Oxygens in DNA are main targets for ethylnitrosourea in normal and xeroderma pigmentosum fibroblasts and fetal rat brain cells.

Abstract

ONE of the major problems in the study of chemical carcinogenesis is to identify in cells or animals the nucleic acid adducts known from in vitro experiments. When normal or xeroderma pigmentosum (XP) human fibroblasts or fetal rat brain cells (FBC) in culture are treated with the carcinogen, ethylnitrosourea (EtNU), to an extent permitting good cell survival (2–5 ethyl groups per 100,000 bases), we find that the nature and proportion of the alkyl products in their DNA are remarkably similar to those found when DNA is alkylated with EtNU to a high extent in vitro (1 ethyl group per 100 bases). The products quantified in this study are ethylphosphotriesters, 7-ethylguanine (7-EtG), 3-ethyladenine (3-EtA), O6-ethyl-deoxyguanosine (O6-EtdGuo), O2-ethylthymidine (O2-EtThd), O4-ethylthymidine and O2-ethylcytosine (O2-EtC). We present here the first experimental evidence that pyrimidine oxygen atoms are major sites of reaction in cells treated with EtNU. A comparison of the present model experiments with purified DNA and poly(dG) · poly(dC) with earlier studies on single-stranded polynucleotides (refs 6, 10, Table 1) indicates that Watson–Crick base-pairing does not significantly hinder alkylation of the oxygens. This is in contrast to previous findings concerning the very low reactivity of base-paired nitrogens. We explain this on the basis that only the oxygens have a pair of electrons not directly involved in hydrogen bonding.