O2- and O4-ethylthymine and the ethylphosphotriester dTp(Et)dT are highly persistent DNA modifications in slowly dividing tissues of the ethylnitrosourea-treated rat.

Male Wistar rats received a single i.p. injection of [3H]ethylnitrosourea (140 mg/kg) and were killed after 2 h, 1, 3, 6, 28 or 56 days. DNA of the following organs was isolated and analysed for the presence of 12 different ethylated bases and ethylphosphotriesters: brain, lung, liver, spleen, kidney, intestine, testis and bone marrow. At 2 h after injection the extent of DNA ethylation was found to be heterogeneous: highest in liver and lowest (3-4 times lower) in testis and brain. The rates at which O2- and O4-ethylthymine and the ethylphosphotriester dTp(Et)dT were lost, were very low in all organs except intestine and spleen. Most likely, loss in the latter organs is exclusively due to cell turnover. The rate of O6-ethylguanine repair strongly varied from organ to organ: high in liver, very low in testis and brain and intermediate in the other organs. In none of the DNAs were significant amounts of imidazole ring-opened 7-ethylguanine found. Our results strengthen the notion that a substantial part of carcinogen-induced DNA damage is of a highly persistent nature and might contribute to the carcinogenic process long after the original exposure has occurred.