Differential sensitivity to ethidium bromide of replicative DNA synthesis and bleomycin-induced unscheduled DNA synthesis in permeable mouse sarcoma cells

Inhibition of O6-alkylguanine-DNA alkyltransferase and DNase I activities in vitro by some alkylating substances and antineoplastic agents

The specificities of the DNA repair enzyme O6-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 different O6-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 approximately 5 micrograms/ml, 15 micrograms/ml, 5 micrograms/ml, 5 micrograms/ml, 100 micrograms/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 micrograms/ml).

Detection of possible DNA repair enzymes on sodium dodecyl sulfate-polyacrylamide gels by protein blotting to damaged DNA-fixed membranes

A novel method for detecting possible DNA repair enzymes on sodium dodecyl sulfate-polyacrylamide gels by blotting them onto a damaged DNA-fixed membrane is presented. To prepare the membrane, highly polymerized calf thymus DNA immobilized on a nylon membrane is damaged chemically. Enzymes, either homogeneous or crude, that are possibly involved in the priming step of DNA repair are fractionated by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and are renatured to active form by incubating the gel in an appropriate buffer. The renatured enzyme is then blotted onto the damaged DNA-fixed membrane, a process during which incision and/or excision are introduced to the damaged DNA by the enzymes. The incision and/or excision provide priming sites for repair DNA synthesis in the subsequent step in which the membrane is incubated with DNA polymerase in the presence of alpha-32P-labeled substrate. The site of substrate incorporation on the membrane reflecting the molecular weight of the repair enzyme is finally visualized by autoradiography. The present technique is established using Escherichia coli exonuclease III and a DNA-fixed membrane treated with bleomycin or acid-depurinated. By application of this method, a priming factor (an exonuclease) involved in the initiation of bleomycin-induced DNA repair is detected in the extract of mouse ascites sarcoma cells, and thus the molecular weight of the enzyme is estimated. Some apurinic/apyrimidinic endonucleases of mammals are also detected by the present procedure.