Mutagenicity of benzo[a]pyrene 7,8-dihydrodiol and 7,12-dimethylbenz[a]anthracene 3,4-dihydrodiol in S. typhimurium mediated by microsomes from rat liver and mouse skin

The mutagenic activities of trans-7,8-dihydro-7,8-dihydroxybenzo[a]-pyrene (BP 7,8-diol) and of trans-3,4-dihydroxy-7,12-dimethylbenz[a]-anthracene (DMBA 3,4-diol) towards S. typhimurium TA100 were measured in assays that were carried out on a micro-scale in liquid medium in the presence of microsomal fractions prepared from mouse skin or rat liver. In the presence of an NADPH-generating system, microsomal enzymes converted both diols into mutagens that were probably the respective 'bay-region' diol-epoxides. The rate of the enzyme-catalysed conversion of the BP 7,8-diol into mutagens by microsomal preparations from mouse epidermis was similar to that occurring with microsomes from rat liver. Pretreatment of mice by the topical application of benz[a]anthracene (BA) or 7,12-dimethylbenz[a]-anthracene (DMBA) increased the mutagenic activity of BP 7,8-diol mediated by mouse skin microsomal preparations by 2-fold and this was paralleled by a 4-fold increase in epidermal aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) activity. The results are discussed in relation to the high susceptibility of mouse skin to polycyclic aromatic hydrocarbon (PAH) carcinogenesis.     

The effect of manganese and ferric ions on the in vitro formation of dihydrodihydroxy metabolites of benzo(a)pyrene

The effect of ferric and manganese ions on the in vitro metabolism of benzo(a)pyrene (BP) to dihydrodihydroxy (diol) metabolites by rat liver microsomal preparations was studied. Of the 3 diols separated by high-pressure liquid chromatography (HPLC) and called diols 1, 2 and 3 in order of elution, diol 1 was identified by its U.V. spectrum as the 9,10-diol; diols 2 and 3 have not yet been identified positively but are probably the 4,5- and 7,8-diols respectively. Higher concentrations of both metals altered the diol profile; 10 and 50 mumol Fe3+ per incubation caused the disappearance of diols 1 and 2 and an increase in diol 3; 10 mumol Mn2+ caused a significant decrease in diol 2 while 50 mumol reduced diol 2 to a negligible amount and inhibited the formation of diol 1; both concentrations caused a relative increase in diol 3. If the tentative identification of diol 3 as the 7,8-diol is correct, manganese and ferric ions could be significant in the metabolism of BP to the active metabolite, the 7,8-diol-9,10-epoxide.     

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.     

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.     

The use of high pressure liquid chromatography to study chemically induced alterations in the pattern of benzo[a]pyrene metabolism.

The metabolism of radiolabeled benzo[a]pyrene (BP) by control, 3-methyl-cholanthrene (3-MC) induced, and 1,1,1-trichloropropene-2,3-oxide (TCPO)-inhibited rat liver microsomes was measured using fluorescence, radiometric, and high-pressure liquid chromatographic (HPLC) assays. Significant differences in the total measurable metabolism of BP by the three microsomal enzyme incubations resulted from the use of the three assay procedures. Appreciable differences in the concentration of the metabolite fractions after 3-MC induction and TCPO inhibition are clearly demonstrated. NMR analysis revealed that while the 3-hydroxy-BP fraction is greater than 90% pure, the 9-hydroxy fraction contains a number of metabolites having essentially identical retention times.     

Nuclear metabolism of benzo(a)pyrene and of (±)-trans-7,8-dihydroxy-7,8,-dihydrobenzo(a)pyrene: Comparative chromatographic analysis of alkylated DNA

Rat liver nuclei were incubated with [¹⁴C]benzo(a)pyrene (BP) or [³H](±)-trans-7,8-dihydrodiol of BP (³H-BP-7,8-diol) in the presence of a NADPH-generating system. The nuclei were able to form from BP the 9,10-, 4,5- and 7,8-dihydrodiols, the 3,6- and 1,6-quinones as well as the 3- and 9-phenols. The total nuclear metabolism was stimulated 11-fold by prior administration to the rats of 3-methylcholanthrene (3MC). BP-7,8-dihydrodiol formation, under these circumstances, was enhanced 29-fold. The rat liver nuclei were also able to form from [³H]BP-7,8-diol, (±)-7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydro BP (diol epoxide 1), (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydro BP (diol epoxide 2), as well as three unknown metabolites. Diol epoxides 1 and 2 represented 23 and 65% of the total metabolites produced during the control nuclear incubation. Pretreatment of the rats with 3MC resulted in 4-fold increase in nuclear metabolic activity. Under the latter circumstances, the diol epoxides 1 and 2 represented 43 and 38%, respectively, of the total nuclear metabolites. Incubation of liver nuclei with labeled BP or BP-7,8-diol in the presence of NADPH resulted in alkylation of DNA. The alkylated deoxyribonucleosides were separated by Sephadex LH-20 chromatography. Two peaks of radioactivity were noted after incubation with the parent polycyclic hydrocarbon while only one peak was seen after incubation with the diol derivative. These results emphasize the importance of nuclei in the metabolism of BP and in the subsequent alkylation of DNA, reactions which may be related to mutagenesis or carcinogenesis.     

Cutaneous metabolism of benzo[a]pyrene: comparative studies in C57BL/6N and DBA/2N mice and neonatal Sprague--Dawley rats

The metabolism of the polycyclic aromatic hydrocarbon (PAH) carcinogen benzo[a]pyrene (BaP) was studied using microsomes prepared from the skin of the mouse and rat. Topical application of the polychlorinated biphenyl (PCB) Aroclor 1254 or the PAH 3-methylcholanthrene (3-MC) to the skin of the C57BL/6N and DBA/2N mouse and the Sprague-Dawley rat caused statistically significant enhancement of cutaneous microsomal aryl hydrocarbon hydroxylase (AHH) activity in each animal. PCB was a more potent inducer of the enzyme than was 3-MC. BaP metabolism by skin microsomes from the same animals was assessed using high performance liquid chromatography (HPLC). The skin of untreated animals metabolized BaP into 9,10-, 7,8- and 4,5-dihydrodiols, phenols and quinones. Skin application of PCB caused greater than 16–18-fold enhancement of BaP metabolism in the C57BL/6N mouse and the rat and 2–5-fold enhancement in the DBA/2N mouse. Skin application of 3-MC enhanced BaP metabolism 2–8-fold in the C57BL/6N mouse and 5–10-fold in the rat and had no effect in the DBA/2N mouse. The formation of procarcinogenic metabolite BaP-7, 8-diol was greatly enhanced (4–12-fold) by treatment with the PCB and 3-MC in the tumor susceptible C57BL/6N mouse and in the tumor-resistant neonatal Sprague-Dawley rat. In contrast, the formation of BaP-7,8-diol was either slightly enhanced (2-fold) or unaffected by treatment with the PCB or 3-MC in the tumor-resistant DBA/2N mouse. Our data indicate that neither the patterns of metabolism nor the amount of BaP-7,8-diol formation in the skin are reliable predictors of tumor susceptibility to the PAH in rodent skin.     

The kinetics of benzo(a)pyrene anti-7,8-dihydrodiol 9, 10-epoxide formation from benzo(a)pyrene and regulatory membrane effects

r-7,c-10,t-8,t-9-Tetrahydroxybenzo(a)pyrene (7,10/8,9-tetrol), which is the principal hydrolysis product of r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene (anti-diol-epoxide), was resolved and measured by HPLC in organic extracts of incubations which contained induced rat liver microsomes and BP. Kinetic analyses showed that: (a) following a 5- to 7-min lag period, anti-diol-epoxide formation was linear, and (b) levels of anti-diol-epoxide formed were highly dependent upon the starting BP concentration. anti-Diol-epoxide production increased at starting BP concentrations of 0–12 μm and decreased in incubations containing 12–25 μm BP. However, between 25 and 100 μm BP, anti-diol-epoxide formation was stable at a level representing 65% of the peak production which occurred at a starting BP concentration of 12 μm. BP oxidation was competitively inhibited by (?)-trans-BP-7,8-dihydrodiol and about five times less effectively by the (+)-trans-BP-7,8-dihydrodiol. The inability of a severalfold excess of BP (25–100 μm) to totally inhibit BP-7,8-dihydrodiol oxidation was explained by the presence of a microsomal substrate compartment which was saturated at only 6–8 μm BP, the remaining BP present as aggregates in the aqueous compartment. Purification of microsomes by Sepharose 2B gel filtration after reaction with [³H]BP also indicated that BP-7,8-dihydrodiol was preferentially concentrated in the microsome compartment leading to a net increase in the ratio of BP-7,8-dihydrodiol to BP in the microsomal compartment, which favored BP-7,8-dihydrodiol oxidation to yield the biologically active anti-diol-epoxide.     

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

• 1.1. 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.
• 2.2. Compared to rat liver microsomes, bullhead liver microsomes produced relatively larger amounts of BP-7,8-diol (predominantly the [−] enantiomer) and smaller amounts of BP-4,5-diol.
• 3.3. BP phase I metabolites were efficiently converted by freshly isolated bullhead hepatocytes to conjugates, predominantly glucuronides.
• 4.4. 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.
• 5.5. 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.

     

Stereoselective aspects of the metabolic activation of benzo[a]pyrene by human skin in vitro

Benzo[a]pyrene (BP) is activated within tissues in both a regio- and a stereoselective manner and, since human skin is sensitive to tumour induction by polycyclic aromatic hydrocarbons (PAH), the steroselective metabolism of BP in this tissue has been investigated. Samples of skin from eleven individuals were treated with [3H]BP in short-term organ culture. Two samples were also treated with mixtures of [14C](+)- and (-)-trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-dihydrodiol) in varying proportions. Following application of [3H] BP, more 7,8-dihydrodiol was recovered from the skin itself than from the culture fluid in ten cases; no 7.8-dihydrodiol was detected in extracts from the eleventh. The 7,8-dihydrodiol metabolite was extracted predominantly (range 74-greater than 99%) as the (-)-enantiomer in nine of these ten patients, although proportionately more (+)-enantiomer was recovered from the culture fluid than from the skin in each case. The relative proportions of [3H]BP tetrols derived from syn- and anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydroxybenzo[a]pyrene (BPDE) detected in these extracts was more variable. When skin samples were treated with [14C]BP-7,8-dihydrodiol, more anti- than syn-BPDE-derived tetrols were extracted, irrespective of the optical purity of the dihydrodiol applied. These findings provide evidence for interindividual variations in the stereoselective metabolism of BP, which may be of some importance in determining individual susceptibility to PAH-induced skin carcinogenesis.     

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

Pretreatment of hamsters with benzo (a) pyrene (BaP) greatly increased the $\text{in vitro}$ metabolism of BaP by lung microsomes from pregnant hamsters, and had less effect on the metabolism of BaP by liver microsomes. The production of various metabolites of BaP by lung microsomes was increased to different extents: 3-hydroxy-BaP (3-OH-BaP) was one of the major metabolites; the metabolic yields of 9, 10-dihydrodihydroxy-BaP (9, 10-diol) and 7,8-diol were increased more than that of the 4,5-diol. In the case of liver microsomes, only the yields of 9,10-diol and 7,8-diol were increased over the control levels. The presence of cyclohexene oxide in the incubation mixtures decreased the production of the diols. Basal-level enzyme activities in placental, fetal liver, and fetal skin microsomes in metabolizing BaP were very low. Pretreatment of pregnant hamsters with BaP induced BaP-metabolizing enzymes in fetal tissue 2–3 fold.     

The oxidation of benzo[a]pyrene-7,8-dihydrodiol mediated by lipid peroxidation in the rat intestine and the effect of dietary lipids

This study has demonstrated that the microsomal fraction of the rat small intestinal mucosa has the capacity to catalyse the oxidation of benzo[a]pyrene(BP)-7,8-diol to BP-diol-epoxides (BPDEs) both by a mechanism involving the mixed-function oxidase system (NADPH-dependent) and as a result of the initiation of peroxidation of the membrane phospholipids by ferrous ions, ascorbate and ADP. The NADPH-dependent reaction was fastest in the proximal part of the intestine and resulted in the formation of approximately equal amounts of BPDE I and BPDE II. The lipid peroxidation-catalysed reaction favoured the production of BPDE I and was maximal in the middle region of the intestine, closely paralleling the rate of lipid peroxidation in the intestinal sections. Feeding rats on a cod liver oil diet, rich in C20:5 and C22:6, significantly increased the incorporation of these fatty acids into the microsomal fractions. This resulted in a greatly increased rate of lipid peroxidation in vitro and a significantly higher rate of lipid peroxidation-catalysed BP-7,8-diol oxidation compared to rats fed fat-free, mono-unsaturated lard or corn oil (58% C18:2) diets. Thus the rate of conversion of BP-7,8-diol to its ultimate carcinogenic forms during lipid peroxidation in the intestinal fractions of rats fed a polyunsaturated fat was quantitatively more important than the NADPH-catalysed reaction as measured in vitro.     

Oxidation of benzo(a)pyrene and 7,8-dihydro-7,8-dihydroxy benzo(a)pyrene by horseradish peroxidase-H2O2 intermediate: fluorometric study

The capacity of oxidation of benzo(a)pyrene (BP) and its analog to be oxidized by peroxidases in several tissues has been studied. The kinetics of the horseradish peroxidase (HRP) oxidation of BP and 7,8-dihydro-7,8-dihydroxy benzo(a)pyrene (BP-7,8-diol) were examined. Effective ratios of H2O2 and HRP for catalytic oxidation were 13.74 for BP and 4.58 for BP-7,8-diol. The maximum ratio was approximately 90 for both hydrogen donors (BP and BP-7,8-diol) to the ES complex. The maximum ratio of oxidized BP and BP-7,8-diol to HRP was 5.7. Ks values for H2O2 were 1.68 and 6.35 microM for BP and BP-7,8-diol, respectively. The mean values of the rate constants, k5, for the oxidation of BP and BP-7,8-diol were 0.56 X 10(5) M-1 sec-1 and 4.1 X 10(5) M-1 sec-1, respectively, at low concentrations. At low concentrations a Hill plot of the oxidation of BP showed a negative value (nH = 0.5) and at high concentrations nH = 1.0. On the other hand, that of BP-7,8-diol showed positive cooperativeness (nH = 1.8). These oxidation reactions caused substrate (donor) inhibition at high concentrations. The inhibition constants, KA', were 9.8 and 5.65 microM for BP and BP-7,8-diol, respectively. The reactivity of the oxidation of BP-7,8-diol was five to six times larger than that of BP.     

Metabolism of benzo[a]pyrene VI. Stereoselective metabolism of benzo[a]pyrene and benzo[a]pyrene 7,8-dihydrodiol to diol epoxides

(±)-7β,8α-Dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (diol epoxide-1) and (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (diol epoxide-2) are highly mutagenic diol epoxide diastereomers that are formed during metabolism of the carcinogen (±)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene. Remarkable stereoselectivity has been observed on metabolism of the optically pure (+)- and (?)-enantiomers of the dihydrodiol which are obtained by separation of the diastereomeric diesters with (?)-α-methoxy-α-trifluoromethylphenylacetic acid. The high stereoselectivity in the formation of diol epoxide-1 relative to diol epoxide-2 was observed with liver microsomes from 3-methylcholanthrene-treated rats and with a purified cytochrome P-448-containing monoxygenase system where the (?)-enantiomer produced a diol epoxide-2 to diol epoxide-1 ratio of 6 : 1 and the (+)-enantiomer produced a ratio of 1 : 22. Microsomes from control and phenobarbital-treated rats were less stereospecific in the metabolism of enantiomers of BP 7,8-dihydrodiol. The ratio of diol epoxide-2 to diol epoxide-1 formed from the (?)- and (+)-enantiomers with microsomes from control rats was 2 : 1 and 1 : 6, respectively. Both enantiomers of BP 7,8-dihydrodiol were also metabolized to a phenolic derivative, tentatively identified as 6,7,8-trihydroxy-7,8-dihydrobenzo[a]pyrene, which accounted for ～30% of the total metabolites formed by microsomes from control and phenobarbital-pretreated rats whereas this metabolite represents ～5% of the total metabolites with microsomes from 3-methylcholanthrene-treated rats. With benzo[a]pyrene as substrate, liver microsomes produced the 4,5-, 7,8- and 9,10-dihydrodiol with high optical purity (>85%), and diol epoxides were also formed. Most of the optical activity in the BP 7,8-dihydrodiol was due to metabolism by the monoxygenase system rather than by epoxide hydrase, since hydration of (±)-benzo[a]pyrene 7,8-oxide by liver microsomes produced dihydrodiol which was only 8% optically pure. Thus, the stereospecificity of both the monoxygenase system and, to a lesser extent, epoxide hydrase plays important roles in the metabolic activation of benzo[a]pyrene to carcinogens and mutagens.     

Metabolites of benzo(a) pyrene produced by placental microsomes from cigarette smokers and nonsmokers

Placental microsomes from a large majority of nonsmokers in this study showed almost undetectable specific activity in metabolizing benzo(a)pyrene (BaP) in vitro. The microsomal fraction of the placentas from individuals who smoked cigarettes during pregnancy had the highest activity in metabolizing BaP than other subcellular fractions. Cigarette smoking during pregnancy induced placental enzymes which converted BaP to a variety of metabolites: the yield of 3-hydroxy-BaP (3-OHBaP) and other phenols of BaP was the largest among the BaP metabolites, 7,8-dihydrodihydroxy-BaP (7,8-diol) having 13–72% the yield of 3-OHBaP. Other metabolites included 9, 10-dihydrodihydroxy-BaP (9,10-diol), 4,5-dihydrodihydroxy-BaP (4,5-diol), quinones of BaP and unidentified metabolites which were more polar than the diols.     

Induction and repair of DNA strand breaks in cultured human fibroblasts exposed to various phenols and dihydrodiols of benzo[a]pyrene

Cultured human fibroblasts from healthy donors were incubated for 30 min with nine different benzo[a]pyrene (BP) derivatives in the presence or absence of liver microsomes from 3-methylcholanthrene treated rats. The induction and repair of DNA strand breaks were analysed by alkaline unwinding and separation of double and single stranded DNA (SS-DNA) by hydroxylapatite chromatography immediately after the incubation or at various times after the treatment. In the absence of microsomes DNA stand breaks were detected in fibroblasts exposed to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP) and the three BP dihydrodiols (BP-4,5-, BP-7,8- or BP-9,10-dihydrodiol). After removal of the BP derivatives from the medium the DNA strand breaks disappeared within 24 h. alpha-Naphthoflavone (alpha-NF) caused a decrease in the induction of strand breaks by 1-, 3- and 9-OH-BP but did not affect the induction of strand breaks in cells exposed to BP-7,8-dihydrodiol. In the presence of microsomes DNA strand breaks were found after exposure to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP), as well as BP-7,8- and 9,10-dihydrodiol. In contrast BP-4,5-dihydrodiol did not induce strand breaks under these conditions. The induction of strand breaks by BP-7,8-dihydrodiol was enhanced in the presence of cytosine-1-beta-D-arabinofuranoside (AraC). In all cases the DNA strand breaks had disappeared 24 h after removal of the BP derivatives and microsomes except after treatment with BP-7,8-dihydrodiol.     

Proteomic profiles of induced hepatotoxicity at the subcellular level

In the present study proteomes of liver samples were analyzed after administration of phenobarbital (PB) or 3-methylcholantrene (3-MC) to mice. Liver cell homogenates were subfractionated by differential ultracentrifugation into cytosol and microsomes, which were subjected to 2-DE to generate the proteomic maps of these fractions. 2-DE yielded 1100 and 800 protein spots for microsomes and cytosol, respectively. General trends of the fraction-specific alterations after 3-MC or PB treatment were evaluated using the Student's t-test and the principal component analysis (PCA). According to the PCA-derived data, the microsomal changes after 3-MC and PB treatment were quite similar. However, in the case of the cytosol data, the specificities of 3-MC- and PB-induced responses could be clearly distinguished from each other. Protein spots, whose expression levels differed from control, were identified by MALDI-TOF PMF. Proteomic studies such as those reported herein can be useful in identifying the molecular-based toxicity of lead drug candidates.     

Microsomal ring-fission of cis- and trans-acenaphthene-1,2-diol

1. The ring-fission of cis- and trans-acenaphthene-1,2-diol by rat liver microsomes was studied. 2. 1,8-Naphthalic acid was detected and isolated after microsomal incubations of the diols. 3. The accompanying reduction of NAD(+) was followed spectrophotometrically. 4. The optimum pH for the microsomal reaction was 9.4 for the oxidation of the cis-diol and 9.8 for that of the trans-diol. 5. p-Chloromercuribenzoate and 2,4-dichlorophenol inhibited the reaction. 6. Possible mechanisms for the microsomal ring-fission, involving 1,8-naphthalic aldehyde, are discussed.     

Host-mediated mutagenicity experiments with benzo[a]pyrene and two of its metabolites

Benzo[a]pyrene (BP) and two of its major metabolites, the ultimate mutagen BP-4,5-oxide and the proximate mutagen trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-diol) were investigated for mutagenicity in Salmonella typhimurium TA1538, TA98 and TA100 using an intrasanguineous host-mediated assay. BP and BP-4,5-oxide were not mutagenic under any experimental conditions. BP-7,8-diol was inactive with the strain TA1538 but was mutagenic with the strains TA98 and TA100. The effect was potentiated by pretreatment of the host mice with the cytochrome P-450 inducer 5,6-benzoflavone. We conclude: (i) one of the reasons for the observed insensitivity of the intrasanguineous host-mediated assay towards BP is that BP-4,5-oxide, which contributes to the microsome-mediated mutagenicity of BP, is inactive in the host-mediated assay; (ii) the finding that BP-7,8-diol is mutagenic in the host-mediated assay demonstrates that the lack of mutagenicity of BP is not intrinsic; (iii) the potentiated mutagenicity after treatment of the hosts with 5,6-benzoflavone suggests that cytochrome P-450 is more important in the activation of BP-7,8-diol in this system than other enzymes (e.g. prostaglandin synthase) that can also activate this compound in vitro.     

Comparison of the formation of benzo[a]pyrene diolepoxide-DNA adducts in vitro by rat and human microsomes: evidence for the involvement of P-450IA1 and P-450IA2

The involvement of cytochrome P-450 isozymes in the activation of benzo[a]pyrene (BP) by human placental and liver microsomes was studied in vitro using monoclonal antibodies (Mab) toward the major 3-methylcholanthrene (MC)-inducible and phenobarbital-inductible rat liver P-450 isozymes (Mab 1-7-1 and Mab 2-66-3, respectively). Microsomes from human placenta and liver and rat liver were incubated with BP and DNA, and BP-diolepoxide-DNA (BPDE-DNA) adducts were measured by synchronous fluorescence spectrophotometry (SFS). The only BP metabolite giving the same fluorescence peak as chemically modified BPDE-DNA was BP-7,8-dihydrodiol. Five (smokers) out of 29 human placentas (smokers and nonsmokers), and five out of nine human livers were able to metabolically activate BP to BPDE-DNA adducts in this system. The Mab 1-7-1 totally inhibited the formation of BPDE-DNA adducts in placental microsomal incubations. Inhibition using rat or human liver microsomes was 50-60% and about 90%, respectively. The Mab 2-66-3 had no effect in any of the microsome types. Adduct formation was inhibited more strongly and at lower concentrations of Mab 1-7-1 compared with the inhibition of AHH activity. This study is a clear indication of the major role of P-450IA1 (P-450c) in human placenta and probably P-450IA2 (P-450d) in human liver in BP activation, while other isozymes also take part in the activation in rat liver. Furthermore, this clearly indicates that AHH activity and BP activation are not necessarily associated.