pyrene-7,8-dihydrodiol was used to study the relative contributions of cytochrome P-450 and peroxyl radical-mediated formation of benzo[a]-pyrene-7,8-dihydrodiol-9,10-epoxide. Peroxyl radical- and P-450-mediated conversion of (+)-trans-benzo[a]pyrene-7,8-dihydrodiol to benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide was proportionately equal in the ferret lung, whereas in the liver the P-450-mediated pathway was predominant. After ETS exposure there was a tendency for P-450-mediated formation of benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide to increase. These results demonstrate significant differences in the metabolism of (-)-trans-benzo[a]pyrene-7,8-dihydrodiol by the lung and liver of juvenile ferrets and suggest a significant role of peroxyl radical-mediated formation of (+)-anti-benzo[a]pyrene-7,8-dihydrodiol-9,10-epoxide in the lung, which may help explain discrepancy between the levels of P-450 and amounts of DNA adducts of polycyclic aromatic hydrocarbons in different organs in smokers.

Fatty acid delta 5 desaturation in rat liver cell nuclei

Activity of one of the key enzymes involved in arachidonic acid (20:4 n-6) biosynthesis, the delta 5 desaturase, was found in rat liver cell nuclei. Up to now, it has been shown that the fatty acid desaturases are located exclusively in the endoplasmic reticulum. Similarly to what happens with microsomal enzyme the nuclear delta 5 desaturase enzyme was only fully active in the presence of a cytosolic factor. In this condition it reached a specific activity of 50 pmol 20:4 n-6 formed/min/mg of protein. This fact would imply that purified nuclei like purified microsomes lack a soluble cytosol factor necessary for the total desaturation reaction expression. Besides the nuclear delta 5 desaturase has an optimal pH of 7.6 and is inhibited by 1 or 10 mM KCN. Low long chain acyl-CoA synthetase activity that catalyzes the formation of 20:3 n-6-CoA, was also found in liver nuclei. This step would be essential in nuclear desaturation since when ATP and/or CoA (necessary for the acylation reaction) are omitted from the incubation mixture, the desaturation reaction does not take place.

Effect of metyrapone on tryptophan binding to rat hepatic nuclei

This study evaluated whether metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), an inhibitor of endogenous adrenal corticosteroid synthesis via inhibition of cytochrome P-450-mediated steroid hydroxylation, would influence the binding of L-tryptophan to rat hepatic nuclei or nuclear envelopes. Previous publications have indicated that binding of L-tryptophan to hepatic nuclear envelope proteins was saturable, stereospecific, and of high affinity. In this study, we investigated whether metyrapone would influence L-tryptophan binding to rat hepatic nuclei or nuclear envelopes as assayed by in vitro L-(5-3H)tryptophan binding. Our results indicate that the addition of metyrapone in vitro has little influence on L-(5-3H)tryptophan binding to hepatic nuclei or nuclear envelopes. On the other hand, when metyrapone (1 mg/100 g body weight) is tube-fed 30 minutes before killing, the isolated hepatic nuclei show decreased specific L-tryptophan binding (total binding minus nonspecific binding [using 2,000-fold excess of unlabeled L-tryptophan]) compared with controls. Also, addition of metyrapone in vitro to rat liver before homogenization and preparation of nuclei caused the nuclei to show decreased specific tryptophan binding compared with controls. Under these in vitro conditions, SKF 525A, another inhibitor of hydroxylation, showed inhibitory effects similar to those of metyrapone. Thus, metyrapone can interfere with rat liver nuclear envelope receptor binding to L-tryptophan, and possibly acts via its effects on hydroxylation. At high doses, metyrapone (20 mg/100 g body weight) appears to inhibit tryptophan-induced stimulation of hepatic protein synthesis.

Differences in phenytoin biotransformation and susceptibility to congenital malformations: a review

The clinical variability of teratogenic response to fetal drug exposure has been well documented. Metabolic differences in biotransformation have been shown to extend to multiple drugs and may involve many steps in drug metabolism with alterations of key intermediates. Although metabolic differences have been reported to be associated with complications of medication use, it has only recently been appreciated that such differences also may be associated in the unborn with the potential for the disruption of normal embryologic development and the production of congenital malformations. It has long been suspected that the teratogenicity of phenytoin may be mediated not only by the parent compound, but also by toxic intermediary metabolites that are produced during the biotransformation of the parent compound. Recent work elucidating differences in isoenzyme forms of cytochrome P-450 enzyme systems, glutathione, and microsomal epoxide hydrolase has provided increased interest in the multiple individual pharmacogenetic differences that may be significant factors affecting increased susceptibility to birth defects in individuals and families with fetal exposure to phenytoin.

Prevention of 2-acetylaminofluorene-induced loss of nuclear envelope cytochrome P450 by the simultaneous administration of 3-methylcholanthrene

Rats fed a basal diet containing 0.05% (w/w) 2-acetylaminofluorene (AAF) for 3 weeks showed a 50% loss of hepatic nuclear envelope cytochrome P450, whereas microsomal P450 remained at control levels. A similar dietary treatment with 0.004% (w/w) 3-methylcholanthrene (MC) caused moderate losses (20-25%) of cytochrome P450 in both nuclear envelopes and microsomes. Administration of the basal diet supplemented with a mixture of AAF (0.05%) plus MC (0.004%) resulted in a preservation of control levels of nuclear envelope cytochrome P450 and a 30% elevation of microsomal P450. Immunoblot analysis revealed that AAF alone, or in concert with MC, induced comparable levels of the P450d form. Induction of cytochrome P450c by dietary MC was detected only when MC was fed together with AAF. As previously found for butylated hydroxytoluene (BHT), the protective effect of dietary MC against hepatocarcinogenesis in AAF-fed rats correlated with a preservation of nuclear envelope cytochrome P450 content and with the induction of cytochrome P450c.

Glucocorticoid repression and basal regulation of the epoxide hydrolase promoter

Through a series of promoter deletions and gene transfer experiments we have examined the basal regulation and glucocorticoid-mediated repression of the rat epoxide hydrolase gene. Three regions of the 5' flanking sequence were found to influence the basal level of promoter function in H4IIE hepatoma cells. Region A (-891 to -355 bp) contains an apparent repressor of epoxide hydrolase expression, while regions B (-271 to -171 bp) and C (-141 to -85) were found to contain important sequences required for optimal promoter activity. Previous work has demonstrated that dexamethasone represses epoxide hydrolase transcription by approximately 50% in isolated rat liver nuclei, and, in this study, we have demonstrated that the ability of the epoxide hydrolase promoter to drive CAT expression is similarly repressed in H4IIE cells treated with 1 microM dexamethasone. Furthermore, the level of endogenous epoxide hydrolase mRNA is decreased by 70-88% in nontransfected H4IIE cells treated with dexamethasone. Interestingly, promoter activity was not decreased by dexamethasone in COS cells, which lack glucocorticoid receptors. The current data show that sequences from -42 to +110 bp are sufficient to support the dexamethasone response, and, furthermore, they suggest that repression may not require direct interaction of the ligand-receptor complex with the promoter region.

Enzymatic oxidation of xenobiotic chemicals

Studies with biomimetic models can yield considerable insight into mechanisms of enzymatic catalysis. The discussion above indicates how such information has been important in the cases of flavoproteins, hemoproteins, and, to a lesser extent, the copper protein dopamine beta-hydroxylase. Some of the moieties that we generally accept as intermediates (i.e., high-valent iron oxygen complex in cytochrome P-450 reactions) would be extremely hard to characterize were it not for biomimetic models and more stable analogs such as peroxidase Compound I complexes. Although biomimetic models can be useful, we do need to keep them in perspective. It is possible to alter ligands and aspects of the environment in a way that may not reflect the active site of the protein. Eventually, the model work needs to be carried back to the proteins. We have seen that diagnostic substrates can be of considerable use in understanding enzymes and examples of elucidation of mechanisms through the use of rearrangements, mechanism-based inactivation, isotope labeling, kinetic isotope effects, and free energy relationships have been given. The point should be made that a myriad of approaches need to be applied to the study of each enzyme, for there is potential for misleading information if total reliance is placed on a single approach. The point also needs to be made that in the future we need information concerning the structures of the active sites of enzymes in order to fully understand them. Of the enzymes considered here, only a bacterial form of cytochrome P-450 (P-450cam) has been crystallized. The challenge to determine the three-dimensional structures of these enzymes, particularly the intrinsic membrane proteins, is formidable, yet our further understanding of the mechanisms of enzyme catalysis will remain elusive as long as we have to speak of putative specific residues, domains, and distances in anecdotal terms. The point should be made that there is actually some commonality among many of the catalytic mechanisms of oxidation, even among proteins with different structures and prosthetic groups. Thus, we see that cytochrome P-450 has some elements of a peroxidase and vice versa; indeed, the chemistry at the prosthetic group is probably very similar and the overall chemistry seems to be induced by the protein structure. The copper protein dopamine beta-hydroxylase appears to proceed with chemistry similar to that of the hemoprotein cytochrome P-450 and, although not so thoroughly studied, the non-heme iron protein P. oleovarans omega-hydroxylase.(ABSTRACT TRUNCATED AT 400 WORDS)

The expression and metabolic activity of cytochrome P-450 isozymes in control and phenobarbital-induced primary cultures of rat hepatocytes

The expression and activity of the phenobarbital (PB)-inducible P-450 isozymes, P-450b and P-450e, and the major 3-methylcholanthrene (MC)-inducible form, P-450c, were studied in primary cultures of adult rat hepatocytes in T1, Leibovitz L-15 (L-15), and a modification of Waymouth 752/1 (Way) media. P-450 isozymes in initially isolated hepatocytes and control and PB-treated cultures were quantitated by Western blot analysis, and activity was determined with 7,12-dimethylbenz[a]anthracene (DMBA) as substrate. Data from the Western blot analysis correlated well with the metabolic activity toward DMBA. P-450b was consistently induced by PB in hepatocytes in T1 and to a lesser extent in Way. P-450e protein was constitutive in initially isolated cells, expressed in control cultures at a reduced level, and increased or maintained by PB in all three media. DMBA metabolite formation associated with P-450b and P-450e activity was induced by PB in hepatocytes in T1 and Way and was inhibited by antibodies to P-450b. P-450c was only infrequently expressed in freshly prepared hepatocytes, but was detected in all control and PB-treated cultures although at a much higher level in T1. Thus, the amounts of P-450 isozymes, their inducibility by PB, and their activity toward DMBA were found to be dependent on the medium. We have demonstrated enzyme induction and increased activity of the major PB-inducible isozymes in hepatocytes in T1; these are also associated with a change in the control of P-450c expression leading to enhanced constitutive expression and inducibility by phenobarbital.

Sodium cholate extraction of rat liver nuclear xenobiotic-metabolizing enzymes

DNA is the purported target of several carcinogenic and mutagenic agents. Nuclear enzymes which could generate or detoxify reactive metabolites are of major concern. Several such enzymes have been identified within nuclei, but obtaining samples with enriched content or activity is difficult, time-consuming, and uses harsh isolation techniques. Extraction of rat liver nuclear suspensions with cholate-containing buffer results in solubilization of 25-30% of the protein. Linear extraction was obtained for total protein and cytochromes P-450 and b5, NADPH-cytochrome P-450 reductase, NADH-cytochrome b5 reductase, DT-diaphorase, and microsomal-like epoxide hydrolase with specific activities comparable to values reported for isolated nuclear membrane, while the yield was five to ten times greater. Detergent extracts of rat liver nuclei were employed to study the comparative response of microsomal and nuclear enzymes to chemical treatment. While the responses to acute inductive (phenobarbital and 3-methylcholanthrene) and toxic (carbon tetrachloride and dibromochloropropane) treatments were qualitatively similar, an initiation-promotion protocol (diethylnitrosamine with phenobarbital promotion) resulted in divergent responses between the enzymes in the two subcellular fractions. Detergent extracts of nuclei offer an efficient means of recovering xenobiotic-metabolizing enzymes from rat liver nuclei, and have been utilized to demonstrate a differential response of nuclear enzymes during preneoplastic development.

Product inhibition of benzo[a]pyrene metabolism in uninduced rat liver microsomes: effect of diol epoxide formation

Conversion of benzo[a]pyrene (BP) to BP 7,8-dihydrodiol 9,10-oxides (DE) (measured as 7,10/8,9-tetrols) by untreated (UT) rat liver microsomes is over 10 times slower than following 3-methylcholanthrene (MC) induction. Time courses have been subjected to a kinetic analysis analogous to that previously reported for metabolism by MC-induced microsomes (J. Biol. Chem., 259 (1984) 13770-13776). Competition between BP and 7,8-dihydrodiol for P-450 is the major determinant of the rate of DE formation. Glucuronidation of quinones and phenols only increases the isolated BP metabolites including DE by 40%. This indicates far less inhibition by these products than for metabolism in MC-microsomes (4-6-fold). Thus stimulation may result from a decreased quinone-mediated oxidation of metabolites. In the presence of DNA, UT-microsomes metabolize BP to approximately equal amounts of 9-phenol-4,5-oxide (9-PO) and DE/DNA adducts. Addition of uridine diphosphoglucuronic acid (UDPGA) fails to enhance modification of DNA by DE, but formation of the 9-PO adduct is reduced as a result of lower free 9-phenol levels. The kinetic characteristics of BP metabolism by UT-microsomes are highly sensitive to the presence of very small but variable amounts (2-25 pmol/mg) of the very active cytochrome P-450c, which is the predominant form in MC-microsomes. The major effect of elevated levels of P-450c is an 8-fold increase in DE formation at low concentrations of BP due to a lowering of Km (7.9-2.6 microM) and an increase in the regioselectivity for DE formation from 7,8-dihydrodiol (5-15% of total BP metabolites). The formation of DE was directly correlated with the content of P-450c (r = 0.94). The presence of increased levels of P-450c in UT-microsomes is probably due to previous exposure of the animals to environmental inducers and is minimized by controlled housing and feeding.

Characterization of multiple epoxide hydrolase activities in mouse liver nuclear envelope

A nuclear envelope-associated epoxide hydrolase in mouse liver that hydrates trans-stilbene oxide has been identified and characterized. This epoxide hydrolase is distinct from the enzyme in nuclear envelopes that hydrates benzo[a]pyrene 4,5-oxide and other arene oxides. This distinction was demonstrated by the criteria of pH optima, response to specific inhibitors in vitro, and precipitation by specific antibodies. The new epoxide hydrolase had a pH optimum of 6.8, was poorly inhibited by trichloropropene oxide, was potently inhibited by 4-phenylchalcone oxide, and did not bind to antiserum against benzo[a]pyrene 4,5-oxide hydrolase. This nuclear enzyme is similar in many of its properties to cytosolic and microsomal trans-stilbene oxide hydrolases and may be nuclear envelope-bound form of these other epoxide hydrolases. It differed from these other trans-stilbene oxide hydrolases in that its affinities for both trans-stilbene oxide (measured as apparent Km) and 4-phenylchalcone oxide (measured as I50) were 4- to 20-fold lower than those of either the cytosolic or microsomal forms.

Complementary DNA and amino acid sequence of rat liver microsomal, xenobiotic epoxide hydrolase

The coding nucleotide sequence for rat liver microsomal, xenobiotic epoxide hydrolase was determined from two overlapping cDNA clones, which together contain 1750 nucleotides complementary to epoxide hydrolase mRNA. The single open reading frame of 1365 nucleotides codes for a 455 amino acid polypeptide with a molecular weight of 52,581. The deduced amino acid composition agrees well with those determined by direct amino acid analysis of the rat protein, and the amino acid sequence is 81% identical to that of rabbit epoxide hydrolase. Analysis of codon usage for epoxide hydrolase, and that of rabbit epoxide hydrolase. Analysis of codon usage for epoxide hydrolase, and comparison to codon usage for NADPH-cytochrome P-450 oxidoreductase and cytochromes P-450b, P-450d, and P-450PCN, suggest that epoxide hydrolase is more conserved than cytochromes P-450b and P-450PCN; comparison of the extent of sequence conservation for 12 homologous proteins between the rat and rabbit, including cytochrome P-450b, supports this hypothesis, and indicates that much of epoxide hydrolase is constrained to maintain its hydrophobic character, consistent with its intramembranous location. The predicted membrane topology of epoxide hydrolase delineates 6 membrane-spanning segments, less than the 8 or 10 predicted for two cytochrome P-450 isozymes; the lower number of membrane-spanning segments predicted for epoxide hydrolase correlates with its lesser dependence on the membrane for maintenance of its tertiary structure and catalytic activity.

Loss of cytochrome P-450 from hepatic nuclear membranes of rats fed 2-acetylaminofluorene

The cytochrome P-450 content of nuclear membranes isolated from the livers of male Sprague-Dawley rats fed a semipurified diet containing 0.05% w/w 2-acetylaminofluorene (AAF) for 3 weeks, was only about 20% of the values in control rats fed the same diet devoid of AAF. This effect was apparent after only 1 week of AAF treatment and persisted in nuclear membranes from isolated hyperplastic nodules (HPN) generated by 4 cycles of interrupted AAF-feeding. The microsomal cytochrome P-450 content, on the other hand, remained at control levels after 1 week of AAF treatment, and it was only slightly decreased after 3 weeks. In contrast, microsomes from HPN generated by prolonged AAF treatment had markedly decreased amounts of cytochrome P-450. The AAF treatment also caused changes in cholesterol epoxide hydrolase activity, which paralleled those observed for cytochrome P-450 content. Nuclear membranes from livers of rats fed AAF for 3 weeks, and from isolated HPN, had only 30-50% of the cholesterol epoxide hydrolase activity present in controls, whereas the microsomal enzyme activity remained at control levels after 3 weeks of AAF feeding but was 50% depressed in microsomes from HPN. The selective loss of cytochrome P-450 and of cholesterol epoxide hydrolase in hepatic nuclear membrane, but not in microsomes, of rats fed AAF for 3 weeks suggests independent control for these enzymes in these two membrane fractions. Cytochrome P-450 plays a role both in the activation of AAF (N-hydroxylation) as well as in its detoxification (ring hydroxylation) whereas cholesterol epoxide hydrolase initiates the detoxification of cholesterol epoxide. Therefore, our findings suggest the hypothesis that AAF treatment causes an early loss, at the surface of the nucleus, of the last line of defense for detoxification of transforming or promoting metabolites generated by microsomal activation of natural substances such as cholesterol and of xenobiotics such as AAF.

Regulation of rat and bovine adrenal metabolism of polycyclic aromatic hydrocarbons by adrenocorticotropin and 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on polycyclic aromatic hydrocarbon (PAH) metabolism and steroidogenesis in primary cultures of bovine adrenal cortical (BAC) and rat adrenal cortical (RAC) cells have been examined. Remarkably TCDD is an ineffective inducer (15-50%) of PAH metabolism in confluent BAC cells and completely antagonizes a 5-fold induction by benz[alpha]anthracene (BA). In the same concentration range (EC50 5 X 10(-11) M) TCDD suppresses steroidogenesis through an effect on cholesterol metabolism. Adrenocorticotropin (ACTH) and cAMP also suppress PAH metabolism at concentrations which stimulate steroidogenesis (10(-7) M). In RAC cells ACTH potently induces PAH metabolism (7-fold) at a comparable concentration to the stimulation of steroidogenesis. Parallel stimulation of PAH metabolism and steroidogenesis by cAMP suggest that ACTH induction of PAH metabolism is mediated by cAMP. TCDD induces PAH metabolism (2.8-fold, EC50 8 X 10(-11) M) at similar concentrations to the inhibitory effect in BAC cells and this action is additive with ACTH induction. In male rats in vivo TCDD induces adrenal microsomal PAH metabolism (72%) and is more effective in this respect than 3-methylcholanthrene (3MC). Rabbit antibodies against rat liver cytochrome P-450c (the major TCDD-inducible liver form) inhibited the TCDD-induced adrenal metabolism of 7,12-dimethylbenz[alpha]anthracene (DMBA), which also exhibited regioselectivity typical of metabolism by P-450c. Constitutive adrenal microsomal metabolism, which exhibited regioselectivity of DMBA metabolism comparable to the ACTH-sensitive cellular metabolism, was not affected by anti-P-450c. It is concluded that ACTH and TCDD induce distinct forms of cytochrome P-450 in RAC cells and that the latter represents a typical Ah-receptor mediated response. The anomalous effect on PAH metabolism in BAC cells that parallels inhibition of steroidogenesis may derive from repression of a distinct adrenal form of P-450 by the TCDD-Ah-receptor complex.

Perinatal development of cytochrome P-450, NADPH-cytochrome c reductase and ethoxycoumarin deethylase in rat liver nuclear membranes

Perinatal development of rat liver nuclear membrane enzymatic activities was investigated with respect to the metabolism of xenobiotica. The qualitative pattern observed was very close to that reported for microsomal enzymes during development. Cytochrome P-450, NADPH-cytochrome c reductase and ethoxycoumarin deethylase are already present in fetuses at 18 days of gestational age. Phenobarbital pretreatment appears to be effective as an inducing agent for all the enzymes studied, but only after birth. The pattern of induction of cytochrome P-450 showed a peak at the 38th day of life three times higher than basal values at that age. NADPH-cytochrome c reductase presented a constant elevation to about twice basal activity throughout the period taken into consideration. Ethoxycoumarin deethylase activity took only 17 days to reach the basal value observed later in adult animals. This enzyme proved highly inducible by phenobarbital (5-fold) early after birth but the increase dropped to 3-fold from the 24th day of life.

Metabolic activation of mitomycin C by liver microsomes and nuclei

Bioreductive alkylating agents require reductive activation prior to exerting their cytotoxic actions. This property results in preferential toxicity to hypoxic cells. Previous data have demonstrated that mitomycin C is activated by hypoxic tumor cells and is selectively cytotoxic to these oxygen-deficient cells. The biotransformation of mitomycin C was studied in liver microsomes and nuclei and in a reconstituted, partially purified cytochrome P-450 drug-metabolizing system to provide information on these reductive processes. Both the metabolism of mitomycin C, measured by disappearance of the quinone portion of the substrate, and the formation of an alkylating metabolite(s), determined by employing 4-(p-nitrobenzyl)pyridine as a trapping agent, required anaerobic conditions and an NADPH-generating system, and were inhibited by O2 and CO in both microsomes and nuclei. A reconstituted enzyme system consisting of NADPH, NADPH-cytochrome P-450 reductase, phospholipid and cytochrome P-450 converted mitomycin C to a reactive metabolite(s) under hypoxic conditions. Omission of N2 or any component of the system decreased the metabolic activation of mitomycin C. These findings support the concept that the cytochrome P-450 system is capable of activating mitomycin C under hypoxic conditions to the alkylating metabolite(s) that is responsible for antineoplastic activity.

Mixed-function amine oxidase of the rat hepatocyte nuclear envelope. Demonstration and effects of phenobarbital and 3-methylcholanthrene

Mixed-function amine oxidase (EC 1.14.13.8) has been demonstrated in highly purified rat hepatocyte nuclear envelope . The enzyme was present in the nuclear envelope at a level 20 percent of that observed in microsomes. Induction studies indicated that nuclear envelope amine oxidase as well as its microsomal counterpart were refractory to the effects of phenobarbital and 3-methylcholanthrene. Phenobarbital administration increased the specific activity of the microsomal N, N-dimethylaniline N-demethylase and benzo[a]pyrene hydroxylase by 600 and 190 percent, respectively, but decreased the specific activity of the nuclear enzymes by 30-50 percent. In contrast, 3-methylcholanthrene increased the specific activity of benzo[a]pyrene hydroxylase in nuclear envelope and microsomes by 42- and 11-fold, respectively. The hydrocarbon also increased the microsomal and nuclear N, N-dimethylaniline N-demethylase by 40 and 60 percent, respectively, but the specific activity of microsomal and nuclear aniline 4-hydroxylase was decreased by 50 percent. Demonstration of amine oxidase in rat hepatocyte nuclear envelope implicates this enzyme in the toxicity and carcinogenicity of certain drugs and chemicals.

Metabolic activation of a tryptophan pyrolysis product, 3-amino-1-methyl-5H-pyrido[4,3-b]indole(TRP-P-2) by isolated rat liver nuclei

The metabolic activation of a tryptophan pyrolysis product, 3-amino-1-methyl-5H-pyrido[4,3-b]indole(Trp-P-2), by rat liver nuclei was studied. Nuclei from the livers of rats treated with polychlorinated biphenyl (PCB) or 3-methylcholanthrene (MC) showed high mutagenic activity with Trp-P-2 in the Ames test, but activities with nuclei of untreated or phenobarbital (PB)-treated rat livers were quite low. The formation of N-hydroxy-Trp-P-2 by nuclei of PCB- or MC-treated rat livers was greater than that by nuclei of untreated or PB-treated rat livers. Similar results were observed with microsomes, which suggests that Trp-P-2 is metabolized by the same type of monoxygenase system in nuclei as in microsomes.

Enzymic synthesis of juvenile hormone in locust corpora allata: evidence for a microsomal cytochrome P-450 linked methyl farnesoate epoxidase

Homogenates of corpora allata from adult Locusta migratoria in phosphate-buffered EDTA have been analysed by sucrose-density-gradient centrifugation. Succinate-cytochrome c reductase activity (mitochondrial) bands between d20/4 1.13-1.15, whereas NADPH-cytochrome c reductase and NADPH-dependent methyl farnesoate 10.11-epoxidase activities band identically between d20/4 1.06-1.12. We conclude that the methyl farnesoate epoxidase is exclusively microsomal. Farnesoic acid O-methyltransferase is an exclusively soluble enzyme which stoichiometrically transfers the S-methyl group from S-adenosylmethionine to farnesoic acid. No carboxyl esterase activity was found. Isolated microsomes were used to obtain an apparent Km = 7.7 X 10-6 M for the epoxidase, although substrate solubility limits the rate to 0.5 V. As expected, the product (juvenile hormone III) is chiral (10 R). The epoxidase is inhibited by excess NADP+ and oxidised cytochrome c, but neither inhibited nor synergised by NADH. NADH supports less than 10% of the NADPH rate of epoxidation. The epoxidase is inhibited by a carbon monoxide/oxygen atmosphere, half-maximal inhibition occurring at a CO/O2 ratio of 4.0. This inhibition is reversed by white-light irradiation.

Induction of nuclear styrene monooxygenase and epoxide hydrolase in rat liver

The apparent Km and Vmax of styrene monooxygenase and styrene epoxide hydrolase were determined in intact nuclear preparations from male rat liver after in vivo treatment with phenobarbital and beta-naphthoflavone, which are known to induce microsomal cytochrome P-450 and cytochrome P-448 respectively. Treatment with phenobarbital does not alter the apparent Km, but greatly increases the Vmax of both nuclear styrene monooxygenase and styrene epoxide hydrolase. Almost the same pattern is observed for styrene monooxygenase after treatment with beta-naphthoflavone, whereas the same treatment slightly increases both the Vmax and Km value of styrene epoxide hydrolase.

Mixed-function oxidases of 25-hydroxycholecalciferol in isolated chick kidney glomeruli: evidence for nuclear localization

Pure chick kidney glomeruli and proximal tubular fragments have been isolated by graded sieving through nylon screens. Electron micrographs revealed that, in distinct contrast to proximal epithelial cells, the glomerular epithelial and endothelial cells are essentially devoid of mitochondria. Glomeruli as well as proximal tubular fragments contain the 1 alpha- and 24R-hydroxylases of 25-hydroxycholecalciferol. The level of 1 alpha-hydroxylase activity was the same in both segments of the nephron. However, the tubular fragments contained twice the 24R-hydroxylase activity found in glomeruli. Glomerular nuclei were purified by sucrose gradient sedimentation and used to confirm the association of the 1 alpha-hydroxylase with this kidney organelle. Almost all of the glomerular 1 alpha-hydroxylase activity was found in the nuclear fraction. Two metabolites, which are produced predominantly by the nuclei, are designated N-1 and N-2. Their structural identity remains unknown. The novel presence of the 1 alpha-hydroxylase in the glomerulus may be important in defining the etiology of bone diseases in patients with glomerulonephritis and similar disorders.

Is nuclear styrene monooxygenase activity a microsomal artifact?

Microsomal contamination in nuclear preparations could represent one of the main sources of bias in the evaluation of the real metabolic capacity of the nuclear envelope. In this paper we present a quantitative study of the level of nuclear styrene monooxygenase enzymatic activity after artificially increasing the native microsomes to nuclei ratio. In one experimental conditions no significant elevation of the nuclear monooxygenase was observed. These data indicate that, if any microsomal contamination is present, it cannot account for more than 30% of the total enzymatic activity found in nuclear preparations.

Plasma membranes from intestinal microvilli and erythrocytes contain cytochromes b5 and P-420

The presence of cytochromes b5, P-450 and P-420 and activities of NADH- and NADPH-cytochrome c redutases were determined in plasma membranes isolated from microvilli of the chick and rat intestinal epithelium and erythrocyte membranes from chick, rat and man. The results are compared with the amounts of these components found in microsomal fractions from intestinal epithelium and in nuclear membranes from chick erythrocytes. Plasma membranes from intestinal microvilli and from erythrocytes contained significant amounts of NADH-cytochrome c reductase activity and of a pigment spectrophotometrically indistinguishable from rat liver microsomal cytochrome b5. In addition, cytochrome b5 fragments were prepared from the membranes by limited trypsin digestion and consisted of two to four components with Mr values in the range 10 000-13 500. In low-temperature difference spectra, the presence of a second cytochrome was noted which was similar to cytochrome P-420. Cytochrome P-450 and NADPH-cytochrome c reductase activities were not detected in plasma membrane fractions in significant concentrations but were present in the corresponding endomembrane fractions. These findings in highly purified, well defined plasma membrane fractions, in which contamination by endomembranes is minimal, strengthen the evidence for the existence of cytochrome-containing redox systems in plasma membranes of various cells and suggest that such redox components are general components of the cell surface. Possible functions and origins of these redox components in plasma membranes are discussed.