Assessing the efficacy of an Aroclor 1254-induced exogenous metabolic activation system for FETAX

The developmental toxicity of N-nitrosodimethylamine (NDMA) and trichloroethylene (TCE) was assessed with Frog Embryo Teratogenesis Assay: Xenopus (FETAX). Late Xenopus laevis blastulae were exposed to NDMA and TCE for 96-h in two separate static-renewal tests with and without the presence of three differently induced exogenous metabolic activation systems (MAS). The MAS consisted of Aroclor 1254-induced (Aroclor 1254 MAS), isoniazid-induced (INH MAS), and a post-isolation mixture (mixed MAS) of Aroclor 1254- and isoniazid-induced rat liver microsomes. Addition of the INH MAS and the mixed MAS increased the Teratogenic Index [TI = LC50/EC50 (malformation)] of NDMA and TCE nearly 2.0- and 2.1-fold and 2.1- and 1.7-fold, respectively. Inclusion of the Aroclor 1254 MAS did not alter the developmental toxicity of either compound. Based on TI values, embryo growth, and types and severity of induced malformations, both NDMA and TCE were developmentally toxic. Use of post-microsome isolation mixtures from differentially induced rat livers increased the efficacy of the exogenous MAS routinely used by FETAX.

Differences between pancreatropic nitrosamine carcinogens and N-nitrosodimethylamine in methylating DNA in various tissues of hamsters and rats

N-Nitrosobis(2-oxopropyl)amine (BOP) and N-nitroso(2-hydroxypropyl)(2-oxypropyl)amine (HPOP) induce pancreatic tumors in the Syrian hamster. BOP and HPOP target the kidneys, esophagus and upper respiratory system in rats, but the pancreas of this species is resistant to the above carcinogens. On the other hand, N-nitrosodimethylamine (DMN) induces hepatic and kidney tumors in the rat, and tumors of the liver and upper respiratory system in the hamster, but it is not known to affect the pancreas of either species. At equimolar doses, ratios of DMN versus BOP or HPOP mediated methylation in hamster liver DNA are 1.6 and 8.1, respectively. Respective ratios in the rat liver are 1.1 and 6.5. However, in both species equitoxic doses of BOP, HPOP and DMN induce similar levels of N7-methylguanine (N7-MeG) in hepatic DNA. At such doses methylation of kidney DNA is 24 and 14 times more extensive in BOP and HPOP than in DMN-treated hamsters. Similarly, ratios of N7-MeG in the pancreas of BOP and HPOP vs. DMN-treated hamsters are 10 and 5, respectively, while in the lung this ratio is 2.2 for both carcinogens. Levels of O6-methylguanine (O6-MeG) in the DNA of extrahepatic tissues are substantially greater in hamsters treated with BOP or HPOP than in those treated with an equitoxic dose of DMN. In rats, equitoxic doses of BOP and DMN induce similar levels of N7-MeG and O6-MeG in hepatic, kidney and lung DNA. However, levels of these adducts in pancreatic DNA are 2 times greater following BOP than DMN administration. Ratios of N7-MeG in pancreas, lung and kidney in HPOP vs. DMN-treated rats are 2.1, 2.7 and 2.1, respectively. Repair of O6-MeG is more effective in rat than in hamster liver, however in other tissues this is not always the case. Levels of O6-MeG in the pancreas of rats are reduced to half of their initial value between 40 and 50 h following the administration of 10, 50 or 20 mg/kg DMN, HPOP or BOP, respectively. However, half-lives for the repair of O6-MeG in hamster pancreas are 28, 62 and greater than 120 h at the respective doses of the above carcinogens. Since the above doses of DMN, HPOP and BOP induce 7, 19 and 41 nmol O6-MeG/mmol of guanine respectively in the hamster pancreas, it is suggested that the rate of repair could be a function of the initial concentration of this adduct. Differences between DMN and BOP or HPOP in methylating pancreatic DNA are sufficient to distinguish the latter two nitrosamines as pancreatic carcinogens for the hamster.

Degradation of dimethyl nitrosamine by Methylosinus trichosporium OB3b

The degradation of dimethyl nitrosoamine (DMNA) by a methanotroph, Methylosinus trichosporium OB3b, was studied using 14C-labelled DMNA. The organism was capable of assimilating DMNA-carbon and converting it to CO2. The rates of CO2 production (VCO2) from DMNA and its cellular uptake (VP) were linearly correlated with DMNA concentrations of 0.03-10 mM, which corresponded to approximately 3% of added DMNA metabolized in 24 h. These rates were two to three orders of magnitude less than the rate of uptake of methane (VCH4. VCH4 was suppressed when the concentrations of DMNA exceeded 0.3 mM. In the presence of 0.1 mM DMNA, VP and VCO2 were essentially the same in the presence or absence of methane in the first 8 h of incubation, but declined sharply thereafter only when methane was absent. These observations suggest that the metabolism of DMNA was carried out by methane monooxygenase (MMO), and that NADH, a cofactor for MMO, may be provided by the oxidation of the stored compounds in the cells when methane is not available.

Roles of cytochrome P450IIE1 in the dealkylation and denitrosation of N-nitrosodimethylamine and N-nitrosodiethylamine in rat liver microsomes

N-Nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) are widely occurring nitrosamines and require enzyme-catalyzed activation for their carcinogenic actions. The low Km forms of the enzyme are generally considered to be important in the activation of environmental carcinogens. In this work we examined the role of cytochrome P450IIE1--a constitutive enzyme that is also inducible by acetone, ethanol, fasting and other factors--in catalyzing the dealkylation and denitrosation of these two carcinogens. The experimentally determined Km value of NDMA demethylase depended upon the experimental conditions and was lower when lower protein concentrations were used. Low Km values of 15-20 microM were observed for NDMA demethylase with different preparations of microsomes. In the deethylation of NDEA, a low Km of approximately 40 microM was observed for both control and acetone-induced microsomes. Immunoinhibition studies indicated that P450IIE1 was responsible for almost all the low Km NDMA demethylase activity in acetone-induced microsomes and greater than 80% in control microsomes. This enzyme was also responsible for about three-quarters of the low Km NDEA deethylase activity in acetone-induced microsomes and about half in control microsomes. The denitrosation of NDMA and NDEA was inhibited to approximately the same extents as the dealkylation reactions under different experimental conditions, suggesting the involvement of the same enzyme and perhaps a common initial intermediate in these two types of reactions. The relevance of this work and its relationship to related information in the literature are discussed.

Induction of N-nitrosodimethylamine metabolism in rat liver and lung by ethanol

The metabolic activation of N-nitrosodimethylamine (NDMA) to an active metabolite is important in the manifestation of its carcinogenic effect. The lung and liver were compared for their responses to the induction of NDMA demethylation by 10% ethanol in the drinking water and by repeated bolus injections. Ethanol in the drinking water increased NDMA metabolism several-fold in both the liver and the lung. Repeated ip injections with 0.6 and 3.0 ml ethanol/kg for 7 days also enhanced this activity in a dose-dependent fashion. These results suggest that in the lung, as in the liver, ethanol may influence the metabolic activation of this nitrosamine.

Stereoselectivity in the microsomal conversion of N-nitrosodimethylamine to formaldehyde

The possibility that N-nitrosodimethylamine (NDMA) might be metabolized preferentially at either the syn (relative to the nitroso oxygen) or the anti methyl group has been examined by comparing the rates of formaldehyde production when unlabeled NDMA, its fully deuteriated analogue (NDMA-d6), and (Z)- or (E)-N-nitrosomethyl(methyl-d3)amine (NDMA-d3) were incubated in turn at concentrations of 0-2.4 mM with acetone-induced rat liver microsomes. The Km values for the conversion of (Z)- and (E)-NDMA-d3 to formaldehyde were identical to each other within experimental error (32 +/- 2 and 35 +/- 1 microM, respectively) but different from those for NDMA (24 +/- 6 microM) and NDMA-d6 (116 +/- 3 microM); similar Vmax values were observed for the four isotopic variants [7.5-8.1 nmol/(mg of protein.min)]. The observed similarity of kinetic parameters for (Z)- and (E)-NDMA-d3 suggested that the isotopic composition of the methyl group is an energetically more important determinant of its rate of oxidation at the NDMA demethylase active site than is its orientation relative to the nitroso oxygen atom. The absence of syn vs anti stereospecificity was confirmed via product isolation studies, in which the formaldehyde generated from each of the four isotopomers was trapped as the dimedone adduct and assayed for deuterium content by mass spectrometry; again, a strong preference for metabolism at CH3 vs CD3 regardless of stereochemistry was observed, though the data on CH2O generation suggested that there may be a slight net excess of anti attack. The results indicate that the microsomal enzymes employed display little regioselectivity in metabolizing the syn vs anti methyl groups of NDMA.

N-nitrosamino phosphates are unlikely transport forms for activated nitrosamines

Some of the target organs for nitrosamine carcinogenicity have a low activating capacity but many carcinogenic nitrosamines can be activated in the liver. Conjugates, such as phosphates, are chemically accessible reaction products of 1-OH-nitrosamines, and are either potential detoxication products or potential transport forms for activated nitrosamines. 14C-labeled 1-(N-ethyl-N-nitrosamino)ethyl phosphate was tested for its ability to enter primary rat hepatocytes but no uptake was detectable. No uptake was observable into fibroblasts and human leukocytes. N-Nitrosomethylbenzylamine is efficiently 1-C-hydroxylated by hepatocytes but the corresponding 1-C-phosphate was detectable neither in the cells nor in the surrounding medium. N-Nitrosamino-1-phosphates, unlike 1-glucuronides, therefore, do not seem to be important for nitrosamine toxicokinetics.

Alterations in dimethylnitrosamine-induced lethality and acute hepatotoxicity in rats during dietary thiamin, riboflavin and pyridoxine deficiencies

The effects of dietary thiamin, riboflavin and pyridoxine deficiencies on dimethylnitrosamine-induced lethality and hepatotoxicity were investigated in the rat. Development of deficiencies was monitored by growth rate, food intake, ratio of liver weight to body weight and the biochemical parameters (thiamin diphosphate effects for thiamin deficiency, glutathione reductase activity coefficient for riboflavin deficiency and erythrocyte glutamate-oxaloacetate transaminase activity for pyridoxine deficiency). Thiamin deficiency slightly increased the acute toxicity of dimethylnitrosamine as observed by the lowering of the LD50 dose and the greater increase in the serum glutamate-oxaloacetate transaminase and serum glutamate-pyruvate transaminase levels. Riboflavin deficiency, on the other hand, slightly increased the LD50 dose of dimethylnitrosamine and resulted in less dimethylnitrosamine-induced damage to the liver. Pyridoxine deficiency did not affect the lethal dose nor significantly alter the transaminases levels.

Toxicokinetics of N-nitrosodimethylamine in the Syrian golden hamster

The single-dose toxicokinetics of N-nitrosodimethylamine (NDMA) has been characterized in 8-week-old male Syrian golden hamsters by analysis using high performance liquid chromatography of serial blood samples. An i.v. bolus dose of 4.2 mumols/kg [14C]NDMA revealed biphasic first-order elimination with a terminal half-life of 8.7 +/- 1.0 min (mean +/- SE) for unchanged NDMA and 31.5 +/- 5.5 min for total radioactivity, and evidence for conversion to polar metabolites was seen in the chromatographic assays. The systemic blood clearance and apparent steady-state volume of distribution for unchanged NDMA were 51.2 +/- 3.0 ml/min/kg and 582 +/- 60 ml/kg, respectively. No unchanged NDMA was detected in the urine following an i.v. bolus dose of 15 mumols/kg [14C]NDMA, but 31% of the total radioactivity was eliminated by that route. A dose of 38 mumols/kg given by gavage indicated a systemic bioavailability of 11 +/- 4% for unchanged NDMA. Reversible binding of NDMA to hamster plasma proteins was found to be negligible. Estimation of the intrinsic hepatic clearance (ClI) in the hamster produced a value of 648 ml/min/kg, which is greater than that previously obtained for the rat, and indicates that the metabolic capacity of the hamster liver is greater than that of the rat. These results suggest that this difference in ClI may play a role in the previously reported (Lijinsky et al. 1987) switch in organotropism from almost exclusivity for liver tumors in hamsters dosed by gavage to additional high incidences of lung and kidney tumors in the rat.

Contribution of DNA methylation and benzylation to N-nitroso-N-benzyl-methylamine-induced mutagenesis in bacteria: effects of rat liver cytochrome P450 isozymes and glutathione transferases

The mutagenicity of N-nitroso-N-benzyl-methylamine (NBzMA), N-benzyl-N-nitrosourea (BzNU) and N-methyl-N-nitrosourea (MNU) in Salmonella typhimurium strains was investigated. BzNU selectively mutated TA100 strain as compared to TA1535, whereas MNU showed an inverse strain response, an effect probably related to the fact that benzylation of DNA is a stronger inducer of SOS DNA repair than methylation, as indicated by the higher activity of BzNU in the SOS chromotest. Benzylation of bacterial DNA by NBzMA, as deduced from the differential strain responsiveness, contributed predominantly to its mutagenicity in the presence of liver preparation from untreated, Aroclor- or ethanol-treated rats. Since benzyl alcohol, a metabolite of NBzMA, was not mutagenic in S. typhimurium, it appears that benzyl carbonium cations responsible for the mutagenicity of NBzMA in TA100 are formed via cytochrome P450-mediated hydroxylation of the methyl group. Neither ferric-EDTA nor desferrioxamine altered the mutagenicity of NBzMA, suggesting that activation occurs mainly within the catalytic site of P450. Experiments with isozyme-specific monoclonal antibodies showed that P450IIE1 did not contribute to N-demethylation of NBzMA at either low or high substrate concentrations and that P450IA contributed only weakly. Debenzylation was catalysed predominantly by P450IA at high NBzMA concentration. Antibodies against rat liver P450IIB enhanced NBzMA mutagenicity in S. typhimurium TA1535 strain up to 17-fold at low substrate concentration, but were without effect at high concentration. In liquid incubation assays, a 100% GSH-dependent reduction of NBzMA mutagenicity was found with liver S9 from untreated Wistar rats. The reducing effect of GSH was less pronounced in the presence of liver S9 from BDVI or Fischer 344 rats.

Human cytochrome P450IIA3: cDNA sequence, role of the enzyme in the metabolic activation of promutagens, comparison to nitrosamine activation by human cytochrome P450IIE1

We report that, in a human cell line, human cytochrome P450IIA3 is capable of metabolizing aflatoxin B1, benzo[a]-pyrene, N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) to cytotoxic and mutagenic species. Cytochrome P450IIA3-mediated activation of NDMA and NDEA was compared with human cytochrome P450IIE1-mediated activation in the same cell system. P450IIE1 was more effective at activating NDMA than P450IIA3, while P450IIA3 was more effective at activating NDEA than P450IIE1. Whole cells and microsomal fractions obtained from control cells and from cells expressing the P450IIA3 cDNA were characterized for expression of P450IIA3. Microsomal coumarin 7-hydroxylase activity was some 40 times greater in the transfected cells than in the control cells and was catalyzed by a protein that was immunochemically related to the rat liver cytochrome P450IIA gene family. Immunoblot analysis demonstrated that this protein was readily detectable in transfected cells but barely detectable in control cells. We also report the DNA and deduced amino acid sequence of the P450IIA3 cDNA isolate used in this study. Our isolate encodes a protein 489 amino acids that is five amino acids shorter at the N terminus but otherwise identical to a previously reported human P450IIA3 cDNA sequence.

N-nitrosamines in the stomach with special reference to in vitro formation, and kinetics after intragastric or intravenous administration in rats

To study the implications of nitrosation in the stomach, the formation of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) from nitrite and secondary amines was examined in vitro under conditions simulating gastric juice. Kinetics of NDMA were investigated after intragastric or intravenous administration of 0.2 mg/kg of NDMA in rats. NDMA and NDEA were measured using combined gas chromatography and thermal energy analyzer. Nitrite levels in human gastric juice were less than 10 micrograms/ml. Optimal pH for nitrosation was between 2.0 to 3.5. Nitrosamine formation reached maximum concentration at 3 to 6 hours. The maximum ratios of nitrosation were 0.15 and 0.11% in NDMA/nitrite and NDEA/nitrite, respectively. In the kinetic study, the highest blood levels of NDMA were observed at 5 min, reaching 174 +/- 40 and 374 +/- 40 ng/ml after administration into the stomach and duodenum, respectively. Then they decreased exponentially and were not detectable after 4 hours. Tissue levels of NDMA in the liver, spleen, kidney, lung and brain showed 70% of the blood levels. Urinary excretion of intravenously administered NDMA during the first 4 hours was less than 0.2%. These results supported the hypothesis that nitrosation occurred in gastric juice under optimal conditions, and indicated that nitrite levels were the limiting factor for nitrosation. Quick disappearance from the gastrointestinal tract suggested that the pathologic implication of nitrosamines formed in the stomach could be important for other organs.

Role of metabolism in dimethylnitrosamine-induced immunosuppression: a review

Dimethylnitrosamine (DMN) has been characterized as a potent hepatotoxin, carcinogen and mutagen. As described below, immunotoxicity should be added to its profile of activity. Although a broad spectrum of immune parameters is affected by DMN, humoral immunity is particularly sensitive. In order for DMN to produce its traditional profile of toxicity it requires metabolic activation to reactive intermediates which alkylate macromolecules. Similarly, DMN also must be metabolized to produce its immunological effects. However, as this review suggests, the metabolism of DMN to an intermediate capable of suppressing the humoral immune response may be qualitatively and/or quantitatively different from that which mediates hepatotoxicity and genotoxicity.

Changes in hepatic xenobiotic-metabolising enzymes in mouse liver following infection with Leishmania donovani

Infection of mice with Leishmania donovani resulted in decreased activities of several liver enzymes involved in the metabolism of xenobiotics. Microsomal membranes from infected livers contained reduced amounts of cytochromes P450 and b5 and NADPH-cytochrome P450 reductase. Several cytochrome P450 isoenzymes (P450-PB1, P450-PB3, P450-PCN and P450-UT1) and P450-mediated reactions (aminopyrine demethylase, aniline hydroxylase, benzphentamine demethylase and ethoxycoumarin deethylase) were affected similarly. The metabolism of two carcinogens (nitrosodimethylamine and 7,12-dimethylbenz[a]anthracene) by liver microsomal membrane preparations was also reduced. Leishmania infection caused an increase of cytosolic epoxide hydrolase and microsomal epoxide hydrolase and NADH-cytochrome b5 reductase were unaffected. The results suggest that Leishmania-infected animals are likely to have altered responses to exogenous toxins compared to uninfected animals.

N-nitrosodimethylamine blood levels in patients with chronic renal failure: modulation of levels by ethanol and ascorbic acid

We measured levels of N-nitrosodimethylamine (NDMA) in peripheral blood from 13 fasting male patients, 30-74 years old, who had chronic renal failure, and in five healthy control subjects (four males and one female) 31-50 years old. In the patients, we found significant (P less than .01) levels of NDMA (mean +/- SD; 201 +/- 111 ng/kg of blood), which is known to be carcinogenic in animals. Five minutes after oral administration of ethanol (0.4 g/kg of body weight), all patients exhibited a significant (P less than .01) rise in blood NDMA levels (338 +/- 125 ng/kg), suggesting continuous endogenous formation of NDMA that was unmasked by ethanol's ability to inhibit first-pass hepatic metabolism of NDMA. In five of six patients, pretreatment with oral ascorbic acid resulted in a blunting, but not statistically significant, effect on maximum blood NDMA levels after consumption of ethanol. Mean levels were 340 +/- 100 ng/kg before treatment with ascorbic acid and 237 +/- 127 ng/kg during treatment. Ethanol administration unmasks increased gastrointestinal formation of NDMA in patients with chronic renal failure. Further studies are required to confirm a possible link between endogenous NDMA formation and the increased incidence of cancer in these patients.

Detoxifying potential of thioproline against N-nitroso compounds, N-nitrosodimethylamine and N-nitrosocimetidine

Thioproline (TPRO), an effective nitrite trapping agent in vivo, was examined for its detoxifying ability in rats against N-nitrosodimethylamine (NDMA) and N-nitrosocimetidine (NCIM). When NDMA (37-101.5 mg/kg) was administered with TPRO (532 mg/kg), no influence of TPRO on NDMA-induced lethality and histological results in liver were observed. NDMA oxygenase activity measured by formaldehyde formation was not affected either. Denitrosation is a route of detoxication of N-nitroso compounds. When NCIM (100 mg/kg), a direct acting mutagen but not carcinogen, was given by gavage with TPRO, urinary excretion of N-nitrosothioproline (NTPRO) in rats apparently increased compared with TPRO alone. This result shows that TPRO is a trapping agent in vivo for nitrosating (NO) species originating from N-nitroso compounds, e.g., NCIM, which are denitrosated non-enzymatically in stomach acidic conditions. Transnitrosation from NDMA to TPRO, where enzymatic denitrosation is required, did not occur in measurable amount after oral administration of NDMA and TPRO.

N-nitrosamines in gastric juice of patients with gastric ulcer before and during treatment with histamine H2-receptor antagonists

The clinical implications of N-nitrosamines (NAs) were studied by analyzing their concentration in the gastric juice of 72 healthy subjects and 279 patients with gastric ulcer before and during treatment with histamine H2-receptor antagonists. NAs were measured by combined gas chromatography and thermal energy analyzer. The detection ratios of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) in the patients were 35.3% and 34.6%, respectively, which were significantly higher than the corresponding values in healthy subjects (19.4% and 16.7%, P less than 0.01). Analysis among the patients showed that this trend was mainly due to higher values in patients who were given histamine H2-receptor antagonists, as their detection ratios increased to 40.2% (NDMA) and 39.9% (NDEA). Patients without histamine H2-receptor antagonists showed moderate increases of detection ratios (NDMA; 24.2% NDEA; 22.6%) compared with healthy controls. The differences in these values between those receiving and not receiving histamine H2-receptor antagonists were statistically significant (P less than 0.01). The maximum concentrations of NDMA and NDEA were 7.9 and 9.8 ng/ml in patients, and 1.2 and 1.3 ng/ml in healthy subjects (the difference between the 2 groups P less than 0.02). These results indicated that patients with gastric ulcer had higher detection ratios and concentrations of NDMA and NDEA in gastric juice and that, while significant increases occurred during treatment with histamine H2-receptor antagonists, the extent of increase was below toxic or experimental carcinogenic levels.

Rat, mouse and hamster isozyme specificity in the glutathione transferase-mediated denitrosation of nitrosoguanidinium compounds

The major isozymes from affinity column-purified glutathione transferases isolated from Sprague-Dawley rat liver, kidney, and testis cytosol and also from BALB/c mouse and Syrian golden hamster liver cytosol have been resolved by chromatofocusing and tested for their ability to denitrosate and thus detoxicate the DNA-methylating agents and potential carcinogens nitrosocimetidine and 1,3-dimethyl-2-cyano-1-nitrosoguanidine (CyanoDMNG). The isozymes have been kinetically characterized using a battery of substrates permitting, in the rat and mouse cases, subunit composition identification. It has been found that the rat and mouse isozymes belonging to the mu class are uniquely and highly active in the denitrosation of nitrosocimetidine and CyanoDMNG. A specific set of hamster glutathione transferase isozymes were also found to be active in these reactions. We have identified the reaction products produced by the rat liver 3-4 isozyme activity. The glutathione transferase-mediated degradations of 1-methyl-2-nitro-1-nitrosoguanidine and CyanoDMNG generate one molecule of S-nitrosoglutathione per molecule of denitrosated guanidinium compound produced. In the CyanoDMNG incubations essentially all degradation was via denitrosation; nitrite and glutathione disulfide were minor products. In the 1-methyl-2-nitro-1-nitrosoguanidine case nonenzymic degradation of the nitroso compound in the presence of reduced glutathione was evident but little of this decomposition produced S-nitrosoglutathione or 1-methyl-2-nitroguanidine. In the presence of rat transferase 3-4 isozyme, glutathione-dependent 1-methyl-2-nitro-1-nitrosoguanidine degradation was shifted markedly towards denitrosation with the concomitant production of S-nitrosoglutathione.

[Risk of endogenous N-nitrosodimethylamine formed by dimethylamine in rats--precursors stability]

N-Nitrosodimethylamine (NDMA) is a potential carcinogen and present in our surroundings widely. NDMA can be formed by many precursors, Dimethylamine (DMA) is the most common precursor in food. To evaluate the risk of DMA nitrosation in vivo, so that men can do their best to reduce endogenous nitrosamine exposure, we carried out this research. This paper is the first part of our research. The stabilities of DMA and nitrite in the stomach were studied. DMA was stable and nitrite was decomposed at the speed of second-order reaction. The stabilities of nitrite in simulate gastric acid and in ascorbic (VC) solution also were studied. Nitrite decomposition in simulate gastric acid was very similar to the one in the stomach. VC solution at the equimolar with nitrite can decompose 53-79% of nitrite in one minute with shaking. We suggested that the efficiency of VC inhibiting endogenous nitrosation can be estimated on the basis of decomposing speed of nitrite and nitrosating speed by nitrite.

Effect of phenethyl isothiocyanate on microsomal N-nitrosodimethylamine metabolism and other monooxygenase activities

1. Phenethyl isothiocyanate (PEITC), a dietary compound derived from cruciferous vegetables, has previously been shown to decrease N-nitrosodimethylamine (NDMA)-induced methylation of hepatic DNA, apparently by inhibition of microsomal activation of the procarcinogen. 2. Using hepatic microsomes from acetone-treated rats, PEITC exhibited competitive inhibition of NDMA demethylase activity with an apparent Ki of 1 microM. In studies using a two-stage incubation protocol, the inhibition by PEITC was time- and metabolism-dependent. 3. Using control rat liver microsomes, PEITC selectively inhibited P450 IIE1-mediated NDMA-demethylase activity as compared to the demethylation of benzphetamine and ethylmorphine. 4. Pretreatment of rats with a single oral dose of PEITC (1 mmol/kg body wt) 24 h before killing caused a marked decrease in hepatic NDMA demethylase activity, but an 11-fold increase in 7-pentoxyresorufin O-dealkylase activity. These trends agreed with immunoblot analysis which indicated that PEITC was a suppressor of P450 IIE1 but an inducer of P450 IIB1. 5. The selective inhibition of P450 IIE1 activity and suppression of its level in microsomes indicates a role for PEITC as a chemopreventive agent against toxic or carcinogenic metabolites of this isozyme.

Metabolic denitrosation of N-nitrosodimethylamine in vivo in the rat

Enzymatic denitrosation is a potentially inactivating metabolic route that has been shown to convert carcinogenic N-nitrosodimethylamine (NDMA) to methylamine (MA) in vitro. To investigate its quantitative course in vivo, groups of 8-week-old male Fischer rats have been given small (8-15 mumol/kg) p.o. or i.v. bolus doses of 14C-labeled NDMA and the subsequent formation of radioactive MA has been monitored by high performance liquid chromatographic analysis of serially collected blood samples from each individual. Adjusting the [14C]MA fluxes observed for the previously measured rates at which MA is itself eliminated from the system after intragastric administration, denitrosation was calculated to represent a rather uniform 21.3 +/- 1.3% (SE) of total NDMA elimination in the four animals studied. By contrast, repetition of the experiment with fully deuterated NDMA (NDMA-d6) revealed a significantly wider variance in the results (39.8 +/- 8.9%). An alternative calculation using values for elimination of i.v. doses of MA and its trideuteromethyl analogue gave an even larger difference for MA formation between NDMA and NDMA-d6, the estimated extents of in vivo denitrosation in this case being 14.5 +/- 0.9% and 48.3 +/- 10.8%, respectively. The results indicate that denitrosation is a major metabolic pathway for NDMA elimination and suggest that deuteration of the carcinogen induces a shift in its metabolism toward increasing denitrosation at the expense of the competing activation pathway. Consequently, denitrosation may be the previously undefined in vivo metabolic route, the existence of which was suggested by the findings that deuteration of NDMA lowered its hepatocarcinogenicity and liver DNA alkylating ability in rats.

The role of yeasts in oral cancer by means of endogenous nitrosation

Oral leukoplakias, particularly non-homogeneous types, are often invaded by yeasts, with Candida albicans being the dominant species. The more advanced precancerous leukoplakia lesions yield more rarely occurring biotypes of C. albicans, suggesting a causal role for these biotypes in the malignant transformation. N-nitroso-benzylmethylamine (NBMA) is a compound able to induce carcinoma of the esophagus and the oral cavity in the rat. The catalytic potential of yeasts, isolated from leukoplakia lesions and from normal mucosa, to produce NBMA from the precursors N-benzyl-methylamine and nitrite was assessed at pH 6.8. The yeast strains differed in nitrosation potential, ranking from 0 to 1.2 micrograms NBMA/10(6) cells. C. albicans strains of the more rarely occurring biotypes showed the highest nitrosation potential, whereas C. tropicalis, C. parapsilosis, and Torulopsis glabrata were ranked lower. Strains with high nitrosation potential were generally isolated from lesions with more advanced precancerous changes. Thus, further evidence is provided supporting the hypothesis that yeasts play a causal role in oral cancer by means of endogenous nitrosamine production.

Influence of dithiocarbamates on the metabolism and toxicity of N-nitrosodimethylamine in rats

Five secondary amines and secondary amino acids were reacted with carbon disulfide to yield dithiocarbamates. These compounds were tested for their influence on biochemical parameters and on some biological effects of N-nitrosodimethylamine (NDMA). The following results were found. (i) All dithiocarbamates tested reduced the activity of N-nitrosodiethylamine-deethylase and completely inhibited the N-nitrosodimethylamine-demethylase in the rat liver. A striking influence on the glutathione content and on the activity of glutathione-S-transferase and glutathione-reductase was not observed. (ii) The excretion of unmetabolized N-nitrosodimethylamine in rat urine during 24 h increased from 0.1% without pretreatment to 3.6% of the given dose on combination with a single dose of dithiocarbamate. (iii) The acute toxicity of NDMA could be reduced with diethanol-dithiocarbamate. After a single simultaneous application of the inhibitor with NDMA, the LD50 was increased from 40 to 66 mg/kg. When the sulfur compound was administered twice, both simultaneously and 24 h after the nitrosamine, lethality was almost completely inhibited. We conclude from these results that dithiocarbamates may be suitable compounds for chemoprevention of nitrosamine-induced tumors.