Characterization and quantitation of 3-alkylthymidines from reactions of mutagenic propylene oxides with thymidine

Propylene oxide: mutagenesis, carcinogenesis and molecular dose

The results from mutagenic and carcinogenic studies of propylene oxide (PO) and the current efforts to develop molecular dosimetry methods for PO-DNA adducts are reviewed. PO has been shown to be active in several bacterial and mammalian mutagenicity tests and induces site of contact tumors in rodents after long-term administration. Quantitation of N7-(2-hydroxypropyl)guanine (7-HPG) in nasal and hepatic tissues of male F344 rats exposed to 500 ppm PO (6 h/day; 5 days/week for 4 weeks) by inhalation was performed to evaluate the potential of high concentrations of PO to produce adducts in the DNA of rodent tissues and to obtain information necessary for the design of molecular dosimetry studies. The persistence of 7-HPG in nasal and hepatic tissues was studied in rats killed three days after cessation of a 4-week exposure period. DNA samples from exposed and untreated animals were analyzed for 7-HPG by two different methods. The first method consisted of separation of the adduct from DNA by neutral thermal hydrolysis, followed by electrophoretic derivatization of the adduct and gas chromatography-high resolution mass spectrometry (GC-HRMS) analysis. The second method utilized 32P-postlabeling to quantitate the amount of this adduct in rat tissues. Adducts present in tissues from rats killed immediately after cessation of exposure were 835.4 +/- 80.1 (respiratory), 396.8 +/- 53.1 (olfactory) and 34.6 +/- 3.0 (liver) pmol adduct/mumol guanine using GC-HRMS. Lower values, 592.7 +/- 53.3, 296.5 +/- 32.6 and 23.2 +/- 0.6 pmol adduct/mumol guanine were found in respiratory, olfactory and hepatic tissues of rats killed after three days of recovery. Analysis of the tissues by 32P-postlabeling yielded the following values: 445.7 +/- 8.0 (respiratory), 301.6 +/- 49.2 (olfactory) and 20.6 +/- 1.8 (liver) pmol adduct/mumol guanine in DNA of rats killed immediately after exposure cessation and 327.1 +/- 21.7 (respiratory), 185.3 +/- 29.2 (olfactory) and 15.7 +/- 0.9 (liver) pmol adduct/mumol guanine after recovery. Current methods of quantitation did not provide evidence for the endogenous formation of this adduct in control animals. These studies demonstrated that the target tissue for carcinogenesis has much greater alkylation of DNA than liver, a tissue that did not exhibit a carcinogenic response.

Genetic toxicology of propylene oxide and trichloropropylene oxide--a review

Aliphatic epoxides are a group of compounds extensively used in industry and as laboratory reagents, and can be produced as metabolic intermediates. An important aliphatic epoxide is propylene oxide, extensively used in the production of propylene glycol and polyols used in the manufacture of polyurethane polymers. The widespread human exposure to propylene oxide is of great health concern. In this review an attempt has been made to evaluate and update the genotoxic effects of propylene oxide and trichloropropylene oxide based on the available literature.

DNA adduct formation by 12 chemicals with populations potentially suitable for molecular epidemiological studies

DNA adduct formation, route of absorption, metabolism and chemistry of 12 hazardous chemicals are reviewed. Methods for adduct detection are also reviewed and approaches to sensitivity and specificity are identified. The selection of these 12 chemicals from the Environmental Protection Agency list of genotoxic chemicals was based on the availability of information and on the availability of populations potentially suitable for molecular epidemiological study. The 12 chemicals include ethylene oxide, styrene, vinyl chloride, epichlorohydrin, propylene oxide, 4,4'-methylenebis-2-chloroaniline, benzidine, benzidine dyes (Direct Blue 6, Direct Black 38 and Direct Brown 95), acrylonitrile and benzyl chloride. While some of these chemicals (styrene and benzyl chloride, possibly Direct Blue 6) give rise to unique DNA adducts, others do not. Potentially confounding factors include mixed exposures in the work place, as well the formation of common DNA adducts. Additional research needs are identified.

Comparison of the isolation of adducts of 2'-deoxycytidine and 2'-deoxyguanosine with phenylglycidyl ether by high-performance liquid chromatography on a reversed-phase column and a polystyrene-divinylbenzene column

2'-Deoxycitidine (dCyd) and 2'-deoxyguanosine (dGuo) were subjected to reaction with phenylglycidyl ether (PGE) in methanol in order to study the formation of the corresponding 2'-deoxynucleoside adducts. Separation methods were developed on analytical and semi-preparative scales using high-performance liquid chromatography with photodiode-array detection on a reversed-phase column and on a polystyrene-divinylbenzene column. The use of the latter column was prompted by decomposition of the preparatively isolated dGuo-PGE adducts on the reversed-phase column. The use of a polystyrene-divinylbenzene column solved this problem and also revealed the presence of one more peak in both the dCyd- and dGuo-PGE reaction mixtures. The adducts of dCyd and dGuo were isolated on preparative reversed-phase and polystyrene-divinylbenzene columns and characterized by UV, fast atom bombardment mass and 360 MHz 1H NMR spectrometry. The adducts of dCyd were the diastereomers of N-3-(2-hydroxy-3-phenoxypropyl)-2'-deoxycytidine and N4-(2-hydroxy-3-phenoxypropyl)-2'-deoxycytidine whereas those of dGuo were the two diastereomers of N-7-(2-hydroxy-3-phenoxypropyl)-2'-deoxyguanosine and a third peak which appeared to be mainly N2-(2-hydroxy-3-phenoxypropyl)-2'-deoxyguanosine.

Adduct formation identification between phenyl glycidyl ether and 2'-deoxyadenosine and thymidine by chromatography, mass spectrometry and nuclear magnetic resonance spectroscopy

Thymidine and 2'-deoxyadenosine were reacted with phenyl glycidyl ether in order to study the formation of the corresponding 2'-deoxynucleoside adducts. Separation methods were elaborated using either reversed-phase high-performance liquid chromatography with photodiode-array detection, or centrifugal circular thin-layer chromatography. The adducts were isolated on a preparative scale and were fully characterized by UV spectroscopy, desorption chemical ionization and fast atom bombardment mass spectrometry and 270- and 360-MHz 1H NMR spectrometry. For thymidine the main adduct was characterized as N-3-(2-hydroxy-3-phenoxypropyl)thymidine. With 2'-deoxyadenosine, predominantly N-1-(2-hydroxy-3-phenoxypropyl)-2'-deoxyadenosine was formed. With longer reaction times, the formation of a minor amount of dialkylated 2'-deoxyadenosine was observed. These nucleoside adducts will be used as marker compounds for studies of DNA adduct formation.

Reactions of propylene oxide with 2'-deoxynucleosides and in vitro with calf thymus DNA

Propylene oxide (PO) is a direct-acting mutagen and rodent carcinogen. We have studied how PO modifies 2'-deoxynucleosides at pH 7.0-7.5 and 37 degrees C for 10 h. PO reacts as an SN2 alkylating agent by forming the following 2-hydroxypropyl (HP) adducts: N6-HP-dAdo (7% yield), 7-HP-Gua (37%) and 3-HP-dThd (4%). Alkylation at N-3 of dCyd resulted in conversion of the adjacent exocyclic imino group at C-4 to an oxygen (hydrolytic deamination) with the formation of a dUrd adduct, 3-HP-dUrd (14%). Ultraviolet spectroscopy and mass spectrometry were used for the structural determination of these adducts. Confirmation of the unexpected 3-HP-dUrd adduct was provided by an accurate mass measurement technique where diagnostic ions in the mass spectra of 3-HP-dUrd were measured to within 0.0005 atomic mass units of the predicted mass. PO was reacted in vitro with calf thymus DNA (pH 7.0-7.5, 37 degrees C, 10 h) and yielded N6-HP-dAdo (1 nmol/mg DNA), 3-HP-Ade (14 nmol/mg DNA), 7-HP-Gua (133 nmol/mg DNA) and 3-HP-dUrd (13 nmol/mg DNA). A mechanism for the hydrolytic deamination of 3-HP-dCyd to 3-HP-dUrd involving the OH on the HP side chain is proposed. This cytosine to uracil conversion may play a role in the mutagenic and carcinogenic activity of this epoxide.

Characterization of reaction products between styrene oxide and deoxynucleosides and DNA

Styrene oxide was reacted with deoxynucleosides and DNA in aqueous buffer at pH 7.4. The products were purified by HPLC, characterized by UV spectroscopy and by chemical ionization mass spectrometry. The main products identified were 7-alkyl-, N2-alkyl- and O6-alkyldeoxyguanosine, 1-alkyl-, and N6-alkyldeoxyadenosine, N4-alkyl-, 3-alkyl- and O2-alkyldeoxycytidine and 3-alkylthymidine. The relative yields of alkylated deoxynucleosides were dG greater than dC greater than dA greater than T. In the reactions of styrene oxide with DNA the dominant product isolated was 7-alkylguanine but N2-alkylguanine was also detected.

Stability of 7-alkyldeoxyguanosines of trichloropropylene oxide, epichlorohydrin and glycidol

Deoxyguanosine was reacted with trichloropropylene oxide (TCPO), epichlorohydrin and glycidol in aqueous solution and the products were isolated by high pressure liquid chromatography (HPLC). For each compound 7-alkyldeoxyguanosine was the main adduct; TPCO reacted about 3 times faster than epichlorohydrin and about 10 times faster than glycidol. The pKa's of the 7-alkyldeoxyguanosines were 6.7, 6.8 and 6.9 for the TCPO-, epichlorohydrin-, and glycidol-adducts, respectively. The respective half-lives of imidazole ring-opening were 6.3, 15 and 27 hrs (pH 8.5, 28 degrees) and those of depurination 1.9, 3.4 and 4.4 hrs (pH 5.5, 28 degrees).

Reactivity of mutagenic propylene oxides with deoxynucleosides and DNA

In an extension of previous studies with deoxycytidine and thymidine reactivities, propylene oxide, glycidol, epichlorohydrin, and trichloropropylene oxide were reacted with deoxyguanosine as well as deoxyadenosine and, except for the trichloro compound, with DNA. Reactivity with the purine deoxynucleosides as well as the four deoxynucleosides in DNA were quantitated by HPLC methods. Correlations were found for the reactivity with individual deoxynucleosides in solution to Taft sigma electron-withdrawing values of the substituents on the epoxides and for reaction with model nucleophiles. In general, these correlations were not as pronounced for the reactivities of the propylene oxides with the nucleosides in DNA. Correlations for reactivity of the propylene oxides with the individual deoxynucleosides in solution and in DNA, except for dThd, were indicated for mutagenicity in TA100 in the liquid-preincubation Ames test. However, this was not the case for mutagenicities determined with the plate incorporation procedure nor with TA1535, where the relative mutagenicity of trichloropropylene oxide was the outstanding difference. Trichloropropylene oxide appeared to depend upon the error-prone system in TA100 for full expression of its mutagenicity.