Metabolic denitrosation of N-nitrosamines: mechanism and biological consequences.

NADPH-dependent microsomal metabolism of N-nitrosamines results in both oxidative dealkylation and denitrosation of the molecule. For denitrosation, two enzymatic mechanisms have been proposed: (i) cytochrome P450 (P450)-dependent one-electron reduction of the nitrosamine molecule, resulting in the formation of nitric oxide (NO) and secondary and primary amine, and (ii) liberation of NO via an oxidative mechanism mediated by a P450-dependent one-electron abstraction. In order to clarify the mechanism of denitrosation, the metabolism and kinetics of N-nitrosodibenzylamine (NDBzA) and its corresponding secondary amine dibenzylamine were studied. The main metabolites of NDBzA are benzaldehyde, the primary amine benzylamine and nitrite. An important finding is that benzaldehyde is generated more rapidly from dibenzylamine than from the parent NDBzA. During reductive denitrosation of NDBzA, the oxygen atom in benzaldehyde is derived from air, while benzaldehyde generated via the oxidative mechanism of denitrosation receives its oxygen atom from water due to hydrolysis of the intermediary benzylidenebenzylamine. Microsomal incubation of NDBzA in buffer containing 18O-H2O resulted in no incorporation of 18O from water into benzaldehyde, which could be related to the formation of the corresponding benzaldehyde, which could be related to the formation of the corresponding benzylidenebenzylamine. It is concluded that NDBzA is denitrosated by the proposed reductive mechanism. Current belief is that denitrosation leads to detoxification of the NA molecule; however, toxic effects cannot be excluded if the conversion of NO into NO2- and NO3- involves intermediary formation of the NO2 radical.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitrogen dioxide induces DNA single-strand breaks in cultured Chinese hamster cells.

Gaseous nitric oxide (NO) and nitrogen dioxide (NO2) were tested for their potential to induce DNA single-strand breaks (SSBs) in Chinese hamster cells (V79 cells). The alkaline elution assay was used for the detection of SSBs. V79 cells were exposed to NO and NO2 in N2 in varying concentrations (0-500 p.p.m.) and over varying periods (5-30 min). NO treatment did not result in any detectable DNA damage. NO2 led to a dose- and time-dependent increase of the rate of SSBs and the amount was dependent on concentration of NO2 and the length of exposure. The lowest observable effective concentration which was statistically different from control values was 10 p.p.m. exposed for 20 min. The metabolites of both gases, nitrite (NO2-) and nitrate (NO3-), had no DNA-damaging activity up to a concentration of 1 mM. The mechanism by which NO2 may generate SSBs is discussed.