[Biotransformation of dimethylnitrosamine]

Abstract

NDMA, one of the most widely occurring carcinogenic compounds in the environment, is present in human food (meat, vegetables, cheese and alcohol beverages), in drinking water, in drugs, in cosmetics, in tobacco and its smokes. Furthermore NDMA may be synthesized from nitrates and nitrites and endogen or exogen amines. Since the first observations of MAGEE and BARNES in 1956 on the carcinogenicity of NDMA, this compound was reported to be carcinogenic in a large number of animal species including mammals, birds, amphibia and fish. NDMA requires metabolic activation for its cytotoxic and carcinogenic actions. The major activation step is believed to be the oxygenation of the alpha-carbon catalysed by a Cytochrome P-450-dependent enzyme system commonly know as NDMA-demethylase. Studies on the enzymology of NDMA metabolism show that some Cytochrome P-450 isozymes exhibit significant NDMA-demethylase activity only at high NDMA concentrations. The form of P-450 inducible by factors such as, fasting, diabetes, ethanol consumption and pretreatment with acetone, pyrazole or isopropanol has the higher affinity for NDMA. The gene coding for this isozyme belongs tho the P-450 II E subfamily. Because NDMA-demethylase activity is decreased by monoamine oxidase inhibitors, some authors have suggested a possible role of MAO in NDMA demethylation. This view is not supported by others who don't find evidence for an involvement of MAO in NDMA metabolism. Likewise, there are contradictory reports about the existence of some NDMA demethylase activity in cytosol, nuclei and mitochondria. NDMA demethylation, followed by nonenzymatic cleavage of the hydroxylated methyl group gives formaldehyde and methyldiazohydroxide and then leads to the formation of, a methonium ion, which is able to methylate nucleophilic sites of cellular macromolecules such as, proteins, RNA and DNA. A lot of studies suggest the existence of an alternative pathway to the formation of a methylating agent, denitrosation. Although the nature of mechanisms of denitrosation is not completely known, authors think that the formation of nitrite may represent a detoxification pathway rather than an activation pathway.