Some aspects of cytochrome P450-dependent denitrosation of N-nitrosamines

The present paper deals with three aspects of cytochrome P450-dependent denitrosation of N-nitrosamines. (1) Nitrate was found in addition to nitrite as a metabolic product of the denitrosation reaction when N-nitrosamines were incubated with a microsomal system. This could also be shown when nitric oxide was added to the microsomes. (2) In order to determine the amount of denitrosation in vivo, the nitroso group of N-nitroso-N-methylaniline was labelled with the 15N isotope and administered to rats; then, the concentrations of 15N-nitrate and 15-N-nitrite in the urine were quantified by measuring the reaction of nitrate and benzene to nitrobenzene. It is estimated from these data that about 33% of the applied dose of 15N-nitroso-N-methylaniline is denitrosated in vivo. (3) Although N-nitrosodiphenylamine (NDPhA) has been classified as a noncarcinogen, recent long-term and short-term studies have cast some doubt. In order to evaluate the mechanism by which NDPhA exerts its possible genetoxic effects, its metabolism was studied in vitro, and NDPhA and its metabolites were tested for induction of DNA single-strand breaks in rat hepatocytes and in Chinese hamster V79 cells. One metabolite was identified as diphenylamine; others were suspected to be the 4-hydroxylated derivative and its corresponding quinoneimine. NDPhA caused DNA damage in rat hepatocytes but not in V79 cells. Diphenylamine also gave negative results in V79 cells, but its putative metabolite, diphenylhydroxylamine, induced a significant increase in DNA single-strand breaks.(ABSTRACT TRUNCATED AT 250 WORDS)