Interaction of PAH-related compounds with the alpha and beta isoforms of the estrogen receptor

The ability of several 4- and 5-ring polycyclic aromatic hydrocarbons (PAHs), heterocyclic PAHs, and their monohydroxy derivatives to interact with the estrogen receptor (ER) alpha and beta isoforms was examined. Only compounds possessing a hydroxyl group were able to compete with 3H-labeled 17beta-estradiol (E2) for binding to either a glutathione-S-transferase and human ERalpha D, E, and F domain fusion protein (GST-hERalphadef) or to the full-length human ERbeta. Competitive binding was comparable for both isoforms, with IC(50) values ranging from 20 to 300 nM (E2 IC(50) approximately 3 nM). However, several compounds were able to induce reporter gene expression preferentially through mERbeta, using MCF-7 cells transiently transfected with either a Gal4-human ERalphadef or Gal4-mouse ERbetadef construct, as well as a Gal4-regulated reporter. These data extend the number and type of PAH-related compounds capable of interacting with ERalpha and ERbeta, and provides additional evidence that even though some compounds may possess a similar affinity for both ER isoforms, the capacity for transcriptional activation can still be isoform-specific.

Tumorigenicity of racemic and optically pure bay region diol epoxides and other derivatives of the nitrogen heterocycle dibenz[a,h]acridine on mouse skin

CD-1 female mice were initiated with a single topical application of 500 nmol dibenz[a,h]acridine (DB[a,h]Acr), its racemic trans-1,2-, 3,4-, 8,9- and 10,11-dihydrodiols, racemic DB[a,h]Acr 3,4-diol 1,2-epoxide-1 and -2 or racemic DB[a,h]Acr 10,11-diol 8,9-epoxide-1 and -2, where the benzylic hydroxyl group is either cis (isomer 1) or trans (isomer 2) to the epoxide oxygen. The mice were subsequently treated twice weekly with 12-O-tetradecanoylphorbol 13-acetate for 25 weeks. High tumorigenicity was observed only for DB[a,h]Acr, its 10,11-dihydrodiol and DB[a,h]Acr 10,11-diol 8,9-epoxide-2 (3.3, 1.2 and 1.6 tumors/mouse, respectively). The tumor-initiating activity of a 50 nmol dose of DB[a,h]Acr and the optically active (+)- and (-)-enantiomers of DB[a,h]Acr 10,11-dihydrodiol and of the optically active DB[a,h]Acr 10,11-diol 8,9-epoxide-1 and -2 were also studied. Only DB[a,h]Acr, (-)-DB[a,h]Acr (10R,11R)-dihydrodiol and the bay region (+)-(8R,9S,10S,11R)-diol epoxide-2 were highly active (1.6, 1.7 and 2.4 tumors/mouse, respectively). These results are consistent with previous studies which showed that the corresponding bay region RSSR diol epoxides of benzo[a]pyrene, benz[a]anthracene, chrysene and benzo[c]phenanthrene as well as the aza-polycyclic dibenz[c,h]acridine are the most tumorigenic isomers.

Metabolism of benzo(a)pyrene by duck liver microsomes

The metabolism of benzo(a)pyrene [BP], a model carcinogenic PAH, by hepatic microsomes of two duck species, mallard (Anas platyrhynchos) and common merganser (Mergus merganser americanus) collected from chemically-contaminated and relatively non-contaminated areas was investigated. The rate of metabolism of BP by liver microsomes of common merganser and mallard collected from polluted areas (2,650 +/- 310 and 2,200 +/- 310 pmol/min per mg microsomal protein, respectively) was significantly higher than that obtained with liver microsomes of the two species collected from non-polluted areas (334 +/- 33 and 231 +/- 30 pmol/min per mg microsomal protein, respectively). The level of cytochrome P-450 1A1 was significantly higher in the liver microsomes of both duck species from the polluted areas as compared to the ducks from the non-polluted areas. The major BP metabolites, including BP-9, 10-diol, BP-4, 5-diol, BP-7, 8-diol, BP-1, 6-dione, BP-3, 6-dione, BP-6, 12-dione, 9-hydroxy-BP and 3-hydroxy-BP, formed by liver microsomes of both duck species from polluted and non-polluted areas, were qualitatively similar. However, the patterns of these metabolites were considerably different from each other. Liver microsomes of ducks from the polluted areas produced a higher proportion of benzo-ring dihydrodiols than the liver microsomes of ducks from the non-polluted areas, which converted a greater proportion of BP to BP-phenols. The predominant enantiomer of BP-7,8-diol formed by hepatic microsomes of the two duck species had an (-)R,R absolute stereochemistry. The data suggest that duck and rat liver microsomal enzymes have different regioselectivity but similar stereoselectivity in the metabolism of BP.

32P-Postlabeling analysis of lipophilic DNA adducts resulting from interaction with (+/-)-3-hydroxy-trans-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene

Bay-region diol epoxides are considered the putative ultimate carcinogens of polynuclear aromatic hydrocarbons. However, the results of studies on tumorigenesis and DNA binding of benzo[a]pyrene (BP) and its bay-region diol epoxide, (+)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyren e [(+)-anti-BPDE] suggest that, in addition to anti-BPDE, other reactive metabolite(s) of BP may also be involved in BP-induced carcinogenesis. Recent studies have demonstrated that 3-hydroxy-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a ]pyrene (anti-BPTE) is another highly reactive metabolite of BP. In order to identify syn- and anti-BPTE-derived DNA adducts and their base selectivity, we synthesized both compounds by two different methods and reacted in vitro with calf thymus DNA and individual nucleotides. The resultant adducts were analyzed by nuclease P1-enhanced 32P-postlabeling. Anti-BPTE produced three major and several minor adducts with DNA; dAp and dGp were the preferred substrates, while dCp and dTp were the least reactive. In contrast, syn-BPTE produced two major adducts each with DNA and dGp; dAp generated only one adduct. Co-chromatography of anti-BPTE-derived DNA adducts with those of mononucleotide adducts revealed that the major adducts in DNA were guanine derived. Further, co-chromatographic results revealed that the anti-BPTE-DNA adducts were distinctly different from that of anti-BPDE-DNA adducts. These observations indicate that both syn- and anti-BPTE can react with DNA bases and these DNA adducts may also contribute to BP-induced carcinogenesis.

Disposition and metabolism of 2,3,7,8-tetrachlorodibenzofuran by channel catfish (Ictalurus punctatus)

The disposition and metabolism of 2,3,7,8-tetrachlorodibenzofuran (TCDF) was was investigated in channel catfish (Ictalurus punctatus) in order to better understand the metabolic and physiological factors that modulate the fate of this extremely toxic compound in channel catfish compared to other species. The fish were dosed orally with [3H]TCDF (1 microgram/kg); tissue were harvested at 3, 7, and 14 days for radioassay. The body burden of TCDF equivalents in catfish at 3 days was 0.36 microgram/kg, which was decreasing with a half-life of 3.6 days. Catfish muscle showed a relatively low capacity to accumulate and retain TCDF, accounting, at 3 days, for only 19.0% of the body burden of TCDF equivalents (half-life in muscle, 5.0 days). Catfish liver, on the other hand, showed a high capacity to accumulate and metabolize TCDF and to secrete TCDF metabolites into the bile. At 3 days, the concentrations of TCDF equivalents in liver and bile were, respectively, 5.7 ng/g liver (19% of the body burden) and 129 ng/ml bile. However, the concentration of TCDF equivalents in liver decreased with a half-life of 1.8 days to 0.04 ng/g (2.0% of the body burden) at 14 days. Thus, the capacity of catfish liver to retain TCDF decreased dramatically as the body burden decreased. The data suggest that the low affinity of lipid poor catfish muscle for TCDF may allow catfish liver to accumulate a concentration of TCDF sufficient to induce the metabolism of this compound by liver monooxygenases. The major TCDF metabolites found in catfish liver and bile were, respectively, 4-OH-TCDF and TCDF-4-O-glucuronide.

Disposition and metabolism of 2,3,7,8-tetrachlorodibenzofuran by rainbow trout (Oncorhynchus mykiss)

The disposition and metabolism of 2,3,7,8-tetrachlorodibenzofuran (TCDF) was investigated in rainbow trout (Oncorhynchus mykiss) in order to better understand the metabolic and physiological factors that modulate the fate of this extremely toxic compound in rainbow trout compared to other species. The fish were dosed orally with [3H]TCDF (1 microgram/kg); fish were terminated at 1-19 days for the determination of whole body half-life or at 0.3-28 days for determination of tissue distribution. Unassimilated TCDF (51.5% of the dose) was eliminated with a half-life of 0.84 days. The assimilated body burden of TCDF equivalents decreased with a half-life of 14.8 days (determined between 3 and 19 days). Trout muscle showed a relatively high capacity to accumulate and retain (unmetabolized) TCDF, accounting, at 3 days, for 32% of the body burden of TCDF equivalents (half-life in muscle, 15.2 days). Trout liver, on the other hand, showed a relatively low capacity to accumulate and metabolize TCDF. At 3 days, the concentrations of TCDF equivalents in liver and bile were, respectively, 0.37 ng/g liver (0.88% of the body burden) and 4.8 ng/ml bile. The data suggest that the relatively high affinity of lipid-rich trout muscle for TCDF limits the ability of the liver to accumulate and metabolize TCDF. The major TCDF metabolites found in trout liver and bile were, respectively, 4-OH-TCDF and TCDF-4-O-glucuronide.

Stereoselective metabolism of dibenz[a,h]acridine to bay-region diol epoxides by rat liver microsomes

The carcinogen dibenz[a,h]acridine (DB[a,h]ACR is metabolized predominantly to trans-3,4-dihydroxy-3,4-dihydro-dibenz[a,h]acridine (DB[a,h]ACR-3,4-diol) and the proximate carcinogen trans-10,11-dihydroxy-10,11-dihydrodibenz[a,h]acridine (DB[a,h]ACR-10,11-diol) [Steward et al. (1987) Carcinogenesis, 8, 1043-1050]. In the present investigation, the stereoselectivity of rat liver enzymes in metabolism of DB[a,h]ACR to its 3,4-diol and 10,11-diol and of DB[a,h]ACR-10,11-diol enantiomers to their bay-region diol epoxides has been examined with liver microsomes from control and 3-methylcholanthrene-treated rats. Both microsomal preparations produced the major metabolites DB[a,h]ACR-3,4-diol and DB[a,h]ACR-10,11-diol containing predominantly R,R-enantiomers with 38-54% optical purity. Metabolism of (-)-(10R,11R)- and (+)-(10S,11S)-enantiomers of DB[a,h]ACR-10,11-diol by liver microsomes from control rats produced predominantly bay-region diol epoxides (46-59% of total metabolites), whereas very little bay-region diol epoxides (14-17% of total metabolites) were produced by liver microsomes from 3-methylcholanthrene-treated rats. The bay-region diol epoxides produced in these studies consisted of predominantly DB[a,h]ACR-10,11-trans-diol epoxide diastereomer in which the benzylic hydroxyl group and epoxide oxygen are trans. However, (-)-DB[a,h]ACR-10R,11R-diol, a major metabolite of DB[a,h]ACR, was metabolized by liver microsomes from 3-methylcholanthrene-treated rats to (+)-[8R,9S,10S,11R]-DB[a,h]ACR-10,11-trans-diol epoxide, a diastereomer which displayed high mutagenic activity in V79 cells, in an amount which was 6.5-fold greater than that of the corresponding cis-diol epoxide diastereomer. The relative amounts of trans-diol epoxide versus cis-diol epoxide in the mixture of bay-region diol epoxides produced from DB[a,h]ACR-10R,11R-diol and DB[a,h]ACR-10S,11R-diol with liver microsomes from control rats and from DB[a,h]ACR-10S,11S-diol with liver microsomes from 3-methylcholanthrene-treated rats were 1.7, 2.1 and 2.3 respectively.

Metabolism of 2-acetylaminofluorene by hepatocytes isolated from rainbow trout

The metabolism of 2-acetyl-[9-14C]aminofluorene (AAF) by hepatocytes isolated from rainbow trout (Oncorhynchus mykiss), Shasta strain, was investigated in order to assess the competing activation and detoxification pathways which may explain the resistance of this species and strain to the initiation of carcinogenesis by this model carcinogenic aromatic amide. Freshly isolated hepatocytes (per milliliter: 1.0 mg dry wt; 1.5 (10(6)) hepatocytes) incubated with 65 microM AAF for 4 hr converted 15.4 nmol AAF to metabolites, including 7.8 nmol of water-soluble compounds. AAF-derived radioactivity extracted from the incubation mixtures, before and after hydrolysis by beta-glucuronidase and arylsulfatase, was analyzed by reversed-phase HPLC. The metabolite profile following incubation of hepatocytes with 6.5 microM AAF for 4 hr included (as percentage of total metabolites); 7-OH-AAF, 5-/8-/9-OH-AAF and 2-aminofluorene (AF) (17, 2.4, and 2.7%, respectively); conjugates of these respective primary metabolites (39, 9, and 4%, respectively). Glucuronides amounted to 49% of the total metabolites. N-OH-AAF and its conjugates always amounted to < 1% of total metabolites. The relative amount of (unconjugated) AF increased considerably (to 26%) following incubation of hepatocytes with 65 microM AAF, with a corresponding decrease in the total amount of glucuronides formed. Following incubation with 65 microM AAF, 1.6% of AAF metabolites was covalently bound to macromolecules, giving a ratio of covalently bound derivatives to detoxification products of 0.028. These data are consistent with the hypothesis that rainbow trout are resistant to AAF-induced hepatocarcinogenesis, in part, because trout liver efficiently detoxifies AAF and forms only relatively small amounts of active intermediates capable of binding to macromolecules, including DNA.

Bacterial and mammalian cell mutagenicity of four optically active bay-region 10,11-diol-8,9-epoxides of the nitrogen heterocycle dibenz[a,h]acridine

The mutagenic activities of the enantiomers of the diastereomeric pair of bay-region 10,11-diol-8,9-epoxides of dibenz[a,h]acridine (DB[a,h]ACR) were evaluated in histidine-dependent strains of Salmonella typhimurium and in cultured Chinese hamster V79 cells. In strains TA98 and TA100 of S.typhimurium, the (-)-[8S,9R,10R,11S] diol-epoxide was the most mutagenic compound, inducing 1200 and 6900 His+ revertants/nmol respectively. The mutagenic activity of each of the remaining three isomers was essentially independent of the bacterial strain used and had 14-72% of the activity of the [S,R,R,S] isomer. However, in Chinese hamster V79 cells, the (+)-[8R,9S,10S,11R] diol-epoxide was the most mutagenic compound (68 8-azaguanine resistant variants/nmol/10(5) cells), inducing from 2 to 11 times as many mutations as the other three isomers. These results are analogous to previous studies with the bay-region diol-epoxides of other polycyclic hydrocarbons in that the isomer with [R,S,S,R] absolute configuration has had variable activity in the bacterial assays, but has generally been the most active in the mammalian cells. Furthermore, this isomer has almost always been highly tumorigenic in the mouse.

Treatment of lactating rats with PCBs induces CYP1A1 and enhances the formation of BP 7,8-dihydrodiol, the proximate carcinogen of benzo(a)pyrene

1. Treatment of pregnant and lactating rats with a single i.p. dose of 250 mg/kg body weight produced 5-fold and 2-fold increases, respectively, in hepatic P-450 concentrations using microsomes isolated from pregnant, neonatal, lactating and foetal rats. 2. Concomitantly, 26-fold, 20-fold and 14-fold increases in neonatal, maternal and foetal ethoxyresorufin-O-deethylation (EROD), respectively were found, but only 2.5-fold increases could be determined using microsomes isolated from lactating rats. 3. The metabolism of [3H]benzo(a)pyrene was increased 9-fold and 2-fold in pregnant and foetal rats, respectively, but only 2-fold increases were measured for lactating rats. 4. Western blot analysis of microsomal proteins obtained from lactating rats showed significant CYP1A1 and CYP1A2 induction and for the same hepatic tissue 62-fold and 44-fold increases in cDNA hybridized CYP1A1 and CYP1A2 mRNA, respectively, were found. 5. Treatment of lactating rats with PCBs resulted in enhanced formation of all BP-metabolites, but the ratio of diol to total BP-metabolites was more than 3-fold greater. 6. The formation of the proximate carcinogen BP-7,8-dihydrodiol was 5-fold increased and a similar 3-fold increase in epoxide hydrolase activity was estimated for lactating rats. 7. The results of the present study indicates that lactation protects, in part, against the inductive effect of PCBs, possibly by enhanced clearance of these chemicals via lactation.

Metabolism of benzo[a]pyrene and (-)-trans-benzo[a]pyrene-7,8-dihydrodiol by freshly isolated hepatocytes from mirror carp

The metabolism of benzo[a]pyrene (B[a]P) and (-)-trans-benzo[a]pyrene-7,8-dihydrodiol [(-)-B[a]P-7,8-diol], a major putative proximate carcinogenic metabolite of B[a]P, was compared in freshly isolated hepatocytes from mirror carp, a strain of common carp (Cyprinus carpio, L.). Hepatocytes incubated with 40 microM [3H]B[a]P produced 1.22 nmol equivalents of B[a]P metabolites/mg dry wt of cells/h. Conjugated derivatives represented approximately 65% of all B[a]P metabolites and included glucuronides (38%), glutathione conjugates (21%) and sulfates (6%). About 14% of the total accumulated metabolites of B[a]P determined after 1 h incubations were identified as unconjugated derivatives, predominantly B[a]P-9,10-dihydrodiol and B[a]P-7,8-diol (7.4 and 3.1% of total metabolites respectively), with only traces of B[a]P tetrols (less than 1%). Hepatocytes incubated with 40 microM (-)-[14C]B[a]P-7,8-diol produced 4.78 nmol equivalents of metabolites/mg dry wt during a 1 h incubation, yielding an average rate of metabolism during this time period approximately 53% of that determined after a 5 min incubation. The profile of (-)-B[a]P-7,8-diol metabolites remained constant with incubation time (glucuronides, 30-33%; conjugates with glutathione, 43-46%; polyhydroxylated B[a]P derivatives plus sulfate conjugates, 22-24%). HPLC analysis revealed that polyhydroxylated metabolites amounted to 18% of the total metabolites; thus sulfate conjugates amounted to only 4% of the total metabolites. The trans-2 B[a]P-tetrol, which is the major hydrolysis product of (+)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (anti-BPDE), represented approximately 11% of the accumulated metabolites of (-)-B[a]P-7,8-diol. Despite the much larger amounts of BPDE formed from (-)-B[a]P-7,8-diol than from B[a]P, the amounts of B[a]P equivalents covalently bound to cellular DNA were the same following 1 h incubations with either substrate (247 +/- 42 or 212 +/- 42 pmol/mg DNA respectively). Thus biochemical and physiological factors other than the production of BPDE are critically involved in determining the level of DNA adducts in hepatocytes as well as the role of these adducts in hepatocarcinogenesis.

Metabolism of benzo[a]pyrene and persistence of DNA adducts in the brown bullhead (Ictalurus nebulosus)

1. The in vitro metabolism of [3H]benzo[a]pyrene (BP) and [14C]benzo[a]pyrene-7,8-dihydrodiol (BP-7,8-diol) by liver of brown bullhead (Ictalurus nebulosus) was characterized, as was the formation and persistence of BP-DNA adducts in vivo. 2. Compared to rat liver microsomes, bullhead liver microsomes produced relatively larger amounts of BP-7,8-diol (predominantly the [-] enantiomer) and smaller amounts of of BP-7,8-diol (predominantly the [-] enantiomer) and smaller amounts of BP-4,5-diol. 3. BP phase I metabolites were efficiently converted by freshly isolated bullhead hepatocytes to conjugates, predominantly glucuronides. 4. BP-7,8-diol was metabolized by hepatocytes 4-fold more rapidly than was BP and was converted to approximately equal amounts of glucuronides, glutathione conjugates and sulfates. 5. BP-DNA adducts formed in bullhead liver with a lag time of several days and maximum adduct formation at 25-30 days. The major adduct was anti-BPDE-deoxyguanosine.

Mutagenicity of dibenz[a,c]anthracene and its derivatives in Salmonella typhimurium TA100

The mutagenic activities of dibenz[a,c]anthracene (DB[a,c]A), and its 11 derivatives, including 3 diols, 6 phenols and 2 oxepines, were studied in the TA100 strain of Salmonella typhimurium at doses varying from 0 to 20 micrograms/plate in the presence of a rat-liver S9 (9000 x g) preparation. Among the diols of DB[a,c]A tested DB[a,c]A-10,11-diol was the most mutagenic compound. However, it was consistently less mutagenic than the parent hydrocarbon. Oxepine-1 and oxepine-2 which are believed to be the photoisomerized products of DB[a,c]A-1,2 oxide and DB[a,c]A-3,4-oxide, respectively, were also less mutagenic than DB[a,c]A. In contrast to these results, 4-hydroxyDB[a,c]A was almost twice as active as DB[a,c]A, and 2-hydroxy- and 3-hydroxyDB[a,c]A were even more (4-6-fold) mutagenic than DB[a,c]A. The remaining phenols were relatively inactive or weakly active in this mutagenicity assay. These results provide initial evidence that the bay-region theory may not be applicable to the mutagenesis of DB[a,c]A, and that the angular ring substituted phenols of DB[a,c]A may be involved in the metabolic activation of this highly mutagenic hydrocarbon.

Metabolism of benzo[a]pyrene and (-)-trans-benzo[a]pyrene-7,8-dihydrodiol by freshly isolated hepatocytes of brown bullheads

The metabolism of [3H]benzo[a]pyrene (BP) and (-)-trans-[14C]7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) was studied in freshly isolated hepatocytes of the wild benthic fish, brown bullhead (Ictalurus nebulosus). Bullhead hepatocytes incubated with 40 microM [3H]BP for 1 h metabolized BP to water soluble metabolites which were separated on silica gel t.l.c. plates to reveal conjugates with glucuronic acid, glutathione, and sulfate (51%, 14% and 4% of total metabolites, respectively). Additional metabolites that were extractable with ethyl acetate were separated by reversed phase HPLC to reveal only two major metabolites: BP-9,10-dihydrodiol and BP-7,8-diol (13% and 2.6% of total metabolites, respectively). Hepatocytes isolated from individual fish displayed an 11-fold variability in the rates at which they metabolized BP (756 +/- 167 pmol x mg dry wt-1 x h-1), which correlated negatively (r = -0.7, P less than 0.01) with an 18-fold variability in the glycogen content of the cells. Hepatocytes isolated from the same fish, in parallel incubations under the same optimum conditions, metabolized BP-7,8-diol 4.5-fold faster than they metabolized BP. The variability in the rate of BP-7,8-diol metabolism was about 7-fold. Major metabolites included glutathione conjugates, glucuronides and sulfates (35%, 25% and 30% of total metabolites, respectively). These conjugates, like those formed from BP, were degradable with gamma-glutamyltransferase, beta-glucuronidase and arylsulfatase, respectively. Ethyl acetate extractable metabolites were predominantly isomeric benzo-ring tetrahydrotetrols (9% of total metabolites). In summary, this study indicates that during short-term incubations bull-head hepatocytes metabolize BP and BP-7,8-diol primarily to conjugated derivatives. The usefulness of thin-layer chromatography for the convenient determination of the rate of BP-7,8-diol metabolism is demonstrated.

Formation and persistence of DNA adducts in the liver of brown bullheads exposed to benzo[a]pyrene

The formation and persistence of benzo[a]pyrene (BP)-DNA adducts in the liver of brown bullheads (Ictalurus nebulosus) treated with the hydrocarbon (20 mg/kg body wt, i.p.) was investigated using the 32P-postlabeling assay. The highest level of covalent binding of BP to liver DNA (188 fmol BP adducts/mg DNA) was observed 25-30 days following treatment. After 70 days, the adduct level in liver DNA had declined to approximately 26% of the maximum adduct level. One major BP-DNA adduct and several minor ones were detected in the liver. The major adduct co-chromatographed with anti-BP-7,8-diol-9,10-epoxide-deoxyguanosine (anti-BPDE-dGuo) adduct. The data suggest that brown bullheads metabolically activate BP by the same mechanism as the mammalian systems susceptible to carcinogenic effects of the hydrocarbon.

Mutagenicity of dihydrodiols and diol epoxides of dibenz[a, h]acridine in bacterial and mammalian cells

Bay-region diol epoxides are ultimate carcinogenic metabolites of a number of polycyclic aromatic compounds. Dibenz[a, h]acridine can form two diastereomeric pairs of these diol epoxides which are not positionally equivalent as a result of the nitrogen atom at position 7. We have assessed the structure-activity relationships resulting from heterocyclic nitrogen substitution by examining the mutagenic activity of these four bay-region diol epoxides of dibenz[a,h]acridine in both bacterial and mammalian cells. In strains TA98 and TA100 of Salmonella typhimurium, the diastereomeric 10,11-diol-8,9-epoxides were 20 to 40 times more mutagenic than the corresponding 3,4-diol-1,2-epoxides. Furthermore, in strain TA100, dibenz[a,h]acridine 10,11-dihydrodiol, the expected metabolic precursor of the 10,11-diol-8,9-epoxide, was metabolically activated by rat hepatic microsomes up to a 12-fold greater extent than the 3,4-dihydrodiol. In Chinese hamster V79 cells, the 10,11-diol-8,9-epoxide diastereomers were 20 to 80 times more mutagenic than their 3,4-diol-1,2-epoxide counterparts. Quantum mechanical calculations of the predicted ease of benzylic carbocation formation at C-1 and C-8 from the diol epoxides indicate that the 3,4-diol-1,2-epoxides should be less reactive due to resonance destabilization of the C-1 carbocation as a result of the electronegative nitrogen atom. Decreased chemical reactivity of 3,4-diol-1,2-epoxides may explain their decreased mutagenic activity.

Comparative metabolism of benzo[f]quinoline by liver microsomes from brown bullheads and rats

1. Liver microsomes from rats were considerably more active in metabolizing benzo[f]quinoline (B f Q) than those from brown bullheads (Ictalurus nebulosus). 2. The main B f Q metabolites formed by both rat and brown bullhead liver microsomes were qualitatively similar and included B f Q-7,8-dihydrodiol, B f Q-9,10-dihydrodiol, B f Q-N-oxide, 7-hydroxy B f Q, and 9-hydroxy B f Q. 3. The liver microsomes from control brown bullheads and rats metabolized B f Q primarily at the 7,8-and 9,10-positions, respectively, whereas in the case of microsomes from 3-methylcholanthrene (3-MC)-treated rats or brown bullheads, the major site of metabolic attack was the 7,8-position. 4. A 3-MC-type of cytochrome P-450 appears to be primarily responsible for the oxidation of B f Q by control brown bullhead liver microsomes, whereas a phenobarbital-inducible type of cytochrome P-450 seems to be involved in the metabolism of B f Q by control rat liver microsomes.

Mutagenicity and tumorigenicity of dihydrodiols, diol epoxides, and other derivatives of benzo(f)quinoline and benzo(h)quinoline

The mutagenic activities of benzo[f]quinoline, benzo[h]quinoline, and a number of their derivatives, including dihydrodiols, K-region oxides, diol epoxides, and tetrahydroepoxides, were assessed in strain TA 100 of Salmonella typhimurium. The dihydrodiol derivatives of benzo[f]quinoline and benzo[h]quinoline were also tested for tumorigenic activity in newborn mice. Benzo[f]quinoline was metabolically activated in the presence of rat liver S-9 preparation to products mutagenic to the bacterial system to a greater extent than was benzo[h]quinoline. However, trans-7,8-dihydro-7,8-dihydroxybenzo[f]quinoline was less mutagenic compared to trans-7,8-dihydroxy-7,8-dihydrobenzo[h]quinoline in the presence of rat liver homogenate. The data on the mutagenic activity of the dihydrodiol derivatives of benzoquinolines were consistent with the intrinsic mutagenicity of the corresponding epoxide derivatives, in that the bay-region diol epoxides and tetrahydroepoxide of benzo[h]quinoline exhibited considerably higher mutagenic activities compared to those of the corresponding derivatives of benzo[f]quinoline at equivalent doses. The K-region oxides of benzo[f]quinoline and benzo[h]quinoline were significantly less mutagenic than their corresponding bay-region diol epoxide and tetrahydroepoxide derivatives. The demonstration that benzo[f]quinoline is significantly more mutagenic than trans-7,8-dihydro-7,8-dihydroxybenzo[f]quinoline, a precursor to the weakly mutagenic bay-region diol epoxide, suggests that the bay-region diol epoxide formation is not the principal pathway for the metabolic activation of benzo[f]quinoline to a mutagen. On the other hand, the isomeric benzo[h]quinoline appears to exert its mutagenic effect via the formation of its bay-region diol epoxide. These results indicate that the position of a nitrogen heteroatom in phenanthrene (the analogous carbocyclic aromatic hydrocarbon) not only has a marked effect on the mutagenic activities of the diol epoxide derivatives, but also can alter the metabolic activation pathways of the parent hydrocarbon. Benzo[f]quinoline, benzo[h]quinoline, and their dihydrodiol derivatives were not tumorigenic in newborn mice.

Comparative metabolism of phenanthrene and benzo[f]quinoline by rat liver microsomes

The metabolism of benzo[f]quinoline (BfQ) and its carbon analog phenanthrene has been compared in incubations with liver microsomes from control, 3-methylcholanthrene (3-MC)- and phenobarbital (PB)-pretreated rats. The rates of phenanthrene metabolism by the three types of microsomes were 0.7, 4.1 and 1.5 nmol/mg protein per min, respectively; the values for BfQ were 0.5, 3.7 and 2.5, respectively. Besides N-oxidation, the metabolism of BfQ by all the above microsomes was almost exclusively at the benzo-ring (49-69%) while that of phenanthrene was predominantly at the K-region (50-71%). Phenanthrene-1,2-dihydrodiol, a precursor of the bay-region diol epoxide of phenanthrene, was produced many times more than phenanthrene-3,4-dihydrodiol by both 3-MC- and PB-induced microsomes. While BfQ-7,8-dihydrodiol, the precursor of the bay-region diol epoxide of BfQ, was the predominant metabolite with 3-MC-induced microsomes, it was a minor metabolite with PB-induced microsomes. The benzo-ring oxidation of BfQ, but not of phenanthrene, was position-specific, i.e. predominantly 7,8-oxidation by 3-MC-induced microsomes and 9,10-oxidation by PB-induced microsomes, and implies that aza-substitution results in a site-specific attack by different cytochromes P-450.

Hepatic DNA adduct formation in rats treated with benzo[f]quinoline

The formation of hepatic DNA adducts in male Sprague-Dawley rats following i.p. administration of benzo[f]quinoline (BfQ) was examined using a 32P-post-labeling assay. BfQ exhibited a low binding (11-27 amol adducts/microgram DNA) to liver DNA. Two BfQ-nucleoside adducts (one major and one minor) were detected. The BfQ-DNA adducts formed in vivo were chromatographically distinct from the adducts formed by the reaction of calf thymus DNA in vitro with BfQ-5,6-oxide, syn-7 beta,8 alpha-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydroBfQ, anti-9 alpha,10 beta-dihydroxy-7 alpha,8 alpha-epoxy-7,8,9,10-tetrahydroBfQ, or anti-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydroBfQ-N- oxide. These results suggest that the bay-region diol epoxide of BfQ, unlike the bay-region diol epoxide derivatives of polynuclear aromatic hydrocarbons, is not involved in the covalent binding of BfQ to DNA.

Metabolism of benzo[f]quinoline by rat liver microsomes

The metabolism of [1,3-14C]benzo[f]quinoline (BfQ) by liver microsomes from control, 3-methylcholanthrene (3-MC)-pretreated and phenobarbital (PB)-pretreated rats has been investigated in order to gain insights into the effect of mixed function oxidase inducers on the types and levels of specific metabolites as formed in vitro. The rates of metabolism of BfQ by liver microsomes from control, 3-MC- and PB-pretreated rats were 0.5, 3.6 and 2.4 nmol/min/mg of respectively. The most predominant metabolite of BfQ detected with liver microsomes from 3-MC-pretreated rats was BfQ-7,8-dihydrodiol, a precursor of the bay-region diol epoxide, constituting 41% of the total ethyl acetate-extractable metabolites. Other metabolites obtained along with their relative proportions were as follows: BfQ-N-oxide, 23% 7-hydroxyBfQ, 15%; 9-hydroxyBfQ, 9%; and BfQ-9,10-dihydrodiol, 6%. BfQ-5,6-dihydrodiol, a K-region dihydrodiol, was a trace metabolite representing approximately 1.0% of the total metabolism. Liver microsomes from PB-pretreated rats oxidized BfQ primarily to BfQ-N-oxide and 9-hydroxyBfQ, which constituted 41% and 20% of the total ethyl acetate-extractable metabolites of BfQ. The relative proportions of BfQ-9,10-dihydrodiol, BfQ-7,8-dihydrodiol and 7-hydroxy-BfQ formed were 12%, 3% and 13% respectively, while the figure for BfQ-5,6-dihydrodiol was 0.5%. The profile of metabolites formed by liver microsomes from control rats was similar to that generated by microsomes from PB-pretreated rats. While benzo-ring metabolites represented a major part of the metabolism of BfQ by liver microsomes from either 3-MC- or PB-pretreated rats, these two types of microsomes exhibited a positional selectivity in the oxidation of BfQ, the former primarily attacking the 7,8-position of BfQ while the latter preferentially oxidizing the 9,10-position. The preponderance of the potentially mutagenic BfQ-7,8-dihydrodiol amongst the metabolites generated by liver microsomes from 3-MC-pretreated rats suggests a possible role for cytochrome P-450c, the major form of rat hepatic cytochrome P-450 induced by 3-MC, in the metabolic activation of BfQ.

Metabolism of dibenz[a,h]acridine by rat liver microsomes

As part of a project to assess the effect of heterocyclic nitrogen in modifying the metabolism and mutagenicity of polycyclic aromatic hydrocarbons, we investigated the metabolism of dibenz[a,h]acridine (DB[a,h]AC) by liver microsomes prepared from male Sprague-Dawley rats. During a 6-min incubation 21, 14, 0.7 or 0.2 nmol DB[a,h]AC per mg protein were metabolized by microsomes from rats pre-treated with DB[a,h]AC, 3-methylcholanthrene (3-MC), phenobarbital (PB) or corn oil, respectively. In each case the predominant metabolites were the dihydrodiols with bay-region double bonds, namely, DB[a,h]AC-3,4-dihydrodiol and DB[a,h]AC-10,11-dihydrodiol, each of which accounted for 21-23% of the total metabolism determined during a 7-min incubation with microsomes from 3-MC-treated rats. Other metabolites produced by these microsomes included DB[a,h]AC-1,2-dihydrodiol (approximately 5% of total metabolites); two K-region oxides [DB[a,h]AC-12,13- and 5,6-oxides (estimated to represent 5% and 2% of total metabolites, respectively)]; several unidentified polar metabolites (10-15%) and several unidentified metabolites which co-eluted with 3-hydroxy-DB[a,h]AC (20%). DB[a,h]AC-8,9-dihydrodiol was not detected (less than 2%). The metabolite profiles produced by microsomes prepared from rats pretreated with DB[a,h]AC, PB or corn oil were very similar to the profile produced by 3-MC-induced microsomes. We conclude that: the potentially mutagenic benzoring dihydrodiols with bay-region double bonds are the predominant metabolite of DB[a,h]AC; the heterocyclic nitrogen atom has little effect in modifying the relative extents of formation of these two benzo-ring dihydrodiols with bay-region double bonds; metabolism at the K-region is only a minor pathway for DB[a,h]AC, as is also true for the carbon analogue dibenz[a,h]anthracene; and induction by a 3-MC-type inducer (e.g. DB[a,h]AC) is required for substantial metabolism to occur.

Induction of hepatic microsomal cytochrome P-448-mediated oxidases by 3,3'-dichlorobenzidine in the rat

Intraperitoneal administration of the hepatocarcinogen 3,3'-dichlorobenzidine (4,4'-diamino, 3,3'-dichlorobiphenyl) to adult male rats caused the induction of hepatic microsomal ethoxycoumarin O-deethylase and p-nitrophenetole O-deethylase activities comparable in magnitudes to those induced by 3-methylcholanthrene; neither aniline hydroxylase nor aminopyrine N-demethylase activity was affected by the pretreatment. The induction was not accompanied by a significant increase in content of hepatic microsomal cytochrome P-450; however, a shift in the absorption maximum of the reduced + CO spectrum of the cytochrome to 448 nm and an increase in the ratio of the 455 nm:430 nm peaks of the reduced + ethylisocyanide spectrum of the hemoprotein was effected. Arylhydrocarbon hydroxylase activity was stimulated 5-fold by dichlorobenzidine pretreatment in comparison with a 12-fold stimulation following 3-methylcholanthrene pretreatment. However, enzymically mediated covalent binding of benzo[a]pyrene to microsomal protein was greater in microsomes from dichlorobenzidine-pretreated rats than in those from methylcholanthrene-pretreated rats. All of the dichlorobenzidine-induced enzymic activities were inhibited by alpha-naphthoflavone but not by SKF-525A. Hepatic microsomes from dichlorobenzidine-pretreated rats appeared to have a higher capacity for metabolizing dichlorobenzidine than those from untreated animals; both sets of microsomes elicited the Type II spectral change on combination with the compound, albeit with different binding affinities and capacities. The results show that dichlorobenzidine, although only a dihalogenated biphenyl derivative, is a potent inducer of cytochrome P-448.

Alteration in cell permeability as a mechanism of action of certain quinone pesticides

The permeability of the Chlorella pyrenoidosa membrane was studied by following the efflux of (14)C-intracellular material from cells which had been allowed to incorporate (14)CO(2) photosynthetically. It was observed that the efflux increased upon treatment with low concentrations (3-30 muM) of 2, 3-dichloro-1, 4-naphthoquinone (dichlone), 2-amino-3-chloro-1, 4-naphthoquinone (06K-quinone), and 2, 3, 5, 6-tetrachloro-1, 4-benzoquinone (chloranil). Dichlone caused a greater loss of intracellular material than chloranil or 06K-quinone. The rate of loss as well as the total loss of (14)C increased with an increase in the concentration of the quinones. In the dichlone-treated cells, the leakage was observed within 1 minute of the addition of the chemical and the effect on cell permeability was irreversible. Cells exposed to dichlone in the light or under anaerobic conditions released significantly greater amounts of (14)C-material than cells treated in the dark or under aerobic conditions. The aqueous ethanol-soluble fraction of the cell was found to be the source of the released material. The proportion of the ethanol-soluble (14)C that leaked out of the cell varied with the time of (14)C-assimilation prior to treatment with dichlone. In the dichlone-treated cells, practically all the (14)C-sucrose, alanine, glutamine, serine, and glycine leaked out, whereas glutamic, aspartic, succinic, and fumaric acids were lost only partially. Essentially no (14)C-lipids were lost from the cells during dichlone treatment.The extreme rapidity of the effect of dichlone on permeability and the low concentrations at which dichlone acted suggest that the cell membrane may be a primary site of action of dichlone, and that the metabolic changes observed in dichlone-treated Chlorella may be due to the changes in the cell membrane structure.

Studies on Effect of Certain Quinones: I. Electron Transport, Photophosphorylation, and CO(2) Fixation in Isolated Chloroplasts

The effect of quinone herbicides and fungicides on photosynthetic reactions in isolated spinach (Spinacia oleracea) chloroplasts was investigated. 2,3-Dichloro-1,4-naphthoquinone (dichlone), 2-amino-3-chloro-1,4-naphthoquinone (06K-quinone), and 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) inhibited ferricyanide reduction as well as ATP formation. Benzoquinone had little or no effect on these reactions. The two reactions showed a differential sensitivity to these inhibitors. Dichlone was a strong inhibitor of both photosystems I and II; photosystem I was more sensitive to 06K-quinone than was photosystem II, whereas the reverse was true of chloranil. Chloranil and 06K-quinone inhibited ferricyanide reduction and the coupled photophosphorylation to the same extent, whereas dichlone affected photophosphorylation to a greater extent than the ferricyanide reduction.CO(2) fixation was inhibited by all the quinones to varying degrees. In chloroplasts treated with 06K-quinone or benzoquinone, CO(2) fixation was inhibited to a greater extent than the photoreduction of ferricyanide or ATP formation, indicating the possibility that the two quinones may also inhibit certain reactions in the carbon reduction cycle. The effect of dichlone and chloranil, but not of 06K-quinone, was overcome by the addition of reduced glutathione. The quinones caused an increase in the proportion of (14)C incorporated into 3-phosphoglyceric acid and a reduction in the amount of glycolic acid.

Studies on Effects of Certain Quinones: II. Photosynthetic Incorporation of CO(2) by Chlorella

The effects of various quinone herbicides and fungicides on the photosynthetic (14)CO(2) fixation and the incorporation of (14)C among the products of photosynthesis in Chlorella pyrenoidosa was investigated. Addition of 30 mum 2,3-dichloro-1,4-naphthoquinone (dichlone), 2-amino-3-chloro-1,4-naphthoquinone (06K-quinone), or 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil) inhibited CO(2) fixation, whereas 1,4-benzoquinone had no effect. Treatment with 3 mum or higher concentrations of dichlone, 06K-quinone or 1,4-benzoquinone also produced marked changes in the pattern of (14)C distribution. A noticeable effect was an increase in the proportion of (14)C in sucrose and glycine accompanied by a reduction in (14)C lipids and glutamic acid. These changes appear to occur as a result of shifts in the flow of carbon along various biosynthetic pathways of photosynthetic CO(2) fixation. It is suggested that inactivation of coenzyme A and shortage of reduced triphosphopyridine nucleotide in the quinone-treated cells inhibited the synthesis of lipids and glutamic acid, thereby diverting more carbon into sucrose and glycine.