Enzymatic removal of O6-ethylguanine versus stability of O4-ethylthymine in the DNA of rat tissues exposed to the carcinogen ethylnitrosourea: possible interference of guanine-O6 alkylation with 5-cytosine methylation in the DNA of replicating target cells.

In order to compare the kinetics of their enzymatic elimination from the DNA of liver, kidney, lung, and brain, the alkylation products O4-ethyl-2'-deoxythymidine (O4-EtdThd) and O6-ethyl-2'-deoxyguanosine (O6-EtdGuo) were quantitated by competitive radioimmunoassay over a period of 48 h after a single pulse of the carcinogen N-ethyl-N-nitrosourea (EtNU) applied i.p. to 10 and 28-day-old BDIX-rats. The content of O4-EtdThd in the DNA of all organs analyzed remained stable, while O6-EtdGuo (initially formed in DNA with 3-4 times higher frequency than O4-EtdThd) was rapidly removed from the DNA of liver, followed by lung and kidney, but persisted strongly in the DNA of brain. At 48 h after the EtNU-pulse, the O4-EtdThd content of liver DNA exceeded the O6-EtdGuo content by about a factor of 4. Since both O6-EtdGuo and O4-EtdThd are miscoding DNA lesions, the lack of enzymatic removal of O4-EtdThd is surprising in view of the apparent concern of cells to restore the integrity of the O6-position of guanine. Genetic consequences more specifically connected with the formation of O6-alkylguanine in DNA might be considered, e.g., possible alterations of gene expression via interference with enzymatic 5-cytosine methylation in 5'-CpG-3' sequences of newly replicated DNA.