Comparative studies on hepatic DNA alkylation in rats by N-nitrosomethylethylamine and N-nitrosodimethylamine plus N-nitrosodiethylamine

Abstract

The purpose of this study was to determine whether quantitative differences in hepatic DNA methylation or ethylation are sufficient to explain differences in the carcinogenicity of N-nitrosomethylethylamine (NMEA), N-nitrosodimethylamine (NDMA), and N-nitrosodiethylamine (NDEA). Methylation and ethylation of hepatic DNA were determined in male Fischer 344 rats following a single IP dose of NMEA, NDMA, NDEA or an equimolar mixture of NDMA plus NDEA. The total nitrosamine dose ranged from 0.05 to 0.25 mmol/kg. After 5 h survival, hepatic DNA was extracted by adsorption onto hydroxyapatite. Acid hydrolysates were analyzed by cation exchange HPLC with fluorescence detection. DNA methylation by NMEA (170 μmol O⁶-methylguanine/mol guanine at 0.1 mmol/kg) was comparable to that observed in animals given an equimolar mixture of NDMA plus NDEA, indicating that NMEA is one-half as effective a methylating agent as NDMA. In contrast, the amount of ethylation by NMEA (5.9 μmol O⁶-methylguanine/mol guanine at 0.1 mmol/kg) was approximately 4 times less than that observed in animals treated with an equimolar mixture of NDMA and NDEA. The presence of NDEA had little effect on DNA methylation by NDMA, suggesting that neither synergism nor competition occurs in the simultaneous activation of these two nitrosamines. The role of β-hydroxylation of NMEA as a metabolic pathway that reduces the extent of DNA ethylation is discussed.