Differences in the Removal of N-Methyl-N-nitrosourea-methylated Products in DNase I-sensitive and -resistant Regions of Rat Brain DNA1

The present study with N-methyl-N-nitrosounea and rat brain DNA was performed in order to study the distribution of alkyl ated products and the difference in the removal of these products from DNase I-sensitive and -resistant regions of DNA. Nuclei were isolated from N-[3H]methyb-N-nitnosourea-treated rats and incubated in the presence of DNase I (5 pg/mI). Digested DNA was further hydrolyzed in 0.1 N HCI, and the alkylated products were identified by chromatography on a cation-exchange column. Resistant DNA was isolated, hydro lyzed, and again the alkylated products were determined. At 4 hr, the specific activity of all the alkybated products in the sensitive regions of DNA was several times higher than the resistant fraction. The rate of loss for the products was greaten in the sensitive than the resistant fractions. 06-Methylguanine was removed from the sensitive regions but was more stable in the resistant regions. These results suggest that DNase 1-sen sitive regions of the DNA are preferentially alkylated by Nmethyl-N-nitrosounea and that the alkybatedproducts, including 06-methylguanine, are selectively removed from the DNase Isensitive regions of the DNA.