Method for estimation of benzo[a]pyrene DNA adducts.

Polycyclic aromatic hydrocarbons, e.g., benzo[a]pyrene (B(a)P) are known carcinogens/mutagens. These compounds may be metabolized by the P450 mixed function monooxygenase to more nucleophilic compounds which may form adducts to the cellular macromolecules, e.g., DNA, RNA, and proteins. We have used synchronous fluorescence scanning for the assay of DNA adduct formation. In our earlier work with in vitro exposed human lymphocytes we estimated the adduct formation (femtomoles B(a)P per microgram DNA) to be higher than that estimated by other workers. We suggested that this difference may be related to the DNA isolation method used. In order to elucidate these differences we compared DNA adduct formation in human lymphocytes where DNA was isolated by the two different methods, i.e., using phenol extraction or the Gene Clean method. The data demonstrate that the phenol extraction procedure gives a yield of adducts per microgram DNA lower than that obtained by the Gene Clean method. The principle of the Gene Clean method for DNA isolation is protein denaturation by means of NaI followed by catching of DNA by absorption on silica particles. In contrast, the phenol extraction method is based upon phenol-mediated denaturation of proteins in the cell lysate leaving the hydrophilic nucleotides in the aqueous phase. However, during adduct formation more lipophilic adducts derived from DNA may redistribute between the aqueous phase and the phenol phase. In support of this theory we found higher adduct concentration per microgram DNA by the Gene Clean method 40 to 60 times than that found by the phenol method.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dioxin suppresses benzo[a]pyrene-induced mutations and DNA adduct formation through cytochrome P450 1A1 induction and (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide inactivation in human hepatoma cells

Benzo[a]pyrene (BaP) is metabolically activated by cytochrome P450 enzymes, and forms DNA adduct leading to mutations. Cytochrome P450 1A1 plays a central role in this activation step, and this enzyme is strongly induced by chemical agents that bind to the aryl hydrocarbon receptor (AhR), which is also known as a dioxin receptor. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a potent AhR ligand has not been shown to form any DNA adduct, but has a possibility to aggravate the toxicity of precarcinogenic polycyclic hydrocarbons through the induction of metabolic enzymes. We treated human hepatoma cells (HepG2) with TCDD, and subsequently exposed them to BaP to elucidate the synergistic effects on mutations. Surprisingly, mutant frequency induced by BaP at the hypoxanthine-guanine phosphribosyltransferase (HPRT) locus was decreased by pretreatment with TCDD. In correlation with decrease in the mutant frequencies, BaP–DNA adduct formation was also decreased by TCDD pretreatment. This suppressive effect of TCDD was more potent when the cells were exposed to (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), a reactive metabolic intermediate of BaP. Among the enzymes catalyzing BaP oxidation and conjugation, cytochrome P450 1A1, 1A2, 3A4 and UDP-glucuronosyltransferase 1A1 mRNAs were induced by the exposure to TCDD. In cytochrome P450 1A1-deficient murine cells and cytochrome P450 1A1-uninducible human cells, TCDD could not suppress BPDE–DNA adduct formation. Further experiments using “Tet-On” cytochrome P450 1A1-overexpressing cells and a recombinant cytochrome P450 1A1 enzyme demonstrated that this is the key enzyme involved in the biotransformation of BaP, that is, both production and inactivation of BPDE. We conclude that TCDD-induced cytochrome P450 catalyzes the metabolism of BPDE to as yet-unidentified products that are not apparently DNA-reactive, thereby reducing mutations in hepatoma cells.     

Differential response to benzo[A]pyrene in human lung adenocarcinoma cell lines: the absence of aryl hydrocarbon receptor activation.

Benzo[a]pyrene (B[a]P) has been shown to produce DNA adducts and to initiate pulmonary carcinogenesis in animals. We observed differential susceptibility to B[a]P in two human lung adenocarcinoma cell lines, A427 and CL3. DNA adducts were induced by B[a]P treatment in CL3 cells, however, A427 cells were much less responsive to B[a]P treatment. Cytochrome P450 1A1 (CYP1A1) is involved in bioactivation of B[a]P in nonhepatic tissues. Cotreatment with alpha-naphthoflavone, a CYP1A1 inhibitor, abolished DNA adduct formation by B[a]P in CL3 cells. Nevertheless, CYP1A1 inducer beta-naphthoflavone, enhanced DNA adduct formation by B[a]P in both A427 and CL3 cells. Both enzyme activity and mRNA levels of CYP1A1 were highly induced by 1 or 10 microM B[a]P treatment for 24 hr in CL3 cells but not in A427 cells. Protein levels of AhR and aryl hydrocarbon receptor nuclear translocator (Arnt) were similar in A427 and CL3 cells before B[a]P treatment. However, B[a]P induced a retarded band with the [32P]-dioxin responsive element in CL3 cells, but not in A427 cells. This study demonstrated that variation in AhR-mediated CYP1A1 induction contributes to differential susceptibility to B[a]P-DNA adduct formation in human lung cells. Since AhR and/or Arnt function is impaired in A427 cells, this cell line offers a model for investigating the repression mechanisms of CYP1A1 induction by B[a]P in lung cells.     

Toxicity of Trp-P-2 to cultured human and rat keratinocytes

Keratinocytes cultured from human and rat epidermis exhibited strongly divergent sensitivities to toxicity from the heterocyclic amine food mutagen Trp-P-2. To find a biochemical basis for this difference, the cultured cells were compared in their expression of phase 1 and 2 biotransformation activities, mutagenic activation and macromolecular adducts. The human and early passage rat cells expressed similar levels of ethoxyresorufin O-deethylase and N-acetyl transferase activities, their microsomes were similarly active in inducing bacterial mutagenesis when incubated with Trp-P-2, and the keratinocytes accumulated similar levels of DNA adducts over a 4-day treatment period. However, the human cells expressed an order of magnitude higher cytosolic glutathione S-transferase activity than the rat cells, likely providing enhanced protection. Late passage rat epidermal cells were insensitive to Trp-P-2 toxicity, attributable to their rapid loss of measured cytochrome P450 activity. Rat esophageal and fore-stomach epithelial cells resembled late passage rat epidermal cells in their lack of sensitivity to Trp-P-2 toxicity and lack of P450 activity. Human esophageal epithelial cells expressed substantial P450 activity but, in contrast to human epidermal cells, were sensitive to Trp-P-2 toxicity. Thus keratinocytes provide a valuable system in which to examine the basis for species- and tissue-specific differences in toxicity from this carcinogenic heterocyclic amine.     

Repair kinetics of trans-4-hydroxynonenal-induced cyclic 1,N2-propanodeoxyguanine DNA adducts by human cell nuclear extracts

trans-4-Hydroxynonenal (HNE) is a major peroxidation product of omega-6 polyunsaturated fatty acids. The reaction of HNE with DNA produces four diastereomeric 1,N(2)-gamma-hydroxypropano adducts of deoxyguanosine (HNE-dG); background levels of these adducts have been detected in tissues of animals and humans. There is evidence to suggest that these adducts are mutagenic and involved in liver carcinogenesis in patients with Wilson's disease and in other human cancers. Here, we present biochemical evidence that in human cell nuclear extracts the HNE-dG adducts are repaired by the nucleotide excision repair (NER) pathway. To investigate the recognition and repair of HNE-dG adducts in human cell extracts, we prepared plasmid DNA substrates modified by HNE. [(32)P]-Postlabeling/HPLC determined that the HNE-dG adduct levels were approximately 1200/10(6) dG of plasmid DNA substrate. We used this substrate in an in vitro repair-synthesis assay to study the complete repair of HNE-induced DNA adducts in cell-free extracts. We observed that nuclear extracts from HeLa cells incorporated a significant amount of alpha[(32)P]dCTP in DNA that contained HNE-dG adducts by comparison with UV-irradiated DNA as the positive control. Such repair synthesis for UV damage or HNE-dG adducts did not occur in XPA cell nuclear extracts that lack the capacity for NER. However, XPA cells complemented with XPA protein restored repair synthesis for both of these adducts. To verify that HNE-dG adducts in DNA were indeed repaired, we measured HNE-dG adducts in the post-repaired DNA substrates by the [(32)P]-postlabeling/HPLC method, showing that 50-60% of HNE-dG adducts were removed from the HeLa cell nuclear extracts after 3 h at 30 degrees C. The repair kinetics indicated that the excision rate is faster than the rate of gap-filling/DNA synthesis. Furthermore, the HNE-dG adduct isomers 2 and 4 appeared to be repaired more efficiently at early time points than isomers 1 and 3.     

Metabolism of and DNA adduct formation by benzo[alpha]pyrene in human skin epithelial cells in vitro pretreated with cytochrome P450 modulators

This study was designed to investigate the effects of four compounds that are shown to influence the cytochrome P450 system, on the metabolism of and DNA adduct formation by benzo[alpha]pyrene (BaP) in human skin epithelial cells in culture. Radiolabeled BaP was used in the metabolism studies, and the levels of metabolites in the ethylacetate extracts of the intracellular and extracellular fractions were determined by HPLC. Among the various metabolites detected BaP-7,8-diol was the only one that was an intermediate on the activation pathway of BaP to the ultimate carcinogen, BPDE I. Both BHA and 7,8-BF pretreatment significantly decreased intracellular production of BaP-7,8-diol compared to cultures treated with only radiolabeled BaP. MeBHA pretreatment greatly increased intracellular BaP-7,8-diol formation compared to BaP treated controls, while disulfiram pretreatment had no effect on the intracellular concentration. Cultures pretreated with BHA, 7,8-BF or disulfiram formed 30-40% less BPDE I-dG adducts than nonpretreated cultures, while cultures pretreated with MeBHA exhibited approximately 200% increase in the BPDE I-dG adduct formation. Thus, BHA and 7,8-BF act similarly in reducing BaP activation and adduct formation. Alternatively, MeBHA increased BaP activation and adduct formation in human keratinocyte cultures in vitro. Disulfiram pretreatment did not reduce BaP-7,8-diol formation, but decreased BPDE I-dG adducts. These studies indicate that modulators of the P450 system act in different fashions at the level of production of an oxygenated procarcinogen metabolite, altering the amount of specific carcinogen-dG adducts that lead to the expression of a transformed phenotype.     

Binding of 3-hydroxybenzo[a]pyrene to bovine hemoglobin and albumin

Previous studies examined the bioavailability and first-pass biotransformation of 3-hydroxy[(3)H]benzo[a]pyrene ([(3)H]-3-OHBaP) in an isolated perfused catfish intestinal model. This work showed that 3-OHBaP, or a metabolite formed in intestine, bound covalently to blood protein. In this study, the blood adducts were characterized in vitro by incubating bovine ferric hemoglobin or albumin with [(3)H]-3OHBaP under various conditions. Incubation of 2 microM [(3)H]-3-OHBaP with hemoglobin for 1 h resulted in 7.49 pmol bound/mg protein, while albumin binding was 1.37 pmol/mg protein. Mild acid hydrolysis released only 5% of the radioactivity from 3-OHBaP-hemoglobin adducts. After gel filtration, the 3-OHBaP-hemoglobin adducts were examined by HPLC analysis. A single peak of radioactivity was detected at the same retention time as the heme component of hemoglobin. Unbound 3-OHBaP was oxidized to BaP-3,6-dione during incubation with ferric hemoglobin. Treatment of hemoglobin with ascorbic acid decreased the formation of hemoglobin adducts by 33%, while hydrogen peroxide treatment increased adduct formation by 44%. Incubation of [(3)H]-BaP-3-beta-D-glucuronide (BaP-3G) with hemoglobin and beta-glucuronidase resulted in greater binding to hemoglobin than incubation with [(3)H]-3-OHBaP alone. The hemoglobin adduct obtained from [(3)H]-BaP-3G also co-migrated with heme. These results indicate that an oxidative process is involved in formation of the heme adduct and that 3-OHBaP or BaP-3G might be a precursor of the bound metabolite.     

Effects of benzo[a]pyrene-DNA adducts on a reconstituted replication system.

We have used a partially reconstituted replication system consisting of T7 DNA polymerase and T7 gene 4 protein to examine the effect of benzo[a]pyrene (B[a]P) adducts on DNA synthesis and gene 4 protein activities. The gene 4 protein is required for T7 DNA replication because of its ability to act as both a primase and helicase. We show here that total synthesis decreases as the level of adducts per molecule of DNA increases, suggesting that the B[a]P adducts are blocking an aspect of the replication process. Polyacrylamide gels indicate that a shorter DNA product is produced on modified templates and this is confirmed by determining the average chain lengths from the ratio of chain initiations to chain elongation. Gene 4 protein primed synthesis reactions display a greater sensitivity to the presence of B[a]P adducts than do oligonucleotide-primed reactions. By challenging synthesis on oligonucleotide-primed B[a]P-modified DNA with unmodified DNA, we present evidence that the T7 DNA polymerase freely dissociates after encountering an adduct. Prior studies [Brown, W. C., & Romano, L. J. (1989) J. Biol. Chem. 264, 6748-6754] have shown that the gene 4 protein alone does not dissociate from the template during translocation upon encountering an adduct. However, when gene 4 protein primed DNA synthesis is challenged, we observe an increase in synthesis but to lesser extent than observed on oligonucleotide-primed synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Non-linear production of benzene oxide-albumin adducts with human exposure to benzene

Benzene is initially metabolized to benzene oxide, which either undergoes further metabolism or reacts with macromolecules including proteins. Previously reported levels of benzene oxide-albumin adducts (BO-Alb) are analyzed from 30 workers exposed to 0.2-302 ppm benzene and 43 controls from Shanghai, China. Although both exposed workers and controls had significant levels of BO-Alb in their blood, exposed subjects' adduct levels (GM=378 pmol/g protein) were much greater than those of controls (GM=115 pmol/g protein). When the natural logarithm of the BO-Alb level was regressed upon the natural logarithm of exposure among the 30 exposed subjects, a strong effect of benzene exposure was observed (R(2)=0.612; p<0.0001). Because the slope of the relationship between BO-Alb and benzene exposure was significantly less than one in log-space, we infer that production of benzene oxide was less than proportional to benzene exposure. Since benzene is a substrate for CYP2E1, these results are consistent with saturation of CYP450 metabolism. They indicate that deviations from linear metabolism began at or below benzene exposures of 10 ppm and that pronounced saturation was apparent at 40-50 ppm. To our knowledge, this is the first study to investigate the linearity of human metabolism of a carcinogen based upon protein adducts.     

Induction of aryl hydrocarbon hydroxylase and DNA adduct formation in parental and carcinogen transformed C3H/10T1/2 clone 8 cells by benzo[a]pyrene

C3H/10T1/2 clone 8 (10T1/2) cells possess aryl hydrocarbon hydroxylase (AHH) activity capable of metabolizing polycyclic aromatic hydrocarbons to ultimate carcinogenic forms. AHH activity in 10T1/2 cells was measured before and after culturing in the presence of benzo[a]pyrene (B[a]P), and compared to the AHH activity found in carcinogen-transformed 10T1/2 cell lines treated similarly. The cell lines were also examined for B[a]P-DNA adduct formation, using the 32P-postlabelling technique. Treatment of parental 10T1/2 cells with B[a]P was found to significantly increase AHH activity and produce substantial numbers of DNA adducts. In addition to a major B[a]P-DNA adduct, 5-6 minor DNA adducts were also detected. Relative to parental 10T1/2 cells, an aflatoxin B1-transformed 10T1/2 cell line (7SA) was found to have significantly depressed AHH activity. In addition, after treatment with B[a]P, 7SA cells had only 8% of the B[a]P-DNA adduct levels found in 10T1/2 cells. This system may provide an in vitro model for investigating mechanisms responsible for the depression of cytochrome P-450 activities by chemical carcinogens.     

Ubiquitous binding of benzo[a]pyrene metabolites to DNA and protein in tissues of the mouse and rabbit

The in vivo formation of benzo[alpha]pyrene (BP) metabolite-DNA adducts in several tissues of mice and rabbits was examined. Included were tissues with widely divergent xenobiotic metabolizing capabilities such as liver and brain. The major adduct identified in each tissue was the (+)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydro-BP (BPDEI)-deoxyguanosine adduct. A 7 beta,8 alpha-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydro-BP (BPDEII)-deoxyguanosine adduct, a (-)-BPDEI-deoxyguanosine adduct and an unidentified adduct were also observed. These adducts were present in all of the tissues of the mice and in the lungs of the rabbits; only BPDEI and BPDEII were seen in the rest of the rabbit tissues. In all of the tissues studied, the DNA adduct levels were unexpectedly similar. For example, the BPDEI-DNA adduct levels in muscle and brain of mice were approx. 50% of those in lung and liver at each oral BP dose examined. After an i.v. dose of BP in rabbits, the BPDEI adduct levels in lung were three times those in brain or liver and twice those in muscle. The binding of BP metabolites to protein was also determined in these tissues. The tissue-to-tissue variation in protein binding levels of BP metabolites was greater than that for BPDEI-DNA adducts. There are several possible explanations for the in vivo binding of BP metabolites to DNA and protein of various tissues. First, oxidative metabolism of BP in each of the examined tissues might account for the observed binding. Second, reactive metabolites could be formed in tissues such as liver and lung and be transported to cells in tissues such as muscle and brain where they bind to DNA and protein. In any case, the tissue-to-tissue variations in protein and DNA binding of BP-derived radioactivity do not correlate with differences in cytochrome P-450 activity.     

Biotransformation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by a rabbit liver cytochrome P450: insight into the mechanism of RDX biodegradation by Rhodococcus sp. strain DN22

A unique metabolite with a molecular mass of 119 Da (C(2)H(5)N(3)O(3)) accumulated during biotransformation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Rhodococcus sp. strain DN22 (D. Fournier, A. Halasz, J. C. Spain, P. Fiurasek, and J. Hawari, Appl. Environ. Microbiol. 68:166-172, 2002). The structure of the molecule and the reactions that led to its synthesis were not known. In the present study, we produced and purified the unknown metabolite by biotransformation of RDX with Rhodococcus sp. strain DN22 and identified the molecule as 4-nitro-2,4-diazabutanal using nuclear magnetic resonance and elemental analyses. Furthermore, we tested the hypothesis that a cytochrome P450 enzyme was responsible for RDX biotransformation by strain DN22. A cytochrome P450 2B4 from rabbit liver catalyzed a very similar biotransformation of RDX to 4-nitro-2,4-diazabutanal. Both the cytochrome P450 2B4 and intact cells of Rhodococcus sp. strain DN22 catalyzed the release of two nitrite ions from each reacted RDX molecule. A comparative study of cytochrome P450 2B4 and Rhodococcus sp. strain DN22 revealed substantial similarities in the product distribution and inhibition by cytochrome P450 inhibitors. The experimental evidence led us to propose that cytochrome P450 2B4 can catalyze two single electron transfers to RDX, thereby causing double denitration, which leads to spontaneous hydrolytic ring cleavage and decomposition to produce 4-nitro-2,4-diazabutanal. Our results provide strong evidence that a cytochrome P450 enzyme is the key enzyme responsible for RDX biotransformation by Rhodococcus sp. strain DN22.     

Short Communication: The effect of CYP1A1 induction on the formation of benzo a pyrene adducts in liver and lung DNA and plasma albumin in rats exposed to benzo a pyrene:adduct quantitation by immunoassay and an HPLC method

Induction of cytochrome P450 enzymes by exposure to polycyclic aromatic hydrocarbons (PAH) can result in both decreased or increased PAH adduct levels. The lung is a main target site for PAH-carcinogenesis. By HPLC determination of B[ a]P-r-7, t-8-dihydrodiol, t-9, 10-epoxide (BPDE-I)-DNA adducts in rat, the level of the ultimate carcinogenic B[a]P-metabolite was higher in lungs than in liver. However, measured by immunoassay, the total benzo[a]pyrene (B[a]P)-DNA adduct levels were higher in liver than in lungs. Induction of CYP1A1 in vivo in rat by repeated i.p. doses of methylcholanthrene (MC) prior to a single dose of B[a]P resulted in a 2.4 times increase in CYP1A1 activity in liver tissue and 1.5 times higher levelsof total B[a]P-DNA adducts in lung and liver compared with controls which only received B[a]P. Increased levels of BPDE-I-DNA adducts were significantly correlated to increased CYP1A1 activity in induced lung tissue but not in liver. The times to reach maximum adduct levels were similar for both controls and MC-induced rats in both lung and liver,and plasma albumin. The BPDE-I-albumin adducts reached a maximum level around 1 day after B[a]P exposure and could not be used as a reliable marker of the short term PAH exposure in this study.     

Short Communication: The effect of CYP1A1 induction on the formation of benzo a pyrene adducts in liver and lung DNA and plasma albumin in rats exposed to benzo a pyrene:adduct quantitation by immunoassay and an HPLC method

Induction of cytochrome P450 enzymes by exposure to polycyclic aromatic hydrocarbons (PAH) can result in both decreased or increased PAH adduct levels. The lung is a main target site for PAH-carcinogenesis. By HPLC determination of B [a]P-r-7, t-8-dihydrodiol, t-9, 10-epoxide (BPDE-I)-DNA adducts in rat, the level of the ultimate carcinogenic B[a]P-metabolite was higher in lungs than in liver. However, measured by immunoassay, the total benzo[a]pyrene (B[a]P)-DNA adduct levels were higher in liver than in lungs. Induction of CYP1A1 in vivo in rat by repeated i.p. doses of methylcholanthrene (MC) prior to a single dose of B[a]P resulted in a 2.4 times increase in CYP1A1 activity in liver tissue and 1.5 times higher levelsof total B[a]P-DNA adducts in lung and liver compared with controls which only received B[a]P. Increased levels of BPDE-I-DNA adducts were significantly correlated to increased CYP1A1 activity in induced lung tissue but not in liver. The times to reach maximum adduct levels were similar for both controls and MC-induced rats in both lung and liver,and plasma albumin. The BPDE-I-albumin adducts reached a maximum level around 1 day after B[a]P exposure and could not be used as a reliable marker of the short term PAH exposure in this study.     

Comparison of the genotoxic activities of the K-region dihydrodiol of benzo[a]pyrene with benzo[a]pyrene in mammalian cells: morphological cell transformation; DNA damage; and stable covalent DNA adducts

Benzo[a]pyrene (B[a]P) is the most thoroughly studied polycyclic aromatic hydrocarbon (PAH). Many mechanisms have been suggested to explain its carcinogenic activity, yet many questions still remain. K-region dihydrodiols of PAHs are metabolic intermediates depending on the specific cytochrome P450 and had been thought to be detoxification products. However, K-region dihydrodiols of several PAHs have recently been shown to morphologically transform mouse embryo C3H10T1/2CL8 cells (C3H10T1/2 cells). Because K-region dihydrodiols are not metabolically formed from PAHs by C3H10T1/2 cells, these cells provide a useful tool to independently study the mechanisms of action of PAHs and their K-region dihydrodiols. Here, we compare the morphological cell transforming, DNA damaging, and DNA adducting activities of the K-region dihydrodiol of B[a]P, trans-B[a]P-4,5-diol with B[a]P. Both trans-B[a]P-4,5-diol and B[a]P morphologically transformed C3H10T1/2 cells by producing both Types II and III transformed foci. The morphological cell transforming and cytotoxicity dose response curves for trans-B[a]P-4,5-diol and B[a]P were indistinguishable. Since morphological cell transformation is strongly associated with mutation and/or larger scale DNA damage in C3H10T1/2 cells, the identification of DNA damage induced in these cells by trans-B[a]P-4,5-diol was sought. Both trans-B[a]P-4,5-diol and B[a]P exhibited significant DNA damaging activity without significant concurrent cytotoxicity using the comet assay, but with different dose responses and comet tail distributions. DNA adduct patterns from C3H10T1/2 cells were examined after trans-B[a]P-4,5-diol or B[a]P treatment using 32P-postlabeling techniques and improved TLC elution systems designed to separate polar DNA adducts. While B[a]P treatment produced one major DNA adduct identified as anti-trans-B[a]P-7,8-diol-9,10-epoxide-deoxyguanosine, no stable covalent DNA adducts were detected in the DNA of trans-B[a]P-4,5-diol-treated cells. In summary, this study provides evidence for the DNA damaging and morphological cell transforming activities of the K-region dihydrodiol of B[a]P, in the absence of covalent stable DNA adducts. While trans-B[a]P-4,5-diol and B[a]P both induce morphological cell transformation, their activities as DNA damaging agents differ, both qualitatively and quantitatively. In concert with the morphological cell transformation activities of other K-region dihydrodiols of PAHs, these data suggest a new mechanism/pathway for the morphological cell transforming activities of B[a]P and its metabolites.     

Thiolate coordination to Fe(II)-porphyrin NO centers

The interaction of the Fe(II)-porphyrin NO model complex [Fe(TPP)(NO)] (1, TPP=tetraphenylporphyrin) with thiophenolate ligands and tetrahydrothiophene is explored both computationally and experimentally. Complex 1 is reacted with substituted thiophenolates and the obtained six-coordinate adducts of type [Fe(TPP)(SR)(NO)](-) are investigated in solution using electron paramagnetic resonance (EPR) spectroscopy. From the obtained g values and (14)N hyperfine pattern of the NO ligand it is concluded that the interaction of the thiophenolates with the Fe(II) center is weak in comparison to the corresponding 1-methylimidazole adduct. The strength of the Fe-S bond is increased when alkylthiolates are used as evidenced by comparison with the published EPR spectra of ferrous NO adducts in cytochromes P450 and P450nor, which have an axial cysteinate ligand. These results are further evaluated by density functional (DFT) calculations. The six-coordinate model complex [Fe(P)(SMe)(NO)](-) (1-SMe; P=porphine ligand used for the calculations) has an interesting electronic structure where NO acts as a medium strong sigma donor and pi acceptor ligand. Compared to the N-donor adducts with 1-methylimidazole (1-MeIm), etc., donation from the pi(h)( *) orbital of NO to Fe(II) is reduced due to the stronger trans effect of the alkylthiolate ligand. This is reflected by the predicted longer Fe-NO bond length and smaller Fe-NO force constant for 1-SMe compared to the 1-MeIm adduct. Therefore, the Fe(II)-porphyrin NO adducts with trans alkylthiolate coordination have to be described as Fe(II)-NO(radical) systems. The N-O stretching frequency of these complexes is predicted below 1600cm(-1) in agreement with the available experimental data. In addition, 1-SMe has a unique spin density distribution where Fe has a negative spin density of -0.26 from the calculations. The implications of this unusual electronic structure for the reactivity of the Fe(II)-NO alkylthiolate adducts as they occur in cytochrome P450nor are discussed.     

Inhibition of Werner syndrome helicase activity by benzo[a]pyrene diol epoxide adducts can be overcome by replication protein A

RecQ helicases are believed to function in repairing replication forks stalled by DNA damage and may also play a role in the intra-S-phase checkpoint, which delays the replication of damaged DNA, thus permitting repair to occur. Since little is known regarding the effects of DNA damage on RecQ helicases, and because the replication and recombination defects in Werner syndrome cells may reflect abnormal processing of damaged DNA associated with the replication fork, we examined the effects of specific bulky, covalent adducts at N(6) of deoxyadenosine (dA) or N(2) of deoxyguanosine (dG) on Werner (WRN) syndrome helicase activity. The adducts are derived from the optically active 7,8-diol 9,10-epoxide (DE) metabolites of the carcinogen benzo[a]pyrene (BaP). The results demonstrate that WRN helicase activity is inhibited in a strand-specific manner by BaP DE-dG adducts only when on the translocating strand. These adducts either occupy the minor groove without significant perturbation of DNA structure (trans adducts) or cause base displacement at the adduct site (cis adducts). In contrast, helicase activity is only mildly affected by intercalating BaP DE-dA adducts that locally perturb DNA double helical structure. This differs from our previous observation that intercalating dA adducts derived from benzo[c]phenanthrene (BcPh) DEs inhibit WRN activity in a strand- and stereospecific manner. Partial unwinding of the DNA helix at BaP DE-dA adduct sites may make such adducted DNAs more susceptible to the action of helicase than DNA containing the corresponding BcPh DE-dA adducts, which cause little or no destabilization of duplex DNA. The single-stranded DNA binding protein RPA, an auxiliary factor for WRN helicase, enabled the DNA unwinding enzyme to overcome inhibition by either the trans-R or cis-R BaP DE-dG adduct, suggesting that WRN and RPA may function together to unwind duplex DNA harboring specific covalent adducts that otherwise block WRN helicase acting alone.     

Biotransformation of Hexahydro-1,3,5-Trinitro-1,3,5-Triazine (RDX) by a Rabbit Liver Cytochrome P450: Insight into the Mechanism of RDX Biodegradation by Rhodococcus sp. Strain DN22

A unique metabolite with a molecular mass of 119 Da (C₂H₅N₃O₃) accumulated during biotransformation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Rhodococcus sp. strain DN22 (D. Fournier, A. Halasz, J. C. Spain, P. Fiurasek, and J. Hawari, Appl. Environ. Microbiol. 68:166-172, 2002). The structure of the molecule and the reactions that led to its synthesis were not known. In the present study, we produced and purified the unknown metabolite by biotransformation of RDX with Rhodococcus sp. strain DN22 and identified the molecule as 4-nitro-2,4-diazabutanal using nuclear magnetic resonance and elemental analyses. Furthermore, we tested the hypothesis that a cytochrome P450 enzyme was responsible for RDX biotransformation by strain DN22. A cytochrome P450 2B4 from rabbit liver catalyzed a very similar biotransformation of RDX to 4-nitro-2,4-diazabutanal. Both the cytochrome P450 2B4 and intact cells of Rhodococcus sp. strain DN22 catalyzed the release of two nitrite ions from each reacted RDX molecule. A comparative study of cytochrome P450 2B4 and Rhodococcus sp. strain DN22 revealed substantial similarities in the product distribution and inhibition by cytochrome P450 inhibitors. The experimental evidence led us to propose that cytochrome P450 2B4 can catalyze two single electron transfers to RDX, thereby causing double denitration, which leads to spontaneous hydrolytic ring cleavage and decomposition to produce 4-nitro-2,4-diazabutanal. Our results provide strong evidence that a cytochrome P450 enzyme is the key enzyme responsible for RDX biotransformation by Rhodococcus sp. strain DN22.     

Plasmodium falciparum: DNA sequence specificity of cisplatin and cisplatin analogues

In this paper, we provided evidence that cisplatin is able to form adducts with cellular DNA in Plasmodium falciparum. The DNA sequence specificity of cisplatin adduct formation was determined in trophozoite-enriched P. falciparum cells and this paper represents the first occasion that the sequence specificity of cisplatin DNA damage has been observed in malaria cells. Utilising a sub-telomeric, 692 bp repeat sequence in the P. falciparum genome, we were able to investigate the DNA adducts formed by cisplatin and five analogues. A run of eight consecutive guanines was the most prominent site of DNA damage in the malarial cells. This study suggests that the mechanism of P. falciparum cell death caused by cisplatin involves damage to DNA and hence inhibition of DNA replication and cell division.