Nitrosamine metabolism in kwashiorkor rats

The in vitro metabolism of nitrosodimethylamine (NDMA) was studied in liver tissue obtained from male weanling kwashiorkor wistar rats. The elimination of this compound and that of nitrosomorpholine (NMOR) from the blood, after a single intravenous dose, was also investigated. N-demethylase activity in liver microsomes of the test animals was not significantly different from that of the controls although the activity of this enzyme per gram wet liver tissue was considerably reduced in the model animals. On the other hand, the glutathione (GSH) content in liver cytosol of the kwashiorkor animals was much higher than that of the controls. The elimination of NDMA and NMOR from the blood of the experimental animals over 8 hr following i.v. administration of the carcinogens, showed that the clearance rate of each nitrosamine was significantly lower in the kwashiorkor rats.

New method for quantitative measurement of N-nitrosodimethylamine formation in the whole mouse

A simple method for the quantitative estimation of the formation of N-nitrosodimethylamine (NDMA) in mice has been developed. Mice were frozen in liquid nitrogen and homogenized. NDMA was then extracted and analyzed by a gas chromatograph equipped with a thermal energy analyzer. In normal mice NDMA (100 nmole) administered orally was rapidly metabolized and recovery of NDMA was about 10% after 60 min. However, when pyrazole (300 mg/kg) was injected i.p. to mice 60 min before the administration of NDMA, more than 80% of the administered NDMA could be recovered within 60 min. This result suggested that in pyrazole pretreated mice the accurate amount of NDMA formed could be estimated. Therefore the NDMA formation was measured in the pyrazole pretreated mice. When 0.25 mumole of aminopyrine and from 0.25 to 2.0 mumole of sodium nitrite were simultaneously administered orally, the amount of the NDMA formation in 20 min was found to be from 8.2 to 60.3 nmole. These values are equal to about from 30 to 200 micrograms/kg of body weight which are nearly daily doses expected to cause the carcinogenic effect on mice or rats. This method of measuring NDMA in pyrazole pretreated mice appears to be useful for investigating the in vivo formation of NDMA quantitatively.

Stability and capacity of dimethylnitrosamine-induced O6-methylguanine repair system in rat liver

Repeated administration of dimethylnitrosamine (DMN) (2 mg/kg for 3 weeks) to BD IV rats results in an increased capacity for the liver to repair O6-methylguanine in DNA, whereas other DNA adducts (7-methylguanine and 3-methyladenine) are not affected. In the experiments reported here, data on the rapidity of action of the enhanced system, its capacity after different challenging doses of [14C]DMN (0.2, 2.0, and 20 mg/kg), and the persistence after cessation of the DMN pretreatment are described. The results show that, after a dose of [14C]DMN (2.0 mg/kg), the increased activity acts very rapidly (10 min) repairing O6-methylguanine as soon as it is formed and that, by 2 hr, 65% of the O6-methylguanine generated in liver DNA has already been removed. Very little removal of O6-methylguanine occurs within this time in control rats not receiving any DMN pretreatment. The DMN-induced repair activity is of limited capacity, since its effect can be detected after DNA damage induced by 0.2 or 2.0 mg of [14C]DMN per kg, whereas this activity has little impact on the O6-methylguanine generated in liver DNA by 20 mg of [14C]DMN per kg. Upon cessation of the DMN pretreatment, the enhanced repair activity, as determined also by the in vitro activity of the O6-methyltransferase, decays slowly, but after 25 days, the repair activity is still higher than control values. No correlation was observed between increased [3H]thymidine incorporation in liver DNA and increased O6-methylguanine repair, indicating that liver cell proliferation is not necessarily coupled with an elevated methyltransferase level.

Evidence for generation of the precarcinogen nitrosodimethylamine in the small intestine in chronic renal failure

We have previously reported raised concentrations of dimethylamine and also bacterial overgrowth in the small intestine in CRF. Evidence for in vivo NDMA formation in the same site in CRF is now presented. Gastroduodenal intubation was performed in 9 healthy volunteers and 7 patients with advanced chronic renal failure. Blood, gastric, and duodenal aspirates were analyzed for NDMA. NDMA levels in control and CRF patients for blood were normal, but for gastric aspirate they were 67 +/- 13 and 312 +/- 68 (P less than 0.001) and for duodenal aspirate they were 70 +/- 21 and 319 +/- 47 (P less than 0.001), respectively. The results of bacterial cultures confirmed small intestinal bacterial overgrowth. We thus demonstrated statistically significant differences between NDMA concentrations in the control subjects and patients for both gastric and duodenal aspirates. This suggests that there is increased intestinal generation of NDMA in uremia. The presence of this precarcinogen may be linked with the reported increase in the incidence of cancer in CRF.

[Mechanism of transplacental penetration of N-nitrosodimethylamine into rat and mouse fetuses]

The authors have studied the time course of changes in N-nitrosodimethylamine (NDMA) concentration in the blood, liver, muscle tissue and embryo of pregnant rats and mice after its intravenous injection in a dose of 50 mg/kg. In the parent organism, the maximal concentration was attained immediately after injection as shown by the measurements made 5 minutes after and then decreased according to the exponential law, with that decrease occurring far rapidly in mice than in rats. In the embryo, the maximal concentration attainment was 30-60 minutes as delayed. The concentration of NDMA in the embryo and tissues of the parent organism ascended linearly up to a definite level as the dose was raised (blood concentration) whereupon its increase became slower and even ceased which forms the basis for an assumption that the carcinogenic substance penetrates via the placenta and cell membranes according to the laws of transport with the aid of carriers.

Metabolism and activation of 1,1-dimethylhydrazine and methylhydrazine, two products of nitrosodimethylamine reductive biotransformation, in rats

Nitrosodimethylamine (DMN) and two of its metabolites, methylhydrazine (MH) and 1,1-dimethylhydrazine (UDMH), were metabolized to CO2 by liver slices obtained from Sprague-Dawley rats. Under the conditions used, DMN and MH produced reactive metabolites that bound covalently to nucleic acids, but UDMH did not. Rat liver microsomes or 9,000 X g supernatants were able to transform DMN, MH, and UDMH to CH2O. In the cases of MH and UDMH, enzymatic and nonenzymatic pathways of CH2O formation were observed in both liver microsomes and 9,000 X g supernatants. DMN, MH, and UDMH led to covalent binding (CB) to proteins in incubation mixtures containing either microsomes or 9,000 X g supernatants. In the case of DMN, the process was enzymatic and required NADPH in both cellular fractions. In the case of MH, the process was enzymatic in microsomes and required NADPH and O2. With UDMH or MH and 9,000 X g supernatants, nonenzymatic interactions resulting in CB to proteins dominated. All these results suggest that part of the CO2 produced during DMN metabolism might be derived from UDMH and MH. Similarly, a significant part of the CB of DMN metabolites to proteins in incubation mixtures containing microsomes or 9,000 X g supernatants might be derived from enzymatic and nonenzymatic reactions of UDMH or MH. Also, a minor part of the CB of DMN-reactive metabolites to nucleic acids might have resulted from MH's further biotransformation to reactive metabolites. Overall, biotransformation of DMN and MH might not be a detoxication process, as previously thought, but one related to some of the DMN toxic effects.

Measurement of 7-methylguanine as an estimate of the amount of dimethylnitrosamine formed following administration of aminopyrine and nitrite to rats

We have demonstrated that there is a dose-related increase in the excretion of 7-[methyl-14C]methylguanine ( [14C]m7Gua) following p.o. administration of di[methyl-14C]methylnitrosamine to rats. Urine was collected for 24 hr after di[methyl-14C]methylnitrosamine administration, and the purines were precipitated from an aliquot of the urine with silver nitrate. Purines were released from the precipitate with HCl, and [14C]m7Gua was quantified by chromatography on an Aminex A-6 column. The excretion of [14C]m7Gua increased linearly with the dose of dimethylnitrosamine. This relationship was used to estimate the amount of di[methyl-14C]methylnitrosamine formed in the reaction of [14C]aminopyrine with sodium nitrite in rats gavaged with these compounds. The dose of dimethylnitrosamine was also estimated from the amount of alkylation of liver DNA in the same animals. These estimates usually differed by less than a factor of 2. [14C]aminopyrine and sodium nitrite were administered. The possibility of using this assay to obtain data on nitrosation in humans is discussed.

[Effect of HF on the hepatic metabolism of dimethylnitrosamine in the rat]

The authors have studied the action of fluorine, administered by inhalation, on the liver metabolism of a chemical carcinogen: dimethylnitrosamine (DMN). The results demonstrate a decrease in the level of cytochrome P450 and in the activity of benzo(a)pyrene hydroxylase in animals treated with DMN or DMN + HF. The greater inhibition in the presence of HF is paralleled by a decrease in the weight of the liver and in the synthesis of liver microsomal proteins. This reduction of activity (with the exception of dimethylnitrosamine demethylase which is unaffected) is supported by the result of the histological examinations showing two different types of lesion-necrotic toxic hepatitis and post-hepatitic cirrhosis - the frequency of which is much higher in the presence of fluorine.

Denitrosation as a determinant of nitrosocimetidine in vivo activity

The widely used drug cimetidine (Tagamet) can be nitrosated in the presence of nitrite and under mild acid conditions to form a compound, nitrosocimetidine (NC), which has a chemical structure very similar to those of the mutagens and laboratory carcinogens N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and methylnitrosourea (MNU). NC has given positive indications in several short-term tests for possible carcinogenic activity and is capable of methylating DNA in vitro and in cultured cells in a manner identical to that of MNNG and MNU. Nevertheless, NC has been found to be a weak carcinogen or a noncarcinogen and to be very poor at modifying DNA in vivo when administered p.o. We have found that NC, like MNNG, decomposes very rapidly when incubated with thiol compound in neutral pH buffer. Much of this decomposition is denitrosation. In the presence of excess reduced glutathione, about 35% of the degradation results in denitrosation to produce cimetidine, and in the presence of excess cysteine about 65% results in denitrosation to produce cimetidine. The compound also rapidly decomposes in whole blood isolated from rats; about 70% of this decomposition produces cimetidine. In solution with purified rat hemoglobin, approximately 90% of the NC degradation proceeds via a denitrosation pathway; hemoglobin cysteine residues have been implicated in the denitrosation reaction. In parallel experiments with MNNG, it has been found that, although a fraction of the decomposition of this agent in the presence of thiol compound, in isolated whole blood, and in solution with purified hemoglobin generates the denitrosated derivative, denitrosation is in the range of one-third to one-half of that found for NC. Denitrosation and degradation to form a methylating species appear to be the major NC and MNNG decomposition pathways in vitro. There is no indication that MNU degradation is sensitive to thiols, nor is the compound susceptible to denitrosation at neutral pH. Molar-equivalent doses of methyl group-radiolabeled NC, MNNG, and MNU were administered via the tail vein to groups of F344 rats, and the DNA methylation yields in lung, liver, kidney, and brain tissue were assessed. Of the organs considered, DNA methylation was greatest in the lungs of MNNG-treated animals, followed by kidney (25% of the lung value). Methylation of lung tissue DNA in MNU-treated animals was about 50% of that observed in the MNNG experiments; DNA methylation in the other organs was about equivalent to that found in the lung.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacokinetic model for N-nitrosodimethylamine based on Michaelis-Menten constants determined with the isolated perfused rat liver

A pharmacokinetic model was constructed to describe the absorption, distribution, and metabolic clearance of N-nitrosodimethylamine. The model is composed of two compartments, total body water and the liver, which are linked by blood flow. Metabolic clearance is presumed to occur only in the liver. Liver clearance kinetics was determined with isolated perfused livers. Clearance appeared to obey Michaelis-Menten kinetics with Km = 8.3 +/- 4.8 microM and Vmax = 0.15 +/- 0.02 mumol/min . liver. The observed value for Km is about 1 order of magnitude lower than any observed when clearance is determined using liver microsome preparations. The model is used to calculate whole-body clearance of N-nitrosodimethylamine and relative tissue exposure as a function of the route of administration. The calculations are compared with previously published experimental data, and it is shown that the accuracy of the model for low doses is a result of the novel value observed for Km in the perfused liver.

Influence of pretreatment with various insecticides on the N-demethylation of dimethylnitrosamine

The effects of various classes of insecticides were studied on the N-demethylation of dimethylnitrosamine (DMN) by mouse liver enzymes. Organochlorine insecticides, represented by lindane, DDT, and endrin, increased the activities of DMN demethylase I and II. The latter enzyme was more susceptible to the inducive action of the tested chlorinated insecticides. On the other hand, the synthetic pyrethroids, fenvalerate and flucythrinate, did not alter the activity of either enzyme. While pretreatment with carbaryl, a carbamate derivative, was without effect, moderate elevation in the activity of both demethylases was observed following administration of carbofuran. Dimethoate, representing organophosphorus compounds, was the only insecticide tested to inhibit the N-demethylation of DMN, with more pronounced effect on DMN demethylase I. Since DMN requires metabolic activation for its hepatotoxic and carcinogenic actions, alterations in the activities of its metabolizing enzymes as a sequela of exposure to certain insecticides may change susceptibility to its toxicity and/or carcinogenicity.

The influence of various factors on the methylation of DNA by the oesophageal carcinogen N-nitrosomethylbenzylamine. I. The importance of alcohol

Alcohol appears to be a predisposing factor for the high incidence of oesophageal cancer in Western France. Therefore, we have investigated the influence of ethanol on the alkylation of DNA by a carcinogen which reacts selectively with oesophageal tissue. Female Wistar rats (approximately 80 g) received a single i.v. injection of N-nitroso-[methyl-14C]benzylamine (2.5 mg/kg body weight). Four hours after injection of the carcinogen, methylation of purine bases in the DNA isolated from various organs was measured. We found that pretreating the rats with alcohol (45-69 micrograms/day/kg) for a period of 3-4 weeks leads to an enhancement of DNA methylation in the oesophagus. An increased amount of methylated purine base was also noted in lung DNA. In contrast, a reduction in the methylation rate of liver DNA was observed.

Effect of phenobarbital and 3-methylcholanthrene on the hepatic microsomal metabolism of N-nitrosodimethylamine, N-nitrosomethylbutylamine and N-nitrosomethylbenzylamine

The effect of phenobarbital (PB) and 3-methylcholanthrene (MC) pretreatment on dealkylations of N-nitrosodimethylamine (NDMA), N-nitrosomethylbutylamine (NMBuA) and N-nitrosomethylbenzylamine (NMBeA) was investigated in rat hepatic microsomes. PB increased the demethylation and the debutylation of NMBuA and the debenzylation of NMBeA. MC increased the debutylation of NMBuA and the debenzylation of NMBeA. However, MC decreased the demethylation of NMBeA. The demethylation of NDMA was not changed by the pretreatment of the inducers. These results suggest that the dealkylations of these nitrosamines are catalyzed by several enzymes, which probably depend on different cytochrome P-450 species.

Carcinogenesis and aging. II. Modifying effect of aging on metabolism of methyl(acetoxymethyl)nitrosamine and its interaction with DNA of various tissues in rats

Young (3 month-old) and old (14 month-old) female outbred rats received a single i.p. dose (13 mg/kg) of N-[14C]methyl-Nacetoxymethylnitrosamine [( 14C]DMN-OAc), which, under these conditions, selectively induces intestinal tumours. Complete DMN-OAc breakdown occurred within 30 min in both young and old rats but exhalation of 14CO2 continued for over 1 h in young rats and over 3 h in old rats. The highest level of methylation in both young and old rats was found in the DNA of epithelial cells of the colon and in other adjacent abdominal organs (liver and uterus). The initial capacity for excision of the O6-methylguanine from liver DNA was greater in young animals, but further this DNA adduct was repaired more efficiently by the liver of old rats. In the DNA of ileal and colonic enterocytes, O6-methylguanine excision was higher in young than in old animals. The DNA tertiary structure, measured by the sedimentation pattern of nucleoids in neutral sucrose gradient, was damaged in old rats, and, to a lesser extent, in young rats. The non-uniform pattern of DNA damage in young and old animals may be associated with differing carcinogenic effects of DMN-OAc in rats of different ages, a hypothesis which is currently under test.

Methylation of liver DNA of rat and mouse by N-nitrosodimethylamine formed in vivo from dimethylamine and nitrite

The extent of formation of N-nitrosodimethylamine (NDMA) in the stomachs of rats and mice after simultaneous oral administration of [14C]dimethylamine and potassium nitrite was determined by measuring the methylation of liver DNA. With doses of around 1 mg dimethylamine hydrochloride/kg body weight and 50 mg potassium nitrite/kg body weight, 0.8% of the amine was nitrosated on average. The individual fluctuations ranged from 0.2 to 1.3% in the rat and from 0.2 to 1.9% in the mouse. Simultaneous administration of 50 mg sodium ascorbate (vitamin C)/kg body weight inhibited the nitrosation by about 80% while 50 mg alpha-tocopherol acetate (vitamin E)/kg body weight reduced the nitrosation by about a half. Assuming similar kinetics and conditions of nitrosation in rats and man, a comparison of the formation of NDMA in vivo from dietary dimethylamine and nitrite with the estimated human uptake of preformed NDMA revealed that in vivo formation in the stomach of man is probably negligible.

Activation of nitrosamines and other carcinogens by mouse-liver S9, mouse hepatocytes and in the host-mediated assay produces different mutagenic responses in Salmonella TA1535

5 indirect alkylating carcinogens, namely, dimethylnitrosamine (DMNA), methylethylnitrosamine (MENA), diethylnitrosamine (DENA), 1,2-dimethylhydrazine (DMH) and cyclophosphamide (CP), were tested in liquid incubation assays for their mutagenic activity towards Salmonella TA1535 in the presence of mouse-liver homogenate (S9) or freshly isolated, single liver-cell preparations. The capacity of these mouse-liver preparations to activate the compounds to mutagens for TA1535 was compared with the mutagenic effect of low doses of the carcinogens in intrasanguineous host-mediated assays, with the same strain of mice as host. Although the mouse hepatocytes retained their activating capacity longer than S9 preparations did during incubation at 37 degrees C, the latter gave much higher yields of mutants with 10 mM (DMNA, MENA, DMH) and 5 mM (CP) of 4 out of the 5 compounds. DENA was not mutagenic in either assay. These differences between whole cell and disrupted cell preparations were reduced or absent when the concentrations of the test compounds were reduced by a factor of 10. It was concluded that hepatocytes at the maximal concentration of cells have a limited capacity to metabolize the mutagens. On the basis of protein concentration, hepatocytes are more effective (nitrosamines) or equally effective (CP and DMH) in activating the compounds. Compared with the host-mediated assays, both liver fractions have only a marginal potential to activate equal low amounts of the carcinogens. The present results do not indicate that hepatocytes take an 'intermediate' position between existing in vitro and in vivo activation systems, although they do suggest that these mouse hepatocyte preparations activate the nitrosamines DMNA and MENA in a quantitatively or qualitatively different way than do mouse-liver homogenates.

The role of highly purified forms of rat liver cytochrome P-450 in the dimethylation of dimethylnitrosamine and its activation to mutagens

Highly purified NADPH-cytochrome P-450 reductase and the major phenobarbital (PB) and beta-naphthoflavone (beta NF) forms of cytochrome P-450 were used in reconstituted systems to study the demethylation and subsequent activation of dimethylnitrosamine (DMN) to mutagenic intermediates. Both forms of cytochrome P-450 were active in the demethylation of DMN, cytochrome P-450 from PB-treated animals being more efficient, generating nearly twice as much formaldehyde per nmol of haemoprotein. Neither form of the cytochrome could activate DMN to mutagens in the Ames test. These findings indicate that DMN demethylation does not lead to its activation to mutagenic products.

DNA breaks induced by micromolar concentrations of dimethylnitrosamine in liver primary cell cultures from untreated and phenobarbital treated rats

Direct genotoxic effects of the alkylating agent dimethylnitrosamine (DMN) have been difficult to detect in several short-term tests. We simplified our method to detect DNA breaks induced by DMN in rat liver primary cell cultures, and increased its sensitivity about 150 times by changing the conditions of ultracentrifugation and exposure to DMN. Additionally we increased 4 times the sensitivity of the improved assay by isolating hepatocytes from rats treated with phenobarbital (PB). Treatment for 24 h with 60 microM and 13.5 microM DMN of hepatocytes isolated from untreated and PB-treated rats, respectively, decreased the molecular weight of DNA by 50%. After 24 h exposure to 13.5 microM [14C]DMN, hepatocytes from PB-treated rats incorporated 3 times more radioactivity into trichloroacetic acid precipitable material than hepatocytes from untreated rats. Also PB-treatment increased remarkably cytotoxic effects of DMN while it did not modify the cytotoxicity nor the genotoxicity of the direct-acting alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine. These results show that DMN is more genotoxic for hepatocytes from PB-treated rats, and suggest that the enhanced genotoxicity is probably due to an augmented metabolism of DMN by these cultures. Our improved assay of DNA breaks as an indicator of DMN genotoxicity is now as sensitive but faster to perform than hepatocyte-mediated mutagenesis. It could be used to explore genotoxic effects of other alkylating agents and the action of microsomal enzyme modifiers on genotoxicity.

An examination of human blood for the presence of volatile nitrosamines

Human blood was examined for the presence of volatile nitrosamines. Nitrosamines were detected by chemiluminescence and mass spectrometry after separation from blood by distillation and solvent extraction. N-nitrosodimethylamine was detected in all but one of 51 blood samples taken from 23 different people, at concentrations from the detection limit (0.1 microgram/litre) to 1.4 microgram/litre with a mean concentration of 0.5 microgram/litre. N-Nitrosodiethylamine was detected in 11 samples, the detection limit being 0.1 microgram/litre. No other volatile nitrosamines were detected. After a test meal of bacon, spinach, bread and beer, the concentration of N-nitrosodimethylamine increased. There was no appreciable difference between the nitrosamine concentrations in the blood of laboratory workers and in the blood of other people. Salivary nitrite concentrations measured semi-quantitatively concurrently with blood sampling varied considerably but showed no apparent correlation with blood nitrosamine levels. Measurements in rabbits given a continuous infusion of N-nitrosodimethylamine gave a clearance rate approximately equal to the blood flow through the liver and a volume of distribution of 1.2 litre/kg body weight. By applying these results to man, the body burden after the meal was calculated as 40-50 microgram. This is substantially higher than the estimated weekly intake of volatile nitrosamines from food.

Influence of a non-synthetic diet with a high fat content on the local occurrence of colonic carcinomas induced by N-nitroso-acetoxymethylmethylamine (AMMN) in Sprague-Dawley Rats

Low doses (0.1 mg/animal administered fortnightly for 48 weeks) of the local carcinogen N-nitrosoacetoxymethylmethylamine (acetoxymethylmethylnitrosamine, AMMN) were administered intrarectally to Sprague-Dawley rats fed a standard diet (Altromin pellets) and resulted in a 22% incidence of colonic cancer. When the carcinogen was administered to rats kept on a nonsynthetic diet with a high fat content for their entire life-time, the colonic cancer risk was identical. The results demonstrate that an unbalanced high-fat diet need not necessarily increase the colonic cancer risk caused by a chemical carcinogen.

Deuterium isotope effect on metabolism of N-nitrosodimethylamine in vivo in rat

The maximal rates of metabolic oxidation of N-nitrosodimethylamine (NDMA) and N-nitrosodimethylamine-d6 (NDMA-d6) in vivo (VH and VD, respectively) have been measured by following 14CO2 exhalation in rats after intraperitoneal injection of the two 14C-labelled carcinogens at high doses (20 or 40 mg/kg). Complete deuteration of NDMA reduced only slightly the maximal rate of metabolism when the two substrates were administered separately (VH/VD approximately 1.2). However, much larger (approximately 4-fold) deuterium isotope effects were observed when mixtures of NDMA with NDMA-d6 were injected. These results are tentatively interpreted as evidence that C-H bond cleavage is not a rate limiting feature of overall metabolism, but that the complex between NDMA and the principal enzyme(s) metabolizing it in vivo freely equilibrates with unbound substrate. Single, large, intraperitoneal doses of NDMA and NDMA-d6 produced a similar alkylation of rat liver DNA and also of kidney DNA. However, a small oral dose (54 micrograms/kg) of NDMA-d6 produced 1/3 less alkylation of liver DNA and 3 times as much alkylation of kidney DNA as did an equimolar dose of NDMA. The reduction in alkylation of liver DNA correlates well with, and possibly explains, the decreased ability of NDMA-d6 to induce liver tumors in rats. The associated increase in the alkylation of kidney DNA suggests that this change is due to a decrease in the amount of nitrosamine removed from the portal blood on the first pass through the liver.

Inhibitory effects of carbon tetrachloride on dimethylnitrosamine metabolism and DNA alkylation

The effect of CCl4 pretreatment (dose range from 0.5 to 2.0 ml/kg body weight) on the pathways of dimethylnitrosamine (DMN) metabolism was investigated. The oxidative-N-demethylation by the enzymes, DMN-demethylase I and II operating at low (4 mM) and high (200 mM) substrate concentration, respectively, was greatly reduced in a dose-dependent manner. In addition, the generation of an electrophilic intermediate capable of methylating DNA, specifically at the N-7 and O-6 positions of guanine, was completely inhibited by CCl4 (0.5 ml/kg body weight) pretreatment. When indole feeding (1% in the diet for 8 days prior to CCl4 administration) was employed as a means to enhance DMN-demethylase activities, it was found that the reduction of DMN-demethylase activities was more pronounced in these rats than in the controls. In agreement with earlier findings, 7-methylguanine and O6-methylguanine were not detectable in the CCl4 pretreated group. These results suggest that CCl4 exerts a strong inhibitory action on hepatic DMN metabolism, in particular on the pathway leading to alkylation of DNA guanine. This phenomenon may explain the protective role of CCl4 against DMN-induced hepatotoxicity and perhaps, carcinogenicity, believed to be closely associated with the abnormal modifications of DNA.