Epoxides in polycyclic aromatic hydrocarbon metabolism and carcinogenesis

Detoxication of carcinogenic fjord-region diol epoxides of polycyclic aromatic hydrocarbons by glutathione transferase P1-1 variants and glutathione

Epidemiological studies suggest that individuals differing in the expression of allelic variants of the human glutathione transferase (GST) Pi gene differ in susceptibility to chemical carcinogens such as polycyclic aromatic hydrocarbons (PAH). This study reports the catalytic efficiencies (k(cat)/Km) of two naturally occurring variants, GSTP1-1/I-105 and GSTP1-1/ V-105, towards a series of fjord-region diol epoxides representing potent biologically active PAH metabolites, and two GSTP1-1 mutants with Ala105 and Trp105 in the active site. The results indicate that individuals who are homozygous for the allele encoding GSTP1-1/V-105 might be more susceptible to PAH carcinogenesis due to other reasons than a reduced capacity for detoxifying diol epoxides.

Differential enantioselectivity of murine glutathione S-transferase isoenzymes in the glutathione conjugation of trans-3,4-dihydroxy-1, 2-oxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene stereoisomers

The kinetics of the glutathione (GSH) conjugation of (+)- and (-)-enantiomers of anti- as well as syn-3,4-dihydroxy-1,2-oxy-1,2,3, 4-tetrahydrobenzo[c]phenanthrene (B[c]PDE) catalyzed by murine GSH S-transferase (GST) isoenzymes has been investigated. Murine GSTs exhibited significant differences in their enantioselectivity toward B[c]PDE stereoisomers. For example, while pi class isoenzyme mGSTP1-1 was virtually inactive toward stereoisomers with 1S configuration [(-)-syn-and (+)-anti-B[c]PDE], these stereoisomers were good substrates for alpha class isoenzyme mGSTA1-2. When GST activity was measured as a function of varying B[c]PDE concentration (10-320 microM) at a fixed saturating concentration of GSH (2 mM), each isoenzyme examined obeyed Michaelis-Menten kinetics with all four B[c]PDE stereoisomers. Alpha class isoenzyme mGSTA4-4 exhibited negligible activity toward all four stereoisomers of B[c]PDE. The catalytic efficiency of mGSTA1-2 was approximately 1.5- to 15-fold higher than other murine GSTs in the GSH conjugation of (-)-anti-B[c]PDE, which among the four B[c]PDE stereoisomers is the most potent pulmonary carcinogen in the newborn mouse model and a potent skin tumor-initiator. While alpha class isoenzymes mGSTA3-3 and mGSTA1-2 were equally efficient in the GSH conjugation of (+)-anti-B[c]PDE, their catalytic efficiencies toward this stereoisomer were significantly higher than those of mGSTP1-1 and mGSTM1-1. Likewise, mGSTA1-2 was relatively more efficient than other GSTs in the GSH conjugation of both enantiomers of syn-B[c]PDE. In summary, our results indicate that (a) murine GSTs significantly differ in their enantioselectivity in the GSH conjugation of B[c]PDE stereoisomers, which may partially account for the observed differences in the carcinogenic potency of B[c]PDE stereoisomers, and (b) mGSTA1-2 and mGSTA3-3 play a major role in the detoxification of B[c]PDE.

ATP-dependent transport of glutathione conjugate of 7beta, 8alpha-dihydroxy-9alpha,10alpha-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene in murine hepatic canalicular plasma membrane vesicles

Glutathione (GSH) S-transferases (GSTs) have an important role in the detoxification of (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9, 10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE], which is the ultimate carcinogen of benzo[a]pyrene. However, the fate and/or biological activity of the GSH conjugate of (+)-anti-BPDE [(-)-anti-BPD-SG] is not known. We now report that (-)-anti-BPD-SG is a competitive inhibitor (Ki 19 microM) of Pi-class isoenzyme mGSTP1-1, which among murine hepatic GSTs is most efficient in the GSH conjugation of (+)-anti-BPDE. Thus the inhibition of mGSTP1-1 activity by (-)-anti-BPD-SG might interfere with the GST-catalysed GSH conjugation of (+)-anti-BPDE unless one or more mechanisms exist for the removal of the conjugate. The results of the present study indicate that (-)-anti-BPD-SG is transported across canalicular liver plasma membrane (cLPM) in an ATP-dependent manner. The ATP-dependent transport of (-)-anti-[3H]BPD-SG followed Michaelis-Menten kinetics (Km 46 microM). The ATP dependence of the (-)-anti-BPD-SG transport was confirmed by measuring the stimulation of ATP hydrolysis (ATPase activity) by the conjugate in the presence of cLPM protein, which also followed Michaelis-Menten kinetics. In contrast, a kinetic analysis of ATP-dependent uptake of the model conjugate S-[3H](2,4-dinitrophenyl)-glutathione ([3H]DNP-SG) revealed the presence of a high-affinity and a low-affinity transport system in mouse cLPM, with apparent Km values of 18 and 500 microM respectively. The ATP-dependent transport of (-)-anti-BPD-SG was inhibited competitively by DNP-SG (Ki 1.65 microM). Likewise, (-)-anti-BPD-SG was found to be a potent competitive inhibitor of the high-affinity component of DNP-SG transport (Ki 6.3 microM). Our results suggest that GST-catalysed conjugation of (+)-anti-BPDE with GSH, coupled with ATP-dependent transport of the resultant conjugate across cLPM, might be the ultimate detoxification pathway for this carcinogen.

Polycyclic aromatic hydrocarbon-DNA adducts in humans: relevance as biomarkers for exposure and cancer risk

The methodology applied for DNA adducts in humans has become more reliable in recent years, allowing to detect even background carcinogenic adduct levels in environmentally exposed persons. Particularly, combinations of the various methods now allow the elucidation of specific adduct structures with detection limits of 1 adduct in 108 unmodified nucleotides or even lower. The quantification of polycyclic aromatic hydrocarbon-DNA (PAH-DNA) adducts in human tissues and cells has been achieved with a number of highly sensitive techniques: immunoassays and immunocytochemistry using polyclonal or monoclonal antisera specific for DNA adducts or modified DNA, the assay, and adduct identification using physicochemical instrumentation. The results summarized in this review show that PAH-DNA adducts have been detected in a variety of human tissues, including target organs of PAH- and tobacco-associated cancers. Although dosimetry has not always been precise, a large number of data now clearly show that lowering exposure to carcinogenic PAH results in decreasing PAH-DNA adduct levels. In most studies, however, bulk DNA of a certain tissue or cell type has been examined, and there were relatively few studies in which mutations as a consequence of DNA damage at specific genes have been investigated. Promising as these biomarker studies seem for epidemiology and health surveillance, future biomonitoring and molecular epidemiological studies should be directed to combine several endpoint measurements: i.e., adduct formation (preferably at specific sites), mutational spectra in cancer-relevant genes, and genetic markers of (cancer) susceptibility in a number of cancer-predisposing genes.

Differential catalytic efficiency and enantioselectivity of murine glutathione S-transferase isoenzymes in the glutathione conjugation of carcinogenic anti-diol epoxides of chrysene and benzo(g)chrysene

The kinetics of the conjugation of carcinogenic anti-diol epoxides of chrysene (anti-CDE) and benzo(g)chrysene [anti-B(g)CDE] with glutathione (GSH) catalyzed by GSH S-transferase (GST) isoenzymes mGSTP1-1, mGSTM1-1, mGSTA3-3, mGSTA4-4, and GST 9.5 of female A/J mouse tissues has been investigated. When GST activity was measured as a function of varying anti-CDE or anti-B(g)-CDE concentrations at a fixed concentration of GSH, each isoenzyme obeyed Michaelis-Menten kinetics. The catalytic efficiencies (k(cat)/Km) of murine GSTs in the GSH conjugation of anti-CDE were in the order of GST 9.5 > mGSTP1-1 > mGSTM1-1 > mGSTA3-3 > mGSTA4-4. While each GST isoenzyme examined in the present study exhibited preference for the GSH conjugation of (+)-anti-CDE with the (R,S)-diol (S,R)-epoxide absolute configuration, which is a far more potent carcinogen than the (-)-anti-CDE [(S,R)-diol (R,S)-epoxide absolute configuration], the enantioselectivity was relatively more pronounced for mGSTP1-1 compared with other murine GSTs. Anti-B(g)CDE was a relatively poor substrate for each GST isoenzyme examined compared with anti-CDE. The catalytic efficiencies of murine GSTs in the GSH conjugation of anti-B(g)CDE were in the order of GST 9.5 > mGSTP1-1 > mGSTM1-1 > mGSTA3-3. With the exception of mGSTM1-1, all other murine GSTs exhibited preference for the GSH conjugation of anti-B(g)CDE enantiomer with the (R,S)-diol (S,R)-epoxide absolute configuration. In summary, the results of the present study indicate that the murine GSTs significantly differ in their catalytic efficiency and enantioselectivity in the GSH conjugation of both anti-CDE and anti-B(g)CDE, and that anti-B(g)CDE is a relatively poor substrate for murine GSTs compared with anti-CDE, which may partially account for the observed relatively higher carcinogenic potency of the former compound.

Differential tolerance to DNA polymerization by HIV-1 reverse transcriptase on N6 adenine C10R and C10S benzo[a]pyrene-7,8-dihydrodiol 9,10-epoxide-adducted templates

To determine the effect of various stereoisomers of benzo[a]pyrene-7,8-dihydrodiol 9,10-epoxide (BPDE) on translesion bypass by human immunodeficiency virus-1 reverse transcriptase and its alpha-helix H mutants, six 33-mer templates were constructed bearing site- and stereospecific adducts. This in vitro model system was chosen to understand the structure-function relationships between the polymerase and damaged DNA during replication. Comparison of the replication pattern between wild type human immunodeficiency virus-1 reverse transcriptase and its mutants, using primers which were 3' to the lesion, revealed essentially similar patterns. While these primers terminated with all three of the C10R and two of the C10S BPDE-adducted templates 1 base 5' and 1 base 3' to the damaged site respectively, (+)-anti-trans-(C10S) BPDE-adducted DNA alone permitted the formation of full-length products. Utilization of a primer with its 3'-hydroxyl 1 base beyond the lesion resulted in full-length products with all the C10S BPDE-adducted templates and the (-)-syn-trans-(C10R)-BPDE-adducted template, following replication with either the wild type or mutant enzymes. However, the other two C10R BPDE-adducted templates failed to allow any primer extension, even with the wild type enzyme. Although T.P depletion studies further confirmed the differential primer extension abilities using the C10R and C10S adducted templates, their binding affinities were similar, yet distinct from the unadducted template.

An alpha class mouse glutathione S-transferase with exceptional catalytic efficiency in the conjugation of glutathione with 7beta, 8alpha-dihydroxy-9alpha,10alpha-oxy-7,8,9,10-tetrahydrobenzo(a)pyrene

The kinetics of the conjugation of glutathione (GSH) with anti-7beta, 8alpha-dihydroxy-9alpha,10alpha-oxy-7,8,9, 10-tetrahydrobenzo(a)pyrene (anti-BPDE) catalyzed by GSH S-transferase (GST) isoenzymes purified from the liver and forestomach of female A/J mouse has been investigated. The GST isoenzymes studied included an alpha class isoenzyme of forestomach (GST 9.5), alpha class hepatic isoenzymes mGSTA3-3 and mGSTA4-4, pi class hepatic isoenzyme mGSTP1-1, and mu class hepatic isoenzyme mGSTM1-1. When the concentration of (+)-anti-BPDE was varied (5-120 microM) at a fixed GSH concentration (2 mM), linear Lineweaver-Burk plots were observed for each isoenzyme. The kcat values for GST 9.5, mGSTA3-3, mGSTP1-1, mGSTM1-1, and mGSTA4-4 were 2.0, 0.02, 0.40, 0. 05, and 0.01 s-1, respectively, with corresponding Km values of 16, 12, 29, 27, and 49 microM. The catalytic efficiency (kcat/Km) of GST 9.5 in the conjugation of GSH with (+)-anti-BPDE, which is believed to be the ultimate carcinogenic metabolite of benzo(a)pyrene, was about 9-625-fold higher as compared with other mouse GST isoenzymes. These results indicate that GST 9.5 of forestomach is different among mammalian alpha class GSTs because (+)-anti-BPDE has been shown to be a poor substrate for alpha class rat or human GST isoenzymes. The catalytic efficiency of GST 9.5 was approximately 4.5-fold higher than that of pi class human isoenzyme (hGSTP1-1), which among human GSTs is reported to be most efficient in the detoxification of (+)-anti-BPDE. Unlike rat GST isoenzymes, linear Lineweaver-Burk plots were observed for mouse GSTs when GSH was used as a variable substrate. The catalytic efficiencies of the mouse GSTs toward (+)-anti-BPDE were about 2-20-fold higher as compared with the (-)-enantiomer of anti-BPDE. The results of the present study suggest that GST 9.5 may play an important role in the detoxification of (+)-anti-BPDE.

Retinoic acid inhibits hydrocortisone-stimulated expression of phenol sulfotransferase in bovine bronchial epithelial cells

The airway epithelium, which is commonly exposed to xenobiotics, contains the conjugative enzyme phenol sulfotransferase (PST). We have previously reported that hydrocortisone (HC) stimulates the expression of PST severalfold in cultured bovine bronchial epithelial cells (Beckmann et al., 1994, J. Cell. Physiol. 160:603-610). Here we report that this stimulation is attenuated by retinoic acid (RA). Dose-response measurements of both enzyme activities and mRNA levels indicated a 50% inhibition of HC-stimulated PST expression with 0.05 nM RA. Varied concentrations of RA had a general repressive effect on HC-stimulated PST expression, with no change in the half-maximal HC stimulatory concentration of 12.5 nM. Steady state kinetic measurements indicated no significant changes in apparent Km values of 3-5 microM for the acceptor substrate, 2-naphthol; only HC- and RA-dependent changes in Vmax were observed. These changes were likely due to altered enzyme expression, as evidenced by immunoblot and Northern blot hybridization analyses. Thus, the expression of PST within bronchial epithelial cells is not merely constitutive, but is subject to both positive and negative controls.

Mechanism of soluble epoxide hydrolase. Formation of an alpha-hydroxy ester-enzyme intermediate through Asp-333

18O-Labeled epoxides of trans-1,3-diphenylpropene oxide (tDPPO) and cis-9,10-epoxystearic acid were synthesized and used to determine the regioselectivity of sEH. The nucleophilic nature of sEH catalysis was demonstrated by comparing the enzymatic and nonenzymatic hydrolysis products of tDPPO. The results from single turnover experiments with greater or equal molar equivalents of sEH:substrate were consistent with the existence of a stable intermediate formed by a nucleophilic amino acid attacking the epoxide group. Tryptic digestion of sEH previously subjected to multiple turnovers with tDPPO in H2 18O resulted in the isolation and purification of a tryptic fragment containing Asp-333. Electrospray mass spectrometry of this fragment conclusively illustrated the incorporation of 180. After complete digestion of the latter peptide it was shown that Asp-333 of sEH exhibited an increased mass. The attack by Asp-333 initiates enzymatic activity, leading to the formation of an alpha-hydroxyacyl-enzyme intermediate. Hydrolysis of the acyl-enzyme occurs by the addition of an activated water to the carbonyl carbon of the ester bond, after which the resultant tetrahedral intermediate collapses, yielding the active enzyme and the diol product.

Regulation of phenol sulfotransferase expression in cultured bovine bronchial epithelial cells by hydrocortisone

One conjugative pathway for the inactivation of endogenous and exogenous hydroxylated aromatic compounds is catalyzed by phenol (aryl) sulfotransferases (PSTs), which esterify phenolic acceptors with sulfate. The tracheobronchial epithelium is commonly exposed to phenolic drugs and pollutants, and metabolic sulfation and PST activity in this tissue have been previously demonstrated. To determine what factors may control PST expression, extracts of serum-free, growth factor-supplemented cultures of bovine bronchial epithelial cells were assayed for PST activity and PST antigen. The most significant finding was dose-dependent, apparent stimulated expression by hydrocortisone (EC50 = 4 nM, maximal stimulation at 20 nM). Time-course experiments, however, revealed progressive loss of PST in the absence of corticosteroid. After decay of extant PST in steroid-free medium, hydrocortisone reinduced the expression of PST three to fivefold. Western blots using mouse anti-bovine PST revealed corresponding increases in 32 kDa PST protein levels in response to hydrocortisone. Steady state kinetic analyses indicated apparent Km values of 1-3 microM for 2-naphthol regardless of culture conditions. These results suggest that detoxification of phenolic compounds by sulfation may be regulated by corticosteroids.

Inhibition of phenol sulfotransferase by pyridoxal phosphate

The biologically abundant cofactor, pyridoxal-5-phosphate (PLP), is a potent inhibitor of bovine phenol (aryl) sulfotransferase (PST). Preincubation of purified enzyme with as little as 1 microM PLP decreased PST activity by 50%. Excess 2-naphthol protected PST from inactivation by PLP, whereas 2-naphthyl sulfate and PAPS were not protective. Although PLP inhibition was apparently competitive with 2-naphthol, a steady-state kinetic Ki value could not be measured due to non-linear Lineweaver-Burk plots in the presence of the inhibitor. Kinetic progress curves revealed that this was due to progressive loss of activity during catalysis. The kinetics of inactivation of PST by PLP were pseudo-first-order and exhibited saturation. The derived KI value for the binding of PLP to PST in the initial reversible step was 23 microM, with a maximal rate of inactivation of 0.077 min(-1). Absorbance spectra of the PST/PLP complex indicated the formation of a Schiff base conjugate, and this is consistent with decreased electrophoretic mobility of the protein-PLP adduct in the presence of dodecyl sulfate only after reduction with borohydride. These results point to the possible regulation of an important detoxification enzyme by a ubiquitous cofactor.

Effects of microsomes and liposomes on glutathione transferase catalysed conjugation of benzo[a]pyrene diol epoxide with glutathione

trans-7,8-Dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE] is rapidly inactivated in aqueous solvents due to hydrolysis to tetraols. No significant effect on the rate of hydrolysis is observed in the presence of glutathione (GSH)-depleted cytosol. However, when the cytosolic fraction is replaced by a mixture of glutathione (GST)-isoenzymes (corresponding to about 10% of the cytosolic protein) a significant inhibition of the rate of hydrolysis is observed, indicating a physical interaction between the diol epoxide and GST. This is compatible with the proposed role of certain GST-isoenzymes as intracellular carriers for lipophilic compounds. Studies on the accessibility of (+)-anti-BPDE to hydrolysis and GST-catalysed conjugation with GSH reveal that the presence of rat liver microsomes or liposomes, in particular those composed of the neutral phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE), very effectively protect the diol epoxide from hydrolysis. Addition of cytosolic fraction depleted of GST to a mixture of GST-isoenzymes and microsomes or liposomes do not significantly increase the rate of GSH-conjugation. This implies the absence of high molecular factors in the cytosol that may increase the accessibility of (+)-anti-BPDE and thus promote conjugation. In contrast to liposomes of PC and PE, those composed of the negatively charged phospholipids phosphatidylserine (PS) and phosphatidylinositol (PI) are considerably less efficient in protecting (+)-anti-BPDE. In fact, these lipids seem to promote hydrolysis, an effect which is lost when PS and/or PI are present together with PC and PE. Taken together, the results presented here suggest that (+)-anti-BPDE and most probably other diol epoxides, are not accessible for GSH-conjugation by direct interaction between GST and the membrane-bound compound. Moreover, there is little support for the existence of cytosolic components that increase the accessibility of (+)-anti-BPDE for conjugation. In agreement with previous results using other compounds, the results indicate that only the fraction of diol epoxide that is free in solution is accessible for conjugation with GSH.

Site-specific benzo[a]pyrene diol epoxide-DNA adducts inhibit transcription elongation by bacteriophage T7 RNA polymerase

Benzo[a]pyrene, an extremely potent procarcinogen and mutagen, is metabolized to a variety of products, including the ultimate carcinogen 7,8-dihydroxy-9,10-epoxy- 7,8,9,10-tetrahydrobenzo[a]pyrene. This product of biotransformation reacts with DNA, forming a series of adducts principally at the N2 position of guanine that differ in their stereochemistry and exhibit unique biological properties. In order to gain a better understanding of the effects on RNA synthesis of these adducts, we used purified bacteriophage T7 RNA polymerase to transcribe a series of templates containing one of four stereoisomerically pure BPDE-guanine lesions--(+)-trans-,(-)-trans-,(+)-cis-anti-N2-BPDE-guanine--or no damaged bases. To construct suitable double-stranded oligodeoxynucleotides for these studies, we annealed an 11-mer containing a site-specific stereoisomerically pure N2-BPDE-guanine adduct, a 37-mer, and a 10-mer to a complementary 58-base sequence of single-stranded DNA. The oligomers were ligated, purified, and reannealed. The resulting DNA template contained the promoter for T7 RNA polymerase and a BPDE adduct at position +16 following the transcription initiation site. The results of the transcription assays clearly demonstrate that each of the adducts inhibits elongation by T7 RNA polymerase, but they do so to significantly different extents, depending on the stereochemical characteristics of the BPDE-modified guanine. The order of inhibition is (+)-trans > (-)-trans > (+)-cis > (-)-cis, when the amount of full-length transcript for each is compared to that obtained for an unmodified template. Furthermore, premature termination of RNA synthesis occurs at or near the site of the BPDE lesion as evidenced by the formation of discrete, truncated transcripts. These results might be related to the fact that the pyrenyl moiety of the trans-BPDE adducts is situated in the minor groove of double-stranded DNA, but is quasi-intercalated into the double helix in the case of the cis stereoisomers.(ABSTRACT TRUNCATED AT 250 WORDS)

Prediction and monitoring of the carcinogenicity of polycyclic aromatic compounds (PACs)

Chemical carcinogenesis is a multistage process that includes initiation, promotion, and progression. Some carcinogenic PACs have been shown to activate proto-oncogenes and deactivate tumor-suppression genes in the carcinogenic process. The function of DNA repair processes appears to be changed in some cases by PACs. Many PACs are well known for their carcinogenic activity, but for this activity to be exerted, metabolic activation by microsomal enzymes must occur. The enzyme system responsible for PAC activation is the mixed-function oxidase system and, in particular, cytochrome P-450. In the case of PAHs, oxidation predominantly produces reactive diol-epoxides that can then be converted to carbonium ions as the reactive electrophiles that can then covalently bind to DNA. Regions of high activity exist in PAHs, namely, the "bay," "K," and "L" regions which are associated with pi electron distribution. The diol-epoxides can exist in either syn or anti forms, each of which has two enantiomers producing four stereoisomers in all. Energy considerations favor the formation of the anti form. Nitrogen-containing PACs can be metabolically activated in a manner similar to that for PAHs, or the nitrogen atom can be oxidized to form hydroxylamines. These reactive electrophiles can then form covalently bound DNA adducts. The monitoring of DNA adducts has been used in risk assessment for human exposure to PACs. This form of biomonitoring has advantages over the monitoring of external exposure or body levels of the chemicals in question. In the case of PACs, binding to DNA is an important step in the multistage carcinogenic process. The estimation of DNA adducts has been used in the monitoring of humans exposed to PAHs in a wide range of industrial situations. Recent research has shown a dose-response relationship between PAH adduct levels and human cancer, thus developing molecular epidemiology as a relevant science for the field of risk assessment. Techniques have been developed for the determination of DNA adducts and these include immunochemical, fluorescence spectroscopic, GC-MS, and 32P-postlabeling methods. The 32P-postlabeling assay is by far the most sensitive, with limits of detection being of the order of one adduct in 10(10) normal nucleotides. The use of HPLC for separation of adducted nucleotides in this postlabeling assay is becoming more common and gives better resolution of adducts than does the TLC technique used in the traditional assay. The detection of adducts on hemoglobin and other proteins has been used as a surrogate for DNA adduct estimation.(ABSTRACT TRUNCATED AT 400 WORDS)

Absence of formation of benzo[a]pyrene/DNA adducts in the cuttlefish (Sepia officinalis, Mollusca: Cephalopoda)

Benzo[a]pyrene (B[a]P) injected intramuscularly into the base of the arms of cuttlefish was released continuously from the injection site and removed from the organism. Only a portion of the compound accumulated in the body. Twenty-four hr after its injection, 75% of B[a]P applied in olive oil was removed from the cuttlefish, and 1.2% was found in the body outside the head, the site of injection. If the carcinogen was dissolved in dimethylformamide, the removal of B[a]P was slower, so that only 18% of the injected B[a]P was removed from the organism and 0.36% accumulated in the body outside the head 24 hr after injection. The high level of B[a]P in gills and hemolymph 4 hr after injection and the kinetics of the decrease of its concentration with time indicate that these two organs could be involved in the excretion of B[a]P from the body. The B[a]P/DNA adducts characteristic for vertebrates could not be demonstrated in gills, skin, brain, hepatopancreas, and lymphocytes of the cuttlefish 24 hr after injection of B[a]P. The dose of the carcinogen injected into the cuttlefish was 2-4 times higher than the dose resulting in the formation of a high level of B[a]P/DNA adducts in the vertebrates. A different metabolism of B[a]P in the tissue of cephalopods, compared to vertebrates, could be less favorable to the process leading to malignant transformation and could explain the absence from the literature of reports of tumors in cephalopods.

Phenol sulfotransferase expression in the airways: enzymological and immunohistochemical demonstration

Phenol (aryl) sulfotransferases (PSTs) provide a conjugative pathway that detoxifies hydroxylated aromatic xenobiotics by esterification with sulfate. Both human and bovine airways have been reported to use this pathway, and in this investigation the bovine system is examined. PST activity in tracheal through fourth generation bronchial mucosal cytosols was 0.1-0.35 nmol/mg protein/min. Activity was generally greater in more distal bronchi and in parenchymal extracts, which contained 0.6-3 nmol/mg/min PST activity. Comparison of the PST activities of bronchial and parenchymal cytosols indicated similar pH activity profiles, although steady state kinetic measurements revealed different Km values for the acceptor substrate 2-naphthol (13.7 microM for bronchial, 31.3 microM for parenchymal). Anion exchange chromatography indicated two PST isoforms being expressed in different ratios. Immunoblot analysis with mouse anti-bovine PST revealed a closely spaced doublet at 32 kDa in both bronchial mucosal and parenchymal cytosolic extracts; however, this doublet was unequally stained in parenchymal extracts. Immunohistochemical analyses revealed faint positive staining of the tracheobronchial epithelium. Greatest immunostaining was observed in the nonciliated secretory epithelial cells of the bronchioles, whereas surrounding smooth muscle, endothelial cells, and alveoli were immunonegative. These results are consistent with the known locations of other detoxification enzymes within the airways.

Metabolism of polycyclic aromatic hydrocarbons to mutagenic species by rat and porcine ovarian granulosa cells: detection by cocultivation with V79 Chinese hamster cells

The capacity of ovarian granulosa cells from rat and pig to release reactive metabolites produced from polycyclic aromatic hydrocarbons with the ability to cause mutations in neighbouring cells has been studied. For this purpose we have used cocultivation with V79 Chinese hamster cells as a detection system. The cells were treated with two different polycyclic aromatic hydrocarbons (PAHs), 7,12-dimethylbenz(a)anthracene (DMBA) or (-)-trans-7,8-dihydroxy-7,8-dihydrobenzo(a)pyrene (BP-7,8-diol). The resulting mutation frequency in the V79 cells after cocultivation, as a function of granulosa cell number, DMBA, or BP-7,8-diol concentration and time, was determined. The progesterone concentration in the medium after cocultivation was also analyzed as a measure of the differentiation of the granulosa cells. These studies demonstrate an increasing frequency of mutations in the V79 cells with an increasing number of granulosa cells. The maximal number of mutations were achieved with a DMBA or BP-7,8-diol concentration of 5 or 2 microM, respectively. The optimal cocultivation time was 24 h. These results clearly show that the granulosa cells can bioactivate PAHs to reactive metabolites with the capacity to migrate into surrounding cells and cause mutations in these cells. Compounds metabolized to mutagenic products by granulosa cells might thus cause mutations in the neighbouring germ cells, with possible consequences for the offspring.

Absence of mutagenicity of benzo[c]phenanthridine alkaloids in somatic cells of Drosophila melanogaster: comparison with 7,12-dimethylbenz[a]anthracene and chrysene

The wing somatic mutation and recombination test (SMART) of Drosophila melanogaster was used to study the mutagenic potential of three benzo[c]phenanthridines with antileukemic properties, fagaronine, nitidine and O-methylfagaronine, as compared with that of two structurally related aromatic polycyclic hydrocarbons: 7,12-dimethylbenz[a]anthracene and chrysene. Although toxic to larvae, the benzo[c]phenanthridines and chrysene gave negative or inconclusive results while 7,12-dimethylbenz[a]anthracene was found to be highly mutagenic and recombinogenic as previously reported. These results suggest that the alkoxy groups and the quaternary nitrogen of the benzo[c]-phenanthridines may reduce or eliminate their mutagenicity in spite of their similarity to methylated polycyclic aromatic hydrocarbons.

Applications of molecular physics ‘biotechnology’ to the rational design of an improved phenytoin analogue

This study exploits molecular physics, in conjunction with a large scale computing environment, as a tool for understanding the clinical phenomenology of phenytoin (PHT) toxicology at a molecular level and for employing this understanding in an attempt to design improved drugs. The application of molecular physics techniques, such as quantum mechanics and molecular force field calculations, to the process of rational anticonvulsant drug design remains virtually unexplored. A 3-step strategy for applying these techniques to the design of an improved PHT molecule is presented. Step 1 employs quantitative structure-activity relationship calculations on 80 PHT analogues to ascertain the portion of the PHT molecule necessary for bioactivity (i.e. the 'bioactive face' of PHT); the N3-C4(O)-C5-R fragment of PHT was identified as the bioactive face. Step 2 employs molecular modelling studies to determine the portion of the PHT molecule necessary for the teratogenic, mutagenic and connective tissue toxicities of PHT (i.e. the 'biotoxic face'); the C2(O)-N3 fragment of PHT was identified as the biotoxic face. Step 3 experiments design an 'improved' PHT analogue, which maintains the bioactive face while eliminating the integrity of the biotoxic face; 2-deoxy-5,5-diphenylhydantoin was designed and synthesized as the improved PHT analogue. This compound had biological activity equivalent to PHT, but was unable to bind to nucleic acids or to chelate metals involved in connective tissue metabolism.

Inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced tumor promotion in murine skin by systemic effects of ultraviolet irradiation

Systemic effects of UVB irradiation (280-320 nm) have been shown to prevent subsequent chemical tumorigenesis induced by an initiation-promotion protocol. The present investigation was designed to determine whether initiation or promotion is prevented by UV irradiation. Groups of 25 B6D2F1/J mice received 12 weeks of intermittent dorsal UVB radiation treatments administered before, or 3 weeks after, initiation with a single application of 7,12-dimethylbenz[a]anthracene on the ventral skin. All mice were promoted ventrally with 5 micrograms 12-O-tetradecanoylphorbol-13-acetate (TPA) applied three times weekly throughout the experiment. UV irradiation consisted of five 30-min exposures per week to a bank of 6 Westinghouse FS40 sunlamps. UV irradiation applied before or after initiation resulted in a decrease of 18-16 tumors per group of 25 mice, for a reduction of 61 and 50%, respectively, at 24 weeks after the first TPA treatment. Thus, prevention of tumor development was similar whether the UV influence was present or not during initiation. This finding suggests that the UV prevention of promotion could account for UV inhibition of skin tumors induced by an initiation-promotion regimen. Consistent with this concept, pretreatment of mice with dorsal UVB radiation was found to reduce DNA synthesis after exposure to TPA by 46%, although it did not decrease tritiated benzo[a]pyrene binding to DNA, in ventral epidermis. Thus, UVB irradiation systemically reduced TPA-induced tumor promotion in murine skin.

Distribution to the fetus and major organs of the rat following inhalation exposure to pyrene

Concentrations of pyrene and total metabolites were determined for individual fetuses and selected maternal organs and tissues immediately and 6 h following a 95-min head-only exposure of pregnant Wistar rats, on gestation day 17, to five levels of pyrene over the range 200-800 mg m-3 as a microcondensation aerosol. The influence of uterine horn, side and position, on distribution to the fetus was assessed. The concentration of both pyrene and its metabolites increased more over the exposure range (eightfold) than did those in the fetus. Concentrations of pyrene or its metabolites in fetal tissues were not found to be related to its position on the uterine horn. There was an unexplained and significant (P less than 0.01) higher concentration of pyrene in fetuses on the right side relative to the left side of the uterine horn for the animals killed immediately following exposure. A comparison of the levels in maternal tissues measured immediately following the exposure and 6 h later showed that there was some redistribution of pyrene and its metabolites to the fat tissues, i.e. levels in the fat increased over the 6 h period following the exposure. Levels in the other tissues diminished during this period. In general, concentrations of pyrene and its metabolites were lowest in the fetal tissues relative to those in the sampled maternal organs and tissues.

Cigarette smoke inhibits cytosolic but not microsomal epoxide hydrolase of human lung

The effect of cigarette smoke exposure on the activity of cytosolic and microsomal epoxide hydrolase (EH) has been investigated in human lung. Patients were classified as 'recent smokers' (n = 9) or 'non-recent smokers' (n = 10) according to whether they were or were not still smoking 1 month before surgery. Cytosolic EH was measured with [3H]trans-stilbene oxide as a substrate, whereas microsomal EH was measured with [7-3H]styrene oxide as a substrate. Microsomal EH activity did not differ between recent smokers (2.51 +/- 0.93 nmol min-1 mg-1) and non-recent smokers (2.74 +/- 1.10 nmol min-1 mg-1), whereas cytosolic EH activity was significantly lower in recent smokers (6.46 +/- 1.79 pmol min-1 mg-1) than in non-recent smokers (8.41 +/- 2.09 pmol min-1 mg-1, P less than 0.05). Cytosolic EH activity was correlated with the number of days that had passed since the cessation of smoking (r = 0.58, P less than 0.05) and the effect was dose-dependent, since the enzyme activity was inversely correlated with the number of cigarettes smoked per day (r = 0.85, P less than 0.01). This suggests that recent smoking exposure inhibits the activity of cytosolic EH but not microsomal EH, and that the inhibition increases with the number of cigarettes smoked per day. The contribution of cytosolic enzymes to xenobiotic metabolism may be remarkable in extrahepatic tissues. The inhibition of cytosolic EH by tobacco smoke may reduce the inactivation of carcinogenic epoxides in human lung tissues and so may increase a person's susceptibility to lung cancer.

Comparative metabolism of 7H-dibenzo[c,g]carbazole and dibenz[a,j]acridine by mouse and rat liver microsomes

The comparative metabolism of the carcinogenic pollutants 7H-dibenzo[c,g]-carbazole (DBC) and dibenz[a,j]acridine (DBA) was investigated in vitro using 3-methylcholanthrene (3MC) induced Sprague-Dawley rat and Hsd:ICR(Br) mouse liver microsomal preparations with benzo[a]pyrene (BaP) as the positive control. Metabolites were isolated and separated by HPLC and identified by spectroscopic and co-chromatographic techniques using synthetic standards. The major metabolites of DBC were the phenols: the 5-OH-DBC, 3-OH-DBC, and 2-OH-DBC. Traces of 1-OH-DBC were also found yet no dihydrodiols were identified. The major metabolites of DBA were the 3,4-diol-DBA and 5,6-diol-DBA, 1,2-diol-DBA, DBA-5,6-oxide and 4-OH-DBA. Treatment of both mice and rats with 3MC resulted in significant (P less than or equal to 0.05) increases relative to control in the microsomal metabolism of DBA to dihydrodiol and phenol metabolites, similar to that observed for BaP. 3MC-induced rat liver microsomes significantly (P less than or equal to 0.05) increased DBC metabolism relative to control microsomes whereas DBC metabolism was not increased with 3MC-induced mouse liver microsomes. These data indicate that different enzymatic pathways are involved in the metabolic activation of DBC in the Hsd:ICR(Br) mouse and Sprague-Dawley rat.

Effects of smoking on benzo(alpha)pyrene- and glutathione-metabolizing enzymes in human lung tissue

The influence of smoking on the metabolism of benzo(alpha)pyrene and glutathione was investigated in 190 patients with primary bronchial carcinoma and 20 patients with benign lung diseases. There were no significant differences in the activities of drug-metabolizing enzymes in the lung tissue of smokers, regardless of whether they smoked low- or high-tar and -nicotine cigarettes; former smokers; or nonsmokers; nor were there such differences between female and male patients. No significant differences existed between patients with squamous cell carcinoma and those with adenocarcinoma. Impaired detoxification due to decreased activity of glutathione S-transferases, rather induction of benzo(alpha)pyrene-metabolizing enzymes, may disturb the delicate balance between the generation and detoxification of reactive metabolites. This impairment may lead to the accumulation of these compounds in the cell.

Endogenous xenobiotic enzyme levels in mammalian cells

The response of mammalian cell lines to chemicals depends, in part, on the exogenous activation system used for the induction of a biological response. This could be attributed to differences in the expression of enzymes involved in xenobiotic metabolism. We have measured the activities of benzo[a]pyrene hydroxylase, dimethylaminoazobenzene N-demethylase, catalase, superoxide dismutase, peroxidase and glutathione-S-transferase in human lymphoblast TK6, mouse lymphoma L5178Y, Chinese hamster ovary (CHO) and lung (V79) and mouse C3H10T1/2 cell lines as well as in primary hepatocytes and S9 preparations of liver from male F344 rats. Nitroreductase was also measured in some of these preparations. Human lymphoblast TK6 and mouse C3H10T1/2 cells had the capacity to metabolize dimethylaminoazobenzene and the latter cell line also metabolized benzo[a]pyrene, indicating the presence of constitutive mono-oxygenase activity. Cytochrome P450 could not be detected spectrophotometrically in the cell lines. Western blot analysis indicated that P450 from the P450IIA family is expressed in C3H10T1/2 cells. Reactivity was also observed with an antibody to P450IA2; however, the identity of this protein remains uncertain. Superoxide dismutase, catalase and peroxidase, which protect cells against oxygen radical damage, were found in all the cell lines and in rat hepatocytes and S9. The human lymphoblast TK6 cell line, however, had the least of each of these three enzymes. Glutathione-S-transferase activity was detected at varying levels in all cell types. Nitroreductase activity was high in S9 and Chinese hamster ovary cells and lower in mouse lymphoma and Chinese hamster V79 cells.

Effect of diallyl sulfide, a naturally occurring anti-carcinogen, on glutathione-dependent detoxification enzymes of female CD-1 mouse tissues

The present studies were undertaken to elucidate the mechanism(s) of the anti-neoplastic effect of diallyl sulfide (allyl sulfide, DAS), a naturally occurring organosulfide abundant in vegetables of the Allium genus, against benzo[a]pyrene (B[a]P)-induced carcinogenesis in the mouse. DAS treatment caused a significant increase in glutathione S-transferase (GST) activity, an enzyme system responsible for detoxification of a variety of electrophilic xenobiotics including several harmful B[a]P metabolites, of mouse stomach in a dose-dependent manner. This activity in the stomach of mice treated with 25, 50 and 75 mumol DAS was higher by 1.13-, 1.20- and 1.58-fold, respectively, when compared to the control. Purification and quantitation of GST from equal amounts (1.2 g) of control and 50 mumol DAS-treated mice stomach tissues demonstrated that elevation in activity occurred as a result of increased de novo synthesis of the enzyme protein. DAS treatment also resulted in increased pulmonary GST activity, but not in a dose-dependent fashion. On the other hand, treatment of mice with DAS did not alter hepatic GST activity. Interestingly, a small but statistically significant (P less than or equal to 0.05) reduction in kidney GST activity was observed in mice treated with 50 or 75 mumol DAS, as compared to the control. The effect of DAS treatment was also assessed on glutathione (GSH) peroxidase activity, another GSH-dependent detoxification enzyme, in mouse tissues. Treatment of animals with 25, 50 and 75 mumol DAS increased stomach GSH peroxidase activity by 1.64-, 1.93- and 2.52-fold, respectively, over the control. This enzyme activity in the lungs of mice treated with 25, 50 and 75 mumol DAS was higher by 1.44-, 1.54- and 1.21-fold, respectively, when compared to the control. On the other hand, GSH peroxidase activity in liver and kidney was unchanged by DAS treatment. These results suggest that DAS and perhaps other naturally occurring organosulfur compounds may exert an anti-neoplastic effect by modulating GSH-dependent detoxification enzymes.

Liver, lung and kidney homogenates used as an activation system in mutagenicity studies of airborne particles and of expectorate and urine samples from exposed workers in a coke plant

A comparison was made between lung and kidney homogenates on the one hand and liver S9 from rats on the other hand in order to compare their ability to activate promutagens. The Salmonella reversion assay was used on extracts of airborne particles from the top of coke oven batteries, and of expectorate and urine samples from exposed workers in the same coke plant. The contents of benzo[a]anthracene and benzo[a]pyrene in the different test solutions were measured by high-resolution gas chromatography/mass spectrometry. Both mutagens were detected in the filter extract and in the expectorates from the exposed workers but not in the expectorates from the control groups or in the urine samples. The liver S9 gave significantly higher mutagenicity than lung and kidney activation with both filter samples and expectorate and urine samples.

Inhibition of meiotic divisions of rat spermatocytes in vitro by polycyclic aromatic hydrocarbons

The toxic effects of polycyclic aromatic hydrocarbons (PAH) on spermatogenic cells undergoing meiotic division were investigated in vitro. Toxicity was assayed as alterations in cell nucleus morphology and cell survival and by DNA flow cytometry. Benzo[a]pyrene (BP) and 7,12-dimethylbenz[a]anthracene (DMBA) inhibited the progression of spermatocytes through meiotic division and were highly cytotoxic at concentrations higher than 1 microM. These results were obtained upon addition of a drug-metabolizing system, indicating that the seminiferous tubules lack the enzymes required for the initiation of PAH metabolism. The spindle poisons, e.g., vincristine and Colcemid, a group of direct-acting agents, affected spermatogenesis during meiotic division in a manner similar to that observed with PAH. In contrast, adriamycin did not inhibit meiotic division, although it did induce the formation of meiotic micronuclei as a result of chromosome breakage. It is concluded that low concentrations, i.e., 0.1 microM of PAH, strongly inhibit meiotic division, presumably after metabolic activation to reactive molecules functionally resembling direct-acting alkylating agents. High concentrations of PAH are cytotoxic.

Exhalation of malondialdehyde from the smoke of high grade cigarettes and effectiveness of filtering it

It has been found that malondialdehyde penetrates during smoking through filters and exhales with smoke from cigarettes such as: "Pall Mall", "Winston", "Camel", "Lucky Strike", "Kansas", "Mustang", "Ronson", "Chesterfield", "Rally", "Oscar" and "Marlboro". These cigarettes exhale malondialdehyde in a different way, in free form in the amount from 8.5 to 23.5 mg/kg cigarettes on an average of 13.3 mg/kg for the whole group and totally (in free and bounded form) from 9.5 to 26.5 mg/kg (average 16.9 mg/kg). Moreover, it has been found that filters retain this aldehyde in free form on an average of 9.9 mg/kg and totally (in free and bounded form) of 19.0 mg/kg. Without filters these amounts would increase aldehyde contents in smoke by 42.7 and 52.9%, respectively. Low malonalogenic properties of tobacco, high effectiveness of cigarette filters, high degree of tar deposition before inhaling into lungs and slow (6 min and longer) smoking decide about low level of exhalation of malondialdehyde in cigarette smoke.

Studies on covalent binding of (-)trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene metabolites to cytochromes P-450 LM2 and LM4 and NADPH-cytochrome P-450 reductase

1. Metabolism of 14C-labelled benzo[a]pyrene (-)trans-7,8-dihydrodiol to protein- and DNA-binding products in a reconstituted enzyme system proceeds 5 to 10 times faster with rabbit cytochrome P-450 LM4 than with LM2. 2. Either cytochrome converts the substrate to ethyl acetate- and water-soluble metabolites, identified by h.p.l.c. Water-soluble metabolites comprise 78% of the total products with cytochrome P-450 LM2, but only 50% of those formed by LM4. The relative proportion of the two types of metabolites is differentially affected by certain modifiers such as 7,8-benzoflavone. 3. Half of the radioactivity in the aqueous phase of reaction mixtures containing cytochrome P-450 LM4 represents (-)trans-7,8-diol metabolites in complex primarily with NADPH and phosphate. The remaining water-soluble products are bound covalently to proteins in the reconstituted system. 4. Polyacrylamide gel electrophoresis, autoradiography, and measurement of the radioactivity in individual bands indicate that a larger fraction of metabolites is bound to cytochrome P-450 LM4 than to NADPH-cytochrome P-450 reductase, and only marginal binding to cytochrome P-450 LM2 is seen. Metabolite binding to added DNA is likewise substantially greater in magnitude when cytochrome P-450 LM4, as opposed to LM2, catalyses (-)trans-7,8-diol oxygenation. Thus, the degree of metabolite binding to monoxygenase proteins and to DNA correlates well with the catalytic activity of cytochrome P-450 LM4 and LM2 towards (-)trans-7,8-diol. 5. DNA causes a dramatic enhancement in the activity of cytochrome P-450 LM4 with (-)trans-7,8-diol, indicating that the cytochrome and/or the reductase may be functionally impaired by metabolites of this substrate. Such an effect may alter the balance between detoxication and activation of the carcinogenic benzo[a]pyrene.

Metabolism and binding of benzo[a]pyrene in randomly-proliferating, confluent and S-phase human skin fibroblasts

The metabolism of benzo[a]pyrene in randomly proliferating and confluent cultures of human skin fibroblast cells was compared with cell cultures in early S phase of the cell cycle after a G1 block. When each cell population was exposed to [G-3H]benzo[a]pyrene for 24 hours and the organic soluble metabolites in the extracellular medium and intracellular components were analyzed by HPLC, a quantitative increase in metabolism was observed in the confluent cell populations. The amount of organic soluble metabolites in the extracellular medium of the confluent dense cultures was 2.7 times the amount found in randomly proliferating cultures and 1.5 times that of the synchronized cultures. The trans-7,8- and 9,10 dihydrodiols and 3-hydroxy benzo[a]pyrene were the major metabolites formed. Small amounts of the sulphate conjugate, 9-hydroxy-benzo[a]pyrene and the tetrols were also detected. Cytoplasmic as well as nuclear extracts from the confluent cell cultures also contained higher amounts of metabolites compared to those from the randomly proliferating and S-phase cells. The levels of DNA modification by metabolically activated benzo[a]pyrene did not differ among the randomly proliferating, confluent and S-phase cells. However, the S-phase cells exhibited approximately 50-fold increase in the frequency of transformation compared to the randomly proliferating cells. Confluent cells were not transformed by benzo[a]pyrene. These data suggest that factors other than random modification of DNA by the carcinogen might have a significant role in the expression of a transformed phenotype and that metabolism and transformation are not directly related. Furthermore, confluent dense cultures with a heightened capability for metabolism of benzo[a]pyrene were more active in the detoxification of benzo[a]pyrene than in the production of the metabolites associated with cellular transformation.

Valpromide is a poor inhibitor of the cytosolic epoxide hydrolase

The effect of the antiepileptics valpromide and sodium valproate on the cytosolic epoxide hydrolase was studied in human fetal liver, kidneys and adrenals and from human adult liver and kidneys. Trans-stilbene oxide was used as substrate. Valpromide (10 mM) lowered the activity of the epoxide hydrolase to one half of the control in all organs studied. Sodium valproate (10 mM) was less powerful as an inhibitor than valpromide; however, it exerted a significant inhibition in all tissues studied.

The mechanisms of action of reproductive toxicants

Successful human reproduction is a complex process which requires normal function of 2 individuals. Reproductive toxicants can impair reproduction by acting in the male, female or both. Reproductive toxicants can produce their adverse effects by several direct and indirect mechanisms. The mechanisms by which reproductive toxicants impair reproduction are reviewed.

Cell-specific metabolic activation of 7,12-dimethylbenz[a]anthracene in rat testis

The binding of metabolites of the polycyclic aromatic hydrocarbon (PAH) 7,12-dimethylbenz[a]anthracene (DMBA) to protein in rat testis seminiferous tubules was studied. Treatment of cultured seminiferous tubule segments with DMBA resulted in very little binding to protein, suggesting that the seminiferous epithelium from rat testis lacks the cytochrome P-450-dependent monooxygenase(s) required for DMBA metabolism. In contrast, Leydig cells from rat testis contain monooxygenase systems which catalyze the metabolism of PAH, such as DMBA. This metabolic activation of DMBA was localized in both mitochondria and microsomes derived from Leydig cells and was decreased by inhibitors of the cytochrome P-450 system and by free radical scavengers, suggesting that the metabolism involved both cytochrome P-450 and free radical-dependent pathways. In the presence of whole Leydig cells or microsomes prepared from Leydig cells, the covalent binding of DMBA metabolites to protein of rat testis seminiferous tubules was increased 5- and 13-fold, respectively. These results suggest that DMBA is metabolized primarily in rat testis Leydig cells and that part of the produced metabolites find their way to the seminiferous epithelium, where they undergo further metabolism producing reactive metabolites, possibly cation radicals and diolepoxides, which interfere with the functions of spermatogonia and spermatocytes by modifying key proteins covalently.

Inhibition of stage-specific DNA synthesis in rat spermatogenic cells by polycyclic aromatic hydrocarbons

Changes in the rate of DNA synthesis in spermatogenic cells after treatment of segments of rat seminiferous tubule at defined stages of epithelial cycle with benzo[a]pyrene (BP) or 7,12-methylbenz[a]anthracene (DMBA) were studied. The incorporation of labeled thymidine into DNA was used as a measure of the rate of DNA synthesis. Very little or no inhibition of DNA synthesis at stages V and VIII of the cycle was observed at BP and DMBA concentrations lower than 100 microM. In contrast, in the presence of added mitochondria and/or microsomes from whole rat testis, 20 microM BP or DMBA inhibited DNA synthesis 5% and 80%, respectively. This inhibition of DNA synthesis was prevented by inhibitors of the cytochrome P-450 system and by free radical scavengers. These results suggest that polycyclic aromatic hydrocarbons (PAH) require metabolic activation in order to inhibit DNA replication in seminiferous tubules. The first step of this biotransformation is cytochrome P-450-dependent and occurs in Leydig cells. However, the metabolites produced in this step may be further metabolized to reactive metabolites by peroxidative pathways in the seminiferous tubules; these latter products may affect DNA replication.