The reactions of the hydroxymethyl radical with 1,3-dimethyluracil and 1,3-dimethylthymine

Thymine carboxylation: nucleophilic addition of carbon dioxide radical anion

PURPOSE

The nucleophilic addition properties of carbon dioxide radical anion (CO2*-) towards N1-substituted thymine derivatives in aqueous solution is studied for comparison with their one-electron reducing reactivity.

MATERIAL AND METHODS

N2O-Saturated aqueous solutions of 1-methylthymine, 1,3-dimethylthymine, and thymidylyl(3'-->5')-thymidine containing excess formate ions were gamma-irradiated at 1.0Gy min(-1). Several carboxylated thymines were isolated by preparative HPLC and identified by GC-MS, NMR and X-ray crystallography.

RESULTS

Along with one-electron reduction yielding N-substituted 5,6-dihydrothymines and C5--C5'-linked dihydrothymine dimers, the addition of CO2 radical anion(s) to the C5--C6 double bond of N-substituted thymines produced several mono- and di-carboxylic acids, among which N-substituted derivatives of 5,6-dihydrothymine-6-carboxylic acid [5-methyldihydroorotic acid (5-methyl-DHO)] were produced in the highest yield. Similar carboxylation by CO2 radical anions was also observed for thymine dinucleoside monophosphate. The X-ray structure of cis-5,6-dihydro-1-methylthymine-6-carboxylic acid (cis-1,5-dimethyl-DHO) was determined to show a chair conformation in the crystal.

CONCLUSIONS

The CO2 radical anion is a nucleophilic radical with rather low reduction potential, thereby possessing a dual reactivity of radical addition preferentially at C6 and one-electron reduction towards thymine-related compounds.

Interaction of trichloromethyl free radicals with thymine in a model system: a mass spectrometric study

In previous studies from our laboratory we found that the CCl4 reactive metabolites produced during enzymatic in vitro or in vivo CCl4 biotransformation covalently bind to DNA. Further, chemically produced.CCl3 produce many adducts of unknown structure with the four DNA bases when the reaction proceeds in model systems. In the present work, we describe our attempt to elucidate by GLC/MS the structures of the adducts resulting when chemically generated.CCl3 interact with thymine. The following reaction products were identified: (i) 5-hydroxymethyl uracil; (ii) thymineglycol; (iii) 5-trichloroethyl uracil (tentative) and (iv) two isomeric 5,6-monochloro monohydroxy adducts of thymine (tentative). Reaction products found do not involve thymine positions directly participating in base-pairing processes. However, alterations in thymine structure reported if they occurred in DNA from livers of CCl4 poisoned animals, might potentially have biological significance that remains to be established.

Alkylation and cleavage of DNA by carbon-centered radical metabolites

Although carbon-centered radicals are formed during the metabolism of several genotoxic compounds, they have received little attention as DNA damaging agents. Carbon-centered radicals, however, can both cleave the DNA backbone and alkylate DNA bases, as has been demonstrated to occur in chemical and biochemical systems. Also, in vivo DNA alkylation by methyl radicals has been evidenced by isolation of C8-methylguanine in hydrolysates of DNA from rats administered 1,2-dimethylhydrazine. While most of the studies related to DNA damage by free radicals have been focused on oxyradicals, further studies on DNA alterations promoted by carbon-centered radicals may be necessary to elucidate the mechanisms of action of chemical mutagens and carcinogens.

The chemistry of free-radical-mediated DNA damage

In the living cell, ionizing radiation can cause DNA damage by the direct effect (ionization of DNA) and the indirect effect (reaction of radicals formed in the neighborhood of DNA with DNA, e.g., OH, eaq-, H, protein- and glutathione-derived radicals). Properties of the base radical cations have been studied in model systems using SO4- radical to oxidize the nucleobases in aqueous solution. The pKa values of some nucleobase radical cations are reported, so are the ensuing reactions of the thymidine radical cation with water. The products of reactions are compared with those formed by OH radical attack. The reaction of eaq- with the nucleobases yields radical anions. Protonation at heteroatom sites and at carbon are discussed, and some recent results regarding the electron transfer to adjacent nucleobases as well as to 5-bromouracil are reported. A brief account is given on the reaction of carbon-centered radicals with the nucleobases. These reactions may mimic the reactions of protein-derived radicals with DNA. Glutathione is present in cells at rather high concentrations and is expected to act as an H- or electron-donor in repairing radiation-induced DNA damage (chemical repair). As thiyl radicals are known to also undergo the reverse reaction, i.e., H-abstraction from suitable solutes, some experiments are reported which probe this type of reaction with dilute DNA solutions. In some polynucleotides radical transfer from the base radical to the sugar moiety occurs with the consequence of strand breakage and base release. Some currently held mechanistic concepts are discussed. Attention is drawn to some important open questions which should be addressed in the near future.