Quantitative analysis of the metabolism of 9,10-dihydrobenzo[a]pyrene by induced rat liver microsomes

The ability of reduced polycyclic aromatic hydrocarbons to be converted to their fully aromatic forms by the microsomal cytochrome P-450 mixed-function oxidases may assist in the explanation of the mutagenic and tumorigenic activities of these agents. The metabolic conversion of 9,10-dihydrobenzo[a]pyrene (9,10-DHB[a]P) to benzo[a]pyrene (B[a]P) and 9- and/or 10-hydroxy-9,10-DHB[a]P (OH-9,10-DHB[a]P) was quantitatively measured. In beta-naphthoflavone-induced rat liver microsomes, 9,10-DHB[a]P was metabolized to B[a]P with a specific activity of 1.51 nmol B[a]P formed/min/mg microsomal protein. The formation of B[a]P was directly related to incubation time and microsomal protein concentration. Similarly, 9,10-DHB[a]P was converted to OH-9,10-DHB[a]P with a specific activity of 4.48 nmol OH-9,10-DHB[a]P formed/min/mg microsomal protein. Its formation was directly related to incubation time and microsomal protein concentration. The possibility of OH-9,10-DHB[a]P as a metabolic intermediate to B[a]P is discussed.

Covalent binding of the carcinogen benzo[a]pyrene diol epoxide to Xenopus laevis 5 S DNA reconstituted into nucleosomes

The sequence-specific interactions of bulky carcinogens with purified DNA might reasonably be expected to be altered when the DNA is organized into chromatin. We have approached this subject by studying the covalent binding of a potent carcinogen, 7r,8t-dihydroxy-9t,10t-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-I) to nucleosomal and free DNA, using a defined fragment of DNA derived from the 5'-end of the 5 S rRNA gene of Xenopus laevis, reconstituted into nucleosomes by salt exchange with unlabeled chicken mononucleosomes. Micrococcal nuclease and hydroxyl radical "footprinting" experiments demonstrated the formation of a uniquely positioned nucleosome covering the transcriptional start point of the gene. The reconstituted nucleosomes or control DNA samples were modified with BPDE-I. DNA was repurified from the nucleosomal and control modification reactions and then irradiated with laser light at 355 nm, causing strand breaks at the positions of adducts. Nucleosomal DNA exhibited a marked decrease in the level of carcinogen binding, especially in the central 80-90 base pairs of the nucleosomal region. Interestingly, although overall binding was inhibited about 2-fold, the sequence-specific pattern of binding to deoxyguanosine residues seen with purified DNA was maintained in the nucleosomal DNA.

The relationship between polycyclic aromatic hydrocarbons in air and in urine of workers in a Söderberg potroom

The relationships between increase of urinary 1-hydroxypyrene over the workweek and the airborne concentrations of benzo(a)pyrene and coal tar pitch volatiles (CTPVs) were studied among groups of workers in a vertical-stud Söderberg potroom of an aluminum smelter. There was a strong correlation between the natural logarithm of the pyrene concentration and the natural logarithm of the total polycyclic aromatic hydrocarbon (PAHs) concentration in personal air samples (r = 0.94). 1-Hydroxypyrene as the major metabolite of pyrene, a polycyclic aromatic hydrocarbon, was used as a marker for exposure to PAHs. A strong positive correlation was found between the natural logarithm of increase of urinary 1-hydroxypyrene and the natural logarithm of the estimated airborne PAH exposure (R2 = 0.84 with CTPV) when the use of facial protective clothing was taken into account. The relationship between increase of urinary 1-hydroxypyrene and PAHs differed for workers who used facial protective clothing under their respirators compared to those who did not. A contradictory fact was found: the use of facial protection seemed to lead to an elevated increase of 1-hydroxypyrene over the workweek. The regression model for the relationship between increase of urinary 1-hydroxypyrene over the workweek and airborne CTPV was: LN(change in 1-hydroxypyrene) = -4.7 + 1.2 LN(CTPV) -0.44 LN(CTPV)*(use of facial protection) + 3.5 (use of facial protection).

Inactivation of plasmid reporter gene expression by one benzo(a)pyrene diol-epoxide DNA adduct in adult rat hepatocytes

Polycyclic aromatic hydrocarbons such as benzo(a)pyrene diol-epoxide (BPDE-I) cause hepatocellular carcinoma. To identify short-term carcinogen effects, we studied hepatocytes transfected with nonreplicating plasmids, adducted covalently with BPDE-I, varying in promoter structure and encoded reporter gene (beta-galactosidase or luciferase). BPDE inactivated gene expression as a first-order function of BPDE concentration in adduction reactions. No evidence of cytotoxicity, diminished coprecipitation and availability, enhanced nicking of supercoiled forms and reduced cellular uptake, or instability of adducted plasmids was observed. At low BPDE:plasmid ratios, inactivation occurred with 1 adduct/plasmid within a target 23-27% of plasmid bases. Using nuclear extracts and BPDE-adducted G-free cassette-encoding plasmids, the fraction of full-length RNA polymerase II-initiated transcripts also declined as a first-order function of BPDE concentration when approximately 3 adducts were distributed among 48% of plasmid bases. These observations suggest that carcinogens such as BPDE block mRNA transcription along DNA templates by forming limited numbers of persistent adducts at coding or noncoding sites.

Formation of the adduct 6-(deoxyguanosin-N2-yl)-3-amino-benzo[a]pyrene from the mutagenic environmental contaminant 3-nitrobenzo[a]pyrene

3-Nitrobenzo[a]pyrene (3-nitro-B[a]P) is a potent bacterial mutagen as a result of nitroreduction. Reaction of N-hydroxy-3-amino-B[a]P, prepared in situ from reduction of 3-nitro-B[a]P with calf thymus DNA, was studied. After enzymatic digestion of the DNA, the resulting modified nucleosides were analyzed by thermospray HPLC-MS and high-resolution proton NMR spectroscopy. The major adduct was identified as 6-(deoxyguanosin-N2-yl)-3-amino-B[a]P. The same adduct was obtained from incubation of DNA with 3-nitro-B[a]P in the presence of the mammalian nitroreductase xanthine oxidase, and hypoxanthine. These data indicate that a mammalian nitroreductase can metabolize 3-nitro-B[a]P to an activated derivative that reacts with DNA to give a novel adduct distant from the site of N-hydroxylation.

The carcinogen benzo(e)pyrene is metabolized by DM15 cells without an uncoupling effect on their gap junctions

Benzo(e)pyrene (B(e)P) promotes carcinogenesis in the skin. Unlike some other promoters however, B(e)P does not produce an uncoupling effect on gap junction permeability in DM15 transformed fibroblasts. This study demonstrates that DM15 cells exhibit a relatively high level of B(e)P metabolism. Moreover, although pretreatment of DM15 cells with benz(a)anthracene results in an 8-fold increase of arylhydrocarbon hydroxylase activity and a 2-fold increase in the rate of B(e)P metabolism, it did not enable B(e)P to affect Lucifer Yellow transfer between DM15 cells. We conclude that neither B(e)P nor its metabolites are capable of uncoupling gap junction permeability in DM15 cells.

Reactivity of benzo[a]pyrene-7,8-dione with DNA. Evidence for the formation of deoxyguanosine adducts

Polycyclic aromatic hydrocarbon (PAH) o-quinones are products of the dihydrodiol dehydrogenase-catalyzed oxidation of trans-dihydrodiols which are proximate carcinogens. The PAH o-quinones are highly reactive molecules and have the potential to alkylate DNA. In this study, the reactivity of [3H](+-)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene ([3H] (+/-)-anti-BPDE), [3H]benzo[a]pyrene-7,8-dione ([3H]BPQ) and [3H](+-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene ([3H](+/-)-B[a]P-diol) with DNA were compared. (+/-)-anti-BPDE reacted equally well with native, deproteinated and deproteinated/sheared calf thymus DNA. In each case DNA adducts were formed which upon digestion to deoxyribonucleosides comigrated on reverse-phase (RP)-HPLC with adducts synthesized by reacting (+/-)-anti-BPDE with oligo-p(dG)10. (+/-)-anti-BPDE also reacted with plasmid (pGEM-3) DNA to yield multiple adducts one of which comigrated with the (+)-anti-BPDE-deoxyguanosine adduct. Under identical conditions [3H]BPQ reacted preferentially with native calf thymus DNA but displayed low reactivity with deproteinated and deproteinated/sheared calf thymus DNA. RP-HPLC analysis of deoxyribonucleoside-BPQ adducts indicated that the predominant adduct formed comigrated with a standard synthesized by reacting BPQ with oligo-p(dG)10. BPQ also reacted with pGEM-3 DNA to yield multiple adducts one of which comigrated with the BPQ-deoxyguanosine adduct. Reactions between [3H]BPQ and poly(dA), poly(dT), poly(dC) and oligo-p(dG)10 indicated that BPQ preferentially formed deoxyguanosine adducts. In this study, [3H]BPQ and [3H](+/-)-anti-BPDE covalently labeled native calf thymus DNA to an equal extent, however, less [3H]BPQ was recovered as deoxyguanosine adducts. By contrast, no covalent modification of calf thymus DNA, pGEM-3 DNA or oligonucleotides was observed with [3H](+/-)-B[a]P-diol. These studies indicate that BPQ has the potential to be genotoxic in vitro; that reactivity is heightened in the presence of protein or circular DNA and that the major adduct formed is a deoxyguanosine adduct.

Relationships between biological potency and electronic states of polychlorinated dibenzofurans and polychlorinated biphenyls

It was found that the differences between the frontier molecular orbital energies (epsilon homo - epsilon lumo = delta epsilon) in polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) have a correlationship with the magnitude of the biological activity which is influenced by both the number and position of chlorine atom substituents on PCDFs and PCBs skeletons. Moreover, it was found that the delta epsilon's values of PCBs are classified into two types which coincide with the well-known classification of PCBs to types of 3-methylcholanthrene and phenobarbital according to their biological activities. The relationship between delta epsilon and biological activity in these xenobiotics suggests that the congeners having small delta epsilon values as 2,3,4,7,8-pentaCDF, 2,3,4,6,7-pentaCDF, 3,4,5,3',4'-pentaCB, and 3,4,5,3',4',5'-hexaCB form stable molecular complexes with an Ah-receptor, e.g. (2,3,4,7,8-PentaCDF-Ah-receptor), while the congeners having large delta epsilon values are strongly suggested to be unstable in a complex formation. Thus, this work presents an explanatory method to help understand the structure-activity relationship of the xenobiotics PCDFs and PCBs.

Biomarkers for individual susceptibility to carcinogenic agents: excretion and carcinogenic risk of benzo[a]pyrene metabolites

In rats exposed to a single intraperitoneal dose of 200 mg/kg of the environmental carcinogen benzo[a]pyrene (BP) in sunflower oil, significant individual variations in excretion of the BP activation (BP-7,8-diol) and deactivation (3-OH-BP) derivatives were found. Most rats developed peritoneal sarcomas. Only the levels of BP-7,8-diol excreted in the urine correlated directly with the latency of tumor formation. After a similar exposure to a dose of 100 mg/kg BP, Macaca fascicularis monkeys excreted smaller quantities than rats of both metabolites. After rats were given 10 intraperitoneal injections each of 10 mg/kg of BP in a water-lipid emulsion, the excreted levels of both metabolites after the first, fifth, and tenth injection were lower than those of the rats that received 200 mg/kg. BP metabolites were also detected in the urine of lung cancer patients who were heavy smokers. The applicability of monitoring the excretion of the BP metabolites to predicting individual cancer risk is discussed.

Evidence of involvement of multiple sites of metabolism in the in vivo covalent binding of dibenzo[a,h]pyrene to DNA

The in vivo formation of dibenzo[a,h]pyrene-DNA adducts in mouse skin was assessed by laser-excited fluorescence spectroscopy at 77 and 4.2 K. Two adducts were identified with fluorescence origin bands at approximately 383.5 and 407.2 nm, and these were shown to possess pyrene and benzo[a]pyrene (B[a]P) chromophores, respectively. Both DNA-bound chromophores displayed considerable electron-phonon coupling and likely assume a highly base-stacked or quasi-intercalated configuration within DNA duplexes. The presence of B[a]P and pyrene aromatic systems indicates that two-electron or monooxygenation metabolism occurred on either the a or h benzo moieties (which are equivalent) in the former case, and on both these rings in the latter case. The presence of two adduct species agrees with 32P-postlabeling analysis of the DNA, which showed the presence of two major adducts in both thin-layer and high-performance liquid chromatographic separations.

Contribution of dihydrodiol dehydrogenase to the metabolism of (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene in fortified rat liver subcellular fractions

Dihydrodiol dehydrogenase (DD; EC 1.3.1.20) purified to homogeneity from rat liver cytosol will catalyze the NAD(P)(+)-dependent oxidation of (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-diol) to yield benzo[a]pyrene-7,8-dione (BPQ). To verify that BPQ is a metabolite of B[a]P-diol in rat liver, an S100 fraction was supplemented with NAD+ and NADP+, and the formation of BPQ was followed by reverse-phase HPLC. The identity of BPQ was established by co-chromatography with an authentic standard (under different solvent conditions) and by RP-HPLC using a diode-array detector which established that the metabolite shared spectral identity with BPQ. The formation of BPQ in the S100 fraction was blocked by either a competitive inhibitor (indomethacin) or a suicide substrate [1-(4-nitrophenyl)-propen-1-ol] for DD, indicating that BPQ was being formed by this enzyme. To assess the contribution of DD to the metabolism of [3H]B[a]P-diol, subcellular fractions obtained from uninduced rat liver were fortified with co-factors to optimize the activity of enzymes that would compete for this proximate carcinogen. Under these conditions, S100 fractions fortified with NAD+ and NADP+ metabolized 25% of the B[a]P-diol, producing 731 +/- 154 pmol of BPQ. In contrast, rat liver microsomes fortified with an NADPH generating system metabolize 75% of the B[a]P-diol producing 2614 +/- 379 pmoles of benzo[a]pyrene-tetrahydrotetrols. Rat liver homogenates (S10) fortified with either uridine diphosphoglucuronic acid or phosphoadenosine phosphosulfate produced 180 +/- 56 and 95 +/- 31 pmoles of conjugates respectively, which were recovered as B[a]P-diol after treatment of the aqueous phase with either beta-glucuronidase or aryl sulfatase. Of the metabolites analyzed BPQ was formed in the second largest amount. These studies show that in uninduced rat liver DD may play a significant role in the metabolism of B[a]P-diol. The metabolic fate of BPQ remains to be determined.

Glutathione S-transferase mu in human lymphocyte and liver: role in modulating formation of carcinogen-derived DNA adducts

Glutathione transferase (GT) activity towards trans-stilbene oxide (tSBO), benzo[a]pyrene-4,5-oxide (B[a]PO) and 1-chloro-2,4-dinitrobenzene (CDNB) was measured in human liver and lymphocytes. GT-tSBO activity is catalyzed by GT mu which has polymorphic expression in human lymphocytes. Our results show that activity of GT-tSBO in lymphocytes correlates with its activity in liver (r = 0.7, P less than 0.001). GT activity towards BPO (GT-BPO) also correlated with GT-tSBO in lymphocytes and liver. However, interindividual variation of GT-BPO is less than that of GT-tSBO, suggesting that BPO may not be as specific a substrate for GT mu and therefore other GT isozymes may contribute to BPO conjugation. Conjugation of CDNB by GT was not different using cytosols from either high or low GT mu individuals. The functional significance of the GT-mu polymorphism was evaluated by measuring its effect on benzo[a]pyrene (B[a]P)- and aflatoxin B1 (AFB1)-DNA adduct formation in vitro. Human liver cytosols prepared from persons having low or high GT-tSBO activity were incubated with human liver microsomes, calf thymus DNA and B[a]P or AFB1. HPLC analysis revealed that the major B[a]P adduct was dG(N2)-7 beta, 8 alpha-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-dG). BPDE-dG adducts were decreased equally by cytosols from either low or high conjugators. In contrast, AFB1-DNA binding was inhibited to a greater extent in high conjugators than low conjugators. HPLC analysis demonstrates that adducts formed were AFB1-FAPyr and AFB1-N7-Gua. The correlation between AFB1-DNA adduct concentrations and GT mu activity was highly significant with a correlation coefficient of r = 0.88 at P less than 0.001. These results suggest that GT mu plays an important role in detoxifying DNA reactive metabolites of AFB1 and this enzyme may be a susceptibility marker for AFB1 related liver cancer. Moreover, our data demonstrate that lymphocytes are a reliable surrogate tissue for detecting liver GT mu polymorphisms.

DNA adducts in rat lung, liver and peripheral blood lymphocytes produced by i.p. administration of benzo[a]pyrene metabolites and derivatives

DNA adducts produced in vivo in rat lung, liver and peripheral blood lymphocytes following the i.p. administration of several synthetic benzo[a]pyrene (B[a]P) metabolites and ring-substituted derivatives have been analyzed by the nuclease P1 version of the 32P-postlabeling assay. These include 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9-, 10-, 11- and 12-hydroxy-B[a]P, (+/-)-B[a]P-trans-4,5-dihydrodiol, (+/-)-B[a]P-trans-7,8-dihydrodiol, (+/-)-B[a]P-trans-9,10-dihydrodiol and B[a]P-7,8-dione. Among the monohydroxy derivatives, only 2-, 9- and 12-hydroxy-B[a]P produced detectable adducts. The only disubstituted derivative studied that produced adducts was the trans-7,8-dihydrodiol. The resulting DNA adducts were compared to those produced in each tissue by administration of B[a]P. 9-Hydroxy-B[a]P and B[a]P-trans-7,8-dihydrodiol each lead to the formation of major B[a]P adducts seen in lung and liver respectively. None of the adducts derived from either 2-hydroxy-B[a]P or 12-hydroxy-B[a]P were observed following administration of B[a]P alone.

Role of isozyme-specific inhibition of cytochrome P450IIB1 activity in m-xylene-induced alterations in rat pulmonary benzo(a)pyrene metabolism

1. m-Xylene (1 g/kg, i.p., 1 h) increased formation of benzo(a)pyrene (BP) mutagenic bay region diols, BP-7,8-diol (66%) and BP-9,10-diol (56%) by rat pulmonary microsomal preparations, while formation of individual BP phenols and quinones was unaltered. 2. m-Xylene administration produced a decrease in cytochrome P450IIB1 activity as measured by pentoxy- and benzyloxy-resorufin O-dealkylation (PROD, BROD), while cytochrome P450IA1 activity, expressed as ethoxyresorufin O-dealkylation (EROD), was unaltered. 3. Pulmonary microsomal epoxide hydrolase activity was also unaltered by m-xylene. 4. In summary, m-xylene alters the relative contribution of P-450 isozymes to BP metabolism resulting in inhibition of BP detoxication and increased production of toxic metabolites.

A new sensitive fluorometric assay for the metabolism of (--)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene by human hair follicles

A new sensitive fluorometric assay was established to measure the stereospecific formation of benzo[alpha]pyrene tetrols formed after cytochrome P450-dependent metabolism of (--)-7,8-dihydroxy-7,8- dihydrobenzo[a]pyrene by human hair follicles. This simple assay requires three human hair follicles and a low (0.5-2.0 microM) substrate concentration and has a limit of detection of approximately 0.3 fmol of tetrols. Freshly isolated human hair follicles from 20 adult volunteers (10 non-smokers and 10 smokers) were assayed. While intersubject and seasonal variations were observed, the assay was found to be reproducible for a given subject. This rapid and non-invasive assay provides a new means for metabolic phenotyping of human subjects for their capacity to metabolize (--)-7,8-dihydroxy-7,8-dihydrobenzo[alpha]pyrene to its carcinogenic form (+)anti-benzo[a]pyrene diolepoxide.

Metabolism and mutagenicity of dibenzo[a,e]pyrene and the very potent environmental carcinogen dibenzo[a,l]pyrene

Dibenzo[a,l]pyrene (DB[a,l]P) is one of the most potent carcinogens ever tested in mouse skin and rat mammary gland. DB[a,l]P is present in cigarette smoke and, presumably, in other environmental pollutants. Metabolism and mutagenicity studies of this compound compared to the weak carcinogen dibenzo[a,e]pyrene (DB[a,e]P) can provide preliminary evidence on its mechanism of carcinogenesis. The mutagenicity of DB[a,l]P, DB[a,e]P, and benzo[a]pyrene (BP) was compared in the Ames assay with Aroclor-induced rat liver S-9. BP was the strongest mutagen. In strain TA100, DB[a,l]P and DB[a,e]P were marginally mutagenic. In strain TA98 both compounds were mutagenic, and DB[a,l]P induced more than twice as many revertants as DB[a,e]P. The mutagenicity of DB[a,l]P does not correlate with its carcinogenicity, since DB[a,l]P is a much stronger carcinogen, but a much weaker mutagen, than BP. The NADPH-supported metabolism of DB[a,e]P and DB[a,l]P was conducted with uninduced and 3-methylcholanthrene-induced rat liver microsomes. Metabolites were analyzed by reverse-phase HPLC and identified by NMR, UV, and mass spectrometry. Uninduced microsomes produced only traces of metabolites with either compound. The major metabolites of DB[a,l]P with induced microsomes were DB[a,l]P 8,9-dihydrodiol, DB[a,l]P 11,12-dihydrodiol, 7-hydroxyDB[a,l]P, and a DB[a,l]P dione. The metabolites of DB[a,e]P with induced microsomes were DB[a,e]P 3,4-dihydrodiol, 3-hydroxyDB[a,e]P, 7-hydroxyDB[a,e]P, and 9-hydroxyDB[a,e]P. Some of these metabolites are very useful in assessing possible pathways of activation in the initiation of cancer.

The strong hepatocarcinogenicity of the electrophilic and mutagenic metabolite 6-sulfooxymethylbenzo[a]pyrene and its formation of benzylic DNA adducts in the livers of infant male B6C3F1 mice

6-Hydroxymethylbenzo[a]pyrene was activated to an electrophilic and mutagenic sulfuric acid ester metabolite by rat and mouse liver sulfotransferase activity. The intrinsic mutagenicity of this reactive ester, 6-sulfooxymethylbenzo[a]pyrene, was inhibited by glutathione and glutathione S-transferase. A single i.p. dose of 2.5 nmol/g body wt of 6-sulfooxymethylbenzo[a]pyrene in infant male B6C3F1 mice induced liver tumors in 35 of 36 mice at 10 months with an average multiplicity of 4.4. A comparable dose of the parent hydrocarbon, 6-hydroxymethylbenzo[a]pyrene, was only a tenth as active. The electrophilic sulfuric acid ester produced high levels of benzylic DNA adducts in the livers of these mice that accounted for about 80% of the total DNA adducts. These results strongly suggest that this sulfuric acid ester is an important ultimate electrophilic and carcinogenic metabolite in carcinogenesis by 6-hydroxymethylbenzo[a]pyrene and possibly even by 6-methylbenzo[a]pyrene and benzo[a]pyrene in mouse liver.

The formation of covalent adducts between benzo[a]pyrenediol epoxide and RNA: structural analysis by mass spectrometry

Racemic 7-r,8-t-dihydroxy-9-t,10-t-epoxy-7,8,9,10-tetrahydrobenzo[a] pyrene was reacted with yeast RNA. Modified nucleosides were isolated and resolved by high-performance liquid chromatography; nine adduct peaks were collected for analysis. The bases in these adducts were identified by comparing their retention times with those of adducts from poly(G), poly(A), and poly(C). These samples gave two major and two minor Guo adducts, four major Ado adducts, and at least four Cyd adducts. The relative efficiencies of adduct formation with the polyribonucleotides were poly(G) greater than yeast RNA greater than poly(A) greater than poly(C). Fluorescence measurements show that emission from Guo adducts is strongly quenched relative to that from Ado adducts. Liquid secondary ion mass spectrometry (LSIMS) of underivatized samples and electron-impact mass spectrometry (EIMS) of permethyl derivatives were used to confirm the base identities and establish the alkylation sites of the RNA adducts. Unique nitrogen-containing hydrocarbon fragments that were observed with all samples by EIMS establish that in each adduct analyzed the C-10 position of the hydrocarbon is linked to the exocyclic amino group of the base. This suggested that the multiple adducts formed with each base are diastereomers derived from cis/trans epoxide ring opening of the (+) and (-) enantiomers of the carcinogen. Several adducts exhibited molecular ions by both LSIMS and EIMS. Large fragments observed by EIMS usually resulted from the loss of CH3OH, CH3O., CH2O, CH3., and H. from the molecular ion. Major fragmentation pathways also resulted in formation of nucleoside, base, ribose, hydrocarbon, and base-hydrocarbon ions. Each of these major ions in turn resulted in further characteristic fragmentation patterns.

Binding of polycyclic and nitropolycyclic aromatic hydrocarbons to specific fractions of rat lung chromatin

Carcinogen-induced damage to nuclear matrix DNA, the site of DNA replication and transcription, could have profound effects on gene regulation and mutation. 1,6-Dinitropyrene (1,6-DNP), 1-nitropyrene (1-NP), 6-nitrobenzo[a]pyrene (6-NBP), benzo[a]pyrene (BP) and benzo[a]pyrene diolepoxide (BPDE) were investigated for their abilities to bind to selected regions of DNA in rat lung cell nuclei. Following in vitro exposure to carcinogen, nuclei were fractionated into active chromatin (AC), nuclear matrix (NM) and bulk (low and high salt) chromatin fractions. At an equivalent molar concentration, the highest binding to unfractionated (total) DNA was obtained with BPDE, followed in order by BP, 1,6-DNP, 6-NBP and 1-NP. BPDE, a direct alkylating compound, was bound approximately 18 times higher than the other compounds. All compounds were bound to AC (mononucleosomal DNA approximately 185 bp) and to NM in greater amounts than to bulk DNA. The binding ratios (AC + NM)/(LS + HS) varied from 2 to 21, depending upon the compound. The selective binding to specific DNA regions did not appear to be significantly related to the structures of the parent compounds or to their inferred metabolites. Thus, it appears that selective binding to specific regions is a general phenomenon that is related to the open state of the chromatin structure.

Effect of nitro-substitution of environmental polycyclic aromatic hydrocarbons on activities of hepatic phase II enzymes in rats

A series of environmentally occurring nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) including those containing nitro-groups oriented coplanarly to the aromatic rings, such as 1- and 3-nitrobenzo[a]pyrene (1- and 3-NBAP), 6-nitrochrysene, and 1- and 4-nitropyrene, and those with a molecular orientation of the nitro-groups perpendicular to the aromatic moieties, such as 7-nitrobenz[a]anthracene and 6-nitrobenzo[a]pyrene (6-NBAP), were used to study the induction of certain rat hepatic phase II conjugating enzymes. Effects of these two different classes of nitro-PAHs on microsomal UDP-glucuronyltransferase (UDPGT), cytosolic glutathione S-transferases (GSTs) and sulfotransferases (STs) were investigated. After three consecutive daily i.p. injections, 1- and 3-NBAP and 6-nitrochrysene significantly increased the activities of UDPGT and GST, whereas their parent PAHs did not induce UDPGT (and GST activity was also unaltered by benzo[a]pyrene). UDPGT and GST activities were also significantly increased by 1-nitropyrene. In contrast, the sulfotransferases directed to 2-naphthol were not significantly induced by any PAH or nitro-PAH when assayed at either pH 5.5 or 7.5; however, the activities of aryl STs III and IV (pH 5.5) were significantly decreased following treatment with pyrene and two nitro-compounds, 6-NBAP and 7-nitrobenz[a]anthracene, in which the nitro-group is oriented perpendicular to the aromatic moiety. These results indicate that a coplanar orientation of the nitro-group of certain nitrated PAHs facilitates the induction of hepatic phase II enzymes by these compounds in rats, and the comparable induction patterns for P450IA1, UDPGT, and GST provide further evidence supporting the coordinate regulation (through the Ah receptor) of these phase I and phase II activities.

Metabolic activation of the carcinogen 6-hydroxymethylbenzo[a]pyrene: formation of an electrophilic sulfuric acid ester and benzylic DNA adducts in rat liver in vivo and in reactions in vitro

Hydroxylation of meso-methyl groups with subsequent formation of reactive electrophilic esters has been proposed as a possible activation pathway in the metabolism, DNA binding and carcinogenicity of some methyl-substituted polycyclic aromatic hydrocarbons. Some data in vitro have been reported in support of this concept. In this study, sulfotransferase activity for 6-hydroxymethylbenzo[a]pyrene (HMBP) in rat and mouse liver cytosols was demonstrated to mediate formation of benzylic adducts from this hydrocarbon with guanosine and with deoxyguanosine and deoxyadenosine in DNA. These benzylic adducts were also obtained from reactions of synthetic 6-sulfooxymethylbenzo[a]pyrene (SMBP) with individual (deoxy)ribonucleosides or DNA. The structure of the major DNA adduct formed from HMBP and SMBP was determined from NMR spectroscopy to be N2-(benzo[a]pyrene-6-methylenyl)-deoxyguanosine. Low levels of a deoxycytidine adduct were also obtained from DNA reacted with SMBP. Covalent modification of DNA by acetyl-CoA- and ATP-dependent activation of HMBP also produced the identical benzylic adducts, but the amounts were smaller than those obtained in the sulfotransferase-mediated reaction. The i.p. administration of HMBP to rats resulted in the formation of a hepatic DNA adduct. After enzymatic hydrolysis to the nucleoside level, this DNA adduct was chromatographically identical with the deoxyguanosine adduct formed in the above in vitro reactions. This adduct accounted for approximately 20-30% of total HMBP residues bound to hepatic DNA and its formation was significantly inhibited by pretreatment of rats with dehydroepiandrosterone, an inhibitor of the sulfotransferase activity for HMBP. The i.p. administration of comparable doses of SMBP to rats led to the formation of much larger amounts of the adducts with the guanine, adenine, and cytosine bases in the liver DNA. The data indicate that the sulfotransferase activity in the rat liver for HMBP plays a major role in the benzylic DNA adduct formation from this hydrocarbon in rat liver in vivo.

Simultaneous determination of cytosolic glutathione S-transferase and microsomal epoxide hydrolase activity toward benzo[a]pyrene-4,5-oxide by high-performance liquid chromatography

Cytosolic glutathione S-transferase (GST) and microsomal epoxide hydrolase (EH) are important detoxification enzymes for many epoxide xenobiotics. We have developed a rapid, simple, and convenient HPLC assay which measures both of these enzyme activities toward benzo[a]pyrene-4,5-oxide (BaPO) in tissue homogenates. Tissue fractions were incubated at 37 degrees C in the presence of 5 mM glutathione. Reactions were initiated by addition of BaPO and terminated by the addition of ice-cold acetonitrile containing 2-methoxynaphthalene as an internal standard. Samples were analyzed directly on a 15-cm C18 reverse-phase column at room temperature, with a ternary solvent program which utilized 0.01% ammonium phosphate buffer (pH 3.5), acetonitrile, and water. The uv absorbance (260 nm) was monitored. Baseline resolution of BaPO, BaPO-GSH, and BaPO-diol and the internal standard was accomplished in 10 min. In rat hepatic S9, production of both BaPO-GSH and BaPO-diol was linear with time and protein up to 15 min and 500 micrograms/ml, respectively. Coefficients of variation for replicate analyses were 2.7 and 3.7% for GST and EH activities in S9, respectively. With fluorescence detection (ex, 241; em, 389 nm), this assay was sensitive enough to measure GST and EH activities in mononuclear leukocytes (MNL). GST and EH activities in 109 human MNL samples were 142 +/- 74 (mean +/- SD; range 21-435) pmol/mg/min and 19 +/- 9 (mean +/- SD; range 3-59) pmol/mg/min, respectively. These results demonstrate the simplicity, high sensitivity, and applicability of this assay for a broad range of tissues.

Modulation of 3-methylcholanthrene toxicity in cultured neoplastic keratinocytes by glucocorticoids and retinoids is not accounted for by macromolecular adduct formation

3-Methylcholanthrene (3-MC) greatly inhibits the growth of two lines of human squamous carcinoma cells, SCC-9 and SCC-12B2. Exposure of the cells to 2,3,7,8-tetrachlorodibenzo-p-dioxin alone was much less effective and, in the presence of 3-MC, did not alter the sensitivity (EC50 = 0.3 microM) or extent of growth inhibition by the latter. The degree of 3-MC-mediated inhibition, however, was markedly alleviated by inclusion of retinoic acid (EC50 greater than or equal to 0.7 microM) and hydrocortisone (EC50 = 40 nM) or dexamethasone (EC50 = 3 nM) in the culture medium. These physiological effectors, which are known to have opposing actions on keratinocyte character in SCC cells, did not significantly alter either aryl hydrocarbon hydroxylase activity or macromolecular adduct formation. Further analysis of the cellular responses indicated that hydrocortisone and, in some experiments, retinoids increased the growth rate in 3-MC-exposed cultures, while 3-MC increased the saturation density in retinoic acid-exposed cultures, an example of interference with a physiological response of the cells. These results indicate that alteration of the differentiated state, regardless of the direction of the change, can alter the sensitivity of the cells to toxic stimuli. Further investigation of the bases of such toxic responses and their modulation by the microenvironment may enhance our understanding of the target cell specificity of polycyclic aromatic hydrocarbons.

Effect of sulfite on the covalent reaction of benzo[a]pyrene metabolites with DNA

Sulfur dioxide (SO2) potentiates the carcinogenicity of polycyclic aromatic hydrocarbons. To investigate the mechanism of SO2 cocarcinogenesis, the effect of sulfite, the hydrated form of SO2, on the covalent reaction of benzo[a]pyrene (BaP) metabolites with DNA in vitro was measured. [14C]BaP was incubated with rat lung or liver post-mitochondrial supernatant (S9), an NADPH generating system, calf thymus DNA and sodium sulfite (0-20 mM). In the presence of lung S9, covalent reaction increased linearly from 0.66 to 1.20 pmol BaP metabolites per mg DNA with increasing sulfite concentrations. Addition of sulfite to rat liver S9 also increased BaP-DNA adduct formation with BaP-DNA adducts increasing from 80 to 120 pmol per mg DNA. Sulfite altered the amount and pattern of BaP metabolites formed by either lung or liver enzyme preparations. BaP was metabolized more extensively and the amount of water soluble BaP metabolites formed increased significantly with sulfite present. With lung S9, the amount of BaP-tetrols, diols, and phenols increased slightly. With liver S9, diol and phenol formation was significantly lower while tetrol formation was unchanged. Incubation of rat lung S9 with sulfite resulted in formation of glutathione S-sulfonate (GSSO3H), a known inhibitor of glutathione S-transferases mediating the conjugation of glutathione (GSH) and BaP epoxides. Our results suggest that sulfite may, by altering the overall metabolic activation and detoxication of BaP, or by reacting directly with DNA, subsequently affect the covalent reaction of BaP metabolites with DNA. These are offered as possible mechanisms to explain the cocarcinogenic effect of SO2.

Nitroreduction of 6-nitrobenzo[a]pyrene: a potential activation pathway in humans

6-Nitrobenzo[a]pyrene, an environmental pollutant, was metabolized by human intestinal microflora to 6-nitrosobenzo[a]pyrene and 6-aminobenzo[a]pyrene. The two-electron reduction product 6-nitrosobenzo[a]pyrene exhibited strong direct-acting mutagenicity in the Salmonella typhimurium assay. These results imply that 6-nitrobenzo[a]pyrene can be hazardous to human health via a nitroreduction activation pathway and opens the possibility that other nitro-polycyclic aromatic hydrocarbons that are not direct-acting mutagens may be activated in vivo by a similar mechanism.

Chromatographic and spectral investigations on the in vivo metabolites of 6-nitrobenzo[a]pyrene

6-Nitrobenzo[a]pyrene (6-NBaP) occurs in the environment, is mutagenic in the Ames assay in the presence of added S9 and is carcinogenic to male but not female mouse liver when injected intraperitoneally (i.p.) into mice. In order to understand what kinds of active metabolites could have been produced in vivo, both male and female mice were injected i.p. with 6-NBaP in dimethyl sulfoxide. Twenty-four hours after injection, urine, feces, blood, liver and spleen (non-target tissue) were examined for metabolites by chromatographic and high-resolution mass spectral means. On the basis of the mass spectral fragmentation patterns of synthetic and metabolic standards, it was observed that both male and female animals excreted ring-hydroxylated metabolites of 6-NBaP in the urine to differing extents. Male animals additionally excreted 6-aminobenzo[a]pyrene and the significance of this observation is discussed.

Microsomal metabolism of 1-nitrobenzo[e]pyrene to a highly mutagenic K-region dihydrodiol

Aerobic metabolism of 1-nitrobenzo[e]pyrene (1-nitro-BeP) by rat liver microsomes produced 1-nitro-BeP trans-4,5-dihydrodiol, 6-hydroxy-1-nitro-BeP, and 8-hydroxy-1-nitro-BeP. When 3,3,3-trichloropropylene 1,2-oxide was incorporated into the metabolism, 1-nitro-BeP 4,5-oxide was the predominant metabolite, and 1-nitro-BeP trans-4,5-dihydrodiol was not detected. All of the metabolites were purified by both reversed- and normal-phase HPLC and characterized by analysis of their mass and 500 MHz proton NMR spectral data. 1-Nitro-BeP was not metabolized under hypoxic conditions. 1-Nitro-BeP and its four metabolites were assayed in Salmonella typhimurium tester strains TA98, TA98NR and TA98/1,8-DNP6, both in the presence and absence of S9 activation. As predicted, 1-nitro-BeP was a weak mutagen without S9 (2 revertants/micrograms in TA98); the addition of S9 resulted in approximately 18, 17 and 4 revertants/micrograms in TA98, TA98NR and TA98/1,8-DNP6 respectively. The two phenolic metabolites were mutagenic both in the presence and absence of S9, producing moderate responses (19-84 revertants/micrograms). In addition, while the 1-nitro-BeP 4,5-oxide was only weakly mutagenic in TA98 (6-14 revertants/micrograms), 1-nitro-BeP trans-4,5-dihydrodiol was unexpectedly potent (approximately 300 revertants/micrograms both with and without S9). These results indicate that microsomal epoxidation of 1-nitro-BeP followed by epoxide hydrolase-catalyzed hydrolysis of the resulting epoxide to the 1-nitro-BeP trans-4,5-dihydrodiol results in the most potent mutagenic derivatives. The weak mutagenicity of 1-nitro-BeP 4,5-oxide demonstrates that not all epoxides of nitrated polycyclic aromatic hydrocarbons (PAHs) are more mutagenic than the corresponding parent nitro-PAHs. Also, the lower S9-mediated mutagenicity of 1-nitro-BeP in TA98/1,8-DNP6 compared with TA98 indicates that the mutagenicity of 1-nitro-BeP is dependent upon nitroreduction and transesterification. Finally, we previously hypothesized that nitrated PAHs with their nitro substituents perpendicular or nearly perpendicular to the aromatic rings are very weak or nondirect-acting mutagens in Salmonella typhimurium tester strains. The results reported in this communication demonstrate that ring-oxidized derivatives of nitro-PAHs do not always follow this structure--mutagenicity correlation.

Radical cations as precursors in the metabolic formation of quinones from benzo[a]pyrene and 6-fluorobenzo[a]pyrene. Fluoro substitution as a probe for one-electron oxidation in aromatic substrates

Three classes of products are formed when benzo[a]pyrene (BP) is metabolized by cytochrome P-450: dihydrodiols, phenols and the quinones, BP 1,6-, 3,6- and 6,12-dione. These products have been thought to arise from attack of a catalytically-activated electrophilic oxygen atom. In this paper we report chemical and biochemical experiments which demonstrate that BP quinones arise from an initial one-electron oxidation of BP to form its radical cation. BP, 6-fluorobenzo[a]pyrene (6-FBP), 6-chlorobenzo[a]pyrene (6-ClBP), and 6-bromobenzo[a]pyrene (6-BrBP) were metabolized by uninduced and 3-methylcholanthrene-induced rat liver microsomes in the presence of NADPH or cumene hydroperoxide (CHP) as cofactor. BP and 6-FBP produced similar metabolic profiles with induced microsomes in the presence of NADPH or 2 mM CHP. With NADPH both compounds produced dihydrodiols, phenols and quinones, whereas with CHP, they yielded only quinones. Metabolism of BP and 6-FBP was also similar with uninduced microsomes and 2 mM CHP, yielding the same BP quinones. With uninduced microsomes in the presence of NADPH, BP produced all three classes of metabolites, whereas 6-FBP afforded only quinones. At a low concentration of CHP (0.10 mM), BP was metabolized to phenols and quinones, whereas 6-FBP gave only quinones. 6-ClBP and 6-BrBP were poor substrates, forming metabolites only with induced microsomes and NADPH. One-electron oxidation of BP by Mn(OAc)3 occurred exclusively at C-6 with predominant formation of 6-acetoxyBP and small amounts of BP quinones. In the one-electron oxidation of 6-FBP by Mn(OAc)3, the major products obtained were 6-acetoxyBP, a mixture of 1,6- and 3,6-diacetoxyBP, and BP quinones. Reaction of BP and 6-FBP radical cation perchlorates with water produced the same BP quinones. Conversely, electrophilic substitution of 6-FBP with bromine or deuterium ion afforded C-1 and/or C-3 derivatives with retention of the fluoro substituent at C-6. These results indicate that metabolic formation of BP quinones from BP and 6-FBP can only derive from their intermediate radical cation.

Kinetic and immunochemical studies of a receptor-like protein that binds aromatic hydrocarbons

A 29-kDa cytosolic protein that binds polycyclic aromatic hydrocarbons (PBP) with high affinity, specificity and saturability was identified in and was purified from C57BL/6J mouse liver (Collins, S., and Marletta, M. A. (1986) Biochemistry 25, 4322-4329). Kinetic studies showed that benzo[a]pyrene had a koff = 0.28 +/- 0.06 min-1 and a kon = 7.0 +/- 0.2 x 10(7) M-1 min-1. The Kd calculated from these rates is 4.0 +/- 0.9 nM which agrees with equilibrium measurements. This provides support for the description of this ligand-protein interaction as simple mass action binding of one ligand per protein molecule. Polyclonal antiserum with a high titer of antibodies specific for this protein was prepared. Western blot analyses from mouse tissues showed highest levels in liver, heart, kidney, and lung with lower levels in intestine, spleen, thymus, testes, and brain. Serum was negative. Liver cytosols from 7-week-old males of eight strains of mice (C57Bl/6J, BALB/cJ, C3H/HeJ, A/J, CD1/Cr, DBA/2J, AKR/J, and SWR/J) had similar concentrations of the PBP. The PBP was also present in liver cytosol from 7-week-old female and 42-week-old male C57Bl/6J mice. High affinity specific binding for benzo[a]pyrene was also found in rat, rabbit, guinea pig, scup, and chicken liver cytosols. This binding had low nM equilibrium binding constants and was not competed by tetrachlorodibenzofuran distinguishing these sites from the Ah receptor. These studies show that the PBP or similar receptor-like proteins are widespread among mouse tissues and strains and evolutionarily diverse animal species.

Metabolism of 1-, 3-, and 6-nitrobenzo[a]pyrene by intestinal microflora

The compounds 1-, 3-, and 6-nitrobenzo[a]pyrene (nitro-BaP) are environmental pollutants and have been shown to be potent bacterial mutagens. The anaerobic metabolism of these isomeric nitro-BaPs was investigated by the incubation of rat intestinal microflora with each isomer for 48 h. Aliquots were removed at several time intervals, extracted, fractionated by high-pressure liquid chromatography (HPLC), and the radioactivity determined. Metabolites were identified by comparison of their chromatographic, ultraviolet-visible absorption, and mass spectral properties with those of authentic standards. The order of the extent of nitroreduction for these isomers was 3-nitro-BaP greater than 6-nitro-BaP greater than 1-nitro-BaP. After 48 h of exposure, 84% of the added 3-nitro-BaP was present as 3-amino-BaP, 51% of the 6-nitro-BaP was metabolized to 6-amino-BaP, and 1-nitro-BaP was reduced to 1-amino-BaP (13%) and 1-nitro-BaP (4%). The order of the extent of microbial nitroreduction for these nitro-BaP isomers is different from the predictions based on electronic and steric hindrance effects. These results suggest that intestinal microflora nitroreductases exhibit a markedly high degree of substrate specificity toward nitro-BaPs that affects the extent of nitroreduction.

Binding characteristics of 4-S proteins from rat and mouse liver. High affinity of ellipticines

The binding characteristics of 4-S components (carcinogen-binding protein) from livers of Sprague-Dawley rats, C57BL/6 and DBA/2 mice have been examined before and after separation from other binding components presents in the cytosol. Competitive potency of 3-methylcholanthrene, benzo[alpha]pyrene, beta-naphthoflavone and 20 ellipticines, a series of compounds differently substituted on the dimethyl-pyrido-carbazole nucleus and deprived of carcinogenic activity, has been determined with [3H]3-methylcholanthrene and/or [3H]benzo[a]pyrene as radioligands. The inducing ability of the same compounds for aryl hydrocarbon hydroxylase and for ethoxyresorufin-O-deethylase has been compared to their affinity for the 4-S protein and the Ah receptor respectively. The main results of this study are as follows. 1. The intrinsic binding characteristics of 4-S proteins were dependent on both the nature of the radioligand used and the presence or absence of other cytosolic binding components. 2. The heterocyclic ellipticines were revealed as strong ligands for the carcinogen-binding protein (stronger than benzo[alpha]pyrene for five derivatives substituted in the A ring of ellipticine), with IC50 values ranging from 0.047 microM (8-hydroxyellipticine) to 5.8 microM (N2-ethyl-9-hydroxyellipticinium). 3. When the affinity of ellipticines was plotted versus their inducing ability of aryl hydrocarbon hydroxylase and ethoxyresorufin-O-deethylase, it appears that a good correlation exists for the Ah receptor but not for the 4-S protein. It is concluded that these data, as well as the lack of enzymatic induction after benzo[alpha]pyrene treatment of DBA/2 mice, which display a high level of 4-S protein, do not support the implication of this binding component in the positive control of cytochrome P-450 induction.

Glutathione transferase activities of cultured human lymphocytes

We have detected glutathione transferase (GST) activity towards 1-chloro-2,4-dinitrobenzene (CDNB) and benzo[a]-pyrene 4,5-oxide (BPO) in freshly isolated peripheral lymphocytes, phytohaemagglutinin (PHA)-stimulated lymphocytes, interleukin-2 (IL-2)-dependent T-cells and lymphoblastoid B-cell lines. The detection of conjugating activity with BPO implies the expression of a neutral, 'mu-like' GST form by both resting and cultured lymphocytes. In a sample of 12 unrelated individuals, BPO conjugating activity of freshly isolated lymphocytes showed a polymorphic distribution. In comparison with freshly isolated cells, BPO activity was increased 2- to 5-fold in IL-2-dependent T-cells and 4- to 10-fold in B-cell lines. The CDNB activity of T-cells was not significantly different to that of freshly isolated cells (P greater than 0.05) but activity in B-cell lines showed a significant increase (P less than 0.01). These data indicate that measurement of BPO activity in freshly isolated lymphocytes may allow the study of the human GST-mu polymorphism. However, IL-2-dependent culture of T-cells or viral immortalization of resting lymphocytes appears to cause the activation or induction of a GST form or forms which conjugate BPO efficiently. In the T-cells this effect may be mediated through addition of IL-2 to the culture since PHA treatment alone did not elevate activity.

Comparison of DNA adducts in mouse and rat epidermis versus dermis after topical application of (+/-)r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene and (+/-) benzo(a)pyrene-4,5-oxide

Mice and rats were treated topically with the BP metabolites, anti-BPDE and BPO. Rat epidermal DNA was extensively modified by BPO, while mouse epidermal DNA was preferentially modified by anti-BPDE. Anti-BPDE-dGuo adducts were observed only in mouse dermal DNA. DNA adducts were absent from the rat dermis. This adduct formation could be the reason for the very different in vivo biological effects of BP metabolites in the two species.

Metabolism of 6-nitrochrysene by intestinal microflora

Since bacterial nitroreduction may play a critical role in the activation of nitropolycyclic aromatic hydrocarbons, we have used batch and semicontinuous culture systems to determine the ability of intestinal microflora to metabolize the carcinogen 6-nitrochrysene (6-NC). 6-NC was metabolized by the intestinal microflora present in the semicontinuous culture system to 6-aminochrysene (6-AC), N-formyl-6-aminochrysene (6-FAC), and 6-nitrosochrysene (6-NOC). These metabolites were isolated and identified by high-performance liquid chromatography, mass spectrometry, and UV-visible spectrophotometry and compared with authentic compounds. Almost all of the 6-NC was metabolized after 10 days. Nitroreduction of 6-NC to 6-AC was rapid; the 6-AC concentration reached a maximum at 48 h. The ratio of the formation of 6-AC to 6-FAC to 6-NOC at 48 h was 93.4:6.3:0.3. Interestingly, compared with results in the semicontinuous culture system, the only metabolite detected in the batch studies was 6-AC. The rate of nitroreduction differed among human, rat, and mouse intestinal microflora, with human intestinal microflora metabolizing 6-NC to the greatest extent. Since 6-AC has been shown to be carcinogenic in mice and since nitroso derivatives of other nitropolycyclic aromatic hydrocarbons are biologically active, our results suggest that the intestinal microflora has the enzymatic capacity to generate genotoxic compounds and may play an important role in the carcinogenicity of 6-NC.

Comparison of highly purified sheep liver and lung NADPH-cytochrome P-450 reductases by the analysis of kinetic and catalytic properties

1. Reductase was purified to electrophoretic homogeneity from sheep liver and lung microsomes. The specific activity of both enzymes ranged from 55 to 66 mumol cytochrome c reduced/min/mg protein. 2. Liver and lung reductases appeared to have similar kinetic and spectral properties. Km (NADPH) and Km (cytochrome c) values were calculated to be 14.3 +/- 1.23 microM and 22.2 +/- 2.78 microM for liver and 11.1 +/- 0.70 microM and 20.0 +/- 2.15 microM for lung reductase, respectively. Kinetic studies showed that cytochrome c can bind the oxidized form of the enzyme as well as its reduced form and both reductases operated through a ping-pong type mechanism. 3. These reductases cannot be distinguished on the basis of monomer molecular weights (Mr 78,000) except that the liver reductase was found to be more susceptible to proteolytic attack. 4. Both reductases supported aniline 4-hydroxylation and ethylmorphine N-demethylation reactions to the same extent in the reconstituted systems. However, sheep lung reductase appeared only 36.5 and 14.8% as effective in catalyzing benzo[a]pyrene reaction as an equivalent amount of reductase from liver in the presence of liver cytochrome P-450 and 3MC-treated rat liver cytochrome P-448, respectively.

Differential induction of human liver UDP-glucuronosyltransferase activities by phenobarbital-type inducers

(1) UDP-glucuronosyltransferase (UDP-GT) activities and their inducibility were investigated in human liver microsomes from a "liver bank". (2) UDP-GT activities were differentially induced in liver microsomes from patients treated with the phenobarbital-type inducers phenytoin or pentobarbital. UDP-GT activity towards bilirubin was induced 3-fold. Enzyme activities towards paracetamol, benzo(a)pyrene-3,6-quinol, 4-methylumbelliferone and 1-naphthol were moderately induced and to similar extents (2-fold). In contrast, morphine and 4-hydroxybiphenyl glucuronidation were not significantly affected. Cytochrome P-450 dependent 7-ethoxycoumarin O-deethylase was increased 5-fold. (3) A human hepatoma cell line (Hep G2) was studied to obtain information on the inducibility of human UDP-GT activities by 3-methylcholanthrene-type inducers. UDP-GT activities towards benzo(a)pyrene-3,6-quinol and 1-naphthol were moderately but significantly induced by 3-methylcholanthrene-treatment of the cells (2-fold), whereas 7-ethoxyresorufin O-deethylase and 7-ethoxycoumarin O-deethylase were increased over 100- and 10-fold, respectively. (4) The results suggest the existence of differentially inducible UDP-GT isoenzymes in human liver. The findings may be useful as a guide to characterize human liver UDP-GT isoenzymes.

Inversion of enantioselectivity in glutathione conjugation of 9,10-dihydrobenzo[a]pyrene 7,8-oxide in hepatic cytosol of rats following induction of hepatic hyperplastic nodules by chemical carcinogens

Enantiomers of 9,10-dihydrobenzo[a]pyrene 7,8-oxide (DBPO) were stereoselectivity conjugated with glutathione (GSH) specifically at benzylic carbon (C7) in normal Sprague--Dawley (SD) rat liver cytosol: (7R,8S)-(+)- greater than (7S,8R)-(-)-DBPOs. In contrast, in liver cytosol of SD rats bearing hepatic hyperplastic nodules induced with chemical carcinogens, (7S,8R)-(-)-DBPO was preferentially conjugated with GSH to (7R,8S)-(+)-DBPO. GSH S-transferases (GSTs) having sub-unit protein 4 were strongly suggested to play an important role in the preferential conjugation of (7R,8S)-(+)-DBPO in the normal rat liver cytosol, while the preferential conjugation of (7S,8R)-(-)-DBPO in the liver cytosol of the rat bearing hepatic hyperplastic nodules, was most likely to be attributable to GST 7-7, a characteristically induced protein in the hepatic hyperplastic nodules.

Reactivity with DNA of three pyrenofuran analogues of benzo(a)pyrene and benzo(e)pyrene

Three pyrenofurans, the pyreno[1,2-b]furan (FP1), the pyreno[2,1-b] furan (FP2) and the pyreno[4,5-b]furan (FP3) have been synthesized as analogues of the mutagenic and carcinogenic benzo(a)pyrene (FP1 and FP2) and of its non-carcinogenic isomer benzo(e)pyrene (FP3). For each of the pyrenofurans, the reactivity with DNA has been tested in presence of liver microsomes of rats induced with 3-methylcholanthrene. Fluorescence spectroscopy showed that only FP2 and FP3 which possess a "bay region" react with DNA. In both cases, metabolites bound to DNA have a fluorescence emission comparable to that of the "bay region" dihydrodiols obtained after the "in vitro" metabolism of initial molecules. FP2 is shown to react similarly to benzo(a)pyrene whereas the reactivity of FP3 is different from that of benzo(e)pyrene, in spite of their structural similarities. This is probably due to reasons of three-dimensional space configuration. The peculiar reactivity of FP3 is predicted by calculations of the bond order values.

Stereoselectivity of cytosolic glutathione S-transferases with arene and alkene oxide substrates in various tissues and isolated hepatic and pulmonary cells of the rabbit

The specific activity and stereoselectivity of cytosolic glutathione S-transferases (GST) from various rabbit tissues and isolated cells were determined as an initial step in characterizing GST isoenzymes in the rabbit. Of the five tissues examined, liver cytosol had the highest specific GST activity with the polycyclic arene oxides (+/-)-benzo[a]pyrene 4,5-oxide (BPO), pyrene 4,5-oxide (PO) and (+/-)-benz[a]-anthracene 5,6-oxide (BAO), and kidney cytosol had the second highest. Lung, intestine and testis had relatively low activities with all three substrates. With the alkene oxide (+/-)-styrene 7,8-oxide (SO), testicular cytosol had the highest GST activity while liver cytosol had only half the activity of the testis. Cytosolic GST from liver and kidney were highly stereoselective for reaction of glutathione with the S-configured oxirane carbon atoms of BPO, PO and BAO, and all tissues but the intestine were enantioselective for (4R,5S)-BPO and (5S,6R)-BAO. With SO, the liver, kidney and testis preferentially catalyzed the reaction of glutathione with the benzylic carbon atom of (7S)-SO. There was virtually no enantioselectivity in lung cytosol with SO but a preference for reaction with (7R)-SO was noted in the intestine. The stereoselectivities found in the intestine with each of the four substrates were markedly different from the other tissues. Cytosol from isolated hepatocytes showed almost identical patterns of stereoselectivity with BPO and PO to those of whole liver cytosol. Similarly, the stereoselectivity of cytosol prepared from alveolar type II cells isolated from rabbit lung was the same as that of whole lung cytosol with these substrates, whereas cytosol of alveolar macrophages differed substantially from lung cytosol in both cases. There were marked differences in stereoselectivity of cytosol from freshly isolated Clara cells with BPO versus PO as substrate. With BPO, Clara cells were very similar to whole lung cytosol, but with PO they were not. The data are consistent with the differential tissue and cellular distribution of multiple GST isoenzymes in the rabbit.

Ring oxidation of 6-nitrobenzo (a) pyrene by female mouse liver

6-Nitrobenzo (a) pyrene (6-NBaP) is an environmental contaminant. In bacterial mutagenesis assays, 6-NBaP requires rat liver S9 enzymes for its activity. Chemical characterization of metabolites of 6-NBaP produced by male rat liver microsomes showed them to be ring-hydroxylated (both mono- and dihydroxy) derivatives. Thus, metabolic activation of 6-NBaP may occur via ring oxidation. It has been shown by others that when injected intraperitoneally into newborn mice, 6-NBaP was carcinogenic to male, but not to female, mouse liver. The nitro-hydrocarbon was not carcinogenic to the lungs of either sex. We have examined the metabolism of 6-NBaP by the liver of a non-susceptible female mouse and observed the formation of ring-hydroxylated metabolites of 6-NBaP. In view of this observation, we suggest that ring hydroxylation alone may not be sufficient in explaining the carcinogenicity of 6-NBaP in male and not in female mice.

Purification of microsomal epoxide hydrolase from liver of rhesus monkey: partial separation of cis- and trans-stilbene oxide hydrolase

Solubilized rhesus monkey liver microsomes were used as the starting material for the purification of epoxide (cis-stilbene oxide) hydrolase. Successive chromatography over DEAE-Sephacel followed by CM-cellulose resulted in two peaks of activity, CM A and CM B. Passage of these two eluates over separate hydroxyapatite columns resulted in two peaks of activity from CM A, HA A1, and HA A2, and one peak from CM B and HA B, with respective recoveries of 1, 7, and 0.2% of cis-stilbene oxide hydrolase activities. A similar recovery was found for benzo[a]pyrene-4,5-oxide hydrolase, while trans-stilbene oxide hydrolase activity coeluted only in HA A2. Fraction HA A1 was homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoblots of the three eluates and solubilized microsomes incubated with anti-HA A1 demonstrated a single band at 49 kDa in each fraction. The three eluates were differentially affected by the inhibitors of epoxide hydrolase, trichloropropene oxide and 4-phenylchalcone oxide, and addition of Lubrol PX and phospholipid. Immunoprecipitation of HA A2 resulted in coprecipitation of cis- and trans-stilbene oxide hydrolase activity. Upon immunoprecipitation of solubilized microsomes, all the cis-stilbene oxide and benzo[a]pyrene-4,5-oxide, but only 50-60% of trans-stilbene oxide hydrolase activity was precipitated. These studies support findings with other species that (i) an immunochemically distinct cytosolic-like epoxide hydrolase exists in microsomes, and (ii) microsomal epoxide hydrolase activity can be separated during ion-exchange chromatography giving proteins with similar molecular weights and immunochemical cross-reactivity. The precipitation of cis- and trans-stilbene oxide hydrolase activity in eluate HA A2 provides convincing evidence that these isozymes are not structurally identical.

Metabolism of 6-nitrobenzo[a]pyrene in rat lung preparations

6-Nitrobenzo[alpha]pyrene (6-NBaP) occurs in our environment. Since human exposure to environmental contaminants may occur via the inhalation route, we examined the metabolites of 6-NBaP formed in lung preparations, and compared the metabolite profile to that which was found with liver. The metabolites formed in both liver and lung preparations consisted of ring-hydroxylated 6-NBaP, 6-hydroxybenzo[a]pyrene (6-OHBaP) and small amounts of benzo[a]pyrene. In the lung experiments, 6-OHBaP most frequently oxidized to quinones. The mechanism for the formation of 6-OHBaP from 6-NBaP remains to be elucidated.

A benzo[a]pyrene metabolite-DNA adduct occurring in the activating cell but not in exogenous DNA

[3H]Benzo[a]pyrene (BP) and salmon sperm DNA were incubated with hepatocytes from 5,6-benzo-flavone-treated rats. The cellular DNA and the exogenously added DNA were separately isolated, hydrolyzed and chromatographed on a Sephadex LH-20 column. The extracellular DNA yielded 3 peaks of radioactivity in the chromatographic eluate. The cellular DNA contained an additional peak suggesting the formation of a DNA adduct from a metabolite that does not leave the cell.

DNA repair synthesis in cultured fish and human cells exposed to fish S9-activated aromatic hydrocarbons

Unscheduled DNA repair synthesis was measured autoradiographically in cultured rainbow trout gonad (RTG) and human fibroblast (HF) cells following exposure to aflatoxin B1 (AFB1), 3,4-benzopyrene (BP), 1,2,5,6-dibenzanthracene (DBA), 1,2-benzanthracene (BA) and pyrene (PY) activated with S9 prepared from rainbow trout liver. S9 from rainbow trout injected with Arochlor 1254 or an oil extract was compared with S9 from Fischer rats injected with Arochlor 1254 for the ability to activate AFB1 and cause DNA repair in RTG and HF cells. All three types of S9 activated AFB1, but the measured DNA repair response was greater in the HF cells. A significant grain count response was found following exposure of HF cells to fish S9-activated BP. Using assay conditions which enhance fish cell grain counts, a significant level of DNA repair was also found in RTG cells exposed to fish S9-activated BP. Marginal but statistically significant amounts of DNA repair were elicited in HF and RTG cells exposed to rainbow trout S9-activated BA and DBA, but no response was detected following PY exposure. Fish S9 was found to be able to activate a series of polycyclic aromatic hydrocarbons (PAH) and cause DNA repair synthesis in both fish and mammalian cells. The magnitude of the repair response roughly parallels the carcinogenic potential of the PAHs. These results elicit trans species and phyla comparisons which help to validate fish as models for aquatic carcinogenesis research, and also demonstrate PAH DNA-damaging effects on fish DNA, adding further credence for studying the effects of these chemicals on aquatic organisms.

Microbial transformation of 6-nitrobenzo[a]pyrene

The fungal metabolism of the potent mutagenic and carcinogenic nitropolycyclic aromatic hydrocarbon (nitro-PAH) 6-nitrobenzo[a]pyrene (6-NO2-BaP) was investigated. Cunninghamella elegans was incubated with 6-NO2-BaP for periods ranging between 1 and 7 d, and the metabolites formed were separated by high-performance liquid chromatography and identified by their UV-visible absorption, mass, and 1H nuclear magnetic resonance spectra. The results of our study indicate that C. elegans metabolized 6-NO2-BaP to glucoside and sulfate conjugates of 1- and 3-hydroxy 6-NO2-BaP and suggests that glycosylation and sulfation reactions may represent detoxification pathways in the fungal metabolism of nitro-PAHs. Experiments using [G3H]-6-NO2-BaP indicated that C. elegans metabolized 62% of 6-NO2-BaP within 168 h. Our data also indicated that the nitro group at the C-6 position of benzo[a]pyrene blocked metabolism at the regions peri to the nitro substituent (C-7, C-8 positions) and enhanced metabolism at the C-1 and C-3 positions. The ability of the fungus C. elegans to metabolize 6-NO2-BaP to biologically inactive compounds may have practical applications in the detoxification of nitro-PAH-contaminated wastes.