3-n(4)-ethanocytosine and chloroacetaldehyde

Etheno (epsilon) adducts are formed in reaction of DNA bases with various environmental carcinogens and endogenously created products of lipid peroxidation. Chloroacetaldehyde (CAA), a metabolite of carcinogen vinyl chloride, is routinely used to generate epsilon-adducts. We studied the role of AlkB, along with AlkA and Mug proteins, all engaged in repair of epsilon-adducts, in CAA-induced mutagenesis. The test system used involved pIF102 and pIF104 plasmids bearing the lactose operon of CC102 or CC104 origin (Cupples and Miller (1989) [17]) which allowed to monitor Lac(+) revertants, the latter arose by GC-->AT or GC-->TA substitutions, respectively, as a result of modification of guanine and cytosine. The plasmids were CAA-damaged in vitro and replicated in Escherichia coli of various genetic backgrounds. To modify the levels of AlkA and AlkB proteins, mutagenesis was studied in E. coli cells induced or not in adaptive response. Formation of varepsilonC proceeds via a relatively stable intermediate, 3,N(4)-alpha-hydroxyethanocytosine (HEC), which allowed to compare repair of both adducts. The results indicate that all three genes, alkA, alkB and microg, are engaged in alleviation of CAA-induced mutagenesis. The frequency of mutation was higher in AlkA-, AlkB- and Mug-deficient strains in comparison to alkA(+), alkB(+), and microg(+) controls. Considering the levels of CAA-induced Lac(+) revertants in strains harboring the pIF plasmids and induced or not in adaptive response, we conclude that AlkB protein is engaged in the repair of epsilonC and HEC in vivo. Using the modified TTCTT 5-mers as substrates, we confirmed in vitro that AlkB protein repairs epsilonC and HEC although far less efficiently than the reference adduct 3-methylcytosine. The pH optimum for repair of HEC and epsilonC is significantly different from that for 3-methylcytosine. We propose that the protonated form of adduct interact in active site of AlkB protein.

Topics: Acetaldehyde; Cytosine; DNA Repair; Escherichia coli; Escherichia coli Proteins; Mixed Function Oxygenases; Mutagenesis; Mutagenicity Tests; Mutagens; Transformation, Bacterial

Oxidative stress converts lipids into DNA-damaging agents. The genomic lesions formed include 1,N(6)-ethenoadenine (epsilonA) and 3,N(4)-ethenocytosine (epsilonC), in which two carbons of the lipid alkyl chain form an exocyclic adduct with a DNA base. Here we show that the newly characterized enzyme AlkB repairs epsilonA and epsilonC. The potent toxicity and mutagenicity of epsilonA in Escherichia coli lacking AlkB was reversed in AlkB(+) cells; AlkB also mitigated the effects of epsilonC. In vitro, AlkB cleaved the lipid-derived alkyl chain from DNA, causing epsilonA and epsilonC to revert to adenine and cytosine, respectively. Biochemically, epsilonA is epoxidized at the etheno bond. The epoxide is putatively hydrolyzed to a glycol, and the glycol moiety is released as glyoxal. These reactions show a previously unrecognized chemical versatility of AlkB. In mammals, the corresponding AlkB homologs may defend against aging, cancer and oxidative stress.

Topics: Acetaldehyde; Adenine; Cytosine; DNA Adducts; DNA Damage; DNA Glycosylases; DNA Repair; Escherichia coli; Escherichia coli Proteins; Lipid Peroxidation; Mixed Function Oxygenases; Mutagenesis

The 3,N(4)-ethenocytosine (epsilon C) residue might have biological role in vivo since it is recognized and efficiently excised in vitro by the E. coli mismatch-specific uracil-DNA glycosylase (MUG) and the human thymine-DNA glycosylase (hTDG). In the present work we have generated mug defective mutant of E. coli by insertion of a kanamycin cassette to assess the role of MUG in vivo. We show that human TDG complements the enzymatic activity of MUG when expressed in a mug mutant. The epsilon C-DNA glycosylase defective strain did not exhibit spontaneous mutator phenotype and did not show unusual sensitivity to any of the following DNA damaging treatments: methylmethanesulfonate, N-methyl-N'-nitro-N-nitrosoguanidine, ultraviolet light, H(2)O(2), paraquat. However, plasmid DNA damaged by 2-chloroacetaldehyde treatment in vitro was inactivated at a greater rate in a mug mutant than in wild-type host, suggesting that MUG is required for the in vivo processing of the ethenobases. In addition, 2-chloroacetaldehyde treatment induces preferentially G.C --> C.G and A.T --> T.A transversions in mug mutant. Comparison of the mutation frequencies induced by the site-specifically incorporated epsilon C residue in E. coli wild-type versus mug indicates that MUG repairs more than 80% of epsilon C residues in vivo. Furthermore, the results show that nucleotide excision repair and recombination are not involved in the processing of epsilon C in E. coli. Based on the mutagenesis data we suggest that epsilon C may be less toxic and less mutagenic than expected. The increased spontaneous mutation rate for G.C --> A.T transition in the ung mug double mutant as compared to the single ung mutant suggest that MUG may be a back-up repair enzyme to the classic uracil-DNA glycosylase.

Topics: Acetaldehyde; Base Pair Mismatch; Cytosine; DNA Damage; DNA Repair; Escherichia coli; Genetic Complementation Test; Humans; Microbial Sensitivity Tests; Mutagenesis, Insertional; Mutagens; Mutation; Plasmids; Thymine DNA Glycosylase

We have previously reported that human cells and tissues contain a 1,N6-ethenoadenine (epsilon A) binding protein, which, through glycosylase activity, releases both 3-methyladenine (m3A) and epsilon A from DNA treated with methylating agents or the vinyl chloride metabolite chloroacetaldehyde, respectively. We now find that both the partially purified human epsilon A-binding protein and cell-free extracts containing the cloned human m3A-DNA glycosylase release all four cyclic etheno adducts--namely epsilon A, 3,N4-ethenocytosine (epsilon C), N2,3-ethenoguanine (N2,3-epsilon G), and 1,N2-ethenoguanine (1,N2-epsilon G). Base release was both time and protein concentration dependent. Both epsilon A and epsilon C were excised at similar rates, while 1,N2-epsilon G and N2,3-epsilon G were released much more slowly under identical conditions. The cleavage of glycosyl bonds of several heterocyclic adducts as well as those of simple methylated adducts by the same human glycosylase appears unusual in enzymology. This raises the question of how such a multiple, divergent activity evolved in humans and what may be its primary substrate.

Topics: Acetaldehyde; Adenine; Animals; Base Sequence; Cattle; Cytosine; DNA; DNA Glycosylases; DNA-Binding Proteins; Guanine; HeLa Cells; Humans; Molecular Sequence Data; N-Glycosyl Hydrolases; Oligonucleotide Probes

Topics: Acetaldehyde; Adenine; Alkylation; Animals; Carcinogens; Cytosine; DNA; DNA Damage; Ethylene Oxide; Guanine; Microsomes, Liver; Rats; RNA; Vinyl Chloride; Vinyl Compounds

Chloroacetaldehyde-modified poly(rC) or poly(dC) was prepared containing either 8-36% 3,N4-ethenocytidine (epsilon C) or 8-36% of a mixture of epsilon C and the hydrated epsilon C (epsilon C . H2O), with the hydrate greatly predominating (greater than 90%). These ribo- and deoxyribonucleotide templates were transcribed with DNA-dependent RNA polymerases from Escherichia coli and calf thymus, in the presence of either Mn2+ or Mg2+ and all four ribonucleoside triphosphates. All the polymers tested were transcribed with either cation present. In an earlier report from this laboratory [Spengler, S., & Singer, B. (1981) Nucleic Acids Res. 9. 365], transcriptional ambiguities resulting from epsilon C residues in enzymatically synthesized poly(rC, epsilon rC) were studied with E. coli DNA-dependent RNA polymerase in the presence of Mn2+. The misincorporations there reported were confirmed when poly(rC, epsilon rC) and poly(dC, epsilon dC), prepared by reaction of poly(rC) and poly(dC) with CAA, were transcribed in the presence of either Mn2+ or Mg2+. We now report that the presence of hydrated epsilon C in polymers also leads to misincorporations but with reproducible differences from those found with epsilon C alone. Nearest-neighbor analysis of the transcription products showed that the hydrate caused misincorporation of A greater than U much greater than C while epsilon C caused misincorporation of U greater than A much greater than C. The extent of misincorporation in transcription was less with Mg2+ than with Mn2+, but the pattern of ambiguity was the same with both cations and with both ribo- and deoxyribocytidylate polymers. Calf thymus DNA-dependent RNA polymerase IIB was also used to transcribe deoxyribocytidine polymers with Mn2+ as the cation. epsilon C and epsilon C . H2O both caused a high level of misincorporation of U , A, and C, but the preferred misincorporations differed slightly from those found with E. coli DNA-dependent RNA polymerase. For both prokaryotic and eukaryotic enzymes, the type of misincorporation resulting from the loss of hydrogen bonding by modification of the N-3 of C not only differed between epsilon C and the hydrated intermediate but also both differed from the transcriptional errors resulting from the presence of 3-methylcytidine in poly(dC) or poly(rC). We conclude that the errors made by these polymerases during transcription do not result primarily from the conditions used (cation, ribo- or deoxyribotemplate) but must b

Topics: Acetaldehyde; Animals; Base Sequence; Cattle; Cytosine; Deoxyribonucleotides; DNA-Directed RNA Polymerases; Escherichia coli; Poly C; Ribonucleotides; RNA, Messenger; Transcription, Genetic