Isolation and characterization of an active DNA-binding metabolite of benzo(a)pyrene from hamster liver microsomal incubation systems

Electrochemiluminescent arrays for cytochrome P450-activated genotoxicity screening. DNA damage from benzo[a]pyrene metabolites

Arrays suitable for genotoxicity screening are reported that generate metabolites from cytochrome P450 enzymes (CYPs) in thin-film spots. Array spots containing DNA, various human cyt P450s, and electrochemiluminescence (ECL) generating metallopolymer [Ru(bpy)2PVP10]2+ were exposed to H2O2 to activate the enzymes. ECL from all spots was visualized simultaneously using a CCD camera. Using benzo[a]pyrene as a test substrate, enzyme activity for producing DNA damage in the arrays was found in the order CYP1B1 > CYP1A2 > CYP1A1 > CYP2E1 > myoglobin, the same as the order of their metabolic activity. Thus, these arrays estimate the relative propensity of different enzymes to produce genotoxic metabolites. This is the first demonstration of ECL arrays for high-throughput in vitro genotoxicity screening.

Hepatocyte-mediated mutagenicity of mononitrobenzo[a]pyrenes in Salmonella typhimurium strains

The mononitro-substituted isomers of benzo[a]pyrene (B[a]P), 1-, 3- and 6-nitrobenzo[a]pyrene (NB[a]P), are environmental pollutants and are metabolized to mutagens in Salmonella by rat-liver homogenate postmitochondrial supernatant (S9) fractions. In this study, activation of these compounds to mutagens was investigated using the hepatocyte-mediated Salmonella mutagenicity assay. Hepatocytes from rats treated with Aroclor 1254 activated both 3-NB[a]P and 1-NB[a]P to mutagens, while 6-NB[a]P was not mutagenic. The positive mutagenicity responses were functions of both the chemical dose and the hepatocyte concentration. By using a nitroreductase-deficient strain (TA98NR) and a transesterificase-deficient strain (TA98/1,8-DNP6), it was verified that the direct-acting mutagenicities of 1- and 3-NB[a]P primarily were due to metabolic processes involving nitroreduction while the S9- and hepatocyte-mediated mutagenicity responses were also dependent on transesterification. When compared with the mutagenic responses produced with S9, the mutations induced by 1- and 3-NB[a]P in the presence of hepatocytes were relatively more dependent upon nitroreductase metabolism and less on transesterification. Thus, intact hepatocytes were capable of activating 1- and 3-NB[a]P to mutagenic metabolites and some of these metabolites appeared to be different from those produced by S9.

Alterations in tissue glutathione and angiotensin converting enzyme due to inhalation of diesel engine exhaust

Quantitative changes in reduced glutathione (GSH) and angiotensin converting enzyme (ACE) of lung and extrapulmonary tissues were determined following exposure of laboratory animals to diesel engine exhaust (DEE). Exposure of male rats and guinea pigs to DEE containing 750 micrograms particulates per cubic meter for 12 wk did not cause any changes in GSH levels of lung, liver, and heart compared to control values. Rats were then exposed for various time periods to 6 mg/m3 DEE. Two weeks of exposure produced statistically significant increases of 21 and 7% in GSH levels of lung and liver, respectively, but not change in the heart. Following 4 wk of exposure, lung showed a 14% increase and heart an 11% increase in GSH level. Furthermore, rats exposed for 4 wk to DEE did not show any particular susceptibility toward the GSH-depleting effect of acetaminophen as compared to controls. A significant depletion (15%) of hepatic GSH was observed after 8 wk of exposure, while lung was still showing an increase of 18% in GSH and heart was unaffected. Time-dependent increases of 18 and 33% in serum ACE activity were noted after 4 and 8 wk of exposure. Pulmonary ACE activity did not decrease until after 8 wk, and then to a small extent (7%). The observed increases in GSH may have been related to the presence of NO2 in the DEE. On the other hand, the depletion of hepatic GSH suggests production of electrophilic compounds due to an induction of metabolic activity of liver. The change in serum ACE activity may be due to time-requiring perturbations in the pulmonary endothelial cells.

Potential carcinogenic and mutagenic industrial chemicals. I. Alkylating agents

A variety of alkylating agents, acylating agents, peroxides, halogenated derivatives, and nitrogen derivatives have been reviewed, principally in terms of their synthesis, areas of utility, stability, distribution, reactivity, levels of exposure, population at risk, metabolism, carcinogenicity, and mutagenicity.

Genetic differences between C57BL/6 and DBA/2 mice in the inductions of UDP-glucuronyl transferases for 3-hydroxybenzo(a)pyrene, p-nitrophenol, and bilirubin by 3-methylcholanthrene

In C57BL/6 mice, aryl hydrocarbon hydroxylase (AHH) increased 1 day after treatment with 3-methylcholanthrene, and induction of UDP-glucuronyl transferases for 3-hydroxybenzo(a)pyrene, p-nitrophenol and bilirubin in the liver microsomes was observed 2 to 5 days later. In DBA/2 mice, neither AHH nor transferase activities were influenced by 3-methylcholanthrene. These results suggest that induction of activating and detoxicating enzymes is genetically linked.

The detoxication of aflatoxin B1 with glutathione in the rat

1. The deactivation of aflatoxin B1 by glutathione (GSH) has been investigated in rat. Binding of metabolites of aflatoxin B1 to [3H]glutathione in vitro with rat liver microsomes is insignificant. Incubation with rat liver 10 000 g supernatant results in increased binding. Under identical conditions, benzo(a)pyrene metabolites are bound to [3H]glutathione much more than is aflatoxin B1. 2. Pre-treatment of rats with aflatoxin 1 (2 mg/kg) caused depletion in GSH of rat liver with a minimum at 6 h but returning to above normal at 24 h. GSH S-transferase activity was marginally increased at 6 h also and returned to normal at 24 h. 3. Kidney GSH was not significantly decreased, but kidney GSH S-transferase activity showed a sudden increase in 6 h, returning to almost normal at 24 h. 4. Pre-treatment with benzo(a)pyrene (2 mg/kg) caused greater depletion of hepatic GSH than occurred with aflatoxin B1 but did not show any effect on kidney GSH. 5. Hepatic and kidney GSH S-transferase in benzo(a)pyrene-treated rats showed greatest activity at 2 h followed by a gradual fall through 24 h. 6. GSH was therefore a less efficient nucleophile for aflatoxin B1 metabolites than for benzo(a)pyrene metabolites.

A comparison of the activities of aryl hydrocarbon monooxygenase in liver microsomes from mice of different strains during prolonged 3,4-benzo(a)pyrene administration

The content and activity of the components of liver microsomal aryl hydrocarbon monooxygenase system change biphasically during long-term 3,4-benzo(a)pyrene administration of C57BL/6 mice as well as to (C57BL/6 X DBA/2)F1 hybrids. The first activity peak (4--14 days) is associated with the induction of aryl hydrocarbon monooxygenase by 3,4-benzo(a)pyrene; the second peak (70--84 days) is related to noninductive mechanism. In DBA/2 mice, the second peak is absent while the slight increase in aryl hydrocarbon monooxygenase activity observed on days 14--28 indicates the aberrant inductive capacity of 3,4-benzo(a)pyrene under its prolonged administration. It is suggested that the weak sensitivity to the blastogenesis caused by 3,4-benzo(a)pyrene observed in C57BL/6 mice and in (C57BL/6 X DBA/2)F1 hybrids is due to the high level of liver aryl hydrocarbon monooxygenase activity at the time of tumor appearance.

Base specificity in the binding of benzo[a]pyrene diol epoxide to simian virus 40 DNA

[14C]Simian virus 40 (SV40) DNA was reacted with [3H]7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene to give 0.60 adducts per genome. The modified DNA was digested to completion with Hind III restriction endonuclease and the 6 fragments isolated by polyacrylamide gel electrophoresis. Hydrocarbon binding to the fragments was proportional to their guanine--cytosine (G--C) content, reflecting selective reaction of the hydrocarbon with deoxyguanosine residues. No sites unusually susceptible to alkylation were detected.

beta-Glucuronidase catalyzed hydrolysis of benzo(a)pyrene-3-glucuronide and binding to DNA

beta-Glucuronidase catalyzes the hydrolysis of benzo[a]pyrene-3-glucuronide to 3-hydroxybenzo[a]pyrene. During the enzymatic hydrolysis, a benzo[a]pyrene derivative is formed which binds to DNA to a far greater extent than either the 3-hydroxybenzo[a]pyrene or its glucuronide. These results suggest that conjugates of benzo(a)pyrene may be converted by beta-glucuronidase at intracellular and organ sites distal to the initial sites of oxygenation and conjugation of benzo(a)pyrene to activated intermediates that are possibly carcinogenic.

Some properties of vicinal diol-epoxides derived from benzo(a)anthracene and benzo(a)pyrene

The alkylating properties of pairs of syn- and anti-isomers of 2 diol-epoxides derived from benzo(a)pyrene (BP) and of 1 derived from benz(a)anthracene (BA) have been investigated. Of the anti-diol-epoxides, anti-BP 7,8-diol-9,10-oxide was the most reactive compound towards DNA, towards sodium p-nitrothiophenolate in a non-aqueous solvent system, and towards 4-(p-nitrobenzyl)pyridine in aqueous solution; anti-BP 9,10,-diol-7,8-oxide was of intermediate reactivity and anti-BA 8,9-diol-10,11-oxide was least reactive. The syn-diol-epoxides gave unsatisfactory results with DNA and 4-(p-nitrobenzyl)pyridine because of their rapid solvolysis in aqueous solution, but with sodium p-nitrothiophenolate showed the order of reactivity syn-BP 7,8-diol-9,10-oxide greater than syn-BA 8,9-diol-10,11-oxide greater than syn-BP 9,10-diol-7,8-oxide. The products of the reaction between diol-epoxides and nucleic acids were examined by Sephadex LH-20 chromatography followed by high-pressure liquid chromatography (HPLC) and the diol-epoxides were shown to react principally with the guanosine and adenosine moieties of RNA.

The in vitro oxidative metabolism of benzo(a)pyrene in human liver measured by different assays

The specific metabolism of benzo(a)pyrene (BP) in adult human liver samples was studied by using triated BP and thin-layer chromatographic separation of metabolites and by using the conventional fluorometric assay. The incubation in vitro of BP with human liver homogenates yielded the usual pattern of metabolites, including several dihydrodiols, phenols and quinones, as well as products bound convalently to protein. Interindividual variation in the production of different metabolites were very large, several ten-fold. The over-all metabolism was inhibited by aminopyrine and SKF 525A but not by 7,8-benzoflavone. The formation of dihydrodiols was inhibited by several epoxide hydratase inhibitors with consequent accumulation of phenols and an unidentified metabolite (possibly BP 4,5-oxide). Radiometric measurement of phenols, total dihydrodiols and individual dihydrodiols correlated well with the fluorometric assay of BP hydroxylase activity.

Metabolic epoxidation of trans-4-acetylaminostilbene: A protective mechanism against its activation to a mutagen

Trans-4-acetylaminostilbene is activated by liver preparations to mutagens for Salmonella typhimurium. Since this compound is metabolized to the trans-alpha,beta-epoxide and since many epoxides are ultimate mutagens, this epoxide was tested for direct mutagenicity. It was, however, found to be non-mutagenic, and, in contrast to the parent compound, the epoxide was no longer activated by liver preparations to mutagens. The same was found for the beta-ketone and for the threo-alpha,beta-dihydrodiol, which are formed metabolically from trans-4-acetylaminostilbene and from its alpha,beta-epoxide. 4-Acetylaminobibenzyl showed a very weak mutagenic activity in the presence of the liver preparation. Thus, it is important to realize that where epoxides are formed from compounds which are known to be metabolized to mutagens, they are not necessarily responsible for the mutagenicity. Epoxidation may even prevent the possibility of bioactivation to mutagens.

Mouse liver and lung glutathione s-epoxide transferase: effects of phenobarbital and 3-methylcholanthrene administration

The effects of 3-methylcholanthrene (3MC) and phenobarbital (PB) administration on the levels of glutathione-S-epoxide transferase activity in supernatant preparations of liver and lung were studied in a number of different strains of mice, C57Bl/6, C3H, C3H-f, Balb/c-, A+ and DBA/2+. Three epoxide substrates, 3MC-11,12-oxide, styrene oxide (SO) and 3,3,3-trichloro-1,2-epoxypropane (TCPO), were employed in this investigation. PB administration (75 mg/kg body weight for 3 days) resulted in 13-57% increases in enzyme activity in the liver supernatant but was ineffective in inducing activity in lung. 3MC administration (40 mg/kg body weight for 2 days) on the other hand was without any effect on glutathione-S-epoxide transferase activity in both liver and lung.

Aryl hydrocarbon hydroxylase and 16alpha-hydroxylase in cultured human lymphocytes

Aryl hydrocarbon hydroxylase and 16alpha-hydroxylase were examined in intact, cultured human lymphocytes. The two microsomal mixed-function oxygenases had different pH optima and showed competitive inhibition for enzyme induction and activity. Population distributions were lognormal for both enzymes, giving apparent evidence for polygenic control. Induced levels of AHH were slightly higher among first-order relatives of lung or colon cancer patients than in the control group. The correlation coefficient (r) for AHH and SAH coinducibility was -0.08, indicating no correlation and suggesting the absence of association between the two enzymes in man.

Elucidation of hydrocarbon structure in an enzyme-catalyzed benzo[a]pyrene-poly (G) covalent complex

The carcinogen, benzo[a]pyrene, was covalently attached to poly (G) by liver microsomes from rats pretreated with 3-methylcholanthrene. The complex was hydrolyzed with enzymes or base and products were isolated by Sephadex chromatography. Absorbance and fluorescence spectra of the products fit that of red-shifted pyrene aromatic system and suggest that metabolism has occurred at the 7-, 8-, 9-, and 10-positions of the hydrocarbon. Benzanthracene or chrysene fluorescence were not observed in these preparations. Benzo[a]pyrene derivatives were synthesized and purified by high-pressure liquid chromatography. Dehydration of 7,8-dihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene resulted in the formation of small amounts of 7-oxo-7,8,9,10-tetrahydrobenzoa[a]pyrene. A 7-keto species was also observed after similar treatment of the hydrocarbon-poly(G) hydrolysis products. Evidence of dehydration at the 9,10-positions was not observed. The hydrocarbon covalently bound to poly(G) is, therefore, a derivative of 7,8-dihydroxy-7,8,9,10-tetrahydrobenzol[a]pyrene with nucleic acid substitution at C-10 or 9.

Glutathione-S-epoxide transferase in mouse skin and human foreskin

Glutathione-S-epoxide transferase activity in skin was determined in six mouse strains as well as in human foreskin (from 2- to 4-day-old neonates) using two substrates, styrene oxide and 3-methylcholanthrene-11,12-oxide. Of the strains used in this study, the highest enzyme activity was noted in the skin of the Balb/c mouse. The enzyme was not inducible in skin by prior administration of 3-methylcholanthrene to the mice. Human foreskin activity was lower than that present in the adult mouse skin and ranged from 1.93 to 5.28 with styrene oxide (at 22 degrees C) and 0.17 to 0.46 nmoles/mg protein/5 min with 3-methylcholanthrene-11,12-oxide (at 37 degrees C) as substrates.

Epoxides metabolically produced from some known carcinogens and from some clinically used drugs. I. Differences in mutagenicity

The epoxide metabolites of two clinically used drugs and an experimental psychotropic agent, carbamazepine 10,11-oxide, cyproheptadine 10,11-oxide and cyclobenzaprine 10,11-oxide, were fully devoid of any mutagenic activity under conditions where K-region-epoxide metabolites of some known carcinogens, such as benzo(a)pyrene, proved to be potent frameshift mutational agents for Salmonella typhimurium TA 1537 and TA 1538. All epoxides tested were non-mutagenic for TA 1535, designed to detect substitution mutations. The 10,11-epoxides of the three drugs, carbamazepine, cyproheptadine and cyclobenzaprine, were not cytotoxic to any of the bacterial tester strains used, precluding that mutagenicity might have been overshadowed by cytotoxicity. When the mutagen, precursor, benzo(a)pyrene, was incubated together with TA 1537 and a mammalian microsomal preparation in the presence of a system generating the co-factor necessary for mono-oxygenase activity, activation to mutagenic species was observed which was dramatically increased in the presence of a potent epoxide hydratase inhibitor, 1,1,1-trichloropropene 2,3-oxide, suggesting epoxide(s) as the (or one of the) mutagenically active species metabolically produced in situ. None of these effects was observed with the three medical drugs. Moreover, the observation that the alkene oxide 4-phenylstyrene 7,8-oxide is mutagenic to the two strains TA 1537 and TA 1538 but the K-region arene oxide derived from 7,12-dimethylbenz(a)anthracene is inactive for the latter strain indicates that epoxidation of an aromatic double bond of a polycyclic hydrocarbon is neither a necessary nor a satisfying condition for frameshift mutagenesis to occur.

Inhibition of hepatic aryl hydrocarbon hydroxylase by 3-methylcholanthrene, 7,8-benzoflavone and other inducers added in vitro

A close correlation has been observed between the ability of aromatic polycyclic hydrocarbons and 7,8-benzoflavone (7,8-BF) to induce hepatic aryl hydrocarbon hydroxylase (AHH) in vivo and to inhibit the induced enzyme system in vitro. The activity of this mono-oxygenase was measured by the conversion of 14C-labeled dimethylbenz(a)anthracene (DMBA) or benzo(a)pyrene (BP) to water-soluble products by rat liver preparations (8000 X g supernatant). DMBA as substrate had the advantage over BP in giving a wider range of ethyl acetate-soluble metabolites and allowing the observation of changes in the pattern of these products following injection or addition of the inducing agents. This property was used to detect low concentration (0.1 muM) of polycyclic hydrocarbons which are strong AHH inducers and which may also be carcinogenic. The liver preparation was active for several months when stored at --20 degrees. A possible mechanism of action for the in vitro behaviour of polycyclic hydrocarbons and 7,8-BF towards AHH is proposed.

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.

Carcinogenesis

An examination has been made of the major chemicals which are known to be carcinogenic, either in animals or in humans. As a result, a generalized type of chemical property seems to be a prerequisite for the carcinogenic activity. In most cases, this is some kind of reactive electrophilic intermediate produced directly from the carcinogen or from one of its immediate metabolites. This electrophilic reagent is generally a positively charged ion of some kind. In the case of the hydrocarbons it seems to be an incipient carbonium ion; in the case of nitrogen compounds, it is a corresponding nitronium ion. There are a number of possible substrate materials (nucleophilic) in a cells, but the dominant one which has been suspected is the nucleic acid component, either the DNA or the RNA. The bases of these materials all have extra pi electrons susceptible to attach by the electrophiles of the carcinogen. Such a modification, particularly of the DNA, could lead to either major or minor changes in the genetic composition of the cell. Minor changes are usually rectifiable or are not visible, and this would include a large variety of point mutations.

Mutagenic and cytotoxic activity of benzol[a]pyrene 4,5-, 7,8-, and 9,10-oxides and the six corresponding phenols

The benzo[a]pyrene 4,5-, 7,8-, and 9,10-oxides and the six corresponding phenols (4-, 5-, 7-, 8-, 9-, and 10-hydroxybenzo[a]pyrene) have been tested for mutagenic and cytotoxic activity in bacteria and in a mammalian cell culture system. Benzo[a]pyrene 4,5-oxide (K-region) was highly mutagenic in two histidine-dependent strains (TA1537 and TA1538) of Salmonella typhimurium which detect frameshift mutagens. In contrast, benzo[a]pyrene 7,8- and 9,10-oxides were less than 1% as mutagenic as the 4,5-oxide. Benzo[a]pyrene 7,8- and 9,10-oxides were unstable in aqueous media, whereas the 4,5-oxide was stable for several hours. This difference in stability could not account for the different mutagenic activities of the three arene oxides. The benzo[a]pyrene oxides were inactive in a strain (TA1535) that is reverted by base pair mutagens such as N-methyl-N'-nitro-N-nitrosoguanidine or in a strain (TA1536) that detects framshift mutagens similar to the acridine half-mustard ICR-191. Benzo-[a]-pyrene and the six phenols were all stable in aqueous media, but they had little or no mutagenic activity in any of the four Salmonella strains. Conversion of 8-azaguanine-sensitive Chinese hamster V79 cells to 8-azaguanine-resistant variants was increased by benzo[a]pyrene 4,5-oxide, whereas the 9,10-oxide was considerably less active. Benzo[a]pyrene and the other derivatives had little or no effect. Benzo[a]yrene 4,5-oxide was more cytotoxic to the Chinese hamster V79 cells than the 7,8- and 9,10-oxides, while 8-hydroxybenzo[a]pyrene was the most cytotoxic of the six phenols.

Effects of inducers and epoxide hydrase on the metabolism of benzo(a)pyrene by liver microsomes and a reconstituted system: analysis by high pressure liquid chromatography

The mobilities of 24 potential metabolites of benzo[a]pyrene were examined with high pressure liquid chromatography. Twelve phenols, five quinones, four dihydrodiols, and three oxides were studied. The chromatographic procedure employed allowed the separation and quantitation of benzopyrene metabolites into three major groups consisting of phenols, quinones, and dihydrodiols. Two of the benzopyrene oxides were unstable during chromatography, whereas the third oxide was more stable and chromatographed in the quinone fraction. Treatment of rats with phenobarbital or 3-methylcholanthrene enhanced the metabolism of benzopyrene by liver microsomes and altered the relative amounts of the various metabolites formed. In the absence of epoxide hydrase (EC 4.2.1.63), benzopyrene was metabolized primarily to phenols and quinones but was not appreciably metabolized to dihydrodiols by a solubilized, reconstituted cytochrome P-448 monooxygenase system. Addition of partially purified epoxide hydrase resulted in the formation of benzopyrene dihydrodiols with a concomitant decrease in the formation of phenolic metabolites, indicating that benzopyrene undergoes metabolism via arene oxides that are precursors for dihydrodiols and phenols.