Metabolism of diethylnitrosamine by human liver slices in vitro

PBPK model-based gender-specific risk assessment of N-nitrosodimethylamine (NDMA) using human biomonitoring data

Despite several screening levels for NDMA reported in water, soil, air, and drugs, the human risk assessment using biomonitoring concentrations has not been performed. In this study, gender-specific exposure guidance values were determined in humans, then biomonitoring measurements in healthy Korean subjects (32 men and 40 women) were compared to the exposure guidance values to evaluate the current exposure level to NDMA. For the human risk assessment of NDMA, the gender-specific physiologically based pharmacokinetic (PBPK) model was developed in humans using proper physiological parameters, partition coefficients, and biochemical parameters. Using the PBPK model, a Monte Carlo simulation was performed to describe the magnitudes of inter-individual variability and uncertainty on the single model predictions. The PBPK modeling and Monte Carlo simulation allowed the estimation of the relationship between external dose and blood concentration for the risk assessment. The procedure for the human risk assessment was summarized as follows: (1) estimating a steady-state blood concentration (Cavg) corresponding to the daily no observed adverse effect level (NOAEL) administration in rats; (2) applying uncertainty factors (UFs) for deriving the human Cavg; (3) determining the exposure guidance values as screening criteria; (4) interpreting the human biomonitoring measurements by forward and reverse dosimetry approaches. Using the biomonitoring concentrations, current daily exposures to NDMA were estimated to be 3.95 μg/day/kg for men and 10.60 μg/day/kg for women, respectively. The result of the study could be used as a basis for implementing further risk management and regulatory decision-making for NDMA.

Systemic Carcinogens (N-Nitroso Compounds) and Synergistic or Additive Effects in Respiratory Carcinogenesis

Systemic carcinogens induce respiratory tract tumors in various animals species. Typical examples of systemic carcinogens are the N-nitroso compounds, which require metabolic conversion to exert their carcinogenic effect. In this review particular emphasis is put on the comparative studies performed in the same species with different nitrosamines, providing an experimental model in which is possible to examine and correlate the biochemical changes preceding the development of detectable tumors and the carcinogenic effect. In addition studies describing synergistic or additive effects in respiratory carcinogenesis are reviewed and discussed.

Genotoxic and carcinogenic risk to humans of drug-nitrite interaction products

The large majority of N-nitroso compounds (NOC) have been found to produce genotoxic effects and to cause tumor development in laboratory animals; four NOC have been classified by the International Agency for Research on Cancer (IARC) as probably and another 15 as possibly carcinogenic to humans. A considerable fraction of drugs are theoretically nitrosatable due to the presence of amine, amide or other groups which by reacting with nitrite in the gastric environment, or even in other sites, can give rise to the formation of NOC, and in some cases other reactive species. This review provides a synthesis of information on the chemistry of NOC formation, the carcinogenic activity of NOC in animals and humans and the inhibitors of nitrosation reactions. It contains information on the drugs which have been tested for the formation of NOC by reaction with nitrite and the genotoxic-carcinogenic effects of their nitrosation products. In an extensive search we have found that 182 drugs, representing a wide variety of chemical structures and therapeutic activities, were examined in various experimental conditions for their ability to react with nitrite, and 173 (95%) of them were found to form NOC or other reactive species. Moreover, 136 drugs were examined in short-term genotoxicity tests and/or in long-term carcinogenesis assays, either in combination with nitrite or using their nitrosation product, in order to establish whether they produce genotoxic and carcinogenic effects; 112 (82.4%) of them have been found to give at least one positive response. The problem of endogenous drug nitrosation is largely unrecognized. Only a small fraction of theoretically nitrosatable drugs have been examined for the possible formation of genotoxic-carcinogenic NOC, guidelines for genotoxicity testing of pharmaceuticals do not indicate the need of performing the appropriate tests, and patients are not informed that the drug-nitrite interaction and the consequent risk can be reduced to a large extent by consuming the nitrosatable drug with ascorbic acid.

Stereochemical effects in the metabolic activation of nitrosopiperidines: correlations with genotoxicity

The genotoxicity of nitrosopiperidine and six alkyl derivatives was studied by use of the Ames tester strains TA100 and TA1535 in the pre-incubation method. Among the compounds investigated, those exhibiting genotoxic activity under the experimental conditions employed were only genotoxic in the presence of S9 mix (10%). The results obtained were correlated with models of the metabolic activation of nitrosamines in an attempt to rationalize the genotoxicity of these compounds. The results show that the presence of substituents in the nitrosopiperidine molecule may be one of the modulating factors affecting the genotoxicity of these cyclic nitrosamines, and may help provide some chemical clues for the identification of risk compounds from among a large group of structurally related molecules.

Elevated mortality from nonalcohol-related chronic liver disease among female rubber workers: is it associated with exposure to nitrosamines?

BACKGROUND

Despite several case reports describing liver toxicity of nitrosamines and the fact that some N-nitroso compounds are used to induce cirrhosis of the liver in animal models, this association has not been investigated in epidemiological studies.

METHODS

A cohort of 2,875 female rubber workers who were active on January 1, 1976, or hired thereafter, and who had been employed for at least 1 year in one of five plants producing tires or technical rubber goods was followed for mortality from January, 1976, through December, 1991. Work histories were reconstructed using routinely documented "cost center codes" and classified into six work areas. Age and calendar year standardized mortality ratios (SMR) and 95% confidence intervals (95% CI) were calculated and stratified by plant, work area, year of hire, and years of employment in the respective work area.

RESULTS

The excess mortality from cirrhosis of the liver was most pronounced for nonalcohol-related cirrhosis of the liver (ICD-9 571.4-571.9: 10 deaths, SMR 202; 95% CI 97-372). Mortality from alcohol-related cirrhosis of the liver (ICD-9 571.0-571.3: 3 deaths, SMR 153; 95% CI 31-446) and from other alcohol-related diseases (organic psychoses, injury, and poisoning) was not statistically significantly elevated. All 10 cases of nonalcohol-related cirrhosis had worked in production of technical rubber goods (SMR 279; 95% CI 134-514) and risks increased with earlier years of hire and with longer duration of employment in this work area.

DISCUSSION

Although our results must be interpreted with caution, they suggest that the observed excess deaths from cirrhosis of the liver are associated with occupational risk factors. In light of additional evidence from case reports and animal data, exposure to nitrosamines may be a plausible risk factor for the observed excess mortality.

Role of N-nitroso compounds (NOC) and N-nitrosation in etiology of gastric, esophageal, nasopharyngeal and bladder cancer and contribution to cancer of known exposures to NOC

The questions of whether and how N-nitroso compounds (NOC) may be inducing cancer in humans are discussed. The principal subjects covered include nitrite-derived alkylating agents that are not NOC, reasons for the wide tissue specificity of carcinogenesis by NOC, the acute toxicity of nitrosamines in humans, mechanisms of in vivo formation of NOC by chemical and bacterial nitrosation in the stomach and via nitric oxide (NO) formation during inflammation, studies on nitrite esters, use of the nitrosoproline test to follow human gastric nitrosation, correlations of nitrate in food and water with in vivo nitrosation and the inhibition of gastric nitrosation by vitamin C and polyphenols. Evidence that specific cancers are caused by NOC is reviewed for cancer of the stomach, esophagus, nasopharynx, urinary bladder in bilharzia and colon. I review the occurrence of nitrosamines in tobacco products, nitrite-cured meat (which might be linked with childhood leukemia and brain cancer) and other foods, and in drugs and industrial situations. Finally, I discuss clues from mutations in ras and p53 genes in human tumors about whether NOC are etiologic agents and draw some general conclusions.

Nitrate, nitrite and N-nitroso compounds

A risk assessment has been made on nitrate, nitrite and N-nitroso compounds encountered in the human diet. Vegetables constitute a major source of nitrate providing over 85% of the average daily human dietary intake. Nitrite and N-nitroso compounds present in the diet contribute relatively small amounts to the body burden and the major source of these biologically reactive compounds is derived from the bacterial and mammalian metabolism of ingested nitrate. Additionally, endogenous synthesis provides an important source contributing to the body burden of nitrate. Data from animal toxicological studies, human effects and epidemiological surveys have been reviewed and evaluated. It is concluded that there is no firm scientific evidence at present to recommend drastic reductions beyond the average levels of nitrate encountered in vegetables grown in keeping with good agricultural practice. Recommendations have also been made for further animal and human studies to be carried out to elucidate the potential risks to man from ingested nitrate.

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)

Relevance of N-nitrosamines to esophageal cancer in China

Studies on the relevance of the N-nitrosamines to esophageal cancer in China are reviewed. Esophageal cancer is a complex and multifactorial problem. Although a causal association between nitrosamines exposure and esophageal cancer in China has not yet been rigorously established, exposure of Lin-Xian subjects to nitrosamines either directly or as a result of their in vivo formation has been detected in our study. Several N-nitrosamines (NDMA, NDEA, NMBzA, NPyr, NPip, and NSAR) in gastric juice collected from Lin-Xian inhabitants have been detected. A correlation was found between the lesions of esophageal epithelium and the amount of nitrosamines present. In addition, the amounts of N-nitrosamino acids (N-nitrosoproline, N-nitrosothiazolidine 4-carboxylic acid, NSAR, and nitrates) excreted in 24-hr urine of subjects in Lin-Xian were significantly higher than those in Fan-Xian, indicating a higher exposure to N-nitroso compound and their precursors of the inhabitants in the high-risk area. The effect of nitrosamines on human esophagus has been investigated at the cellular levels. The amounts of O6-MedG in DNA of esophageal or stomach mucosa of patients from Lin-Xian were higher than that from Europe (Lyon and Essen). The presence of O6-MedG in the human fetal esophagus cultured with NMBzA was also detected. These findings indicate that the elevated levels of O6-MedG in esophageal DNA could be the result of a recent exposure to N-nitroso compounds or a genetically determined reduced cellular capacity for repair of O6-MedG from DNA. The hyperplasia was induced in the esophagus of human fetus that cultured with NMBzA for 2 weeks to 2 months. The intervention studies of esophageal cancer in Lin-Xian have been pursued. Intake of moderate doses of ascorbic acids by Lin-Xian subjects effectively reduced the urinary levels of N-nitrosamino acids to those found in un-dosed subjects in the low-risk area. If N-nitroso compounds are formed in vivo and are among the causative factors of esophageal cancer in Lin-Xian, ascorbic acid appears to be effective in lowering the body burden of these carcinogenic compounds. Thus, the plan of chemoprevention is carried out in Lin-Xian.

O(6)-methyldeoxyguanosine in oesophageal DNA among individuals at high risk of oesophageal cancer

A radioimmunoassay using monoclonal antibodies (MAb) against 0(6)-medGuo and 0(6)-etdGuo enabled the presence of these DNA adducts to be detected in human tissues. With this methodology and with 1 mg of hydrolysed DNA it is possible to detect approximately 25 fmol 0(6)-medGuo or 12.5 fmol 0(6)-etdGuo per mg DNA. The analysis comprised a total of 37 human tissue specimens derived from patients who underwent surgery for cancer of the oesophagus in Linxian County (People's Republic of China); for comparison 12 tissue samples obtained from hospitals in Europe were similarly analysed. Seventeen samples from Linxian County showed a level of 0(6)-medGuo ranging from 15 to 50 fmol/mg DNA, 10 showed higher levels up to 160 fmol/mg DNA, and the remaining 10 samples were below the limit of detection. All the tissue samples from Europe showed levels below 45 fmol 0(6)-medGuo/mg DNA, 7 being below the limit of detection. No 0(6)-etdGuo was detected in any of the samples. 0(6)-alkylguanine DNA alkyltransferase activity observed in tissue extracts from Linxian County ranged from 190 to 326 fmol (mean values) of 0(6)-medGuo removed per mg protein and lower values were obtained for the removal of 0(6)-etdGuo. The findings obtained using this approach appear promising in assessing the role of alkylating agents, e.g. nitrosamines, in the aetiology of human cancer.

Toxicokinetic and metabolic study of dimethylnitrosamine and diethylnitrosamine in crayfish Austropotamobius pallipes

The acute toxicities of dimethylnitrosamine and diethylnitrosamine have been evaluated in adult male crayfish Austropotamobius pallipes; LD50 values are 2250 mg/kg and 230 mg/kg, respectively. Toxicokinetic studies of 14C-dimethylnitrosamine and 14C-diethylnitrosamine in crayfish, administered by i.v. injection, show high concentrations of 14C in abdominal muscle and hepatopancreas. Excretion is greater with dimethylnitrosamine, and retention in the tissues, especially the hepatopancreas, is greater with diethylnitrosamine. Metabolites identified in excreta include monomethylnitrosamine from dimethylnitrosamine, and hydroxyethyl-ethyl-, bishydroxyethyl- and carboxyethyl-ethylnitrosamine from diethylnitrosamine.

Hepatic microsomal enzymes in Schistosoma mansoni-infected mice: I. Effect of duration of infection and lindane administration on dimethylnitrosamine demethylases

The prevalence of schistosomiasis in Egypt, its suspected relationship to carcinoma of the bladder, and the inevitable exposure of schistosomal patients to insecticides prompted the undertaking of this study. Dimethylnitrosamine (DMN) demethylases, responsible for the activation of DMN, were assessed in the hepatic microsomes from albino mice under the influence of Schistosoma mansoni infection and/or lindane administration. DMN-demethylase I responded to infection by an early increase in activity, reaching a peak 30 days after disease induction and declining sharply afterwards. DMN-demethylase II followed the same general pattern, but it reached its peak 20 days after infection and declined at a slower rate than DMN-demethylase I. The effects of lindane, an organochlorine insecticide, were also studied on these two enzymes in uninfected mice and in S. mansoni-infected animals after different durations of disease induction. Given orally, in a dose of 40 mg/kg/day for 3 consecutive days, lindane increased the activity of both enzymes. Since DMN requires metabolic activation for its hepatotoxic and carcinogenic actions, alterations in its activating enzymes, as a consequence to schistosomiasis and/or exposure to lindane, may change susceptibility to its toxic and carcinogenic potentials.

Carcinogenic N-nitroso compounds and their environmental significance

A short review is given on the potent and organ-specific carcinogenic effects of N-nitroso compounds. Like many other chemical carcinogens, nitrosamino compounds require activation in vivo via enzymatic alpha-hydroxylation to form alkylating agents as ultimate carcinogens. Since no epidemiological data in man are available, extrapolation of animal data to man are important as well as dose-response studies in risk evaluations. Important aspects of these problems are presented. Finally the environmental impact of N-nitroso compounds is summarized.

Removal of O6-methylguanine from DNA by human liver fractions

In in vitro assays using methylated DNAs as substrates, human liver fractions were shown to be able to catalyze the removal of O6-methylguanine. The amount of removal was proportional to the amount of protein added, and the loss of O6-methylguanine occurred with stoichiometric formation of guanine in the DNA and S-methylcysteine in protein. This indicates that human liver contains a protein similar to that previously found in bacteria exposed to alkylating agents. This protein acts as a transmethylase, transferring the intact methyl group from O6-methylguanine in DNA to a cysteine residue on that protein. A similar activity is present in rodent liver, but it was found that human liver was about 10 times more active in carrying out this reaction. In contrast, there was no difference between the human and rat liver extracts in catalyzing the loss of another methylation product, 7-methylguanine, from alkylated DNA. The liver is the organ most likely to be alkylated after exposure to exogenous potential alkylating agents such as dimethylnitrosamine. The present results show that human liver has a significant capacity to repair O6-methylguanine in DNA, which has been implicated as a critical product in carcinogenesis and mutagenesis.

Fatal nitrosamine poisoning

A case is reported in which progressive liver symptoms with rise in bilirubin concentration, hemorrhagic diathesis, and signs of portal hypertension developed three years before death in liver coma. The pathologic and neuropathologic findings are described. The case was clarified after dimethylnitrosamine was demonstrated in food intended for the patient and after it was established that small amounts of nitrosamine could have been repeatedly ingested by the patient over a period of years. Comparable cases of human dimethylnitrosamine poisonings published in the literature are presented. The relatively typical morphologic alterations in the liver are described. Problems involved in the histological interpretation of such liver changes as well as the forensic conclusions to be drawn are discussed.

The polymorphic 4-hydroxylation of debrisoquine in a Saudi arab population

1. Debrisoquine 4-hydroxylation was polymorphic in 102 Saudi arab volunteers, the population comprising one phenotypically poor metabolizer and 101 phenotypically extensive metabolizers of debrisoquine. 2. Mean urinary recoveries of drug and metabolite were low in Saudis (15 +/- 10% dose, mean +/- S.D.), which compared well with previously studied populations of Egyptians (16%) and Ghanaians (18%), but which were lower than those seen in a UK white population (41%). 3. Saudis, like Egyptians, were more extensive metabolizers of debrisoquine than UK whites, as judged by the metabolic ratio (% dose as debrisoquine/% dose as 4-hydroxy-debrisoquine eliminated in the urine). 4. Neither sex, urine collection period nor urinary recovery of drug and metabolite had any statistically significant effect upon the distribution of metabolic ratios in Saudis. 5. The frequency of the DL allele controlling the recessive poor metabolizer trait was 0.099 +/- 0.049 (+/- S.E.M.) in Saudis, which compared well to Egyptians (0.118 +/- 0.059) but was significantly lower than that for UK whites (0.298 +/- 0.030). 6. These findings raise questions regarding the efficacy and safety in Saudis of drugs undergoing oxidative metabolism which have been evaluated for usage in European white subjects.

Methods for analysis of the mutagenicity of indirect mutagens/carcinogens in eukaryotic cells

The review discusses the variety of methods for activation of indirect mutagens/carcinogens and testing them in cell cultures, especially in mammalian cell cultures. After the necessity for including metabolizing components in mutagenicity tests has been pointed out, the enzymes that transform foreign compounds metabolically, and the factors influencing them, are described. In the main section the various methods of activating indirect mutagens/carcinogens are presented. The methods of including in vivo metabolism in mutagenicity tests are: Analysis of cells from organisms contaminated with a chemical (III.l.a); body fluid-mediated mutagenesis (III.l.b); host-mediated assay (III.l.c). The following activation systems are suitable for including in vitro metabolism of test compounds in mutagenicity tests: Liver and lung perfusion (III.2.a.alpha); organ slices and homogenates (III.2.a.beta); subcellular fractions (III.2.a.gamma); cultivated cells (cell-mediated mutagenesis) (III.2.b); nonenzymatic activation systems (III.2.c). Finally the main factors that influence the metabolism of test substances are summarized. Two factors illustrate the mutagenicity tests with regard to the metabolism of mammalian livers and the methods of performing mutagenicity tests in man.

Alpha-hydroxylation of N-nitrosopyrrolidine and N'-nitrosonornicotine by human liver microsomes

The environmental carcinogens N-nitrosopyrrolidine (NPYR) and N'-nitrosonornicotine (NNN) were incubated with human liver microsomes to test for the presence of enzymes catalyzing alpha-hydroxylation, which is likely to be an activation mechanism for these compounds. Both nitrosamines underwent alpha-hydroxylation; rates were higher for NPYR than for NNN, as observed in rats. The results indicate that both NPYR and NNN can be metabolically activated by human liver microsomes.

Problems of dose-response studies in chemical carcinogenesis with special reference to N-nitroso compounds

Dose-response studies in chemical carcinogenesis are a valuable method for evaluating the risk involved by a certain carcinogen. This article deals with the methods that are presently applied for dose-response studies. It is emphasized that there are many possible mistakes that are already hidden in the method itself; a critical approach is made to the indicative value of dose-response studies. It is also emphasized that the conclusions which may be drawn from dose-response studies can always only be extrapolative but never mathematically calculable. Finally, some practical examples for dose-response studies with n-nitroso compounds are described.

N-nitroso compounds: their chemical and in vivo formation and possible importance as environmental carcinogens

Brief reviews are presented on the occurrence of N-nitroso (NNO) compounds, the chemistry and kinetics of NNO compound formation from nitrite and amines or amides, the in vivo formation of these compounds (as detected by tumor induction) on feeding nitrite with amines or amides to rodents, and the carcinogenicity for rats of some new nitrosamides. The possible human hazard caused by exposure to specific readily nitrosated compounds is reviewed. Whether NNO compounds might be causing human cancer of various organs (e.g., pancreatic, nasopharyngeal, and esophageal cancer) is discussed. Some of our results in [3H]thymidine incorporation in the rat esophageal epithelium are presented. Nitrosamines that cause esophageal cancer in rats were found to inhibit [3H]thymidine incorporation, both in vivo and in vitro, when esophagi were incubated with nitrosamines. With reference to the hypothesis that human gastric cancer is caused by nitrosamides (e.g., nitrosoureas), certain correlations were examined between gastric cancer and environmental exposure to nitrate, nitrite, and nitrosatable amides. In studies from our laboratory, dried, salted fish, which was treated with excess nitrite at pH 1 and then "denitrosated" at pH O, yielded 16 mg methylurae/kh fish, possibly derived from methylguanidine.

The absorption and metabolism in rats of small oral doses of dimethylnitrosamine. Implication for the possible hazard of dimethylnitrosamine in human food

1. Groups of rats were given one dose of the carcinogen dimethylnitrosamine by gastric intubation. The dose was varied between 10mg/kg body wt. and 1 microgram/kg body wt. 2. The dose was rapidly absorbed. 3. The methylation of liver DNA resulting from the administration of this carcinogen was proportional to dose. This suggests that small doses are absorbed from the gut with no more loss than large doses. 4. As the dose was decreased there was a disproportionately greater decrease in the alkylation of kidney DNA, and when the dose was less than 40 microgram/kg body wt. the methylation of kidney DNA was no longer detectable. This possibly explains why small amounts of dimethylnitrosamine in the diet do not induce kidney tumours. 5. Comparison of the relative alkylation of liver DNA and kidney DNA resulting from an oral and from an intravenous dose of dimethylnitrosamine suggest that small amounts of dimethylnitrosamine absorbed into the portal blood from the gut are completely metabolized by the liver and do not enter the general circulation. 6. The implications of these results for the possible hazard of dimethylnitrosamine in human food is discussed.

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.

Carcinogenic N-nitro-dimethylamine from the reaction of the analgesic amidopyrine and nitrite extracted from foodstuffs

The reaction of the analgesic amidopyrine (100 mg) with nitrite extracted from cured meats and from spinach in varying degrees of spoilage was studied. Unde physiological conditions the carcinogenic dimethylnitrosamine was formed at milligram levels at nitrite concentrations as low as 4 mg (in 175 ml extracted from 100 g boiled ham). The rate of decrease in concentration in the human stomach after ingestion of amidopyrine and of nitrite contained in boiled ham or in a broth from boiled ham was also measured.

In vitro production of N-nitrosodimethylamine and other amines by proteus species

N-nitrosodimethylamine, a potent carcinogen, was produced by three strains each of Proteus mirabilis, P. morganii, and P. rettgeri, but not by three strains of P. vulgaris grown under the same conditions. Many of the alkaline-extractable volatile metabolites elaborated by these organisms are the same, but there are some qualitative and quantitative differences among species. Representative gas-liquid chromatographic profiles of the four species are presented, and the significance of the differences is discussed. Primary emphasis, however, is given to the importance of the production of N-nitrosodimethylamine by these microorganisms and the conditions under which it is produced.

Metabolic interactions among environmental chemicals and drugs

It is evident that metabolic interactions can occur among drugs, insecticides, food additives, carcinogenic hydrocarbons, and a variety of environmental chemicals. A common denominator governing these effects is the versatile nature of the liver microsomal enzymes that metabolize chemicals with diverse structures and biological activities, and the fact that these enzymes can be stimulated or inhibited by other chemicals administered simultaneously. The discovery of these particular enzymes in the 1950's laid the groundwork for the current research on metabolic interactions. Such research provides information that is helpful in the evaluation of the safety and efficacy of drugs and environmental chemicals, and suggests new directions for further research. Some examples are as follows.

Cellular injury and carcinogenesis. Inhibition of metabolism of dimethylnitrosamine by aminoacetonitrile

1. Aminoacetonitrile, a lathyrogenic agent known to decrease the hepatotoxic action of dimethylnitrosamine, inhibited the metabolism of this compound by rats in vivo and by rat liver slices in vitro. 2. Methylation of nucleic acids in rat liver and kidney by dimethylnitrosamine in vivo was inhibited by treatment of the animals with aminoacetonitrile. 3. These findings are discussed in relation to the hypothesis that dimethylnitrosamine requires metabolism to exert its hepatotoxic and carcinogenic action.