DNA alkylation and tumor induction in regenerating rat liver after cell cycle-related continuous N-nitrosodimethylamine infusion

Synchronized regenerating rat liver after partial hepatectomy was used to study cell cycle-related DNA base alkylation and liver carcinogenesis. A continuous iv infusion of [14C]N-nitrosodimethylamine (DMN) at a dose of 0.5 mg/kg/hour was given to inbred male Wistar Af/Han rats over a period of 8 hours either during the G1 phase, hydroxyurea-synchronized DNA synthesis, or the G2+M-phase of regenerating liver or to untreated rats (G0-phase liver--carcinogen dose, 1.5 mg/kg/hour). Two hours after the end of the infusion, the amount of 7-methylguanine was highest in the G0-phase liver, with a decrease in the G1 phase, the S-phase, and the G2+M-phase. After continuous DMN exposure, the O6-methylguanine:7-methylguanine ratio was lower in the S-phase and G2+M-phase livers than in the G0-phase and G1-phase livers, indicating an increased O6-methylguanine repair during DNA synthesis and the G2+M-phase. Liver tumors in rats treated by continuous DMN infusion either during the G0 phase or the S-phase developed only after carcinogen exposure during DNA synthesis.

[Action of F- on liver metabolism of dimethylnitrosamine and benzo(a)pyrene in the rat]

The authors carried out this study according to three parameters (dose, level, age of the rats and way of administration) on the action of fluor study in liver metabolism of dimethylnitrosamine and benzo(a)pyrene. The results showed that, in parallel with the increase of the dose level, at certain concentrations, it occurs generally an induction of dimethylnitrosamine-demethylase and a decrease in the amount of cytochrome P450 either by intraperitoneal injection, by ingestion or by inhalation. On the other hand, fluoride does not affect benzo(a)pyrene metabolism. Moreover, inhalation of FH looks the most suitable for this study. It allows to keep a relatively constant rate of F' in the blood and bring about more important changes in the metabolism of dimethylnitrosamine (30 to 70% induction) and in the amount of cytochrome P450 (10 to 40% decrease), as compared to the controls. The results suggest that, in the conditions of our experiments, fluoride would exert an favourable effect on the carcinogenic power of dimethylnitrosamine without affecting benzo(a)pyrene one.

The induction of chromosomal damage in rat hepatocytes and lymphocytes. II. Alkylation damage and repair of rat-liver DNA after diethylnitrosamine, dimethylnitrosamine and ethyl methanesulphonate in relation to clastogenic effects

Rat-liver DNA alkylation by diethylnitrosamine (DEN), dimethylnitrosamine (DMN) and ethyl methanesulphonate (EMS) was studied in an attempt to relate chromosome-damaging effects of these agents (the formation of micronuclei in hepatocytes; see preceding paper) to specific alkylation patterns. No correlation was observed between the induction of micronuclei and liver DNA N-alkylation, measured as 3- and 7-alkyl-purines. O6-Alkylguanine is probably not involved in micronucleus induction because it is lost from DNA too rapidly to explain the much more persistent clastogenic effects. In contrast, both the initial amounts of alkylphosphotriesters and the persistencies of these products roughly paralleled the respective effects on micronucleus induction. The possible involvement of alkylphosphotriesters or other O-alkylation products of comparable stabilities is discussed. Results with DMN suggest that part of the primary DNA methylation damage is converted into a secondary (DNA) lesion and that both the primary and secondary lesion(s) contribute to the process of micronucleus formation.

Biological activity of benzylating N-nitroso compounds. Models of activated N-nitrosomethylbenzylamine

Unsymmetrically substituted N-nitrosomethylbenzylamine is an oesophageal carcinogen with potential methylating and benzylating properties. Whereas the methylating activity of the compound has been investigated, little is known of its potential benzylating properties. In order to elucidate the biological consequences of benzylation, related model compounds which are presumed benzylating agents were synthesized and tested for mutagenicity. N-nitrosobenzylurea and its structural analogue N-nitroso-p-methylbenzylurea were direct acting mutagens in Salmonella typhimurium TA 98. Activity was also present in TA 1535, but it was less pronounced. N-nitroso-alpha-acetoxybenzyl-benzylamine was equally mutagenic in S. typhimurium TA and TA 1535. N-nitroso-acetoxymethyl-benzylamine and N-nitrosoacetoxy-methyl-p-methylbenzylamine are two model compounds which may decompose by hydrolysis or through esterases to yield intermediates also though to arise after alpha-C hydroxylation of the methyl group of the parent nitrosamines. These compounds needed additional activation by enzymes present in the post-mitochondrial supernatant of rat liver. They were distinctly mutagenic in TA 98. Furthermore, all compounds also caused the induction of phage lambda in a qualitative assay with Escherichia coli Br 513. Thus, benzylation of DNA clearly results in a biological consequence. These findings are supportive of the theory that if enzymic attack occurs on the methyl group of N-nitrosomethylbenzylamine, benzylation may also contribute to the overall biological activity of the compound.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. III. Effect of cobaltous chloride treatment

1. The effects of CoCl2 administration to rats on xenobiotic metabolism, dimethylnitrosamine (DMN) metabolism to formaldehyde and methanol, and monoamine oxidase (MAO) enzyme activities in hepatic subcellular fractions have been studied. 2. CoCl2 treatment markedly decreased hepatic mixed-function oxidase enzyme activities and microsomal cytochrome P-450 content. In contrast, the N-oxidation of N, N-dimethylaniline and the activity of microsomal NADPH-cytochrome c reductase was unaffected. 3. The metabolism of DMN to formaldehyde by postmitochondrial supernatant fractions was decreased at substrate concn. of 0 . 5, 5 and 50 mM by CoCl2 treatment but the metabolism of 5 and 50 mM DMN to methanol was affected less. 4. CoCl2 had little effect on MAO activities in whole homogenates, but microsomal MAO activities were markedly inhibited. 5. The inhibition of microsomal MAO indicates that CoCl2 is not a specific inhibitor of cytochrome P-450-dependent biotransformations and consequently the inhibition of DMN metabolism is not evidence of a wholly cytochrome P-450-dependent process.

The effects of dietary corn oil on the metabolism and mutagenic activation on N-nitrosodimethylamine (DMN) by hepatic microsomes from male and female rats

Microsomes from male and female rats fed a diet containing 10% corn oil metabolized N-nitrosodimethylamine (DMN) more rapidly than microsomes from rats fed a similar diet devoid of corn oil. The daily administration of phenobarbital for 4 days prior to harvesting the microsomes resulted in significant induction of the Vmax for N-demethylation of DMN in rats fed the fat-free diet but resulted in no induction (females) or suppression (males) of this enzyme in rats fed the diet containing corn oil. Using concentrations of DMN ranging from 12 to 100 mM, microsomes from rats fed the high fat diet activated DMN to produce mutagenesis in S. typhimurium (TA100) more rapidly than those from rats fed the fat-free diet. Phenobarbital administration induced this activation more effectively in rats fed the corn oil diet than in rats fed the fat-free diet. Phenobarbital induces DMN N-demethylation in rats fed both fat-free and 10% corn oil diets when the DMN concentration is above 10 mM and explains, at least in part, this enhanced mutagenic activation.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. II. Inhibition by substrates and inhibitors of monoamine oxidase

1. The metabolism of dimethylnitrosamine (DMN) to formaldehyde by rat-hepatic postmitochondrial supernatant fractions has been compared with the activities of several cytochrome P-450-dependent mixed-function oxidase enzymes and the Ziegler mixed-function amine oxidase enzyme (EC 1.14.13.8). 2. A variety of monoamine oxidase (MAO, EC 1.4.3.4) inhibitors of diverse chemical structure inhibited the metabolism of DMN. In parallel studies a number of MAO substrates, but not their deaminated products, also inhibited DMN metabolism, whereas substrates of diamine oxidase were ineffective. 3. At concentrations which inhibited DMN metabolism several MAO substrates and inhibitors did not inhibit the N-oxidation of N, N-dimethylaniline and an inhibitor and an activator of the Ziegler enzyme had no corresponding effect on DMN metabolism. 4. The metabolism of DMN and a number of MAO enzyme activities were stable to storage under conditions where mixed-function oxidase enzymes were not. 5. These results are consistent with the suggestion that DMN may, at least in part, be metabolized by hepatic enzyme(s) not dependent on cytochrome P-450 and that a microsomal amine oxidase enzyme, unrelated to the Ziegler enzyme, may be involved in the hepatic degradation of this nitrosamine. The present data does, however, suggest a role for microsomal NADPH-cytochrome c reductase in hepatic DMN metabolism.

Inhibition of dimethylnitrosamine metabolism by some heterocyclic compounds and by substrates and inhibitors of monoamine oxidase in the rat

Pretreatment of rats with a number of nitrogen-containing heterocyclic compounds was found to inhibit markedly the metabolism of dimethylnitrosamine (DMN) in terms of both CO2 excretion and decline in blood DMN concentration. However, many of these compounds had either much less or no inhibitory effect on the in vivo metabolism to CO2 of a typical mixed-function oxidase substrate, aminopyrine. In addition, a number of model inhibitors of monoamine oxidase (MAO) activity also inhibited DMN metabolism in the intact animal, and a number of primary amines, known substrates of hepatic MAO, inhibited DMN metabolism but not that of aminopyrine in the isolated perfused liver system. These results, together with in vitro data and previously reported studies on the effect of MAO inhibitors and substrates on the mutagenicity of DMN, suggest that the metabolism and bioactivation of DMN may be in part mediated by a MAO type of enzyme activity.

Esophageal and hepatic microsomal metabolism of N-nitrosomethylbenzylamine and N-nitrosodimethylamine in the rat

The metabolism of the rat esophageal carcinogen N-nitrosomethylbenzylamine (NMBzA) was studied using microsomes prepared from liver and esophageal mucosa of untreated male Sprague-Dawley rats. NMBzA was extensively metabolized to benzaldehyde, benzyl alcohol, and formaldehyde by hepatic microsomes. The rate of metabolism at the benzyl moiety was 10-fold higher than that at the methyl moiety. Mucosal microsomes metabolized NMBzA to benzaldehyde and formaldehyde at rates one-fifth and one-sixtieth of those in the liver, respectively; benzyl alcohol formation was undetectable. Esophageal metabolism of NMBzA was exclusively located in the mucosa, preferentially in the microsomal fraction, was reduced nicotinamide adenine dinucleotide phosphate dependent, and was inhibited by CO and 2-diethylaminoethyl-2,2-diphenylvalerate. A low level of cytochrome P-450 was detected in the mucosal microsomes. Whereas hepatic metabolism of NMBzA was inducible by phenobarbitone pretreatment, mucosal metabolism was not altered by either phenobarbitone or 3-methylcholanthrene pretreatment. The hepatocarcinogen N-nitrosodimethylamine was extensively metabolized by hepatic microsomes to formaldehyde, but demethylation was not detected in the microsomes from esophageal mucosa, a nontarget tissue. The results indicate that rat esophageal mucosa contains an enzyme system which metabolizes NMBzA at a high rate and exhibits properties typical of cytochrome P-450. This enzyme may play a role in determining which compounds induce tumors in rat esophagus.

Mutagenesis in Salmonella after NADH-dependent microsomal activation of dimethylnitrosamine

The mutagenic activity of dimethylnitrosamine activated by rat-liver microsomes in the presence of NADH was compared with that obtained with NADPH. 3 histidine auxotrophic strains of Salmonella underwent reversions after activation with NADH as the sole coenzyme. All 3 tester strains showed a dose-response relationship with dimethylnitrosamine (10-125 mumoles per plate) after NADH-supported activation. With NADH as the sole coenzyme, the most sensitive strain, hisG46, showed a 105-fold increase in mutagenesis frequency as compared with the 230-fold increase obtained with NADPH. Activation of dimethylnitrosamine in the presence of NADH and NADPH, in combination, produced mutagenesis at frequencies above those seen with NADH alone, but less than or equal to those seen with NADPH as the only coenzyme during the activation step. Experiments in vitro showed that microsomal incorporation of carbon from [14C]dimethylnitrosamine was highest in the presence of NADPH, lowest with NADH and reached intermediate levels when both coenzymes were present. The source of the microsomes in all experiments was liver from rats pre-treated with Aroclor 1254.

Mechanism of esophageal tumor induction in rats by N-nitrosomethylbenzylamine and its ring-methylated analog N-nitrosomethyl(4-methylbenzyl)amine

The metabolism of the esophageal carcinogen N-nitrosomethylbenzylamine (MBN) and its ring-methylated analog N-nitrosomethyl(4-methylbenzyl)amine (4-MeMBN) was investigated in male Wistar rats. When given in the drinking water, both compounds have been shown to induce a high incidence of esophageal carcinomas but, after systemic administration of equimolar doses, 4-MeMBN is considerably less toxic and carcinogenic than is MBN. Following a single i.v. injection, 4-MeMBN disappeared from serum faster than did MBN. After 5 hr, neither compound was detectable in serum. Within 12 hr after a single i.v. injection (0.017 mmol/kg) of [methyl-14C]-MBN, 49% of the radioactivity was exhaled as 14CO2, and less than 5% was in the urine, compared with only 13% as 14CO2 and 65% in the urine after an equimolar dose of 4-Me[methyl-14C]MBN. The urinary metabolite of 4-MeMBN was identified as its benzoic acid derivative. Methylation of DNA purines 4 hr after a single i.v. injection (0.017 mmol/kg) of [methyl-14C]MBN was highest in the esophagus (344 mumol 7-methylguanine per mol guanine), followed by liver, lung, and forestomach. Considerably less DNA methylation was produced by an equimolar dose of 4-MeMBN, with highest values in liver, followed by esophagus (22 mumol 7-methylguanine per mol guanine) and lung. However, s.c. injections of equitoxic doses of MBN (18 mg/kg) and 4-MeMBN (394 mg/kg) produced similar amounts of 7-methylguanine in esophageal nucleic acids. These data indicate that the lower toxicity and carcinogenicity of 4-MeMBN after systemic administration are due to the rapid formation (mainly in the liver) and excretion via the urine of its benzoic acid derivative. The strong carcinogenic effect of orally administered 4-MeMBN can be explained by direct uptake from the drinking water into the esophageal mucosa. Following a single i.v. injection (0.017 mmol/kg) of [methylene-14C]MBN and 4-Me[methylene-14C]MBN, no benzylated bases were detectable in rat tissues. This indicates that the bioactivation of these compounds is initiated predominantly by hydroxylation at the methylene bridge leading to a methylating rather than a benzylating intermediate as the ultimate carcinogen.

Metabolism of N-nitrosomethylbenzylamine in the mongolian gerbil (meriones unguiculatus)

Adult male gerbils (Meriones unguiculatus) received a single s.c. injection of N-nitroso(methyl-14C) methylbenzylamine (2.5 mg/kg body weight). Of the radioactivity injected 46% was exhaled as 14CO2 within 8-10 hours and an additional 21% was excreted as urinary metabolites. Six hours after injection of the carcinogen, methylation of DNA bases was highest in lung (81.5 mumol 7-methylguanine/mol guanine) followed by liver, esophagus, kidney, forestomach, glandular stomach, spleen, colon and small intestine. The brain DNA no alkylation products were detectable. These data differ significantly from those obtained in rats and mice which under similar experimental conditions metabolized N-nitrosomethylbenzylamine preferentially in esophagus and liver respectively.

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

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

Metabolic activation of benzo[a]pyrene, aflatoxin B1, and dimethylnitrosamine by a human hepatoma cell line

The metabolism of chemical carcinogens by a human hepatoma cell line, huH-1, was studied. The huH-1 line has been derived from a hepatoma of a 57-year-old HBs-antigen carrier and cultivated for several years. The hepatoma cells metabolized about 90% of 5 microM benzo[a]pyrene into water-soluble products within 24 h. Aryl hydrocarbon hydroxylase activity in huH-1 cells was induced to 24 times higher than the basal level by treatment with 13 microM benz[a]anthracene for 24 h. Metabolic activation of benzo[a]pyrene, dimethylnitrosamine and aflatoxin B1 by huH-1 cells was observed by cell-mediated sister-chromatid exchange assay. Sister-chromatid exchanges in human diploid fibroblasts were observed in the cultures mixed with or without huH-1 cells. All 3 chemicals induced sister-chromatid exchanges in human fibroblasts far more efficiently in the cultures mixed with huH-1 cells than in those without huH-1 cells. Some characteristics of huH-1 cells as a human cell-mediated metabolic activation system for carcinogens are discussed.

Relationship between carcinogenicity and in vitro metabolism of nitrosomethylethylamine, nitrosomethyl-N-butylamine, and nitrosomethyl-(2-phenylethyl)amine labeled with deuterium in the methyl and alpha-methylene positions

With the use of rat liver preparations, the in vitro microsomal metabolism of methylethylnitrosamine, methyl-n-butylnitrosamine, and methyl(2-phenylethyl)nitrosamine labeled with deuterium in the methyl and alpha-methylene positions has been compared with that of the parent (unlabeled) compounds. All three forms of the liver carcinogen methylethylnitrosamine are metabolized with two sets of kinetic constants. Examination of these kinetic constants suggests that both methylation and ethylation of cellular nucleophiles might be important in the carcinogenic action of these nitrosamines. The esophageal carcinogen, methyl(2-phenylethyl)nitrosamine, gave only one set of kinetic constants during metabolism. The metabolism of the three methylbutylnitrosamines gave results similar to that of the three methylethyl nitrosamines. Except for metabolism of d2-methylbutylnitrosamine to butyraldehyde, two sets of kinetic constants were found. Approximately equivalent amounts of methylating species were produced from d3-methylbutylnitrosamine and d0-methylbutylnitrosamine.

Metabolism of N-nitrosamines by cultured human and rat esophagus

The metabolism of several N-nitrosamines (N-nitrosodimethylamine, N-nitrosoethylmethylamine, N-nitrosodiethylamine, N-nitrosobenzylmethylamine, and N-nitrosopyrrolidine) in cultured human and rat esophagus has been investigated by measuring (a) CO2, (b) metabolites with an oxo group, and (c) metabolites bound to DNA. Both acyclic and cyclic N-nitrosamines were metabolized by rat esophagus. The highest level of metabolite binding was seen with N-nitrosobenzylmethylamine, an organotrophic carcinogen for the rat esophagus. The binding level was about 100-fold higher than in human esophagus. This compound methylated rat esophageal DNA at positions 7 and O6 of guanine. The level of benzylation in rat was one-tenth of the level of methylation. Formation of benzaldehyde exceeded that of formaldehyde plus CO2 by a factor of six, indicating that the methylene group was preferentially oxidized. N-Nitrosoethylmethylamine, another unsymmetrical N-nitrosamine, was preferentially oxidized by rat esophagus in the ethyl group, as shown by higher formation of CO2 and acetaldehyde from the compound labeled in the ethyl group. The highest binding level to DNA from this compound was observed with the methyl group. No binding was detected to human esophagus. N-Nitrosopyrrolidine was oxidized by both rat and human esophagus in the alpha position, as measured by the formation of 2,4-dinitrophenylhydrazone derivative of 4-hydroxybutanal. Binding of metabolites of N-nitrosopyrrolidine to DNA was detected only in rat esophagus. As measured by the formation of both CO2 and formaldehyde, N-nitrosodimethylamine was metabolized by both human and rat esophagus. While most of the radioactivity associated with DNA was found to be incorporated into guanine and adenine, methylation of the guanine positions 7 and O6 was detected by chromatography of the hydrolyzed rat DNA. The results indicate significant quantitative and perhaps qualitative differences between cultured rat and human esophagus in their ability to activate N-nitrosamines, although unknown physiological differences after culture may contribute to this difference.

Metabolic activation of dimethylnitrosamine to mutagens: role of cytochromes P-450 and P-448

Pretreatment of hamsters with phenobarbitone, 3-methylcholanthrene and Arochlor 1254 induced the hepatic microsomal mixed function oxidases, yet decrease the efficiency of activation of dimethylnitrosamine (DMN) to intermediates mutagenic to the Salmonella typhimurium strain TA-100. Furthermore, no correlation was obtained between cytochrome P-450 content, microsomal demethylation of DMN and its activation to mutagens. These results indicate that the demethylation of DMN by the mixed-function oxidases is not the rate-limiting step in the metabolic activation of the carcinogen to mutagen(s), and that other microsomal or soluble enzymes may be involved.

Induction of 6-thioguanine-resistant mutations by rat-liver homogenate (S9)-activated promutagens in human embryonic skin fibroblasts

Most normal human fibroblasts grown in culture do not metabolize promutagens/procarcinogens. Thus screening assays employing normal human fibroblasts have only been successful for direct-acting chemical mutagens and various radiations. In this report we describe a mutation assay (HGPRT locus) employing a normal human embryonic skin fibroblast and a rat-liver homogenate (S9) mixture. 3 model promutagens, benzo[a]pyrene (B[a]P), 3-methylcholanthrene (3MC), and dimethylnitrosamine (DMN) have been utilized in these studies. In addition to discussing conditions for optimizing the response of this assay, our results indicate that at constant amount of S9 protein concentration, there exists a linear correlation between mutagenicity and dose. At 50% survival, the mutant frequencies induced by B[a]P and 3MC (5 micrograms/ml) are 60 and 30 times the background mutant frequency, respectively. Similarly, at 50% survival, DMN (5 mg/ml) induced 6-TGr mutant frequencies are 25-fold over the background frequency. The increase in cytotoxicity resulting from exposure of cells to these 'activated' chemicals is also a linear dose response. At high S9 concentrations a deactivation or detoxification phenomenon occurs. However, the mutagenic efficiency of S9-activated chemicals when plotted as the number of induced mutations versus log survival is unaffected by the deactivating capacity of S9 proteins. This study demonstrates a quantitative mutation assay using an early passage human culture with an exogenous rat-liver microsomal preparation providing activating enzymes.

Detection of metabolites of N-nitrosopyrrolidine and N-nitrosoethylmethylamine in cultures of human bladder epithelial cells of normal origin

Cultured human bladder epithelial cells of normal origin are capable of metabolizing N-nitrosamines. Cultures from three different cell lines were incubated with NPYR and NEMA in order to investigate the formation of volatile N-nitrosamines. Seven volatile compounds with positive TEA response, including NDMA and NEMA, were detected in the culture media incubated with NPYR. The identity of NEMA, which was the main component of this group of metabolites, was confirmed by mass spectrometry. Culture media incubated with NEMA showed formation of NDMA. In four of five incubation experiments with NPYR, NHPYR was detected as a metabolite. The formation of metabolites containing the N-nitroso group presumably occurs via a beta-hydroxylation pathway.

In vitro formation of methyldiethyldithiocarbamate after the reaction of nitrosoacetoxymethylmethylamine or methylnitrosourea with disulfiram

Analysis by scintillation measurement, mass spectrometry and h.p.l.c. showed that diethyldithiocarbamic acid (DDTC), the main metabolite of disulfiram (DSF), forms methyldithiocarbamate (MeDDTC) when incubated with [14C]nitrosoacetoxymethylmethylamine ([14C]NAMM) or [14C]methylnitrosourea ([14C]MNU) in different media (bacteria, esterases, rat liver 9000 x g supernatant fraction and microsomes). When DSF instead of DDTC was used, MeDDTC was formed only when soluble enzymes were present which are required to split DSF into two DDTC moieties. No physiological methylation of DDTC takes place as was shown by experiments with [3H]MNU. [14C]Methanol, formed by the decay of [14C]NAMM and [14C]MNU was shown to have no alkylating properties.

Mutagenicity and synthesis of alpha-substituted N-nitrosamines: derivatives with dithiocarbamic acid

Disulfiram (DSF) can considerably alter the organotropy of chemical carcinogens. For N-nitrosodimethylamine and for N-nitrosodiethylamine the organotropy is shifted from the liver to the nasal cavity or the oesophagus, respectively. Whereas the influence of DSF or its metabolites on enzyme systems has been studied, little is known about its interaction with the carcinogens at a molecular level. Therefore, postulated reaction products of a series of alpha-hydroxylated N-nitroso-dialkylamines and dithiocarbamate were synthesized and tested for mutagenicity in Salmonella typhimurium TA 1535. The results show that the compounds conjugated at a primary alpha-C-atom are not mutagenic, whereas those conjugated at a secondary alpha-C-atom are active. The primary N-nitroso-dithiocarbamates represent unique examples of inactivated dialkyl-nitrosamine derivatives. In addition, their formation in vitro was indirectly demonstrated. The possible role these inactivated compounds may play during the DSF-modulation of carcinogenesis will be discussed.