Inhibition ofO 6-alkylguanine-DNA alkyltransferase and DNase I activities in vitro by some alkylating substances and antineoplastic agents

Summary The specificities of the DNA repair enzymeO ⁶-alkylguanine-DNA alkyltransferase from brain and liver cells of the chick embryo and of DNase I were demonstrated in vitro by their response to substrate DNA pretreated with monofunctional alkylating agents of differentO ⁶-guanine alkylating ability and some antineoplastic agents. Treatment of DNA with ethidium bromide, Hoechst 33258, doxorubicin, Fe²⁺/bleomycin, and suramin resulted in a dose-dependent diminution of alkyltransferase activity (DE₅₀ 5 g/ml, 15 g/ml, 5 g/ml, 5 g/ml, 100 g/ml, respectively). Apart from bleomycin, comparable results were obtained with DNase I. Thermal denaturation of the substrate DNA reduced both alkyltransferase and DNase I activity. No effect was seen with X-irradiation. Cisplatin decreased only DNase I activity. Some topoisomerase II and/or gyrase inhibitors remained without significant effects on the alkyltransferase reaction whereas DNA catabolism by DNase I was diminished in a dose-dependent manner (DE₅₀ between 6.5 and 19 g/ml).Abbreviations AT alkyltransferase - BB bisbenzimide - EB ethidium bromide - DOX doxorubicin - CDDP cis-diamminedichloroplatinum (II) - MMS methylmethanesulphonate - EMS ethylmethanesulphonate - MNU methylnitrosourea - ENU ethylnitrosourea