Trapping of a cross-link formed by a major purine adduct of a metabolite of the carcinogen N-nitrosomorpholine by inorganic and biological reductants.

3-Hydroperoxy-N-nitrosomorpholine in buffered aqueous media in the presence of calf thymus DNA was treated with a phosphine reductant to generate the transient α-hydroxynitrosamine and subsequent diazonium ion that alkylated the DNA, as previously reported. Subsequent addition of hydride donors, for 30 min, followed by acid hydrolysis of the mixture allowed detection and quantification of 6-(2-{2-[(9H-purin-6-yl)amino]ethoxy}ethoxy)-9H-purin-2-amine, the reduced cross-link formed from deposition, via the diazonium ion, of a 3-oxapentanal fragment on O(6)-Gua, and condensation with N(6)-Ade, presumably in the vicinity. Decreasing the temperature of the reaction mixtures and decreasing the pH modestly increased the yields of the trapped cross-link. Among three borohydride reductants, NaNCBH3 is superior, being ∼4 times more effective on a molar basis, as opposed to a hydride equivalent basis, than NaBH4 or Na(AcO)3BH. For trapping with NaNCBH3, it is deduced that the reaction likely occurs with the iminium ion that is in protonic equilibrium with its conjugate base imine. In an experiment in which the hydroperoxide was decomposed and NaNCBH3 was introduced after various periods of time, the amount of cross-link was observed to increase, nearly linearly, by ∼4-fold over 1 week. These data indicate that there are a minimum of two populations of cross-links, one that forms rapidly, in minutes, and another that grows in with time, over days. Reduced nicotinamide cofactors and ascorbate are observed to effect reduction (over 3 days) of the cross-links, confirming the possibility that otherwise reversible cross-links might be immortalized under biological conditions.

Analogs of iso-azepinomycin as potential transition-state analog inhibitors of guanase: synthesis, biochemical screening, and structure-activity correlations of various selectively substituted imidazo[4,5-e][1,4]diazepines.

Guanase is an important enzyme of the purine salvage pathway of nucleic acid metabolism and its inhibition has beneficial implications in viral, bacterial, and cancer therapy. The work described herein is based on a hypothesis that azepinomycin, a heterocyclic natural product and a purported transition state analog inhibitor of guanase, does not represent the true transition state of the enzyme-catalyzed reaction as closely as does iso-azepinomycin, wherein the 6-hydroxy group of azepinomycin has been translocated to the 5-position. Based on this hypothesis, and assuming that iso-azepinomycin would bind to guanase at the same active site as azepinomycin, several analogs of iso-azepinomycin were designed and successfully synthesized in order to gain a preliminary understanding of the hydrophobic and hydrophilic sites surrounding the guanase binding site of the ligand. Specifically, the analogs were designed to explore the hydrophobic pockets, if any, in the vicinity of N1, N3, and N4 nitrogen atoms as well as O(5) oxygen atom of iso-azepinomycin. Biochemical inhibition studies of these analogs were performed using a mammalian guanase. Our results indicate that (1) increasing the hydrophobicity near O(5) results in a negative effect, (2) translocating the hydrophobicity from N3 to N1 also results in decreased inhibition, (3) increasing the hydrophobicity near N3 or N4 produces significant enhancement of inhibition, (4) increasing the hydrophobicity at either N3 or N4 with a simultaneous increase in hydrophobicity at O(5) considerably diminishes any gain in inhibition made by solely enhancing hydrophobicity at N3 or N4, and (5) finally, increasing the hydrophilic character near N3 has also a deleterious effect on inhibition. The most potent compound in the series has a Ki value of 8.0±1.5µM against rabbit liver guanase.

Structure-based drug design and potent anti-cancer activity of tricyclic 5:7:5-fused diimidazo[4,5-d:4',5'-f][1,3]diazepines.

Judicial structural modifications of 5:7-fused ring-expanded nucleosides (RENs), based on molecular modeling studies with one of its known targets, human RNA helicase (hDDX3), led to the lead, novel, 5:7-5-fused tricyclic heterocycle (1). The latter exhibited promising broad-spectrum in vitro anti-cancer activity against a number of cancer cell lines screened. This paper describes our systematic, albeit limited, structure-activity relationship (SAR) studies on this lead compound, which produced a number of analogs with broad-spectrum in vitro anti-cancer activities against lung, breast, prostate, and ovarian cancer cell lines, in particular compounds 15i, 15j, 15m and 15n which showed IC(50) values in submicromolar to micromolar range, and are worthy of further explorations. The SAR data also enabled us to propose a tentative SAR model for future SAR efforts for ultimate realization of optimally active and minimally toxic anti-cancer compounds based on the diimidazo[4,5-d:4',5'-f][1,3]diazepine structural skeleton of the lead compound 1.

Lactone metabolite common to the carcinogens dioxane, diethylene glycol, and N-nitrosomorpholine: aqueous chemistry and failure to mediate liver carcinogenesis in the F344 rat.

1,4-Dioxan-2-one, 1, was synthesized, and the equilibrium constant between it and the hydrolysis product 2-(2-hydroxyethoxy) acetic acid, 2, was determined as K(O) = 70 ± 4 in acidic aqueous media, 25 °C, ionic strength 1 M (KCl), and 5% by volume acetonitrile. The carboxylic acid dissociation constant of 2 was determined under the same conditions to be pK(a) = 3.31 ± 0.02. On the basis of these two determinations, the equilibrium constant between 1 and carboxylic acid anion, 3, and the proton was calculated to be K(OA) = 0.034 ± 0.002 M. The stability of 1 was determined in the range of pH between 1 and 8.5 in buffered aqueous solutions under the conditions above by UV spectrophotometric methods and exhibited simple first order kinetics of decay. On the basis of buffer dilution plots, the values of k(o), the rate constant for solvent mediated decomposition, were determined. The plot of log k(o) against pH is consistent with a three term rate law for solvolysis with a hydrogen ion catalyzed rate constant k(H+) = 1.1 (±0.1) M(-1) min(-1), a water catalyzed rate constant, k(w) = 9.9 (±0.5) × 10(-4) min(-1), and a hydroxide ion catalyzed rate constant, k(OH) = 4.1 (±0.3) × 10(4) M(-1) min(-1). The t(1/2) for decay at pH 7.0, at 25 °C, is ∼2 h. Treatment of F344 rats with aflatoxin B(1) (AFB(1)) (positive control) elicited the expected preneoplastic foci in the livers of each rat tested, while subsequent administration of 1 (a total of 1.32 g over 12 weeks) failed to statistically increase focal number or focal volume percent. In 8 rats administered 1 (1.32 g, 12 weeks) alone, no increase above background foci was detected. This study does not support compound 1 as a common carcinogen.

Investigations into specificity of azepinomycin for inhibition of guanase: discrimination between the natural heterocyclic inhibitor and its synthetic nucleoside analogues.

In our long and broad program to explore structure-activity relationships of the natural product azepinomycin and its analogues for inhibition of guanase, an important enzyme of purine salvage pathway of nucleic acid metabolism, it became necessary to investigate if the nucleoside analogues of the heterocycle azepinomycin, which are likely to be formed in vivo, would be more or less potent than the parent heterocycle. To this end, we have resynthesized both azepinomycin (1) and its two diastereomeric nucleoside analogues (2 and 3), employing a modified, more efficient procedure, and have biochemically screened all three compounds against a mammalian guanase. Our results indicate that the natural product is at least 200 times more potent toward inhibition of guanase as compared with its nucleoside analogues, with the observed K(i) of azepinomycin (1) against the rabbit liver guanase=2.5 (±0.6)×10(-6) M, while K(i) of Compound 2=1.19 (±0.02)×10(-4) M and that of Compound 3=1.29 (±0.03)×10(-4) M. It is also to be noted that while IC(50) value of azepinomycin against guanase in cell culture has long been reported, no inhibition studies nor K(i) against a pure mammalian enzyme have ever been documented. In addition, we have, for the first time, determined the absolute stereochemistry of the 6-OH group of 2 and 3 using conformational analysis coupled with 2-D (1)H NMR NOESY.

A novel transition state analog inhibitor of guanase based on azepinomycin ring structure: Synthesis and biochemical assessment of enzyme inhibition.

Synthesis and biochemical inhibition studies of a novel transition state analog inhibitor of guanase bearing the ring structure of azepinomycin have been reported. The compound was synthesized in five-steps from a known compound and biochemically screened against the rabbit liver guanase. The compound exhibited competitive inhibition profile with a K(i) of 16.7±0.5µM.

Products of the direct reaction of the diazonium ion of a metabolite of the carcinogen N-nitrosomorpholine with purines of nucleosides and DNA.

A number of putative purine nucleoside and nucleobase adducts of the diazonium ion derived from 3-hydroxy-N-nitrosomorpholine have been synthesized as dimethylacetals. These are converted, in most cases nearly quantitatively, to the aldehydes, or in two cases to their derivatives, on treatment with mild acid to yield standards for a quantitative investigation of alkylation of purine nucleosides and DNA by the above metabolite of the powerful carcinogen N-nitrosomorpholine. The stability of the resulting nucleobase ethoxyacetaldehyde (EA) adducts has been characterized under a number of conditions with respect to their propensity to decompose. The stabilities, compared to that of the previously characterized adduct of the model benzimidazole, are generally unexceptional. Deposition of adducts on purine nucleosides and DNA were quantified in reactions in which 3-hydroperoxy-N-nitrosomorpholine was reduced to the hydroxy metabolite by a water-soluble phosphine at 21 +/- 2 degrees C. The adduct profile is highly similar to that observed from simpler alpha-hydroxy metabolites of acyclic dialkylnitrosamines, with the three most abundant ethoxyacetaldehyde (EA) adducts in reactions of duplex DNA being N7-EA-Gua approximately O(6)-EA-Gua > N3-EA-Ade. The initial rate kinetics of formation of hydroxyethyl (HE) lesions from the initially formed EA lesions have been determined in the case of the major products in the cases of both the nucleoside and DNA adducts. The rates of formation of HE adducts are accelerated in DNA, relative to the nucleosides in the cases of the N7-EA-Ade, N7-EA-Gua, and O(6)-EA-Gua adducts by factors of 7, 14, and 54, respectively. The initial rates of depurination of the N3-EA-Ade, N7-EA-Gua, and N7-EA-Gua adducts have also been quantified, and they are unexceptional in comparison with what has been previously reported for simple alkyl adducts. The adduct profiles reported here stand in significant contrast to what has been reported previously for structurally closely related alpha-substituted cyclic nitrosamines. In part or whole, this may be due to methodological differences in the conduct of the present and previous reports.

Hydrogen peroxide is a second messenger in phase 2 enzyme induction by cancer chemopreventive dithiolethiones.

The ability of three dithiolethione cancer chemopreventives, oltipraz 1, anetholedithione (ADT) 2, 1,2-dithiole-3-thione (D3T) 3, and the major metabolite, 4, of 1, to induce the cytoprotective enzyme NQO1 in Hepa 1c1c7 cells and the inhibition of this induction by catalase are demonstrated. The ability of 1, 3, and 4 to form O(2)(*) has been reported, and it is here demonstrated that 2 decomposes in the presence of GSH to form, upon addition of the nitrone spin trap DMPO, the DMPO-OH adduct that is detectable by EPR. Decomposition of 2 in the presence of GSH elicits, upon the addition of hydroethidine and excitation at 510 nm, fluorescence at 580 nm that is diminished by the addition of superoxide dismutase. The compound 4, is a product of the reduction of 1, and it is demonstrated that 2 and 3 decompose in the presence of reductants such as thiolates and NaBH(4), followed by addition of CH(3)I, to form the dimethylated products of reductive cleavage of the S(1)-S(2) bond. The same products are isolated subsequent to lysis in buffer containing CH(3)I of Hepa 1c1c7 cells treated with 2 or 3. Reductive cleavage of 2 and 3 in aqueous ethanol by NaBH(4) in an argon atmosphere, followed by acidic destruction of remaining borohydride and neutralization and introduction of O(2) results in the reformation of 2 and 3 to the extent of 80 and 33%, respectively. The data in toto are consistent with a model in which dithiolethiones, generally, undergo reductive cleavage in Hepa 1c1c7 cells, thereby resulting in the generation of O(2)(*) that dismutates to H(2)O(2), that subsequently, by direct or indirect means, effects the nuclear translocation of transcription factor Nrf2, that upregulates phase 2 enzyme expression.

Prospective type 1 and type 2 disulfides of Keap1 protein.

Experiments were carried out to detect cysteine residues on human Keap1 protein that may be sensors of oxidative stress that gives rise to changes in the GSH/GSSG redox couple. Human Keap1 protein, at a final concentration of 6 microM, was incubated for two hours in aqueous buffer containing 0.010 M GSH, pH 8, in an argon atmosphere. Subsequently, excess iodoacetamide and trypsin were added to generate a peptide map effected by LCMS analysis. Peptides containing all 27 carboxamidomethylated cysteines were identified. Replacement of GSH by 0.010 M GSSG yielded a map in which 13 of the original carboxamidomethylated peptides were unperturbed, while other caboxamidomethylated cysteine-containing peptides were undetected, and a number of new cysteine-containing peptide peaks were observed. By mass analysis, and in some cases, by isolation, reduction, carboxamidomethylation, and reanalysis, these were identified as S-glutathionylated (Type 1) or Cys-Cys (Type 2) disulfides. Such peptides derived from the N-terminal, dimerization, central linker, Kelch repeat and C-terminal domains of Keap1. Experiments were carried out in which Keap1 was incubated similarly but in the presence of various GSH/GSSG ratios between 100 and 1 ([GSH + GSSG] = 0.010 M), with subsequent caraboxamidomethylation and trypsinolysis to determine differences in sensitivities of the different cysteines to the type 1 and type 2 modifications. Cysteines most sensitive to S-glutathionylation include Cys77, Cys297, Cys319, Cys368, and Cys434, while cysteine disulfides most readily formed are Cys23-Cys38 and Cys257-Cys297. The most reducing conditions at which these modifications are at GSH/GSSG = 10, which computes to an oxidation potential of E h = -268.5 mV, a physiologically relevant value. Under somewhat more oxidizing, but still physiologically relevant, conditions, GSH/GSSG = 1 ( E h = -231.1 mV), a Cys319-Cys319 disulfide is detected far from the dimerization domain of the Keap1 homodimer. The potential impact on protein structure of the glutathionylation of Cys434 and Cys368, the two modified residues in the Kelch repeat domain, was analyzed by docking and energy minimizations of glutathione residues attached to the Kelch repeat domain, whose coordinates are known. The energy minimizations indicated marked alterations in structure with a substantial constriction of Neh2 binding domain of the Keap1 Kelch repeat domain. This alteration appears to be enforced by an extended hydrogen-bonding network between residues on the glutathione moiety attached to Cys434 and amino acid side chains that have been shown to be essential for repression of Nrf2 by Keap1. The modifications of Keap1 detected in the present study are discussed in the context of previous work of others who have examined the sensitivity of cysteines on Keap1 to electrophile assault.

Interactions of the major metabolite of the cancer chemopreventive drug oltipraz with cytochrome c: a novel pathway for cancer chemoprevention.

The major metabolite of the cancer chemopreventive agent oltipraz, a pyrrolopyrazine thione (PPD), has been shown to be a phase 2 enzyme inducer, an activity thought to be key to the cancer chemopreventive action of the parent compound. In cells, mitochondria are the major source of reactive oxygen species (ROS) and cytochrome c (cyt c) is known to participate in mitochondrial electron transport and confer antioxidant and peroxidase activities. To understand possible mechanisms by which PPD acts as a phase 2 enzyme inducer, a study of its interaction with cyt c was undertaken. UV-visible spectroscopic results demonstrate that PPD is capable of reducing oxidized cyt c. The reduced cyt c is stable for a long period of time in the absence of an oxidizing agent. In the presence of ferricyanide, the reduced cyt c is rapidly oxidized back to its oxidized form. Further, UV-visible spectroscopic studies show that during the reduction process the coordination environment and redox state of iron in cyt c are changed. Low-temperature EPR studies show that during the reduction process, the heme iron changes from a low-spin state of s = 1/2 to a low-spin state of s = 0. Room-temperature EPR studies demonstrate that PPD inhibits the peroxidase activity of cyt c. EPR spin trapping experiments using DMPO show that PPD inhibits the superoxide radical scavenging activity of oxidized cyt c. From these results, we propose that PPD interacts with cyt c, binding to and then reducing the heme, and this may enhance ROS levels in mitochondria. This in turn could contribute to the mechanism by which the parent compound, oltipraz, might trigger the cancer chemopreventive increase in transcription of phase 2 enzymes. The modifications of cyt c function by the oltipraz metabolite may have implications for the regulation of apoptotic cell death.

Mutagenesis by exocyclic alkylamino purine adducts in Escherichia coli.

Exocyclic alkylamino purine adducts, including N(2)-ethyldeoxyguanosine, N(2)-isopropyldeoxyguanosine, and N(6)-isopropyldeoxyadenosine, occur as a consequence of reactions of DNA with toxins such as the ethanol metabolite acetaldehyde, diisopropylnitrosamine, and diisopropyltriazene. However, there are few data addressing the biological consequences of these adducts when present in DNA. Therefore, we assessed the mutagenicities of these single, chemically synthesized exocyclic amino adducts when placed site-specifically in the supF gene in the reporter plasmid pLSX and replicated in Escherichia coli, comparing the mutagenic potential of these exocyclic amino adducts to that of O(6)-ethyldeoxyguanosine. Inclusion of deoxyuridines on the strand complementary to the adducts at 5' and 3' flanking positions resulted in mutant fractions of N(2)-ethyldeoxyguanosine and N(2)-isopropyldeoxyguanosine-containing plasmid of 1.4+/-0.5% and 5.7+/-2.5%, respectively, both of which were significantly greater than control plasmid containing deoxyuridines but no adduct (p=0.04 and 0.003, respectively). The mutagenicities of the three exocyclic alkylamino purine adducts tested were of smaller magnitude than O(6)-ethyldeoxyguanosine (mutant fraction=21.2+/-1.2%, p=0.00001) with the N(6)-isopropyldeoxyadenosine being the least mutagenic (mutant fraction=1.2+/-0.5%, p=0.13). The mutation spectrum generated by the N(2)-ethyl and -isopropyldeoxyguanosine adducts included adduct site-targeted G:C-->T:A transversions, adduct site single base deletions, and single base deletions three bases downstream from the adduct, which contrasted sharply with the mutation spectrum generated by the O(6)-ethyldeoxyguanosine lesion of 95% adduct site-targeted transitions. We conclude that N(2)-ethyl and -isopropyldeoxyguanosine are mutagenic adducts in E. coli whose mutation spectra differ markedly from that of O(6)-ethyldeoxyguanosine.

Evidence for the Formation of alpha-Hydroxydialkylnitrosamines in the pH-Independent Solvolysis of alpha-(Acyloxy)dialkylnitrosamines.

A kinetic study of the decay of alpha-(acyloxy)dialkylnitrosamines in aqueous solutions, at 25 degrees C, ionic strength 1 M (NaClO(4)), and 4% acetonitrile by volume, is reported. Rate constants for disappearance of the N-NO chromophore or appearance of benzaldehyde product increase with the introduction of electron-withdrawing groups into the acyloxy moiety. For some compounds, in some regions of pH, the kinetics are non-first-order. For these compounds, in other regions of pH, the rate constants are coincident with those previously reported for the decay of the corresponding alpha-hydroxydialkylnitrosamines. These data provide direct evidence that alpha-hydroxydialkylnitrosamines are intermediates in the pH-independent decomposition of the alpha-(acyloxy)dialkylnitrosamines studied.

Bypass of N²-ethylguanine by human DNA polymerase κ.

The efficiency and fidelity of nucleotide incorporation and next-base extension by DNA polymerase (pol) κ past N(2)-ethyl-Gua were measured using steady-state and rapid kinetic analyses. DNA pol κ incorporated nucleotides and extended 3' termini opposite N(2)-ethyl-Gua with measured efficiencies and fidelities similar to that opposite Gua indicating a role for DNA pol κ at the insertion and extension steps of N(2)-ethyl-Gua bypass. The DNA pol κ was maximally activated to similar levels by a twenty-fold lower concentration of Mn(2+) compared to Mg(2+). In addition, the steady state analysis indicated that high fidelity DNA pol κ-catalyzed N(2)-ethyl-Gua bypass is Mg(2+)-dependent. Strikingly, Mn(2+) activation of DNA pol κ resulted in a dramatically lower efficiency of correct nucleotide incorporation opposite both N(2)-ethyl-Gua and Gua compared to that detected upon Mg(2+) activation. This effect is largely governed by diminished correct nucleotide binding as indicated by the high K(m) values for dCTP insertion opposite N(2)-ethyl-Gua and Gua with Mn(2+) activation. A rapid kinetic analysis showed diminished burst amplitudes in the presence of Mn(2+) compared to Mg(2+) indicating that DNA pol κ preferentially utilizes Mg(2+) activation. These kinetic data support a DNA pol κ wobble base pairing mechanism for dCTP incorporation opposite N(2)-ethyl-Gua. Furthermore, the dramatically different polymerization efficiencies of the Y-family DNA pols κ and ι in the presence of Mn(2+) suggest a metal ion-dependent regulation in coordinating the activities of these DNA pols during translesion synthesis.

Chemistry of the cysteine sensors in Kelch-like ECH-associated protein 1.

The protein Kelch-like ECH-associated protein 1 (Keap1) is a cysteine-rich regulatory and scaffold protein. Human Keap1 contains 27 cysteines. Some of these cysteines are believed to mediate derepression of the transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2), which subsequently upregulates phase 2 enzymes, in response to electrophilic/oxidative assault. Some current models depict a highly select group of two and possibly a few more cysteine residues as key sensors. The assumptions and approaches undergirding these models are commented upon. The chemical reactivity of the cysteines of Keap1 toward an array of electrophiles and one oxidant is reviewed. A number of reports in the recent literature of molecules that putatively modify cysteines of Keap1 are also included. Insights into the current molecular basis of electrophile/oxidant activation of the Nrf2 pathway via reaction at cysteines of Keap1 are discussed. Finally, important knowns and unknowns are summarized.

Novel, Broad Spectrum Anti-Cancer Agents Containing the Tricyclic 5:7:5-Fused Diimidazodiazepine Ring System.

Synthesis of a series of novel, broad-spectrum anti-cancer agents containing the tricyclic 5:7:5-fused diimidazo[4,5-d:4',5'-f][1,3]diazepine ring system is reported. Compounds 1, 2, 8, 11, and 12 in the series show promising in vitro antitumor activity with low micromolar IC(50)'s against prostate, lung, breast, and ovarian cancer cell lines. Some notions about structure-activity relationships and a possible mechanism of biological activity are presented. Also presented are preliminary in vivo toxicity studies of 1 using SCID mice.

Lesion bypass of N2-ethylguanine by human DNA polymerase iota.

Nucleotide incorporation and extension opposite N2-ethyl-Gua by DNA polymerase iota was measured and structures of the DNA polymerase iota-N2-ethyl-Gua complex with incoming nucleotides were solved. Efficiency and fidelity of DNA polymerase iota opposite N2-ethyl-Gua was determined by steady state kinetic analysis with Mg2+ or Mn2+ as the activating metal. DNA polymerase iota incorporates dCMP opposite N2-ethyl-Gua and unadducted Gua with similar efficiencies in the presence of Mg2+ and with greater efficiencies in the presence of Mn2+. However, the fidelity of nucleotide incorporation by DNA polymerase iota opposite N2-ethyl-Gua and Gua using Mn2+ is lower relative to that using Mg2+ indicating a metal-dependent effect. DNA polymerase iota extends from the N2-ethyl-Gua:Cyt 3' terminus more efficiently than from the Gua:Cyt base pair. Together these kinetic data indicate that the DNA polymerase iota catalyzed reaction is well suited for N(2)-ethyl-Gua bypass. The structure of DNA polymerase iota with N2-ethyl-Gua at the active site reveals the adducted base in the syn configuration when the correct incoming nucleotide is present. Positioning of the ethyl adduct into the major groove removes potential steric overlap between the adducted template base and the incoming dCTP. Comparing structures of DNA polymerase iota complexed with N2-ethyl-Gua and Gua at the active site suggests movements in the DNA polymerase iota polymerase-associated domain to accommodate the adduct providing direct evidence that DNA polymerase iota efficiently replicates past a minor groove DNA adduct by positioning the adducted base in the syn configuration.

Synthesis and aqueous chemistry of alpha-acetoxy-N-nitrosomorpholine: reactive intermediates and products.

[reaction: see text] Alpha-acetoxy-N-nitrosomorpholine (7) has been synthesized starting by the anodic oxidation of N-acetylmorpholine in methanol. The 55% yield of N-nitrosomorpholinic acid, after cyanide-for-methoxy group exchange and hydrolysis, is an improvement of approximately 10-fold over our original 10-step method, and this is readily converted to 7. A study of the kinetics of decomposition of 7 in aqueous media at 25 degrees C and 1 M ionic strength was conducted over the pH range from 1 to 12. The reaction exhibited good first-order kinetics at all values of pH, and a plot of the log of k0, the buffer-independent rate constant for decomposition, against pH indicated that a pH-independent reaction dominates in the neutral pH region whereas acid- and base-catalyzed reactions dominate in the low and high pH regions, respectively. Reaction at neutral pH in the presence of increasing concentrations of acetate ion results in a decrease in the value of k(obsd), to an apparent limiting value consistent with a common-ion inhibition by the capture, and competing base-catalyzed hydration of, an N-nitrosiminium ion intermediate. The 100-fold smaller reactivity of 7 at neutral pH compared with its carbon analogue, alpha-acetoxy-N-nitrosopiperidine, is also consistent with the electronic effects expected for such a reaction. The dinitrophenylhydrazones derived from pH-independent and acid-catalyzed reactions are identical in kind and quantity, within experimental error, to those observed in the decay of alpha-hydroxy-N-nitrosomorpholine. Decay of 7 in the presence of benzimidazole buffer results in the formation of 2-(2-(1H-benzo[d]imidazol-1-yl)ethoxy)acetaldehyde (12) and 2-(1H-benzo[d]imidazol-1-yl)ethanol (13). Independent synthesis and study of 12 indicates that it is stable at 80 degrees C in 0.1 M DCl, but it slowly decomposes to 13 in neutral and basic media in a reaction that is stimulated by primary and secondary amines, but not by tertiary amines and carbonate buffer. The benzimidazole trapping studies and those of the stability of 12 indicate the possibility that metabolic activation of N-nitrosomorpholine by hydroxylation alpha to the nitroso nitrogen can result in the deposition of a metastable ethoxyacetaldehyde adduct on the heteroatoms of DNA.

Replication of N2-ethyldeoxyguanosine DNA adducts in the human embryonic kidney cell line 293.

N(2)-Ethyldeoxyguanosine (N(2)-ethyldGuo) is a DNA adduct formed by reaction of the exocyclic amine of dGuo with the ethanol metabolite acetaldehyde. Because ethanol is a human carcinogen, we assessed the biological consequences of replication of template N(2)-ethyldGuo, in comparison to the well-studied adduct O(6)-ethyldeoxyguanosine (O(6)-ethyldGuo). Single chemically synthesized N(2)-ethyldGuo or O(6)-ethyldGuo adducts were placed site specifically in the suppressor tRNA gene of the mutation reporting shuttle plasmid pLSX. N(2)-EthyldGuo and O(6)-ethyldGuo were both minimally mutagenic in double-stranded pLSX replicated in human 293 cells; however, the placement of deoxyuridines on the complementary strand at 5'- and 3'-positions flanking the adduct resulted in 5- and 22-fold enhancements of the N(2)-ethyldGuo- and O(6)-ethyldGuo-induced mutant fractions, respectively. The fold increase in the N(2)-ethyldGuo-induced mutant fraction in deoxyuridine-containing plasmids was similar after replication in 293T cells, a mismatch repair deficient variant of 293 cells, indicating that postreplication mismatch repair has little role in modulating N(2)-ethyldGuo-mediated mutagenesis. The mutation spectrum generated by N(2)-ethyldGuo consisted primarily of single base deletions and adduct site-targeted transversions, in contrast to the exclusive production of adduct site-targeted transitions by O(6)-ethyldGuo. The yield of progeny plasmids after replication in 293 cells was reduced by the presence of N(2)-ethyldGuo in parental plasmids with or without deoxyuridine to 39 or 19%, respectively. Taken together, these data indicate that N(2)-ethyldGuo in DNA exerts its principal biological activity by blocking translesion DNA synthesis in human cells, resulting in either failure of replication or frameshift deletion mutations.

Cancer chemopreventive oltipraz generates superoxide anion radical.

The cancer chemopreventive actions of oltipraz, a member of a class of 1,2-dithiolethiones, have been primarily associated with the induction of phase 2 enzymes mediated by a 41bp enhancer element known as the anti-oxidant response element in the promoter regions of many phase 2 genes. It has been suggested that oxygen radical formation by oltipraz may be a critical mechanism by which it exerts chemoprevention. Therefore, in the present work, studies were performed to directly determine if oltipraz generates oxygen free radicals. Electron paramagnetic resonance (EPR) spin trapping demonstrated that oltipraz slowly reacts in the presence of oxygen to generate the superoxide anion radical. This formation of superoxide by oltipraz was concentration- and time-dependent. EPR oximetry studies showed that oxygen was also slowly consumed paralleling the process of superoxide formation. Thus, oltipraz induced superoxide formation occurs and could be involved in the mechanism by which it exerts chemoprotection.

Polymerization past the N2-isopropylguanine and the N6-isopropyladenine DNA lesions with the translesion synthesis DNA polymerases eta and iota and the replicative DNA polymerase alpha.

The effects of N2-isopropylGua and N6-isopropylAde adducts in template DNA on polymerization by the human replicative DNA polymerase alpha (B-family) and the translesion synthesis DNA polymerases eta and iota (Y-family) were investigated. A direct comparison between the accuracies of DNA synthesis using catalytic fragments of the human DNA polymerases eta and iota is reported. We show that the N2-isopropylGua adduct is a powerful block to polymerization by DNA polymerase alpha. In contrast, the DNA polymerases eta and iota synthesize DNA past the N2-isopropylGua lesion with efficiencies and accuracies opposite the lesion comparable to the unadducted Gua. All three DNA polymerases bypass the N6-isopropylAde adduct with only modest effects on efficiencies and accuracy. These results illustrate the dramatically different consequences to polymerization conferred by the position of the isopropyl adduct when catalyzed by DNA polymerase alpha and the lack of this effect on polymerization by the translesion synthesis DNA polymerases eta and iota. A steady-state kinetic analysis of nucleotide insertion opposite the N2-isopropylGua and the N6-isopropylAde by the DNA polymerases eta and iota was performed to measure the accuracy of DNA synthesis at these lesions. This analysis showed that the DNA polymerases eta and iota preferably insert the correct nucleotide Cyt opposite the N2-isopropylGua lesion and the correct nucleotide Thy opposite the N6-isopropylAde with levels of accuracy similar to those detected opposite the unadducted nucleotides, thus, demonstrating minimal blocking and mutagenic potential by these lesions to the translesion synthesis polymerases. Similarly, a kinetic analysis of polymerization opposite the N6-isopropylAde by the DNA polymerase alpha showed comparable levels of insertion accuracy relative to the unadducted Ade. These results suggest that positioning of the isopropyl adduct on the purine ring to locate this group into the minor groove of the DNA is an important determinant to effect blocked replication by a replicative (B-family) polymerase, but not to affect replication by a translesion synthesis (Y-family) polymerase.