N7-guanine adducts of the epoxy metabolites of 1,3-butadiene in mice lung

Influence of the methyl group in isoprene epoxides on reactivity compared to butadiene epoxides: Biological significance

Reactions of the epoxides of 1,3-butadiene and isoprene (2-methyl-1,3-butadiene) with oxygen, nitrogen and sulfur nucleophiles have been compared to enable a better molecular understanding of the relative human toxicities of these epoxides. Hydrolysis of rac.-ethenyloxirane in (¹⁸O)water gave 77% (2-¹⁸O)but-3-ene-1,2-diol and 23% (1-¹⁸O)but-3-ene-1,2-diol. The R:S ratio for but-3-ene-1,2-diol from hydrolysis of (S)-ethenyloxirane was 75:25. Hence, hydrolysis of ethenyloxirane occurs by competing S_N2 attack at C-2 and C-3 in 3:1 ratio, with no S_N1 component. Hydrolysis of rac.-2-ethenyl-2-methyloxirane gave 2-hydroxy-2-methylbut-3-en-1-ol (73%) and 27% of a 2:1 mixture of the E- and Z-isomers of 4-hydroxy-2-methylbut-2-en-1-ol. In (¹⁸O)water (2-¹⁸O)2-hydroxy-2-methylbut-3-en-1-ol was obtained. Formation of these products occurs via S_N1 ionisation to resonance-stabilised allylic cations which are captured by water. Reaction of rac.-ethenyloxirane with l-valine methyl ester gave diastereoisomeric adducts from S_N2 attack of the valine amino at both C-2 (substituted position) and C-3 of the oxirane. The corresponding reaction of rac.-2-methyl-2-ethenyloxirane gave diastereoisomeric adducts, (R, S)- and (S, S)-N-(2-hydroxy-2-methyl-3-buten-1-yl)-l-valine methyl ester, from S_N2 attack of the valine amino solely at C-3. Reactions of rac.-2-ethenyl-2-methyloxirane with cysteine derivatives occurred at C-2 in neutral polar media (S_N1 reaction) or at C-3 in basic media (S_N2), whereas for ethenyloxirane products arose from attack at both C-2 and C-3. Reaction of meso-butadiene diepoxide (meso-2,2'-bioxirane) with l-valine methyl ester gave mainly 2:1 adducts, dimethyl 2,2'-(((2R,3S)-2,3-dihydroxybutane-1,4-diyl)bis(azanediyl))-(2S,2'S)-bis(3-methyl-butanoates), whereas 2-methyl-2,2'-bioxirane gave a mixture of 1:1 [methyl 2-(3,4-dihydroxy-3-methylpyrrolidin-1-yl)-3-methylbutanoates] and 2:1 adducts. Meso-2,2'-bioxirane reacted with N-acetylcysteine methyl ester in methanol to afford meso-thiolane-3,4-diol, by elimination of N-acetyldehydroalanine methyl ester from a precursor cyclic adduct. Similarly, 2-methyl-2,2'-bioxirane gave solely 3-methylthiolane-3,4-diols. Thus, the methyl group of isoprene has a subtle effect on the reactivity of its epoxides relative to those of butadiene and therefore, in the context of their toxicology, could abrogate crosslinking of nitrogen functions in biomolecules related to mutagenicity and carcinogenicity.

Wear of dental materials: Clinical significance and laboratory wear simulation methods -A review

This review focusses on tribological aspects of teeth during function, the clinical significance of wear, wear of natural teeth and restorative materials and laboratory methods to simulate wear of restorative materials. Ceramic, metal alloy and amalgam show low material wear, whereas resin-based materials demonstrate substantial wear in the long term. The clinical wear shows a high variability with the patient factor accounts for about 50% of the variability. Wear as such seldomly compromises the function of the stomatognath system or individual teeth and is in most cases an esthetic problem. Particles that are ingested due to attrition and abrasion wear may pose a health risk to the patient, especially those from composite resin materials. However, systematic clinical studies on that issue are not available. For laboratory research many wear simulation devices and methods have been developed but only few are validated and have a moderate correlation with clinical wear.

Dental composite components induce DNA-damage and altered nuclear morphology in gingiva fibroblasts

OBJECTIVE

Released dental composite components can damage human gingival fibroblasts (HGFs) and their DNA. The cytotoxicity, chromatin condensation and the induction of DNA double strand breaks (DSBs) by different compounds of dental composites was investigated using an improved γ-H2AX focus assay.

METHODS

HGFs were incubated with the monomers: bisphenol-A-ethoxylate-dimethacrylate (Bis-DMA), bisphenol-A-glycerolate-dimethacrylate (BisGMA), ethyltriethylen glycol methacrylate (ETEGMA), glycidyl methacrylate (GMA), 1,6-hexandiol-dimethycrylate (HDDMA), trimethylolpropane ethoxylate triacrylate (TMPTA), and acrylamide (ACR). DSBs were determined by enumerating γ-H2AX and 53BP1 foci colocalized at DSBs.

RESULTS

A concentration-dependent induction of DSBs was found in the order: GMA>BisGMA>ACR>Bis-DMA>HDDMA>TMPTA>ETEGMA. HGFs exposure to GMA (0.3mM) and to BisGMA (0.09mM) induced the highest rate of DSB foci, i.e. 12-fold and 8-fold, respectively, relative to control (0.33 DSB foci/cell). At the highest concentrations (EC50) prominent changes in the chromatin morphology of HGF cell nuclei, i.e. compaction of nuclear chromatin and reduction of the area covered by the ovoid fibroblast nuclei, were observed. Nuclear condensation was significantly induced by GMA (1.7-fold at 0.3mM) and BisGMA (1.6-fold at 0.09mM), which correlated with the highest numbers of induced DSB foci (GMA, BisGMA, 3.9 and 2.6 foci/cell, respectively).

SIGNIFICANCE

The improved γ-H2AX/53BP1 focus assay revealed a concentration-dependent increase in DSBs for all tested substances. Furthermore, concentration-dependent changes in HGF cell nucleus morphology was noted, demonstrating genotoxic effects of the substances tested.

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

Mutagenicity of stereochemical configurations of 1,2-epoxybutene and 1,2:3,4-diepoxybutane in human lymphblastoid cells

The carcinogenicity of 1,3-butadiene (BD) is related to its bioactivation to several DNA-reactive metabolites; accumulating evidence suggests that the stereochemistry of these BD intermediates may play a significant role in the mutagenic and carcinogenic actions of the parent compound. The objective of this study was to evaluate the cytotoxicity and mutagenicity of stereochemical forms of 1,2-epoxybutene (EB) and 1,2:3,4-diepoxybutane (DEB), two genotoxic BD metabolites, in a human lymphoblastoid cell line, TK6. Cytotoxicity was measured by comparing cloning efficiencies in chemical-exposed cells versus those in control cells. The hypoxanthine-guanine phosphoribosyltransferase (HPRT) and thymidine kinase (TK) mutant frequencies (MFs) were measured using a cell cloning assay. HPRT mutants collected from cells exposed to the three forms of DEB were analyzed by PCR to characterize large genetic alterations. All the three stereoisomers of DEB caused increased HPRT and TK MFs compared to the concurrent control samples. There were no significant differences in cytotoxicity or mutagenicity among the three isomers of DEB in TK6 cells. Molecular analysis of HPRT mutants revealed similar distributions of types of mutations among the three isomers of DEB. There were also no statistically significant differences in mutagenic efficiencies between the two isomers of EB in TK6 cells. These results were consistent with the in vivo findings that there was little difference in the mutagenic efficiencies of racemic-DEB versus meso-DEB in rodents. Thus, in terms of mutagenic efficiency, stereochemical configurations of EB and DEB are not likely to play a significant role in the mutagenicity and carcinogenicity of BD.

Identification of 2,3-epoxymethacrylic acid as an intermediate in the metabolism of dental materials in human liver microsomes

OBJECTIVES

In previous studies it could be demonstrated that methacrylic acid (MA) is an intermediate in the metabolism of unpolymerized dental comonomers, released from dental restorative materials. This study was performed to identify the possible dental material intermediate 2,3-epoxymethacrylic acid (2,3-EMA) from MA in human liver microsomes. Most epoxy compounds are regarded as highly toxic substances.

METHODS

The formation and hydrolysis were studied in defined systems containing only MA and human liver microsomes at 37 degrees C. Hydrolysis was inhibited by cyclohexene oxide, a competitive inhibitor of epoxide hydrolase. The reaction product 2,3-EMA was analyzed by the headspace gas chromatography-mass spectrometry. After 5, 30, and 60 min samples were taken and analyzed.

RESULTS

For the reaction of MA to 2,3-EMA the average conversion rate was about 5% within 1h. It was found that without cyclohexene oxide the rate constant of enzymatic hydrolysis at pH 7.4 was about 10 times higher than the rate constant of the formation from MA in combination with cyclohexene oxide (k=8.3 versus 0.83 micromol/l min), indicating an instability and thus a high reactivity of 2,3-EMA. The formation of the MA intermediate 2,3-EMA was not observed when heat-inactivated liver microsomes were used (controls).

SIGNIFICANCE

It could be clearly demonstrated that 2,3-EMA is a product of dental material metabolisms in biological systems. Therefore, increased toxicity might occur on dental restorative materials which are able to release (co)monomers which can be metabolized to MA.

Dose responses for DNA adduct formation in tissues of rats and mice exposed by inhalation to low concentrations of 1,3-[2,3-[(14)C]-butadiene

Male Sprague-Dawley rats and B6C3F1 mice were exposed to either a single 6h or a multiple (5) daily (6h) nose-only dose of 1,3-[2,3-(14)C]-butadiene at exposure concentrations of nominally 1, 5 or 20 ppm. The aim was to compare the results with those from a similar previous study at 200 ppm. DNA isolated from liver, lung and testis of exposed rats and mice was analysed for the presence of butadiene related adducts, especially the N7-guanine adducts. Total radioactivity present in the DNA from liver, lung and testis was quantified and indicated more covalent binding of radioactivity for mouse tissue DNA than rat tissue DNA. Following release of the depurinating DNA adducts by neutral thermal hydrolysis, the liberated depurinated DNA adducts were measured by reverse phase HPLC coupled with liquid scintillation counting. The guanine adduct G4, assigned as N7-(2,3,4-trihydroxybutyl)- guanine, was the major adduct measured in liver, lung and testis DNA in both rats and mice. Higher levels of G4 were detected in all mouse tissues compared with rat tissue. The dose-response relationship for the formation of adduct G4 was approximately linear for all tissues studied for both rats and mice exposed in the 1-20 ppm range. The formation of G4 in liver tissue was about three times more effective for mouse than rat in this exposure range. Average levels of adduct G4 measured in liver DNA of rats and mice exposed to 5 x 6 h 1, 5 and 20 ppm 1,3-[2,3-(14)C]-butadiene were, respectively, for rats: 0.79 +/- 0.30, 2.90 +/- 1.19, 16.35 +/- 4.8 adducts/10(8) nucleotides and for mice: 2.23 +/- 0.71, 12.24 +/- 2.15, 48.63 +/- 12.61 adducts/10(8) nucleotides. For lung DNA the corresponding values were for rats: 1.02 +/- 0.44, 3.12 +/- 1.06, 17.02 +/- 4.07 adducts/10(8) nucleotides, and for mice: 3.28 +/- 0.32, 14.04 +/- 1.55, 42.47 +/- 13.12 adducts/10(8) nucleotides. Limited comparative data showed that the levels of adduct G4 formed in liver and lung DNA of mice exposed to a single exposure to butadiene in the present 20 ppm study and earlier 200 ppm study were approximately directly proportional across dose, but this was not observed in the case of rats. From the available evidence it is most likely that adduct G4 was formed from a specific isomer of the diol-epoxide metabolite, 3,4-epoxy-1,2-butanediol rather than the diepoxide, 1,2,3,4-diepoxybutane. Another adduct G3, possibly a diastereomer of N7-(2,3,4-trihydroxybutyl)-guanine or most likely the regioisomer N7-(1-hydroxymethyl-2,3-dihydroxypropyl)-guanine, was also detected in DNA of mouse tissues but was essentially absent in DNA from rat tissue. Qualitatively similar profiles of adducts were observed following exposures to butadiene in the present 20 ppm study and the previous 200 ppm study. Overall the DNA adduct levels measured in tissues of both rats and mice were very low. The differences in the profiles and quantity of adducts seen between mice and rats were considered insufficient to explain the large difference in carcinogenic potency of butadiene to mice compared with rats.

Clerocidin alkylates DNA through its epoxide function: evidence for a fine tuned mechanism of action

Clerocidin (CL) is an effective topoisomerase II-poison, which has been shown to produce DNA depurination and strand breaks per se at the guanine (G) level. To elucidate the roles played by the different functional groups of CL in the reactivity towards nucleic acids, we investigated CL derivatives with key structural modifications. The derivatives were reacted with plasmid, single-/double-stranded DNA and isolated 2'-deoxy-guanosines (dG). We show here that an intact oxirane ring is essential to achieve DNA modification and depurination. Through HPLC/MS and MS/MS techniques we were able to unambiguously characterize adducts obtained by reacting isolated dG and single-/double-stranded DNA with the drugs, indicating beyond reasonable doubt that the structure of a typical adduct is formed by epoxide alkylation at N7 of G with subsequent loss of the pentose unit. Further, we showed that reduction of vicinal carbonyl functions affect drug activity to a large extent. Our findings demonstrate that the characteristic DNA-alkylating properties of CL arise from mutual action of the functional groups present in this molecule. Its oxidation state seems crucial to modulate the rates of reactivity by finely tuning the strain applied on the oxirane ring.