DNA METHYLATION IN SPECIFIC CELLS OF RAT-LIVER BY N-NITROSODIMETHYLAMINE AND N-NITROSOMETHYLBENZYLAMINE

Dose - response curves for the O6-methylation of guanine in the hepatic DNA of Wistar and Sprague-Dawley rats were determined after administration of N-nitrosomethylbenzylamine (NMBzA) or N-nitrosodimethylamine (NDMA). Similar results were obtained for both rat strains but methylation of hepatic DNA by NDMA was approximately 9-fold more efficient than with NMBzA when doses were compared on a molar basis. Comparison by immunohistochemical analysis of the distribution of nuclei containing O6-methylguanine within the liver lobules showed that both agents tended to alkylate cells close to the central veins at the lower doses. With increasing doses, the band width of alkylated cells around the central vein increased, spreading in the case of NDMA virtually into the portal zones, whereas with NMBzA the zone of alkylated nuclei reached little more than halfway from the central vein to the portal zone. These differences in the distribution of alkylated cells may explain the differing hepatic responses to these two nitrosamines.