Methylation of DNA in target and non‐target organs of the rat with methylbenzylnitrosamine and dimethylnitrosamine

The sites of labelling of DNA with [14C]methyl groups from methylbenzylnitrosamine and dimethylnitrosamine were studied in rat oeso‐phageal epithelium and liver. All four combinations of tissue and carcinogen were studied. Tissues were labelled in vitro and the DNA contained therein purified and hydrolysed (pH 1, 37° C) to free purines and apurinic acid. Quantitative analysis was performed with the aid of thin‐layer chromatography. The apurinic acid and 7‐methylguanine fractions were found to be extensively labelled. Smaller amounts of radioactivity were found in O6‐methyl‐guanine and some of the methylated adenines. The same carcinogen produced different patterns of labelling in oesophageal and liver DNA. The proportion of O6‐methylguanine to 7‐methylguanine was higher when the methylating agent was a carcinogen specific for the organ.