The metabolic activation of benzo(a)pyrene and 9-hydroxybenzo(a)pyrene by liver microsomal fractions

Elevated urinary mutagenicity among those exposed to bituminous coal combustion emissions or diesel engine exhaust

Urinary mutagenicity reflects systemic exposure to complex mixtures of genotoxic/carcinogenic agents and is linked to tumor development. Coal combustion emissions (CCE) and diesel engine exhaust (DEE) are associated with cancers of the lung and other sites, but their influence on urinary mutagenicity is unclear. We investigated associations between exposure to CCE or DEE and urinary mutagenicity. In two separate cross-sectional studies of nonsmokers, organic extracts of urine were evaluated for mutagenicity levels using strain YG1041 in the Salmonella (Ames) mutagenicity assay. First, we compared levels among 10 female bituminous (smoky) coal users from Laibin, Xuanwei, China, and 10 female anthracite (smokeless) coal users. We estimated exposure-response relationships using indoor air concentrations of two carcinogens in CCE relevant to lung cancer, 5-methylchrysene (5MC), and benzo[a]pyrene (B[a]P). Second, we compared levels among 20 highly exposed male diesel factory workers and 15 unexposed male controls; we evaluated exposure-response relationships using elemental carbon (EC) as a DEE-surrogate. Age-adjusted linear regression was used to estimate associations. Laibin smoky coal users had significantly higher average urinary mutagenicity levels compared to smokeless coal users (28.4 ± 14.0 SD vs. 0.9 ± 2.8 SD rev/ml-eq, p = 2 × 10-5 ) and a significant exposure-response relationship with 5MC (p = 7 × 10-4 ). DEE-exposed workers had significantly higher urinary mutagenicity levels compared to unexposed controls (13.0 ± 10.1 SD vs. 5.6 ± 4.4 SD rev/ml-eq, p = .02) and a significant exposure-response relationship with EC (p-trend = 2 × 10-3 ). Exposure to CCE and DEE is associated with urinary mutagenicity, suggesting systemic exposure to mutagens, potentially contributing to cancer risk and development at various sites.

Identification and characterization of a novel benzo[a]pyrene-derived DNA adduct

The aim of this study was to generate and identify a novel benzo[a]pyrene (BP)-derived DNA adduct found both in vitro and in vivo. To date, the majority of studies have focused on N(2)-[10 beta(7 beta,8a,9a-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene)yl]-deoxyguanosine (anti-BPDE-dG), the major adduct generated following bioactivation of BP. However, a second adduct is also formed following bioactivation of BP which has been speculated to result from further metabolism of 9-OH-BP. In order to identify this second reaction pathway, the ultimate DNA binding species, and the DNA base involved, we have synthesized and characterized a dG-derived DNA adduct arising from further bioactivation of 9-OH-BP in the presence of rat liver microsomes. Analysis of the adducted nucleotides was conducted using both the (32)P-postlabeling assay and capillary electrophoresis-mass spectrometry (CE-MS).

Preferential activation of 6-aminochrysene and 2-aminoanthracene to mutagenic moieties by different forms of cytochrome P450 in hepatic 9000 X g supernatants from the rat

6-Aminochrysene and 2-aminoanthracene were activated to metabolites which were mutagenic to Salmonella typhimurium TA98 by hepatocytes or hepatic 9000 X g supernatants (S9s) from control or xenobiotic-treated rats. Hepatocytes from Aroclor-1254-treated rats were more efficient than hepatocytes from untreated rats at activating these aromatic amines. When plate-incorporation and liquid-incubation bacterial mutagenesis assays were performed in the presence of limiting amounts of rat hepatic S9, 2-aminoanthracene was activated to a greater extent in both cases, as judged by his+ revertant formation, by 3-methylcholanthrene-induced hepatic S9 than by phenobarbital-induced or control S9s. In contrast, 6-aminochrysene was activated more efficiently by phenobarbital-induced S9 than by 3-methylcholanthrene-induced or control S9s. This unexpected finding was confirmed employing polyclonal antibodies directed against specific forms of rat cytochrome P450. Thus, when employing Aroclor-1254-induced S9 as a source of metabolic activation, antibody directed against cytochrome P450IA1 inhibited the activation of 2-aminoanthracene but not of 6-aminochrysene. In contrast, antibody directed against cytochrome P450IIB1 inhibited the activation of 6-aminochrysene but not of 2-aminoanthracene. These results suggest that under conditions in which the amounts of S9 added are rate-limiting, the two aromatic amines are preferentially activated by different induced forms of cytochrome P-450.

Effects of dietary Schizandra chinensis, brussels sprouts and Illicium verum extracts on carcinogen metabolism systems in mouse liver

Ethanol extracts of Brussels sprouts (BRX), Schizandra chinensis (SZX) or Illicium verum (IVX) were added to a semi-purified basal diet and fed to adult male and female C57B1/6 mice for 14 and 10 days, respectively. Other groups received the unsupplemented basal diet or a mouse chow. Liver fractions were prepared from these mice to investigate the effects of the diets on the enzyme systems involved in benzo[a]pyrene (BP) and aflatoxin B1 (AFB1) metabolism. The effects of the microsomal or cytosolic liver fractions on the in vitro mutagenicity of BP and AFB1 and on the DNA binding of AFB1 were also studied. There were several apparently sex-related differences in the responses of the monooxygenase system components measured. In males, cytochrome P-450 levels were significantly increased only in the chow group, while significant increases in both 7-ethoxycoumarin O-deethylase (ECD) and aryl hydrocarbon hydroxylase (AHH) activities were seen only in the SZX group. In females, cytochrome P-450 levels were significantly increased in both the BRX and SZX groups, whereas AHH activity was significantly increased only in the chow and BRX groups and ECD activity was increased in the SZX and IVX groups. Microsomal epoxide hydratase (EH) was induced in males in the SZX and IVX groups and in females only in the SZX group, while cytosolic EH was significantly increased only in IVX males. Diet-induced changes in monooxygenase activities were found to be the best indicators of changes in microsome-mediated BP mutagenesis and AFB1 mutagenesis and binding to DNA in vitro, with a direct correlation between high AHH and/or ECD activities and the levels of mutagenic response to BP or AFB1 in the Ames assay and of DNA binding of AFB1.

The effects of dietary brussels sprouts and Schizandra chinensis on the xenobiotic-metabolizing enzymes of the rat small intestine

After an initial equilibration period of 7 days on a semi-synthetic basal diet, male Sprague-Dawley rats were fed for 2 wk on either the basal diet (controls), the basal diet containing 5% Schizandra chinensis or 25% Brussels sprouts, or on rat chow. One group of chow-fed rats was pretreated with 20 mg 3-methylcholanthrene (3-MC)/kg body weight, 24 hr before they were killed. Microsomal and cytosolic fractions were prepared from small intestine mucosa. Microsomes were assayed for cytochrome P-450, aryl hydrocarbon hydroxylase (AHH), ethoxycoumarin O-deethylase (ECD) and epoxide hydrolase (EH) activities, and for metabolism of benzo[a]pyrene (BaP), Cytosols were assayed for glutathione S-transferase (GST) activity. The largest increase in intestinal mixed-function oxidase activity over levels in the controls was seen in the 3-MC-treated group. However, EH and GST activities in these animals were not significantly increased. Increases in cytochrome P-450 levels and significant increases in AHH, ECD, EH and GST activities occurred in the rats fed Brussels sprouts. Rats in the S. chinensis group showed inhibition of AHH activity relative to controls, but increased activity of ECD, EH and GST. In the rats fed chow there were significant increases in the activities of all the enzymes assayed except GST. The percentage conversion of BaP to metabolites reflected the results of the AHH assay and the groups were ranked in the following order: 3-MC greater than Brussels sprouts greater than rat chow greater than basal diet greater than S. chinensis. The profile of BaP metabolites showed a larger proportion of the BaP diols and 3-hydroxybenzo[a]pyrene, and a smaller proportion of BaP-4,5-epoxide and the BaP quinones, for the Brussels sprouts- and S. chinensis-fed groups. The significance of these results is discussed in regard to the role of the small intestine as a mediator of toxicity induced by ingested chemicals.

Kinetics of benzo[a]pyrene induced mutagenesis in a highly sensitive salmonella/microsome assay

The kinetics of benzo[a]pyrene induced mutagenesis in Salmonella typhimurium TA98 and TA100 were studied using a liquid-phase assay and were compared to kinetics of metabolism of benzo[a]pyrene in the same system. Mutagenesis was terminated by the addition of pyrene + menadione at selected times. Under conditions of the assay using hepatic microsomes from 3-methyl-cholanthrene pretreated rats, metabolism of benzo[a]pyrene was over 50% complete in 5 min, and metabolism of initial metabolites became appreciable after this time. The revertant frequency (revertants/10(8) survivors) and the production of tertiary metabolites increased sharply after 5 min and leveled off at 40 min.

A benzo[alpha]pyrene-7,8-dihydrodiol-9,10-epoxide is the major metabolite involved in the binding of benzo[alpha]pyrene to DNA in isolated viable rat hepatocytes

Benzo[alpha]pyrene is metabolised by isolated viable hepatocytes from both untreated and 3-methylcholanthrene pretreated rats to reactive metabolites which covalently bind to DNA. The DNA from the hepatocytes was isolated, purified and enzymically hydrolysed to deoxyribonucleosides. The hydrocarbon-deoxyribonucleoside products after initial separation, on small columns of Sephadex LH-20, from unhydrolysed DNA, oligonucleotides and free bases, were resolved by high pressure liquid chromatography (HPLC). The qualitative nature of the adducts found in both control and pretreated cells was virtually identical; however pretreatment with 3-methylcholanthrene resulted in a quantitatively higher level of binding. The major hydrocarbon-deoxyribonucleoside adduct, found in hepatocytes co-chromatographed with that obtained following reaction of the diol-epoxide, (+/-) 7 alpha,8 beta-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydrobenzo[alpha]pyrene with DNA. Small amounts of other adducts were also present including a more polar product which co-chromatographed with the major hydrocarbon-deoxyribonucleoside adduct formed following microsomal activation of 9-hydroxybenzo[alpha]-pyrene and subsequent binding to DNA. In contrast to the results with hepatocytes, when microsomes were used to metabolically activate benzo[alpha]-pyrene, the major DNA bound-product co-chromatographed with the more polar adduct formed upon further metabolism of 9-hydroxybenzo[alpha]pyrene. These results illustrate that great caution must be exercised in the extrapolation of results obtained from short-term mutagenesis test systems, utilising microsomes, to in vivo carcinogenicity studies.