Hepatic microsomal metabolism of N-nitrosodimethylamine to its methylating agent

An important role for cytosol in the microsomal metabolism of N-nitrosodimethylamine to a mutagen: evidence for two different mutagenic metabolites

Microsomal-mediated mutagenesis induced by N-nitrosodimethylamine (NDMA) in Salmonella TA100 at neutral pH was only slightly affected by cytosol and was similar in its threshold type dose-response curve to mutagenesis induced by direct-acting N-nitroso-N-methyl compounds. However, mutagenesis in strain TA104 was greatly enhanced by cytosol and this mutagenesis did not exhibit a threshold. In the presence of microsomes alone NDMA was more potent in TA100 than TA104, but in the presence of microsomes plus cytosol (S-9 fraction) this order was reversed at the doses tested. A possible explanation for these results is that NDMA is metabolized by microsomes to a mutagen (presumably methyldiazonium ion; MDI) that is more potent in TA100 than in TA104, but in the presence of S-9 fraction a fraction of the NDMA is metabolized by a pathway leading to a different mutagen with a different specificity. The ratio of metabolism via these pathways appears to be dependent on pH.

Role of the transfer of metabolites from hepatocytes to splenocytes in the suppression of in vitro antibody response by dimethylnitrosamine

The metabolism and subsequent immunosuppressive effects of dimethylnitrosamine (DMN) were investigated in mixed cultures of mouse hepatocytes and mouse splenocytes. Hepatocytes were shown to activate DMN to an immunosuppressive form that caused the suppression of the in vitro antibody response to the T-dependent antigen, sheep erythrocytes (SRBC). A significant increase in the binding of DMN metabolites to trichloroacetic acid (TCA) precipitable material in splenocytes was induced when 94 microM [14C-methyl]DMN was added to the co-culture medium, indicating that reactive intermediates of DMN were transferred from hepatocytes to splenocytes and resulted in alkylation of macromolecules in splenocytes. The amount of [14C]DMN bound to TCA precipitable material in splenocytes increased in a time-dependent manner up to 4 hr of incubation. Aminoacetonitrile (AAN), a high-affinity DMN demethylase inhibitor, reversed the suppression by low concentrations of DMN (0.5 to 5 mM), but not by high concentrations of DMN (greater than 5 mM). AAN also inhibited the binding of [14C]DMN to both hepatocytes and splenocytes. These results suggest that reactive metabolites of DMN are released from hepatocytes and that the suppression of the antibody response by DMN is mediated via these reactive intermediates.

N-nitrosamines: bacterial mutagenesis and in vitro metabolism

Many nitrosamines are potent mutagens. The rate-limiting step in their in vitro metabolism to mutagens is usually a single enzymatic reaction catalyzed by one or more of the many cytochrome P-450-dependent mixed-function oxidases present in the microsomal cell fraction. Current evidence indicates that this reaction activates nitrosamines to alpha-hydroxynitrosamines, which have half-lives on the order of seconds. This product decomposes to an aldehyde and a much shorter-lived ultimate metabolite which is probably an alkyl diazonium ion or an alkyl carbocation. This may react with DNA leading to premutagenic adducts. Such adducts represent a very small fraction of the ultimate mutagen, with the rest reacting with water to yield the corresponding alcohol. Evidence for this pathway includes (1) the observation of deuterium isotope effects in metabolism and mutagenesis, (2) products (aldehydes, alcohols, and N2) consistent with this pathway, (3) studies on metabolism of nitrosamines using purified cytochrome P-450, (4) formation of DNA adducts such as O6-alkylguanines which are consistent with those expected from the ultimate mutagen, (5) expected products and genotoxic effects of other sources of activated nitrosamines, e.g., alpha-acetoxynitrosamines, alkanediazotates and related compounds. Hydroxylation of nitrosamines at other positions also occurs in vitro (usually to a lesser extent), but these products are generally stable and must be further metabolized to exert mutagenic effects (with the exception of N-nitrosoalkyl(formylmethyl)amines, which are direct-acting mutagens). Because only low percentages of nitrosamines are metabolized in vitro, the contribution to mutagenesis by secondary metabolism is small. In this respect, in vitro metabolism can differ significantly from in vivo metabolism. Bacterial mutagenesis by nitrosamines has most often been studied in Salmonella typhimurium and to a lesser extent E. coli. Mutagenesis by nitrosamines generally requires a source of microsomes (a 9000 X g supernatant fraction is often used), and NADPH. Liver fractions from Aroclor-1254- or PB-induced rodents have been most frequently employed but liver fractions from untreated animals, and homogenates of other organs (lung, kidney, nasal mucosa, and pancreas) have also been utilized. Liver homogenates from humans are generally similar to those from untreated rats in metabolizing nitrosamines to mutagens but large interindividual variations are observed. Mutagenesis is often most effective using a liquid preincubation, a slightly acidic incubation mixture and hamster liver fractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduced glutathione inhibits the alkylation by N-nitrosodimethylamine of liver DNA in vivo and microsomal fraction in vitro

The transfer of radioactivity from N-nitroso-[14C]dimethylamine to trichloroacetic acid precipitable macromolecules in the microsomal fraction of rat liver was investigated. This transfer was found to depend on N-nitrosodimethylamine being metabolized. Cytosolic fraction and cytosol enriched with reduced glutathione inhibited the binding of radioactivity to acid insoluble proteins. Depletion of glutathione in rat liver with diethylmaleate prior to i.v. administration of 10 mg N-nitroso-[14C]dimethylamine/kg led to an increase in O6-methylguanine and N-7-methylguanine in DNA. If rats were fed disulfiram for 6 days (2 g/kg feed), glutathione and glutathione S-transferase were enhanced, and the degree of methylation of guanine by N-nitrosodimethylamine was greatly reduced, as was the metabolism of N-nitrosodimethylamine in the intact animal. Fasting rats for 24 h did not change the N-nitrosodimethylamine-demethylase activity in vitro but greatly enhanced the methylation of guanine in vivo, while the glutathione content and glutathione S-transferase activity were not changed compared to fed animals.

Factors influencing the activity of dimethylnitrosamine demethylase in hamster, rat and chicken liver microsomes

In vitro metabolism of dimethylnitrosamine (DMN) by liver microsomal fractions of hamster, rat and chicken revealed that the three species under certain assay conditions, were capable of metabolizing DMN at different rates (hamster greater than rat greater than chicken). The magnitude of the demethylase activity was found to be dependent on the nature of the buffer, the concentration of cytochrome P-450 (P-450) and the concentration of the substrate DMN. Enzyme activity was higher in Hepes buffer than in the phosphate buffer. Concentrations of phosphate higher than 20 mM inhibited the activity of the rat and chicken enzymes. This effect of phosphate was not a consequence of increase in ionic strength since KCl over a wide range of concentration failed to inhibit the activity.

Indole-3-carbinol protects against covalent binding of benzo[a]pyrene and N-nitrosodimethylamine metabolites to mouse liver macromolecules

Benzo[a]pyrene (BaP) and N-nitrosodimethylamine (NDMA) are carcinogens and indirect acting mutagens. A naturally occurring dietary indole, indole-3-carbinol (I-3-C), has been shown to decrease the incidence of aryl hydrocarbon induced neoplasia in experimental animals. We examined the relationship between the ability of I-3-C to alter the rate of carcinogen oxidation and its ability to decrease the rate of covalent binding of carcinogen metabolites to DNA and protein. We found that I-3-C inhibited the covalent binding of NDMA oxidation products to DNA in vitro in proportion to its ability to inhibit carcinogen metabolism. Pretreatment of mice by gavage with I-3-C resulted in no change in the rate of aryl hydrocarbon hydroxylase or NDMA demethylase in hepatic post-mitochondrial supernatant. However, this pretreatment resulted in a 60-90% decrease in the ability of carcinogen oxidative metabolites to bind covalently to DNA or protein in vitro. Similarly, in in vivo experiments, gavage with I-3-C, followed by gavage with BaP or NDMA, resulted in a 63-85% decrease in covalent binding to macromolecules, with no concomitant change in carcinogen metabolism. The results suggest that the in vivo administration of I-3-C may confer protection for hepatic macromolecules against covalent binding of the metabolites of these two indirect acting mutagens.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. I. Comparison with mixed-function oxidase enzymes

1. The metabolism of dimethylnitrosamine (DMN) to formaldehyde by rat-liver preparations has been studied at substrate concn. of 0.5, 5 and 50 mM and compared with mixed-function oxidase enzyme activities. 2. The microsomal metabolism of low (0.5 and 5 mM) and high (50 mM) substrate concn. of DMN was differentially affected by acetone addition or KI treatment. 3. A series of heterocyclic compounds related to pyrazole were potent inhibitors of metabolism of 0.5 and 5 mM DMN at concn. which had little effect on mixed-function oxidase activities. In contrast, purine addition slightly stimulated the metabolism of low but not high concn. of DMN. 4. The results are consistent with the suggestion that multiple enzymic pathway(s) are involved in hepatic DMN metabolism and that some of these pathway(s) may be independent of cytochrome P-450.