Reactivity of propylene oxides towards deoxycytidine and identification of reaction products

32P-postlabelling of propylene oxide 1- and N(6)-substituted adenine and 3-substituted cytosine/uracil: formation and persistence in vitro and in vivo

Propylene oxide, a widely used monofunctional alkylating agent, has been shown to be genotoxic in in vitro test systems and induces tumors in the nasal tissues of experimental animals. Propylene oxide, like related alkylating agents, forms several different adducts with DNA bases, but predominantly at the 7-position of guanine. We have previously described the in vitro and in vivo formation and stability of this major adduct. The aim of the present study was to perform a similar investigation of other adducts of propylene oxide. 1-(2-Hydroxypropyl)adenine (1-HP-adenine) and 3-(2-hydroxypropyl)cytosine (3-HP-cytosine), as well as their rearrangement products to N(6)-(2-hydroxypropyl)adenine (N(6)-HP-adenine) and 3-(2-hydroxypropyl)uracil (3-HP-uracil), respectively, were analysed by a very sensitive (32)P-postlabelling method involving nuclease P1 enhancement and radioisotope detector-coupled HPLC separation. All four adducts could be detected in DNA treated in vitro with propylene oxide. The sum of the levels of 1- and N(6)-HP-adenine amounted to 3.5% and the sum of 3-HP-cytosine and 3-HP-uracil to 1.7%, respectively, of 7-(2-hydroxypropyl)guanine (7-HP-guanine). In male Fischer 344 rats exposed to 500 p.p.m. propylene oxide by inhalation for 20 days, 1-HP-adenine was detected in all analysed tissues, including nasal epithelium, lung and lymphocytes, whereas N(6)-HP-adenine was only found in the tissues of the nasal cavities. The highest level of 1-HP-adenine (2.0 mol/10(6) mol of normal nucleotides, i.e. 2% of 7-HP-guanine) was found in the respiratory nasal epithelium, which also represents the major target for tumour induction in the rat following inhalation of propylene oxide. The levels of this adduct in the lung and in the lymphocytes were considerably lower, amounting to 15 and 9%, respectively, of that of the respiratory nasal epithelium. In rats killed 3 days after cessation of exposure, practically no decrease in 1-HP-adenine was observed, indicating no or very slow repair. 3-HP-uracil could only be detected in the respiratory nasal epithelia of propylene-exposed rats and its concentration was as low as 0.02 mol/10(6) mol of normal nucleotides (0.02% of 7-HP-guanine). Since 3-HP-uracil was chemically much more stable than the latter, the obtained animal data suggest repair of the cytosine and/or uracil adducts. Incubation of propylene oxide-reacted DNA with a protein extract from mammalian cells indicated that an enzymatic repair mechanism exists for removal of 3-HP-cytosine, but not for 3-HP-uracil or 1- and N(6)-HP-adenine. Another finding was that uracil glycosylase is probably not involved. The level of 1-HP-adenine in the propylene oxide-exposed rats was approximately 50 times lower than that of 7-HP-guanine. Nevertheless, this adduct is conveniently analysed and has high chemical stability and recovery, which results in high sensitivity (detection limit 0.3 mol/10(9) mol of normal nucleotides using 10 microgram DNA). 1-HP-adenine might, therefore, be considered as an alternative to 7-HP-guanine for monitoring exposure to propylene oxide.

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.

Reaction of epichlorohydrin with 2'-deoxynucleosides: characterization of adducts

Epichlorohydrin (ECH) is a simple 3-carbon epoxide of industrial importance and thus has the potential for human exposure in the workplace. It has been shown to be genotoxic in several systems and is a compound capable of reacting with biological nucleophiles. This study details the products formed from the reaction of ECH with 2'-deoxynucleosides at pH 7 and 37 degrees C for 6 h. Reaction with 2'-deoxyguanosine yielded 7-(3-chloro-2-hydroxypropyl) guanine (7-CHP-Gua) resulting from alkylation at N-7 of 2'-deoxyguanosine followed by depurination. Two unusual adducts were also partially characterized which resulted from further reaction of 7-CHP-Gua with another molecule of ECH to yield 1,7-bis(3-chloro-2-hydroxypropyl)guanine (1,7-bis-CHP-Gua) which could then cyclize with the exocyclic amino group to yield 1,N2-(2-hydroxypropano)-7-(3-chloro-2-hydroxypropyl) guanine (1,N2-HP-7-CHP-Gua). Reaction with 2'-deoxyadenosine gave only one product, namely 1,N6-(2-hydroxypropano)-2'-deoxyadenosine (1,N6-HP-dAdo). The reaction of 2'-deoxythymidine with ECH also yielded one product which was identified as 3-(3-chloro-2-hydroxypropyl)-2'-deoxythymidine (3-CHP-dThd). A 3-(3-chloro-2-hydroxypropyl)-2'-deoxyuridine (3-CHP-dUrd) product was isolated from the reaction of ECH with 2'-deoxycytidine. This product most likely resulted from the deamination of an initially formed 3-(3-chloro-2-hydroxypropyl) -2'-deoxycytidine (3-CHP-dCyd), a phenomenon which we have previously reported to occur during the reaction of 2'-deoxycytidine with other aliphatic epoxides. Evidence is also presented that 3-CHP-dUrd is converted to 3-(2,3-dihydroxypropyl)-2'deoxyuridine (3-DHP-dUrd) under physiological conditions, with a half-life of 213 h. Reaction of ECH with calf thymus DNA (pH 7.0, 37 degrees C, 3 h) resulted in the formation of 7-CHP-Gua (200 nmol/mg DNA.

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.

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

Thymidine was reacted in methanol with four epoxides of varying mutagenicities: propylene oxide, glycidol, epichlorohydrin and trichloropropylene oxide. A single product was detected with each epoxide, and these products had the same retention times on silica high pressure liquid chromatography (HPLC). UV spectra of the products identified them as 3-alkylthymidines, and this was confirmed by infrared (IR) and nuclear magnetic resonance (NMR) spectra. Mass spectra (MS) analysis showed the products to be consistent with attachment at the least substituted carbon of the epoxide. Formation of 3-alkylthymidines correlated to Taft sigma electron withdrawing values for the substituents on the epoxides and mutagenicities in strain TA100 of the Ames Assay.