Identification of adducts formed in the reaction of alpha-acetoxy-N-nitrosopyrrolidine with deoxyribonucleosides and DNA.

N-Nitrosopyrrolidine (NPYR) is a well-established hepatocarcinogen in the rat. NPYR requires metabolic activation by cytochrome P450-catalyzed alpha-hydroxylation to express its carcinogenic activity. This produces alpha-hydroxyNPYR (2), which spontaneously ring opens to 4-oxobutanediazohydroxide (4), a highly reactive intermediate, which may itself modify DNA or yield a cascade of electrophiles that react with DNA to produce adducts. Multiple dGuo adducts formed in this reaction have been previously characterized, but there are no examples of adducts formed with other DNA nucleobases. In this study, we used alpha-acetoxyNPYR (3) as a stable precursor to 2 and 4. Compound 3 was allowed to react with DNA. The DNA was enzymatically hydrolyzed to deoxyribonucleosides, and the products were analyzed by LC-ESI-MS and LC-ESI-MS/MS. Reactions of 3 with individual deoxyribonucleosides were also carried out. The products were identified by their MS, UV, and NMR spectra as N6-(tetrahydrofuran-2-yl)dAdo (16) and N4-(tetrahydrofuran-2-yl)dCyd (17) in addition to the previously characterized N2-(tetrahydrofuran-2-yl)dGuo (13). Unstable dThd adducts were also formed. Further characterization of the adducts was achieved by NaBH3CN reduction of the reaction mixtures of 3 with deoxyribonucleosides or DNA. This produced N6-(4-hydroxybut-1-yl)dAdo (21), N4-(4-hydroxybut-1-yl)dCyd (22), O2-(4-hydroxybut-1-yl)dThd (23), O4-(4-hydroxybut-1-yl)dThd (24), and 3-(4-hydroxybut-1-yl)dThd (25). Adducts 21 and 22 were characterized by their spectral properties, while the dThd adducts 23-25 were identified by comparison to synthetic standards. The results of this study demonstrate that 3 forms adducts with dAdo, dCyd, and dThd in DNA, in addition to the previously characterized dGuo adducts. These newly characterized standards can be used to investigate DNA adduct formation in rats treated with NPYR.

Reactions of alpha-acetoxy-N-nitrosopyrrolidine with deoxyguanosine and DNA.

We investigated the reactions of alpha-acetoxy-N-nitrosopyrrolidine (alpha-acetoxyNPYR) with dGuo and DNA. Alpha-acetoxyNPYR is a stable precursor to the major proximate carcinogen of NPYR, alpha-hydroxyNPYR (3). Our goal was to develop appropriate conditions for the analysis of DNA adducts of NPYR formed in vivo. Products of the alpha-acetoxyNPYR-dGuo reactions were analyzed directly by HPLC or after treatment of the reaction mixtures with NaBH3CN. Products of the alpha-acetoxyNPYR-DNA reactions were released by enzymatic or neutral thermal hydrolysis of the DNA, then analyzed by HPLC. Alternatively, the DNA was treated with NaBH3CN prior to hydrolysis and HPLC analysis. The reactions of alpha-acetoxyNPYR with dGuo and DNA were complex. We have identified 13 products of the dGuo reaction-6 of these were characterized in this reaction for the first time. They were four diastereomers of N2-(3-hydroxybutylidene)dGuo (20, 21), 7-(N-nitrosopyrrolidin-2-yl)Gua (2), and 2-(2-hydroxypyrrolidin-1-yl)deoxyinosine (12). Adducts 20 and 21 were identified by comparison to standards produced in the reaction of 3-hydroxybutanal with dGuo. Adduct 2 was identified by its spectral properties while adduct 12 was characterized by comparison to an independently synthesized standard. With the exception of adduct 2, all products of the dGuo reactions were also observed in the DNA reactions. The major product in both the dGuo and DNA reactions was N2-(tetrahydrofuran-2-yl)dGuo (10), consistent with previous studies. Several other previously identified adducts were also observed in this study. HPLC analysis of reaction mixtures treated with NaBH3CN provided improved conditions for adduct identification, which should be useful for in vivo studies of DNA adduct formation by NPYR.

Reactions of alpha-acetoxy-N-nitrosopyrrolidine and crotonaldehyde with DNA.

alpha-Acetoxy-N-nitrosopyrrolidine (alpha-acetoxyNPYR) is a stable precursor to alpha-hydroxyNPYR, the initial product of metabolism and proposed proximate carcinogen of NPYR. Crotonaldehyde (2-butenal) is a metabolite of NPYR and also a mutagen and carcinogen. Both alpha-acetoxyNPYR and crotonaldehyde are known to form DNA adducts, but these reactions have not been completely characterized. In previous studies, we detected substantial amounts of unidentified radioactivity in hydrolysates of DNA that had been reacted with radiolabelled alpha-acetoxyNPYR. We have now characterized these products as 2-hydroxytetrahydrofuran, the cyclic form of 4-hydroxybutanal, and paraldol, the dimer of 3-hydroxybutanal. They were characterized by comparison with standards and by comparison of their derived 2,4-dinitrophenylhydrazones with standards. [3H]H2O was also identified. 2-Hydroxytetrahydrofuran is the major product in neutral thermal hydrolysates of alpha-acetoxyNPYR-treated DNA and is derived predominantly from N2-(tetrahydrofuran-2-yl)deoxyguanosine 8. Paraldol is present to a lesser extent than 2-hydroxytetrahydrofuran in these reactions and is formed from paraldol-releasing adducts, which in turn are produced by the reaction of crotonaldehyde or paraldol, solvolysis products of alpha-acetoxyNPYR, with DNA. Paraldol is a major product in hydrolysates of crotonaldehyde-treated DNA, being present in amounts 100 times greater than those of previously identified adducts. These results provide a more complete picture of the reactions of alpha-acetoxyNPYR with DNA and yield some new insights on possible endogenous DNA adducts formed from crotonaldehyde.

Lactols in hydrolysates of DNA treated with alpha-acetoxy-N-nitrosopyrrolidine or crotonaldehyde.

alpha-Acetoxy-N-nitrosopyrrolidine (alpha-acetoxyNPYR) is a stable precursor to alpha-hydroxyNPYR, the initial product of metabolism and proposed proximate carcinogen of N-nitrosopyrrolidine (NPYR). Crotonaldehyde (2-butenal) is a metabolite of NPYR and also a mutagen and carcinogen. Both alpha-acetoxyNPYR and crotonaldehyde form DNA adducts, but these reactions have not been completely characterized. In previous studies, we detected substantial amounts of unidentified radioactivity in hydrolysates of DNA that had been treated with radiolabeled alpha-acetoxyNPYR. In this study, we have characterized these products as 2-hydroxytetrahydrofuran, the cyclic form of 4-hydroxybutanal, and paraldol, the dimer of 3-hydroxybutanal. These products were identified by comparison to standards and by conversion to 2,4-dinitrophenylhydrazones. 2-Hydroxytetrahydrofuran is the major product in neutral thermal hydrolysates of alpha-acetoxyNPYR-treated DNA and is derived predominantly from N2-(tetrahydrofuran-2-yl)deoxyguanosine 8. Paraldol is present to a lesser extent than 2-hydroxytetrahydrofuran in these reactions and is formed from paraldol-releasing adducts, which in turn are produced in the reaction of crotonaldehyde, a solvolysis product of alpha-acetoxyNPYR, with DNA. Other products in hydrolysates of alpha-acetoxyNPYR-treated DNA are N7-substituted guanines 5 and 6, cyclic N7-C8 guanines 4, 11, and 12, and 1, N2-propanodeoxyguanosines 9 and 10. Paraldol is a major product in hydrolysates of crotonaldehyde-treated DNA, being present in amounts 100 times greater than those of previously identified adducts 9 and 10. The results of this study provide a more complete picture of the reactions of alpha-acetoxyNPYR with DNA and yield some new insights about possible endogenous DNA adducts formed from crotonaldehyde.

Reactions of alpha-acetoxy-N-nitrosopyrrolidine and alpha-acetoxy-N-nitrosopiperidine with deoxyguanosine: formation of N2-tetrahydrofuranyl and N2-tetrahydropyranyl adducts.

The goal of this study was to compare the reactions of alpha-acetoxy-N-nitrosopyrrolidine (alpha-acetoxyNPYR) and alpha-acetoxy-N-nitrosopiperidine (alpha-acetoxyNPIP) with deoxyguanosine (dG). alpha-AcetoxyNPYR and alpha-acetoxyNPIP are stable precursors to the alpha-hydroxynitrosamines which are formed metabolically from NPYR and NPIP. These alpha-hydroxynitrosamines are believed to be the proximate carcinogens of NPYR and NPIP. NPYR and NPIP, although structurally similar, have remarkably different carcinogenic properties, and a comparison of the reactions of their metabolically activated forms with dG and ultimately DNA could provide insights on their mechanisms of carcinogenicity. Reactions of alpha-acetoxyNPYR and alpha-acetoxyNPIP with dG were carried out at 37 degrees C and pH 7.0. The products were analyzed by HPLC and characterized by their spectral properties and by comparison to standards. In each reaction, one of the major products was a new type of dG adduct: N2-(tetrahydrofuran-2- yl)dG (THF-dG) from alpha-acetoxyNPYR and N2-(3,4,5,6-tetrahydro-2H-pyran-2-yl)dG (THP-dG) from alpha-acetoxyNPIP. THF-dG was synthesized independently by reaction of either 2-chlorotetrahydrofuran or 2,3-dihydrofuran with dG. Similarly, THP-dG was prepared by reaction of 2-chloro-3,4,5,6-tetrahydro-2H-pyran with dG. The structures of THF-dG and THP-dG were established by their UV and 1H-NMR spectra. THF-dG was less stable than THP-dG, but could be readily converted to a stable derivative, N2-(4-hydroxybutyl)dG, by reaction with NaBH4. THF-dG and THP-dG were converted to dG and 2-hydroxytetrahydrofuran or 2-hydroxy-3,4,5,6-tetrahydro-2H-pyran, respectively, upon neutral thermal or acid hydrolysis. This reaction was found to be reversible, with the adducts being produced in substantial amounts by reaction of 2-hydroxytetrahydrofuran or 2-hydroxy-3,4,5,6-tetrahydro-2H-pyran with dG. The latter reaction accounts for part of the THF-dG and THP-dG produced from the alpha-acetoxynitrosamines; stable oxonium ion-derived electrophiles may also be involved in the formation of THF-dG and THP-dG. Comparisons of the yields of various adducts in the reaction of alpha-acetoxyNPYR and alpha-acetoxyNPIP with dG showed some major differences. Whereas yields of THF-dG and THP-dG were similar, adducts formed from open chain diazonium ion or related intermediates were formed more extensively from alpha-acetoxyNPYR than from alpha-acetoxyNPIP. Adducts formed from enal products of the two nitrosamines were also different. Adduct formation as characterized in this study may account for some of the contrasting carcinogenic properties of NPYR and NPIP.

Formation of N2-tetrahydrofuranyl and N2-tetrahydropyranyl adducts in the reactions of alpha-acetoxy-N-nitrosopyrrolidine and alpha-acetoxy-N-nitrosopiperidine with DNA.

We studied the reactions with DNA of alpha-acetoxy-N-nitrosopyrrolidine (alpha-acetoxyNPYR) and alpha-acetoxy-N-nitrosopiperidine (alpha-acetoxyNPIP) in order to obtain more information on adduct formation by metabolic activation via alpha-hydroxylation of two cyclic nitrosamines, N-nitrosopyrrolidine (NPYR) and N-nitrosopiperidine (NPIP). Enzyme hydrolysis and HPLC analysis of DNA that had been reacted with unlabeled, [14C]-, or [3H]alpha- acetoxyNPYR permitted the positive identification of N2-(tetrahydrofuran-2-yl)deoxyguanosine (THF-dG). It was identified by comparison of its UV spectrum and retention time to those of a standard, by conversion upon NaBH4 treatment to N2-(4-hydroxybutyl)deoxyguanosine, and by neutral thermal hydrolysis to 2-hydroxytetrahydrofuran (THF-OH). The levels of THF-dG in DNA exceeded that of other adducts of alpha-acetoxyNPYR. Reaction of alpha-acetoxyNPIP with DNA followed by enzyme hydrolysis and HPLC analysis resulted in the positive identification of two diastereomers of N2-(3,4,5,6-tetrahydro-2H-pyran-2-yl)deoxyguanosine (THP-dG) by comparison of their retention times and UV spectra to those of standards. The levels of THP-dG were similar to those of THF-dG formed from alpha-acetoxyNPYR. Neutral thermal hydrolysis of DNA that had been reacted with alpha-acetoxyNPIP produced 2-hydroxy-3,4,5,6-tetrahydro-2H-pyran (THP-OH). Studies on the mechanism of formation of THF-dG and THP-dG indicated that stable cyclic oxonium ion-derived electrophiles could be their major precursors. Our data provide the first evidence for the formation of cyclic oxonium ion-derived DNA adducts from cyclic nitrosamines and indicate some potential differences in DNA binding between alpha-acetoxyNPYR and alpha-acetoxyNPIP.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of thiols on DNA modifications via alkylation and Michael addition by alpha-acetoxy-N-nitrosopyrrolidine.

The hepatocarcinogen NPYR is metabolically activated by alpha-hydroxylation mediated by cytochrome P-450 enzymes to yield a 4-oxobutylating agent and 2-butenal (crotonaldehyde). Both are reactive intermediates capable of modifying DNA with guanine either by simple alkylation or by Michael type addition, respectively. In order to assess the roles of these pathways in NPYR tumorigenesis, we are interested in identifying agents which can selectively modify one of these two pathways. In this study, we examined the effects of three thiols--(mesna), glutathione (Glu), and N-acetylcysteine (Nac)--on DNA adduct formation by alpha-acetoxyNPYR, a stable precursor of alpha-hydroxyNPYR. Calf thymus DNA isolated from incubation of alpha-acetoxyNPYR with or without thiol was hydrolyzed and analyzed for the adducts formed by alkylation (adducts 1 and 2) and Michael addition (adducts 3-5). The results showed that the addition of mesna completely blocked the formation of the crotonaldehyde-derived adducts 3-5, whereas it exerted little effect on the formation of the alkylated adducts 1 and 2. These results indicate the preferential conjugation of mesna with crotonaldehyde. In contrast, Nac had little selectivity on adduct formation; levels of adducts 1 to 5 were were reduced by 36-75%. These results suggest that Nac conjugated with both alkylating agent and crotonaldehyde. Similar to mesna, Glu blocked the formation of the crotonaldehyde-derived adducts (adducts 3-5) efficiently. However, unlike mesna, Glu inhibited the formation of adduct 1, while it did not inhibit the formation of adduct 2, although both adducts are presumably derived from the 4-oxobutylating agent.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative metabolism and urinary excretion of N-mononitrosopiperazine and N,N'-dinitrosopiperazine in the rat.

The metabolism of N-mononitrosopiperazine (NPz) and N,N'-dinitrosopiperazine (DNPz) was investigated in Sprague-Dawley rats. When an oral dose of 50 micrograms/rat NPz was administered, 10.5% of the dose was recovered unchanged in urine together with N-nitroso-3-hydroxypyrrolidine (NHPYR, 1.3% of administered NPz dose) and N-nitrosodiethanolamine (NDELA, 0.3%). For DNPz, 7.7% of the administered dose together with NPz (0.04%), NHPYR (2.9%), NDELA (6.7%) and N-nitroso(2-hydroxyethyl)glycine (NHEG, 20.2%) was recovered in urine after 24 h. These results show extensive metabolism of both NPz and DNPz and suggest that both NPz and NHPYR could be analysed in urine as markers for the endogenous nitrosation of piperazine in patients receiving antihelminthic treatment with piperazine.

Formation of acyclic and cyclic guanine adducts in DNA reacted with alpha-acetoxy-N-nitrosopyrrolidine.

This paper describes the reaction of alpha-acetoxy-N-nitrosopyrrolidine with DNA to produce six adducts: two new acyclic adducts, 7-(4-oxobutyl)guanine (6) and 7-(3-carboxypropyl)guanine (7), and four cyclic adducts--the exocyclic 7,8-guanine adducts 5, 11, and 12 and an exocyclic 1,N2-guanine adduct 13--which we have previously characterized. The initial purpose of this study was to carry out an independent synthesis to verify the structure of adduct 5, which is formed in liver DNA of rats treated with N-nitrosopyrrolidine. This was accomplished by the reaction of 2',3',5'-triacetylguanosine with 4-iodobutyraldehyde. This reaction also produced 7-(4-oxobutyl)guanine (6), which underwent air oxidation to 7. The new adducts were characterized by their proton NMR, UV, and mass spectral properties, by chemical transformations, and by independent syntheses. The six adduct standards were used to develop HPLC systems for their analysis as products of the reaction of alpha-acetoxy-N-nitrosopyrrolidine with DNA. Studies on their rates of formation and stability in DNA were carried out. The major products were 7-(4-oxobutyl)guanine (6) and the exocyclic 7,8-guanine adduct 5, which apparently were both formed mainly by reaction with DNA of 4-oxobutanediazohydroxide (4). Their concentrations were maximal after 6 h and subsequently decreased due to depurination. Little evidence was obtained for cyclization of 6 to 5, at the base level or in DNA. The concentrations of adducts 11-13, which were formed by reaction with DNA of crotonaldehyde (10), increased gradually over the 36-h time period studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of exocyclic guanine adducts in hydrolysates of hepatic DNA of rats treated with N-nitrosopyrrolidine and in calf thymus DNA reacted with alpha-acetoxy-N-nitrosopyrrolidine.

This report describes the isolation and characterization of DNA adducts formed in vitro from alpha-acetoxy-N-nitrosopyrrolidine and in rats treated with the hepatocarcinogen N-nitrosopyrrolidine. Esterase-catalyzed hydrolysis of alpha-acetoxy-N-nitrosopyrrolidine in the presence of calf thymus DNA, followed by neutral thermal hydrolysis of the DNA, resulted in formation of three previously unknown Adducts 1-3. They were isolated and characterized by their UV, mass, and proton magnetic resonance spectra as the exocyclic 7,8-guanine adducts 2-amino-6,7,8,9-tetrahydro-9-hydroxypyrido[2,1-f]purine-4(3H)-one (Adduct 1), and cis- and trans-2-amino-7,8-dihydro-8-hydroxy-6-methyl-3H-pyrrolo[2,1-f] purine-4(6H)-one (Adducts 2 and 3). Adduct 1 was formed by addition of 4-oxobutyl diazohydroxide, or a related carbonium ion, to the 7 and 8 positions of guanine. Adducts 2 and 3 resulted from Michael addition of 2-butenal to the 7 and 8 positions of guanine. Esterase-catalyzed hydrolysis of alpha-acetoxy-N-nitrosopyrrolidine in the presence of DNA also produced the exocyclic 1,N2-propanodeoxyguanosine Adducts 4a and 4b which we have previously described. Neutral thermal hydrolysates of hepatic DNA isolated from rats treated with N-nitrosopyrrolidine contained a fluorescent adduct, as previously reported (E.J. Hunt and R.C. Shank, Biochem. Biophys. Res. Commun., 104: 1343, 1982). This fluorescent adduct was shown to be identical to Adduct 1. Adducts 2, 3, 4a, and 4b were not detected in hepatic DNA hydrolysates from these animals. The results of this study provide the first example of a structurally characterized DNA adduct formed in vivo from a cyclic nitrosamine and support the alpha-hydroxylation hypothesis of cyclic nitrosamine metabolic activation.

Interaction of alpha-acetoxy-N-nitrosopyrrolidine with DNA assessed by loss of restriction endonuclease recognition sites and formation of apurinic/apyrimidinic sites.

alpha-Acetoxy-N-nitrosopyrrolidine (AcO-NPYR), in the presence of esterase, was reacted with the plasmid pBR322. The AcO-NPYR-treated plasmid was nicked by reaction with spermidine, suggesting the presence of apurinic sites. The treated plasmid also blocked the action of the restriction endonuclease BglI and, to a lesser extent, DraI. The recognition sequences of these restriction enzymes suggest that AcO-NPYR interacts preferentially with guanine and/or cytosine bases in DNA.

The metabolism of nitrosopyrrolidine by hepatocytes from Fischer rats.

Nitrosopyrrolidine (NO-PYR), an hepatocellular carcinogen, is rapidly metabolized to CO2 by hepatocytes freshly isolated from the livers of male Fischer rats. Using CO2 evolution as a measure of NO-PYR metabolism, we observed two kinetic constants; a high affinity component (Km = 0.11 mM), and a lower affinity component (K m = 3.2 mM). The high affinity component has similar kinetic constants to those observed for in vitro reactions with microsomes plus cytosol (Km = 0.36 mM). Therefore, it is probable that the microsomal reaction is the limiting factor in the metabolism of NO-PYR in hepatocytes. NO-PYR may be metabolized to CO2 through normal anaplerotic sequences. Some metabolites of NO-PYR which have been tentatively identified are gamma-hydroxybutyrate, succinic semialdehyde, 3,4-dihydroxybutyric acid lactone, lactate, acetate, pyruvate, glyoxylate, gamma-aminobutyrate and alanine. 2-Hydroxytetrahydrofuran (2-hydroxy-THF). a product of alpha-hydroxylation was detected at low levels in only one of four reactions. 3-Hydroxy-NO-PYR is present but represents only a small percentage of the total metabolism and is probably of little significance in the overall catabolism of NO-PYR in hepatocytes.

Mutagenicity of N-nitrosopyrrolidine derivatives in Salmonella (Ames) and Escherichia coli K-12 (343/113) assays.

The mutagenicity of nitrosopyrrolidine (NPYR) and its derivatives was determined by use of the Ames Salmonella assay. A clear specificity to revert the missense stain of TA1535 and a requirement for the phenobarbital-induced rat-liver activation system (S9 mix) were noted. 3,4-Dichloronitrosopyrrolidine was more mutagenic than NPYR, whereas 3-hydroxynitrosopyrrolidine was weakly mutagenic. The carcinogenic nitroso-3-pyrrolidine was not mutagenic under the test conditions. The noncarcinogenic derivatives (2,5-dimethylnitrosopyrrolidine, nitrosoproline and 4-hydroxynitrosoproline) were not mutagenic. Liquid preincubation assays were not any more effective than the pour-plate assays. Selected derivatives of NPYR were tested in the Escherichia coli K-12 (343/113) assay A specificity to revert the missense mutation at the arg locus and a dependence on phenobarbital-induced rat-liver S9 mix were noted with NPYR and its derivatives. 3,4-Dibromonitrosopyrrolidine, which was not mutagenic in Salmonella, was effective in E. coli, and the weakly carcinogenic NPRL was a weak mutagen resulting in a 2-fold enhancement in the E. coli arginine reversion assay.