Molecular basis for the sex-related difference in renal N-nitrosodimethylamine demethylase in C3H/HeJ mice

Previous work with rat and rabbit liver enzymes has demonstrated that cytochrome P450IIE1 is responsible for the metabolism of N-nitrosodimethylamine (NDMA), a widely occurring carcinogen. The present study demonstrated that a similar enzyme also exists in the mouse kidney and is regulated by testosterone. These results can account for the reported sex-related difference in the renal metabolism of NDMA in mouse strains such as C3H/HeJ. NDMA demethylase activities (expressed as pmol/min/mg protein) in kidney microsomes of female and male C3H/HeJ mice were 3.0 +/- 0.7 and 51.9 +/- 11.2, respectively. After testosterone treatment (500 mg/kg b.w. in olive oil, s.c.) for 2 days, the renal NDMA demethylase activity of the female mice was elevated 17-fold. The difference and change in NDMA demethylase activity were accompanied by corresponding differences and changes in P450IIE1 as quantified by immunoblot analysis (using antibodies prepared against rat P450IIE1) as well as in the mRNA level for P450IIE1 as determined by Northern and slot blot analyses (using a cDNA probe containing the coding sequence of rat P450IIE1 gene). Based on gel electrophoresis, the molecular weight of mouse renal P450IIE1 was 52,000 and the size of mouse renal P450IIE1 mRNA was approximately 1.8 kilobases; both were similar to those found in rat liver and kidney. Renal P450IIE1 mRNA levels in female, male, and testosterone-treated female mice were at a ratio of 1:22:20. On the other hand, this testosterone-related difference was not observed in hepatic P450IIE1. In liver microsomes, there were no significant differences in NDMA demethylase activity, P450IIE1 content, and P450IIE1 mRNA level between male and female mice or between untreated and testosterone-treated female mice. The apparent Km value of NDMA demethylase in mouse kidney microsomes (22 to 27 microM NDMA) were similar to that in rat liver microsomes. Renal NDMA demethylase activity was inhibited by a monoclonal antibody prepared against rat P450IIE1. These results suggest that mouse renal P450IIE1 is similar to rat P450IIE1 and is responsible for the low Km form of NDMA demethylase activity. Nevertheless, only the mouse renal enzyme is regulated by testosterone.     

Metabolism of azoxymethane, methylazoxymethanol and N-nitrosodimethylamine by cytochrome P450IIE1

The metabolism of azoxymethane (AOM), methylazoxymethanol (MAM) and N-nitrosodimethylamine (NDMA) by liver microsomes from acetone-induced rats as well as by a reconstituted system containing purified cytochrome P450IIE1 was examined. The products consisted of MAM from AOM; methanol and formic acid from MAM; and methylamine, formaldehyde, methanol, methylphosphate and formic acid from NDMA. Compared to liver microsomes from untreated rats, the metabolic activity of acetone-induced microsomes was approximately 4 times higher for all three carcinogens. Using the reconstituted system, the enzyme activities (nmol substrate metabolized/nmol P450/min) for AOM, MAM and NDMA were 2.88 +/- 1.14, 2.87 +/- 0.59 and 9.47 +/- 2.24 respectively. Incubations carried out in the presence of a monoclonal antibody to cytochrome P450IIE1 resulted in a 85-90% inhibition of all three reactions in this system. These results provide conclusive evidence that AOM, MAM and NDMA are metabolized by the same form of rat liver cytochrome P450. In addition, the stoichiometry of NDMA products formed in these reactions indicates that denitrosation, a presumed detoxication process, and alpha-hydroxylation, an activation reaction, are also catalyzed by the same cytochrome P450 isozyme.     

Metabolism of N-nitrosodialkylamines by human liver microsomes

The metabolism of N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine, N-nitrosobenzylmethylamine, and N-nitrosobutylmethylamine was investigated in incubations with human liver microsomes. All of the 16 microsomal samples studied were able to oxidize NDMA to both formaldehyde and nitrite at NDMA concentrations as low as 0.2 mM; the rates of product formation of the samples ranged from 0.18 to 2.99 nmol formaldehyde/min/mg microsomal protein (median, 0.53 nmol). At a concentration of 0.2 mM NDMA, the rates of denitrosation (nitrite formation) were 5 to 10% (median, 6.3%) those of demethylation (formaldehyde formation); the ratio of denitrosation to demethylation increased with increases in NDMA concentration, in a similar manner to rat liver microsomes. Immunoblot analysis with antibodies prepared against rat P-450ac (an acetone-inducible form of cytochrome P-450) indicated that the P-450ac [P-450j (isoniazid-inducible form)] orthologue in human liver microsomes had a slightly higher molecular weight than rat P-450ac and the amounts of P-450ac orthologue in human liver microsomes were highly correlated with NDMA demethylase activities (r = 0.971; P less than 0.001). Analysis of four selected microsomal samples showed that human liver microsomes exhibited at least three apparent Km and corresponding Vmax values for NDMA demethylase. This result, suggesting the metabolism of NDMA by different P-450 enzymes, is similar to that obtained with rat liver microsomes, even though most of the human samples had lower activities than did the rat liver microsomes. The high affinity Km values of the four human samples ranged from 27 to 48 microM (median, 35 microM), which were similar to or slightly lower than those observed in rat liver microsomes, indicating that human liver microsomes are as efficient as rat liver microsomes in the metabolism of NDMA. The human liver microsomes also catalyzed the dealkylation and denitrosation of other nitrosamines examined. The rates of product formation and the ratios of denitrosation to dealkylation varied with the structures and concentrations of the substrates as well as with the microsomal samples tested. The results indicate that human liver microsomes are capable of metabolizing N-nitrosodialkylamines via the pathways that have been established with rat liver microsomes.     

Metabolism and activation of N-nitrosodimethylamine by hamster and rat microsomes: comparative study with weanling and adult animals

It has been reported that hamster liver preparations are more effective for the metabolic activation of N-nitrosodimethylamine (NDMA) to a mutagen than rat liver preparations. The enzymatic basis for this phenomenon, however, has not been clearly elucidated. The present study was undertaken to examine the enzymology of NDMA metabolism by different hepatic subcellular fractions prepared from hamsters and rats of two different ages, and to investigate the correlation between the metabolism and the activation of NDMA to a mutagen for Chinese hamster V79 cells. The content of cytochrome P-450 was approximately 1.5-fold higher in hamster microsomes than in rat microsomes from both ages (1.19-1.38 versus 0.73-0.83 nmol P-450/mg protein). Weanling hamster microsomes exhibited multiple apparent Km values for NDMA metabolism as did weanling rat microsomes. The apparent Km I value of NDMA demethylase (NDMAd) in hamster microsomes was about one-half that in rat microsomes (36 versus 83 microM) with corresponding Vmax values of 2.09 and 2.57 nmol/min/nmol P-450. The Km I values for denitrosation did not differ from the corresponding values for NDMAd with Vmax values of 0.17 and 0.22 nmol/min/nmol P-450 for hamster and rat microsomes, respectively. These apparent Km values were affected neither by sonication nor by the presence of cytosolic proteins in S9 fractions. Adult rat liver microsomes showed less than one-half the NDMAd activity in weanling rat liver microsomes, whereas such age difference was not observed in hamster liver microsomes. This result was confirmed by Western blotting showing the levels of P-450ac (an acetone-inducible form of P-450) of these microsomes at comparable levels to their NDMAd activities. NDMAd was highly correlated to the metabolic activation of NDMA to a mutagen for V79 cells in an activation system mediated by microsomes prepared from hamsters and rats of different ages. The results from this study clearly demonstrate the enzymatic basis for the more effective metabolism of NDMA in adult hamsters than in adult rats.     

Deuterium isotope effect on denitrosation and demethylation of N-nitrosodimethylamine by rat liver microsomes

In an attempt to elucidate the molecular basis for the decrease in rat liver carcinogenicity and DNA-alkylating ability that accompanies deuteration of N-nitrosodimethylamine (NDMA), NDMA and its fully deuterated analogue ([2H6]NDMA) were incubated with acetone-induced rat liver microsomes. Rates for the competing metabolic routes, denitrosation and demethylation, were determined from colorimetric data on nitrite and formaldehyde generation, respectively. The Vmax calculated for demethylation of NDMA was 7.9 nmol/min/mg, while that for denitrosation was 0.83 nmol/min/mg. Deuteration of NDMA did not significantly change the Vmax for either pathway, but it did increase the Km for demethylation from 0.06 to 0.3 mM. The Km for denitrosation was also increased from 0.06 to 0.3 mM on deuteration, as determined by incubating an equimolar mixture of amino-15N-labeled NDMA with [2H6]NDMA and measuring the methyl[15N]amine:[2H3]methylamine ratio by derivatization-gas chromatography-mass spectrometry. The fact that the Km values for denitrosation were so similar to those for demethylation suggested that the two pathways were catalyzed by the same enzyme. The isotope effects calculated from these data [VmaxH/VmaxD approximately 1 and (Vmax/Km)H/(Vmax/Km)D approximately 5] show that microsomal metabolism of NDMA is not significantly shifted from demethylation to denitrosation on deuteration of substrate and may indicate a low commitment to catalysis for the enzyme. The results are consistent with the view that the metabolism of NDMA is initiated by formation of an alpha-nitrosamino radical which either combines with a hydroxyl radical to form the alpha-hydroxynitrosamine as the initial product of the demethylation pathway or fragments to nitric oxide and N-methylformaldimine as the first products of denitrosation.     

Enzymatic mechanisms in the metabolic activation of N-nitrosodialkylamines

The metabolism of several N-nitrosodialkylamines was studied using rat liver microsomes and purified cytochrome P450 isozymes in a reconstituted monooxygenase system. With purified acetone/ethanol-inducible cytochrome P450 (P450ac), high N-nitrosodimethylamine (NDMA) demethylase activity was observed. Cytochrome b5 was also involved in NDMA metabolism by decreasing the Km of NDMA demethylase. A close relationship between the demethylation and denitrosation of this substrate was observed. P450ac was also active in the metabolism of N-nitrosoethylmethylamine (NEMA), but was less active than phenobarbital-inducible cytochrome P450 (P450b) in the metabolism of N-nitrosobutylmethylamine (NBMA), especially in catalysing the debutylation reaction. Similar substrate specificity was demonstrated with liver microsomes from rats treated with other inducers. With different P450 isozymes and microsomes, a close relationship between metabolism and activation of nitrosamines to mutagens to V79 cells was demonstrated. DNA alkylation by NDMA in vitro was correlated with the rate of metabolism of these compounds, whereas DNA alkylation in vivo was more complex and was dose-dependent. The work demonstrates the importance of knowledge of the substrate specificity of cytochrome P450 isozymes in understanding the mechanisms of the metabolic activation of nitrosamines.     

Enzyme kinetics of N-nitrosodimethylamine demethylase in rodents and humans

A variety of Km values have been reported for hepatic microsomal N-nitrosodimethylamine demethylase (NDMAd). We demonstrated previously that the biologically important, high affinity (KmI) form of microsomal NDMAd is manifested by cytochrome P450IIE1 (also known as P450ac and P450j). The KmI value of NDMAd was, however, affected greatly by assay conditions: the possible presence of inhibitors and the presence of cytochrome b5. We re-examined the KmI value by testing the effect of enzyme concentrations and of different types of enzyme preparations on the Km. The KmI value ranged from 15 to 22 microM, as estimated by the direct linear plot, using a microsomal protein concentration in the range of 0.1 to 0.8 mg/ml with correction for substrate utilization. A slight yet significant dependency of microsomal protein concentration on the Km (r = 0890; p less than 0.05) was seen. When five different microsomal preparations were compared, the KmI value ranged from 14 to 24 microM (median, 20 microM), as estimated by the direct linear plot. The Km estimated by the commonly used Eadie-Hofstee plot did not differ from that by the direct linear plot. These Km values are close to the values obtained in studies with isolated cells and tissue slices. The KmI form of NDMAd (P450IIE1 and its orthologues) is present in rats, mice, rabbits, hamsters and guinea-pigs. It is responsible for the age-dependent differences between rats and hamsters and for the sex-related differences in mouse kidneys, and for the bioactivation and toxicity of NDMA. This enzyme also exists in human liver microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic isotope effect on the demethylation and denitrosation of N-nitrosodimethylamine in vitro

Deuteration of N-nitrosodimethylamine (NDMA) has been shown to decrease the carcinogenicity of this compound. This result is believed to be due to a kinetic isotope effect on the metabolic activation of this carcinogen, but conflicting views exist concerning whether the isotope substitution affects the Km or Vmax of the reaction. In order to elucidate the molecular basis of these observations, as well as the mechanisms of the demethylation and denitrosation reactions, the metabolism of NDMA and deuterated NDMA (NDMA-d6) was studied using acetone-induced rat-liver microsomes. The demethylation of NDMA displayed a Km of 0.06 mM and a Vmax of 7.9 nmol/min per mg protein. Deuteration of NDMA increased the Km value by five fold but did not appreciably affect the Vmax. The denitrosation of NDMA also displayed a Km of 0.06 mM, but the Vmax was 0.83 nmol/min per mg; deuteration again increased the Kmax several fold but had no effect on the Vmax. The results indicate that deuteration inhibits the metabolism of NDMA by increasing the Km but not the Vmax and suggest that there is a close relationship between the demethylation and denitrosation reactions.     

Metabolism of carcinogenic nitrosamines by rat nasal mucosa and the effect of diallyl sulfide

Rat nasal cavity is one of the target organs for carcinogenesis induced by N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The present work investigated the metabolism of these nitrosamines by rat nasal microsomes, as well as the possible modulating factors. Microsomes prepared from rat nasal mucosa were efficient in metabolizing these nitrosamines. In general, the metabolism of the nitrosamines was slightly higher in 9-week-old rats than in 4-week-old animals, and there was no sex-related difference. Fasting of rats for 48 h, which is known to induce hepatic cytochrome P450IIE1 and NDMA metabolism, did not increase the nasal metabolism of NDMA, NDEA, or NNK. Pretreatment of rats with acetone, another inducer of hepatic P450IIE1, did not increase the metabolism of NDMA. Furthermore, it decreased the nasal metabolism of NDEA and NNK. Immunoinhibition studies suggest that, in the nasal mucosa, P450IIE1 is only partially responsible for the oxidation of NDMA and other P450 isozymes are responsible for the metabolism of NDEA. A single p.o. pretreatment of male rats with diallyl sulfide (DAS), a component of garlic oil, caused a significant decrease in the oxidative metabolism of NDEA and NNK in rat nasal mucosa. Whereas the nasal metabolism of NDMA was reduced by DAS pretreatment, there was no change in the amount of the nasal microsomal proteins immunoreactive with the antibodies against P450IIE1. The inhibitory effect of DAS on the nasal oxidative metabolism of NDMA, NDEA, and NNK was also observed in experiments in vitro. The results demonstrate the ability of nasal mucosa to metabolically activate these nitrosamines and the inhibition of this process by DAS, suggesting that DAS may be effective in inhibiting the related nasal tumorigenesis.     

Effects of ethanol on the DNA-damage induced by 2 carcinogenic nitrosamines

We studied the DNA single-strand breaks (DNA SSBs) induced by two nitrosamines using rat hepatocytelin situ nick translation assay. In the hepatocytes treated with 20 mu M of N-nitrosodimethylamine (NDMA), 100 mM ethanol enhanced DNA SSBs 3 times higher than those of control. However, there was no significant difference between the DNA SSBs with and without ethanol in 300 mu M of N-nitrosodiethylamine (NDEA) treated groups. Pretreatment of 100 mM ethanol increased P450IIE1 levels determined by Western blotting, whereas the amount of total P450 was not affected. Although NDMA is possibly activated by P450IIE1, there could be other isozymes responsible for the activation of NDEA. Phenobarbital inducible isozymes such as P450IIB1 and IIB2, or P450IIA3 may be primarily responsible.     

High variability of nitrosamine metabolism among individuals: Role of cytochromes P450 2A6 and 2E1 in the dealkylation of N-nitrosodimethylamine and N-nitrosodiethylamine in mice and humans

We undertook this study to answer several questions regarding nitrosamine metabolism. Kinetics of nitrosamine metabolism showed the involvement of at least two enzymes in the dealkylation of N-nitrosodiethylamine (NDEA) and N-nitrosodimethylamine (NDMA) in mouse liver microsomes. Coumarin inhibited both reactions competitively. On the other hand, microsomal coumarin 7-hydroxylase was inhibited by NDMA (K_i 2.7 mM) and NDEA (K_i 0.013 mM). The big difference in the K_i values suggests a higher affinity of NDEA than NDMA to Cyp2a-5 (mouse cytochrome P450_coh). A specific antibody against Cyp2a-5 inhibited more of the microsomal NDEA (up to 90%) than NDMA (up to 40%) dealkylation. The converse was true with anti-Cyp2e-1 antibody. These results suggest that the primary substrate for Cyp2a-5 is NDEA and for Cyp2e-1, NDMA. Western blot analysis of human liver microsomes showed a great interindividual variation in the amounts of CYP2A6 (human cytochrome P450_coh) and CYP2E1. Also, courmarin 7-hydroxylation and nitrosamine dealkylation varied greatly among individuals. A high correlation (r = 0.93, P < 0.001) was found between NDEA and coumarin metabolism. Both activities were associated with CYP2A6. On the other hand, little or no correlation was found between microsomal CYP2A6 and CYP2E1 or between CYP2E1 and NDEA dealkylation. Immunoinhibition of human microsomal NDEA metabolism by CYP2a-5 antibody varied greatly among individuals (10–90%), suggesting, as in the case of mice, that NDEA is metabolized primarily by CYP2A6, at least in some individuals. Taken together the data suggest that (1) the metabolic activation of nitrosamines in humans varies greatly among individuals; (2) different nitrosamines may partially be metabolized by different cytochrome P450 isozymes; and (3) because of similarities between nitrosamine metabolism in mice and humans, inbred strains of mice would be relevant experimental models for studying nitrosamine activation.     

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.     

Enzyme mechanisms in the metabolism of nitrosamines.

Many nitrosamines are metabolized by cytochromes P450, one of which (P450IIE1) has received much attention because of its role in the metabolic activation of N-nitrosodimethylamine. This enzyme exists in man, rat, mouse, hamster and other animal species. It is inducible by fasting, diabetes and exposure to ethanol, acetone, isoniazid, benzene and other chemicals. P450IIE1 is responsible for the low Km form of N-nitrosodimethylamine demethylase and is the major enzyme catalysing the metabolic activation of this carcinogen. In addition, P450IIE1 is the most active P450 species known in the metabolism of N-nitrosoethylmethylamine and N-nitrosopyrrolidine. In the metabolism of N-nitrosobutylmethylamine, P450IIE1 preferentially oxidizes the methyl group over the butyl group, whereas P450IIB1 efficiently oxidizes both the methyl and butyl groups. P450IIB1 also catalyses the alpha-oxygenation of both the pentyl and methyl groups of N-nitrosopentylmethylamine, forming pentaldehyde and formaldehyde at a rate ratio of 2:1, as well as oxygenation at other carbons of the pentyl group. Many nitrosamines are effectively activated in nonhepatic target tissues. The metabolism of 4-(N-nitroso-methylamino)-1-(3-pyridyl)-1-butanone in lung and nasal microsomes is discussed.     

Participation of rat liver cytochrome P450 2E1 in the activation of N-nitrosodimethylamine and N-nitrosodiethylamine to products genotoxic in an acetyltransferase-overexpressing Salmonella typhimurium strain (NM2009).

The possible roles of cytochrome P450 (P450) enzymes in the metabolic activation of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) by rat liver microsomes have been examined in a system containing the bacterial tester strain Salmonella typhimurium NM2009, a newly developed strain showing high O-acetyltransfer activities. The DNA-damaging activity could be determined by measuring expression of the umu gene in a plasmid containing the fused umuC-lacZ gene construct in the bacteria. The following lines of evidence support the view that both NDMA and NDEA are principally oxidized to reactive products by P450 2E1 in rat liver microsomes. First, NDMA and NDEA were activated by rat liver microsomes in a protein- and substrate-dependent manner and the former chemical was more active than the latter; both activities were induced in rats treated with P450 2E1 inducers such as ethanol, acetone and isoniazid and by starvation. Second, activation of NDMA and NDEA were both inhibited significantly by antibodies raised against rat P450 2E1 and by P450 2E1 inhibitors such as diethyldithiocarbamate and 4-methylpyrazole in rat liver microsomes. Finally, in reconstituted monooxygenase systems containing purified rat P450 enzymes, P450 2E1 gave the highest rates of the activation of both NDMA and NDEA; the addition of rabbit cytochrome b5 to the system caused about a 1.5-fold increase in both reactions. In separate experiments we also found that N-nitrosomethylacethoxymethylamine, a compound that reacts with DNA after ester cleavage, is more genotoxic in S.typhimurium NM2009 than in S.typhimurium NM2000, a strain that is defective in O-acetyltransferase activity. Part of the pathway involved in the activation of nitrosamines is suggested to be acetylation of alkyldiazohydroxides formed by P450 or acetylesterase, because the genotoxic activity of N-nitrosomethylacethoxymethylamine in S.typhimurium NM2009 could be inhibited by the O-acetyltransferase inhibitor pentachlorophenol. These results indicate that NDMA and NDEA are oxidized to gentoxoic products by rat liver microsomes and that a P450 2E1 enzyme plays a major role in the activation of these two potent carcinogens. The activation pathway of N-nitrosodialkylamines through acetylation by O-acetyltransferase has been proposed. This simple bacterial system for measuring genotoxicity should facilitate studies on the activation of N-nitroso alkylamines.     

Mechanism and control of denitrosation of N-nitrosodimethylamine

The NADPH-dependent microsomal denitrosation of N-nitrosodimethylamine (NDMA) has been investigated using a new procedure which was devised for the determination of nitric oxide under aerobic conditions. On the basis of the results obtained with rat-liver microsomes it is concluded that nitric oxide is formed as a precursor of nitrite in a superoxide dismutase (SOD)-insensitive reaction. The enzyme involved in the denitrosation was found to correspond to the cytochrome P450 isoenzyme responsible for the dealkylation of NDMA. The chemical mechanism of the liberation of nitric oxide is proposed to be of an oxidative nature.     

Effect of diallyl sulfide on rat liver microsomal nitrosamine metabolism and other monooxygenase activities

It has been reported that p.o. administration of diallyl sulfide (DAS), a naturally occurring component of garlic (Allium sativum), inhibits 1,2-dimethylhydrazine-induced colon and liver cancer in rodents. A possible mechanism for this protective effect is inhibition of hepatic activation of the procarcinogen. The effect of DAS on P450IIE1, an isozyme of cytochrome P-450 which is active in the oxidative metabolism of dimethylhydrazine, was conveniently assayed in the present study by determination of N-dimethylnitrosamine demethylase (NDMAd) activity at 1 mM N-dimethylnitrosamine in Sprague-Dawley rat liver microsomal incubations. DAS was found to be a competitive inhibitor of NDMAd, in contrast to the irreversible inactivation of NDMAd produced by carbon tetrachloride incubated under similar conditions. The inhibition by DAS of the demethylation of several substrates was selective. The thioether was most potent against N-dimethylnitrosamine, less effective against N-nitrosomethylbenzylamine, and essentially ineffective against benzphetamine and ethylmorphine. Microsomes prepared at 3 h after DAS administration (200 mg/kg in corn oil intragastrically) showed moderate inhibition (less than 30% inhibition compared to control microsomes) of several demethylase activities; however, microsomes prepared 18 h posttreatment showed a marked decrease (about 80% inhibition compared to controls) in NDMAd activity, minor effects on other demethylase activities, and a 6-fold increase in pentoxyresorufin dealkylation. These trends at 18 h agreed with immunoblot analyses which showed suppression in the level of P450IIE1 and an elevation in P450IIB1. The selective inhibition of P450IIE1 activity and suppression of its level in microsomes may contribute to the reported chemoprotective effects of DAS.     

Substrate specificity and alkyl group selectivity in the metabolism of N-nitrosodialkylamines

Metabolic activation may be a key step in determining the tissue specificity of carcinogenic nitrosamines. In previous work, we characterized P450IIE1 (an acetone/ethanol-inducible form of cytochrome P-450) as the major enzyme for the metabolic activation of N-nitrosodimethylamine. In this work, we investigated the metabolism of other N-nitrosodialkylamines in rat liver microsomes and in reconstituted monooxygenase systems containing purified cytochrome P-450 isozymes. The enzyme specificities in the metabolism of N-nitrosoethylmethylamine and N-nitrosodiethylamine were similar to those of N-nitrosodimethylamine; i.e., these substrates were more efficiently metabolized by acetone- or ethanol-induced microsomes than by other types of microsomes. However, substituting one methyl group with a benzyl or butyl group, as in N-nitrosobenzylmethylamine or N-nitrosobutylmethylamine (NBMA), substantially changed the enzyme specificity. P450IIE1 efficiently catalyzed the demethylation but not the debutylation of NBMA, whereas P450IIB1 (a phenobarbital-inducible form) efficiently catalyzed both the debutylation and demethylation reactions. In the demethylation of NBMA by P450IIE1, the addition of cytochrome b5 markedly increased the activity at low but not at high substrate concentrations, suggesting a decrease in Km value. This effect, however, was not observed in the debutylation of NBMA by P450IIE1 or P450IIB1, and in the demethylation of NBMA by P450IIB1. These studies demonstrate the substrate specificity and alkyl group selectivity in the metabolism of nitrosamines by cytochrome P-450 isozymes.     

Analysis of the inhibition of N-nitroso-dimethylamine activation in the liver by N-nitro-dimethylamine using a new non-linear statistical method

N-nitro-dimethylamine (NTDMA) is carcinogenic to rats: it induces nasal cavity tumours. It can be demethylated to N-nitromethylamine and formaldehyde and reduced to N-nitroso-dimethylamine (NDMA): a potent liver carcinogen and also of the nasal cavity if activation in the liver is blocked. To explain the mechanism of NTDMA carcinogenicity we compared its demethylation with that of NDMA in liver microsomes from female and male rats, untreated, fasted or treated with ethanol to induce cytochrome P450 2E1 (CYP2E1). Kinetic parameters were analysed by nonlinear statistical methods, which yielded unbiased parameter estimates for the calculated Km and Vmax values. Km for both compounds was very similar in females (24-47 microM) whereas Vmax for NTDMA was consistently higher than for NDMA as substrate: 1.07-4.70 nmol formaldehyde/mg microsomal protein x min and 0.52-2.76 nmol, respectively. In liver microsomes from induced male rats NTDMA was found to be a much more effective inhibitor of NDMA activation (KEI 39.6-73.6 microM) than NDMA of NTDMA demethylation (KEI 224-286 microM). Nasal microsomes can demethylate both NDMA and NTDMA but the kinetics are vastly different. NTDMA is demethylated at a linear rate and approximately 10-fold more effectively than NDMA. The mechanism of carcinogenicity of ingested NTDMA, we propose, is a partial reduction to NDMA in the liver and inhibition of NDMA activation in the liver by residual NTDMA, which enables NDMA to reach the nasal mucosa where it is activated to DNA-alkylating species and the observed tumours are formed.     

High affinity diethylnitrosamine-deethylase in liver microsomes from acetone-induced rats

The effects of acetone treatment on microsomal cytochrome P-450-dependent mono-oxygenases of the rat liver have been investigated to elucidate the role of this system in the metabolism of diethylnitrosamine (DEN). Acetone markedly enhanced the hepatic P-450 content and the activities of p-nitrophenol hydroxylase, acetone hydroxylase, ethoxycoumarin deethylase and DEN deethylase (DENd), whereas activities of pentoxy-resorufin O-deethylase and ethoxy-resorufin O-deethylase were not affected. Two distinct apparent Km values (0.43 and 9.1 mM), dependent on the substrate concentration, were observed for the DENd of acetone-induced microsomes. Only one Km value (8.4 mM) was observed for the DENd of control microsomes. In control microsomes at a DEN concentration of 1 mM, the N-deethylation of DEN was undetectable whereas in acetone-induced microsomes the N-deethylation rate was approximately 2.3 nmol/mg protein per min. The results suggest that acetone-induced microsomes of rat liver contain a high affinity form of DEN-deethylase which should be the P-450j isozyme (known to catalyze the oxidation of dimethylnitrosamine at low Km). P-450j is strongly enhanced by acetone treatment as indicated by the increase of the specific acetone hydroxylase. The treatment also enhanced the metabolism of DEN at substrate concentrations higher than 1 mM, suggesting that other P-450(s) catalyse DEN-deethylation although with lower substrate affinity. The low Km form of DENd is a P-450-dependent mono-oxygenase. It requires NADPH and O2, is inhibited by CO, but not by mannitol, superoxide dismutase, catalase or desferrioxamine. Its action therefore appears not to be mediated by oxygen radical species. Many solvents such as dimethylsulfoxide, dioxolane, chloroform and butanol when present at 10 mM in the incubation mixture inhibited the low Km form of DENd. However, pyrazole and piperonylbutoxide were found to be the strongest inhibitors. These results establish that acetone affects the metabolism of DEN, particularly at low concentrations, in a fashion somewhat similar to dimethylnitrosamine.