Kinetics of formation of O6-ethylguanine in,and its removal from liver DNA of rats receiving diethylnitrosamine

The ethylation of rat liver DNA by a single dose of diethylnitrosamine and the stability of O⁶-ethylguanine in vivo were studied. Whereas the dose response relations for 7-ethylguanine, 3-ethyladenine, the pyrimidine oligonucleotide fraction containing ethylphosphotriesters and an as yet unreported Fraction X corresponded with a first-order process of formation, the results suggested a steeper dose-response relation for O⁶-ethylguanine formation. In the dose range 0.5–10 mg/kg diethylnitrosamine, the O⁶-ethylguanine/7-ethylguanine ratio increased progressively with the dose, under conditions in which the in vivo stability (removal rate) of O⁶-ethylguanine was not affected. This led to the hypothesis that the formation of O⁶-ethylguanine, but not that of the other ethylated products, was facilitated by some dose-dependent process or condition. Support for this view was obtained by the markedly enhanced O⁶-[¹⁴C]ethylguanine content of DNA following pretreatment of the rats with non-radioactive diethylnitrosamine which was allowed to be metabolized completely prior to the administration of a tracer dose of [¹⁴C]diethylnitrosamine. Since neither the amounts of the other ethylation products nor the stability of the labelled O⁶-ethylguanine were affected by the pretreatment, changes in carcinogen metabolism or excision rate could be excluded as causes of the observed increase in O⁶-ethylguanine content. The half-life of the condition that facilitates O⁶-ethylguanine formation following pretreatment, may approximate that of O⁶-ethylguanine itself. The nature of the facilitating process and the possible role of O⁶-alkylguanine in hepatocarcinogenesis are discussed.