A Kinetic Approach to the Mechanism of Deoxyribonucleic Acid Cross-Linking by HNO2

Demonstration of an alternative mechanism for G-to-G cross-link formation

The cross-link dG-to-dG is an important product of DNA nitrosation. Its formation has commonly been attributed to nucleophilic substitution of N2 in a guaninediazonium ion by guanine, while recent studies suggest guanine addition to a cyanoamine derivative formed after dediazoniation, deprotonation, and pyrimidine ring-opening. The chemical viability of the latter mechanism is supported here by the experimental demonstration of rG-to-aG formation via rG addition to a synthetic cyanoamine derivative. Thus, all known products of nitrosative guanine deamination are consistent with the postulate of pyrimidine ring-opening. This postulated mechanism not only explains what is already known but also suggests that other products and other cross-links also might be formed in DNA deamination. The study suggests one possible new product: the structure isomer aG(N1)-to-rG(C2) of the classical G(N2)-to-G(C2) cross-link. While the formation of aG(N2)-to-rG(C2) has been established by chemical synthesis, the structure isomer aG(N1)-to-rG(C2) has been assigned tentatively based on its MS/MS spectrum and because this assignment is reasonable from a mechanistic perspective. Density functional calculations show preferences for the amide-iminol tautomer of the classical cross-link G(N2)-to-G(C2) and the amide-amide tautomer of G(N1)-to-G(C2). Moreover, the results suggest that both cross-links are of comparable thermodynamic stability, and that there are no a priori energetic or structural reasons that would prevent the formation of the structure isomer in the model reaction or in DNA.

Products of the reaction between a diazoate derivative of 2'-deoxycytidine and L-lysine and its implication for DNA-nucleoprotein cross-linking by NO or HNO(2)

Recently, we have reported that a stable diazoate intermediate (dCyd-diazoate) is produced upon the reaction of dCyd with nitrous acid and nitric oxide [Suzuki, T., Nakamura, T., Yamada, M., Ide, H., Kanaori, K., Tajima, K., Morii, T., and Makino, K. (1999) Biochemistry 38, 7151-7158]. In this work, the reaction of dCyd-diazoate with L-Lys was investigated. When 0.4 mM dCyd-diazoate was incubated with 10 mM L-Lys in sodium phosphate buffer (pH 7.4) at 37 degrees C, two unknown products were formed in addition to dUrd. By spectrometric measurements, the products were identified as dCyd-Lys adducts with C4(dCyd)-N(alpha)(Lys) and C4(dCyd)-N(epsilon)(Lys) linkages (abbreviated as dCyd-alphaLys and dCyd-epsilonLys, respectively). The yields at the reaction time of 72 h were 28.0% dCyd-alphaLys, 13.4% dCyd-epsilonLys, and 11.1% dUrd with 33.9% unreacted dCyd-diazoate. When 0.4 mM dCyd-diazoate was incubated with 22 mg/mL poly(L-Lys) at pH 7.4 and 37 degrees C for 24 h, 82% of the free dCyd-diazoate disappeared, indicating adduct formation with the polymer. At pH 7.4 and 37 degrees C, dCyd-alphaLys and dCyd-epsilonLys were fairly stable and gave rise to no product after incubation for 7 days. At pH 4.0 and 70 degrees C, both adducts disappeared with the same first-order rate constant of 1.7 x 10(-)(6) s(-)(1) (t(1/2) = 110 h), which was approximately (1)/(3) of that of dCyd. These results suggest that if dCyd-diazoate is formed in DNA in vivo, it may react with free L-Lys and the side chain of L-Lys in nucleoproteins, resulting in stable adducts and DNA-protein cross-links, respectively.

Chemical synthesis of cross-linked purine nucleosides

[reaction: see text] An efficient route to all of the possible cross-linked 2'-deoxypurines 1-3 has been developed by means of the Pd-mediated C-N bond formation in the key step. Utilizing this protocol, the synthesis of the first unnatural protected purine trimeric adduct 4 has been accomplished.

Molecular models that may account for nitrous acid mutagenesis in organisms containing double-stranded DNA

Nitrous acid (NA) is often presumed to cause base substitutions in organisms with double-stranded DNA as a direct consequence of oxidative deamination of adenine and of cytosine residues. Here we summarize evidence indicating that other mechanisms are involved in the case of NA-induced G/C-->A/T transition mutations. We present several models for pathways of NA mutagenesis that may account for our experimental results and overlapping data noted in the literature. One model proposes that the base substitution mutations observed are due to DNA alkylation damage mediated via nitrosation of polyamines and/or other ubiquitous cellular molecules. Other models assume that predisposing lesions, such as G-to-G cross-links, are first formed. The cross-links are pictured as leading to perturbations in DNA structure that allow subsequent opportunity for NA-induced deaminations of cytosine residues in their immediate vicinity. The deaminations preferentially result in G/C-->A/T transition mutations at sites highly dependent on adjoining base sequence context (i.e., in NA "mutational hotspots"). A final model proposes that NA-induced G/C-->A/T transition mutations arise mainly from oxidative deamination of guanosine residues and not from deamination of cytosine residues in duplex DNA.

Effect of in vitro cleavage of apurinic/apyrimidinic sites on bleomycin-induced mutagenesis of repackaged lambda phage

Previous studies have revealed bleomycin to be a potent base-substitution mutagen in repackaged phage lambda. In order to assess the role of apurinic/apyrimidinic (AP) sites in bleomycin-induced mutagenesis, bleomycin-damaged lambda DNA was treated with putrescine or endonuclease IV to effect cleavage of bleomycin-induced AP sites. The DNA was then packaged, the phage grown in SOS-induced E. coli, and the frequency of clear-plaque mutants in the progeny was determined. Bleomycin-induced mutagenesis was decreased approx. 2-fold by treating the DNA with putrescine, but was unaffected by endonuclease IV. The results are consistent with the production of bleomycin-induced mutation at certain AP sites having a closely opposed single-strand break, since such sites are cleaved by putrescine but not by endonuclease IV.

Molecular dosimetry of 8-MOP + UVA-induced DNA photoadducts in Saccharomyces cerevisiae: correlation of lesions number with genotoxic potential

Acid hydrolysis of purified DNA extracted from cells of a haploid repair-proficient (RAD) yeast strain that had been treated with 8-MOP + UVA revealed the existence of two major and one minor thymine photoproduct. At survival levels of the RAD strain between 100% and 1% furanside monoadducts constituted the major DNA lesion, followed by diadducts that, at the lowest survival level, nearly reached 50% of the thymine photoproducts; pyrone-side monoadducts were only detectable at the highest UVA exposure dose applied and clearly constitute a minority photoproduct. The number of induced diadducts was verified by determination of interstrand cross-links via denaturation and renaturation of 8-MOP + UVA-treated DNA from RAD and rad2 yeast strain. 8-MOP + UVA was shown to induce two types of locus-specific mutations: reversion of the lys1-1 ochre allele was between 20- to 50-fold higher than that of the his4-38 frameshift allele. Mutant yield for the lys 1-1 reversion was the same in RAD and excision repair-deficient rad2-20 strains whereas frameshift mutagenesis was about eightfold higher in the rad2-20 background.

Biological and chemical effects of mustard gas in yeast

Mustard gas induces inactivation and mutation in yeast. Both effects are dose-proportional, indicating single-hit events. Induction of both effects is influenced by the cell's capacity for DNA dark-repair, whereby the probability of reversion is highest in repair-proficient cells. Binding of mustard gas to cells and probably to DNA is independent of DNA-repair systems. The number of inter-strand cross-links, as determined by assaying for renaturability of alkalidenatured DNA, increases in a dose-proportional manner. At 37% survival an excision-deficient strain contains 55 inter-strand cross-links. Chromatographic analysis yields several alkylation products of DNA. Their relative frequencies resemble the values reported for E. coli and bacteriophage T7.

Formation and fate of cross-links induced by polyfunctional anticancer drugs in yeast

A method to detect low levels of interstrand cross-links in DNA of Saccharomyces cerevisiae is described. Isopycnic ultracentrifugation of alkali-treated, unpurified Eaton press homogenates allows the detection of less than one cross-link per yeast chromosome. Efficient separation of single- and double-stranded DNA requires low cell density and addition of glycerol during homogenization. Using a yeast strain defective in excision repair, a dose dependent formation of interstrand cross-links after treatment of cells with biological doses of nitrogen mustard, Triaziquone and Chloramubil could be demonstrated. The most powerful of these alkylating agents is Triziquone: half of the DNA molecules are shown to be cross-linked after a 12 min exposure to 9 X 10(-9) g/ml of the drug. The cross-linking reaction continues after excessive alkylating agent is removed. After having reached a maximum the fraction of renaturable DNA decreases upon further incubation. The speed of this "after-reaction" depends on temperature: 48 h after the end of treatment renaturability of DNA has almost completely disappeared when cells are kept at 36 degrees C.