Studies on lung tumours. III. Oxidative metabolism of dimethylnitrosamine by rodent and human lung tissue

The oxidative metabolism of the carcinogen dimethylnitrosamine (DMN) was studied in mouse, rat, hamster and human respiratory tissue. [14C]DMN was purified by Dowex-1-bisulfite column chromatography to remove a contaminant (probably [14C]formaldehyde) interfering with the enzyme assay. Since formaldehyde and methyl carbonium ions - yielding methanol with water - are considered to be the primary products of DMN metabolism, tissue slices were assayed for the production of [14C]CO2 from 14C-labelled methanol, formaldehyde, formate, and DMN. Oxidation of formaldehyde to formate was not, but oxidation of formate to CO2 was very much rate-limiting. This rate-limiting step was circumvented by introducing quantitative chemical oxidation of formate to CO2 by mercury(II)chloride following the enzymic reaction. Since oxidation of methanol to CO2 proved to be insignificant, production of CO2 from DMN by lung tissue enzymes and HgCl2 may serve as a parameter for N-demethylating activity and the production of the suspected carcinogenically active methyl carbonium ions. The DMN-N-demethylating activities of lung tissue slices of two mouse strains with widely different susceptibilities to formation of lung adenomas by DMN differed significantly, but the difference seemed too small to explain the divergence in tumourigenic response. The enzymatic activities decreased in hamster bronchus, hamster trachea, hamster lung, GRS/A mouse lung, C3Hf/A mouse lung, human lung, Sprague-Dawley rat lung, in that order. The reported resistance of the hamster respiratory system to tumour induction by DMN may therefore not be due to poor DMN-N-demethylating capacity.

Metabolism of dimethylnitrosamine to mutagenic intermediates by kidney microsomal enzymes and correlation with reported host susceptibility to kidney tumors

Purified kidney microsomal enzymes from C3H/HeJ and Ha/1CR mice exhibited an unusually high capacity to generate mutagenic metabolites from dimethylnitrosamine (DMNA) when compared with similar enzyme preparations from BALB/cJ and RF/J mice. These results suggested that the DMNA-activating enzymes involved in mutagen formation were either present at higher levels or were more active in the kidneys of C3H/HeJ and Ha/ICR mice than in those of the other two strains. This strain difference correlated with the established susceptibility of these four strains to the neoplastic activity of DMNA.

Partial hepatectomy and toxicity of dimethyl-nitrosamine and carbon tetrachloride, in relation to the carcinogenic action of dimethylnitrosamine

The yield of tumours in the liver of rats was increased when dimethylnitrosamine was given 1, 6 or 12 h after partial hepatectomy and still further increased if it was given after an interval of 24-72 h. The increase was greater after two-thirds than after one-third hepatectomy. An increase in the number of kidney tumours was also found. Microsomal DMN-demethylase activity was depressed after partial hepatectomy for up to 6 days in mice and rats. The LD50 of DMN on the other hand was decreased for 3 days, after which it returned to normal. The extent of liver necrosis produced by DMN was increased at 6 and 24 h after partial hepatectomy but was within the usual range at longer intervals. These results suggest that prolonged exposure of the tissues to DMN after partial hepatectomy played a significant role in the development of liver tumours as well as those in the kidney, in addition to the role of regeneration of the liver, and that the relative roles were still to be elucidated.

The binding of dimethylnitrosamine to nucleic acids catalyzed by liver enzymic fractions

The in vitro binding of a metabolite of dimethylnitrosamine (DMNA) to calf thymus DNA catalyzed by a microsomal system from rat liver is reported. The amount ob binding is unaffected by using microsomes induced by 3-methylcholanthrene and/or normal or induced pH 5 enzymes. The microsomal system is also effective in catalyzing covalent binding of the nitroso compound with synthetic polyribonucleotides.

Identification of dimethylnitrosoamine metabolites in vitro

The incubation of dimethylnitrosoamine (DMNA) in the presence of rat liver microsomes leads to production of formaldehyde, formic acid, methylamine, and N-methylhydrazine. When pH 5-enzymes are added to the medium there is also the formation of N-methylhydroxylamide and N,N-dimethylhydrazine. The last compound is the only metabolite produced, to a lesser extent, by the pH 5-enzymes. Thus, the denitrosated or non-denitrosated metabolites are produced either by an oxidative dealkylation and by a reduction of DMNA, catalysed by microsomal and cellular soluble enzymes.

Factors affecting metabolism and mutagenicity of dimethylnitrosamine and diethylnitrosamine

For exploration of the factors affecting dimethylnitrosamine (DMN) mutagenicity, for gathering of information on the metabolism of DMN, a frequently used and relatively well-understood carcinogen, and for explanation of metabolic variations in DMN carcinogenicity, parallel in vitro assays of the microsomal activation of DMN to a mutagen and of DMN demethylation were performed. Salmonella typhimurium G46 reversions to histidine independence increase linearly with time of incubation for 30 min. At low concentrations of microsomal protein, increases in protein yield a more than proportional increase in mutations. Increasing DMN concentration saturates the enzyme, yielding less demethylation and fewer mutations proportionately. Mutagenesis is completely inhibited by 1 mM 2-diethyl-aminoethyl-2,2-diphenylvalerate. When both DMN and microsomal protein are varied at high concentrations, there is a simple linear relationship between mutagenicity and DMN demethylase activity. Thus DMN demethylase activity may be the primary controlling factor in the metabolism of DMN to a mutagen, and probably to a carcinogen; other simultaneous pathways of DMN metabolism proportional to demethylation have not been ruled out. Induction with both phenobarbital and 3-methylcholanthrene (3-MC) increased rat and mouse liver DMN demethylase activity. Mouse liver microsomes from the C57BL/6 strain demethylate DMN at a markedly lower rate than do microsomes from the C3H strain, but after 3-MC induction the relationship is reversed. Strain differences in activation of DMN were not found in the activation of diethylnitrosamine to a mutagen. Hepatic dealkylation of DMN and diethylnitrosamine to active mutagenic metabolites is increased in both rats and mice by both 3-MC and phenobarbital induction, which is in contrast to the findings of others that 3-MC and phenobarbital induction, which is in contrast to the findings of others that 3-MC decreases the incidence of DMN-induced hepatic tumors in rats, and phenobarbital decreases the incidence of diethylnitrosamine-induced hepatic tumors in mice.

Phosphotriesters in rat liver deoxyribonucleic acid after the administration of the carcinogen NN-dimethylnitrosamine in vivo

After treatment with NN-di[14C]methylnitrosamine, samples of DNA were isolated from rat livers by a conventional phenol procedure and examined for the presence of phosphotriesters. A method of capable of detecting relatively small amounts of 14C-labelled phosphotriesters was developed and used to establish that these products account for 10-12% of the total methylation pattern found after treatment with this agent in vitro. The significance of the presence of phosphotriesters in DNA is discussed.

Transformed liver cells obtained in culture from hepatectomized rats treated with dimethylnitrosamine (dMN)

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