Hepatic mitochondrial cytochrome P-450 system. Distinctive features of cytochrome P-450 involved in the activation of aflatoxin B1 and benzo(a)pyrene

Rat liver mitoplasts containing less than 1% microsomal contamination contain cytochrome P-450 at 25% of the microsomal level and retain the capacity for monooxygenase activation of structurally different carcinogens such as aflatoxin B1 (AFB1), benzo(a)pyrene (BaP), and dimethylnitrosamine. Both phenobarbital (PB) and 3-methylcholanthrene (3-MC) induce the level of mitochondrial cytochrome P-450 by 2.0- to 2.5-fold above the level of control mitoplasts. The enzyme activities for AFB1 (3-fold) and BaP (16-fold) metabolism were selectively induced by PB and 3-MC, respectively. Furthermore, the metabolism of AFB1 and BaP by intact mitochondria was supported by Krebs cycle substrates but not by NADPH. Both PB and 3-MC administration cause a shift in the CO difference spectrum of mitoplasts (control, 448 nm; PB, 451 nm; and 3-MC, 446 nm) suggesting that they induce two different forms of mitochondrial cytochromes P-450. Mitoplasts solubilized with cholate and fractionated with polyethylene glycol exhibit only marginal monooxygenase activities. The activity, however, was restored to preparations from both PB-induced and 3-MC-induced mitochondrial enzymes (AFB1 activation, ethylmorphine, and benzphetamine deamination and BaP metabolism) by addition of purified rat liver cytochrome P-450 reductase, and beef adrenodoxin and adrenodoxin reductase. The latter proteins failed to reconstitute activity to purified microsomal cytochromes P-450b and P-450c that were fully active with P-450 reductase. Monospecific rabbit antibodies against cytochrome P-450b and P-450c inhibited both P-450 reductase and adrenodoxin-supported activities to similar extents. Anti-P-450b and anti-P-450c provided Ouchterlony precipitin bands against PB- and 3-MC induced mitoplasts, respectively. We conclude that liver mitoplasts contain cytochrome P-450 that is closely similar to the corresponding microsomal cytochrome P-450 but can be distinguished by a capacity to interact with adrenodoxin. These inducible cytochromes P-450 are of mitochondrial origin since their levels in purified mitoplasts are over 10 times greater than can arise from the highest possible microsomal contamination.

Ethanol and dimethylnitrosamine and diethylnitrosamine metabolism and disposition in the rat. Possible relevance to the influence of ethanol on human cancer incidence

Alcohol consumption is associated with an increase in human cancer, notably of the oesophagus. We have found that ethanol will alter profoundly the distribution of two carcinogenic nitrosamines in the rat. Small oral doses of dimethylnitrosamine (NDMA) are absorbed from the portal blood as it passes through the liver, and do not reach the extrahepatic organs. Ethanol, equivalent to a man drinking 1 pint (0.5 l) of beer, prevents this first pass clearance in the rat and exposes sensitive extrahepatic organs to this carcinogen. As a consequence the alkylation of kidney DNA by 35 micrograms NDMA/kg body weight was increased 4.6-fold by concurrent administration of 240 mg ethanol/kg, and smaller doses of [14C]-NDMA produced detectable alkylation of kidney DNA only if the rats were given ethanol. Measurement of metabolism of NDMA by liver slices confirmed that this action of ethanol is the result of inhibition by ethanol of NDMA metabolism in liver (Ki = 0.5 mM). Comparison of urinary excretion in man and rat suggests that ethanol also inhibits first pass clearance of NDMA in man. There was no complete first pass clearance of diethylnitrosamine (NDEA), but while ethylation of kidney DNA was decreased by ethanol, that of oesophageal DNA was increased between 1.8- and 4.6-fold. Measurement of the metabolism of NDEA to CO2 by liver slices, kidney slices, and oesophageal epithelium suggest that the changes in alkylation of kidney and oesophageal DNA are the result of selective inhibition of NDEA metabolism in liver and kidney. The ethylation of oesophageal DNA was greater relative to liver after a small dose than after a large dose possibly because of the low Km of the oesophageal metabolic activating system relative to that in liver and kidney. These results explain experiments showing that concurrent administration of ethanol increases the carcinogenicity and alters the organs affected by these nitrosamines. It is tentatively proposed that the effect of ethanol on human cancer incidence is mediated through similar influences on the metabolism and disposition of the nitrosamines to which man is exposed.

Effect of protein diet on age-dependent methylation of liver nuclear components of mice administered N-nitrosodimethylamine

Protein diets deficient or enriched in DL-methionine were studied for the effect of methylation of nuclear components in subadult and adult outbred albino male mice given N-nitrosodimethylamine. After the mice were fed diets for 6 days, they were given ip injections of [14C]N-nitrosodimethylamine (5 mg/kg body wt) and killed 45 minutes later. Liver nuclei were isolated; incorporation of 14C into nuclear proteins and methylation of DNA and RNA were determined. In the subadult and adult animals the nuclear protein content was diminished by shortage of a single essential amino acid. Incorporation of 14C into the total protein was reduced. The reduction amounted to 55% in the subadult and 23% in the adult animals. Neither an age-dependent nor a diet-dependent change in the specific methylation of DNA and RNA was observed. Analysis of methylation of DNA and RNA was observed. Analysis of methylated purine bases showed an age-related rise in O6-methylguanine in the adult as compared with the subadult mice.

Organ specificity in the microsomal activation and toxicity of N-nitrosomethylbenzylamine in various species

The microsomal metabolism of the rat esophageal carcinogen N-nitrosomethylbenzylamine (NMBZA) at the methylene carbon atom to yield benzaldehyde was studied in various organs of a number of species to determine the role of metabolic activation in the carcinogenicity or toxicity of the nitrosamine. In the Sprague-Dawley rat, NMBZA was metabolized by microsomes from liver, lung, and esophageal mucosa. In the F344 rat and rabbit, metabolic activity was present in both liver and esophageal mucosa, the only tissues studied in these species. In contrast, in the Syrian hamster and BALB/cByJ mouse, NMBZA debenzylation was undetectable in the esophagus but occurred at relatively high rates in liver, lung, and kidney. The forestomach mucosa exhibited undetectable levels of activity in the Sprague-Dawley rat and BALB/cByJ mouse, although in the hamster, it was present at a very low level. Administration of a dose of NMBZA acutely toxic to the rat (18 mg/kg i.p.) resulted in significant cellular damage only to the rat esophageal mucosa, no other tissues examined in the rat, hamster, or mouse being affected. These observations, together with the available data on carcinogenicity of the nitrosamine in the rat and rabbit, suggest that in the esophagus, at least, metabolic activation of NMBZA is necessary to elicit its toxic and/or carcinogenic effect. However, NMBZA is also metabolized at a high rate in the liver of all species but is not toxic or carcinogenic in this tissue, suggesting that other factors besides metabolic activation must be involved in the resistance of hepatocytes to the effects of the nitrosamine. Microsomes prepared from human esophageal mucosa from six patients metabolized NMBZA at rates that were either undetectable or approximately 70 times lower than in the Sprague-Dawley rat.

Differential effect of chronic ethanol consumption by the rat on microsomal oxidation of hepatocarcinogens and their activation to mutagens

The effect of chronic ethanol consumption by rats on hepatic microsomal activation of the hepatocarcinogens dimethylnitrosamine (DMN) and 2-acetylaminofluorene (2-AAF) was investigated. There was a marked increase in the rate of the oxidative demethylation of DMN and its activation to a mutagen by microsomes following ethanol intake. N- and C-hydroxylation of 2-AAF were measured at substrate concentrations ranging from 2 to 70 microM. The ratio of formation of N-hydroxy-2-acetylaminofluorene to C-hydroxy-2-acetylaminofluorenes increased with decreasing substrate concentration, suggesting enhanced carcinogenic potential of 2-AAF with diminishing levels of carcinogen. Kinetic analysis indicated that N-hydroxylation as well as 7-, 5- and 3-hydroxylation of 2-AAF do not follow Michaelis-Menten kinetics. In contrast to the marked inductive effect of ethanol consumption on the metabolic activation of DMN, only a minimal random effect on the N-hydroxylation of 2-AAF was demonstrable in two separate experiments. Furthermore, N-hydroxylation of 2-AAF by microsomes from control and ethanol-treated rats followed similar kinetics. While ethanol consumption enhanced the mutagenic activation of DMN by hepatic microsomes, no such effect of ethanol consumption on the conversion of 2-AAF to a mutagen was observed. The data indicate that chronic ethanol consumption does not have a general inductive effect on the microsomal activation of hepatocarcinogens.

Measurement of nitrosodimethylamine by capillary gas chromatography-mass spectrometry with the 15N-labelled compound as an internal standard

Artifactual formation of N-nitrosodimethylamine during the extraction procedure from aminopyrine and nitrite was examined. The use of the basic pH condition was the most effective in preventing artifactual formation. Sulfamic acid or ascorbic acid was partially effective in preventing artifactual formation. Since significant losses of volatile N-nitrosodimethylamine occur during the extraction and concentration steps, we analyzed N-nitrosodimethylamine by combined gas chromatography mass spectrometry with 15N-nitrosodimethylamine as an internal standard. The use of a fused silica capillary column enabled us to obtain a fine separation of the chromatogram. This methodology was applied to our model experiment, which was performed to locate the formation of N-nitrosodimethylamine when a rabbit was exposed to NO2 after the administration of aminopyrine. SO3 inhaled together with NO2 was found to increase the nitrosation.

Metabolism of, and DNA methylation by, N-nitrosomethylbenzylamine in chicken

The metabolism of 14C-nitrosomethylbenzylamine (NMBA) was studied in chicken. Following a single IV dose of 2 mg/kg, 14C-NMBA was cleared from the blood with a half-life of 3.8 min. At 10 min after administration 14C-NMBA was totally metabolized in the liver, whereas in the esophagus no measurable metabolic degradation had taken place. Maximum exhalation of radioactive CO2 occurred 1 h after IV administration of NMBA, and 11% of the total radioactivity had been exhaled as CO2 by 8 h. These results are compared with data on the metabolism of NMBA in the rat. The analysis of methylated bases in the DNA of different organs of chicken revealed that 7-me guanine was formed in all organs. The highest amount of 0(6)-me guanine was found in liver DNA, followed by kidney DNA. O6-me guanine was not detectable in any other organ. The O6-/7-me guanine ratio in DNA was calculated to be 0.05 and 0.02 for liver and 0.01 for kidneys.

Potentiation of dimethylnitrosamine genotoxicity in rat hepatocytes isolated following ethanol treatment in vivo

Unscheduled DNA synthesis (UDS), following exposure to dimethylnitrosamine (DMN), was potentiated in cultured hepatocytes isolated following treatment of rats for 14 or 28 days with 20% ethanol/5% sucrose solution. Ethanol treatment was associated with increased UDS, a concomitant increase in hepatic microsomal protein concentration and DMN N-demethylase activity. Increased aniline hydroxylase activity of hepatic microsomes from ethanol-treated rats preceded the measured increase in microsomal protein content or DMN metabolism. The increase in metabolism of DMN in vitro and potentiation of DMN-induced UDS associated with ethanol treatment may contribute to a synergistic effect of ethanol on DMN hepatotoxicity and carcinogenicity. In contrast, ethanol pretreatment did not increase the cytotoxicity of DMN as characterized by enzyme release.

Quantitative studies on the in vitro metabolic activation of dimethylnitrosamine by rat liver postmitochondrial supernatant

The metabolic activation of dimethylnitrosamine (DMN) to mutagenic and/or cytotoxic intermediates in vitro has been characterized and the relationship between DMN demethylase and ethoxyresorufin-O-deethylase (EROD) or ethylmorphine-N-demethylase (EMND) has been evaluated. A mammalian assay system which uses the postmitochondrial supernatant (S-15 fraction) prepared from a rat liver homogenate as an enzyme source and V79 Chinese hamster cells as targets for chemically induced damage was used. The enzyme pattern of the S-15 fraction was altered by pretreatment of experimental animals in vivo and/or by the use of enzyme inhibitors in vitro. The results of these studies indicate that the concentration of S-15 fraction in the reaction mixture can markedly influence the degree of DMN-induced cytotoxicity when it is metabolized in vitro and that the degree of DMN-induced cytotoxicity and mutagenicity are linearly related. The degree of cytotoxicity and mutagenicity induced in V79 cells by DMN does not correlate with EROD activity (a measure of 3-methylcholanthrene-inducible mixed-function oxidases) nor with EMND activity (a measure of phenobarbital-inducible mixed function oxidases) in the S-15 fraction.

Free radicals in vivo. Covalent binding to lipids

As one means of determining the extent to which free radical metabolites are involved in the interaction of hepatotoxic drugs with target tissues, we have measured the covalent binding to hepatic lipids of carbon tetrachloride, acetaminophen, 2-furamide, furosemide, dimethylnitrosamine, and bromobenzene. Transesterification of the Folch lipid fraction was required to distinguish radioactive label present but not covalently bound to alkyl residues through radical addition or combination reactions. Although all hepatotoxins were covalently bound to hepatic protein in the range of 1-2 nmoles/mg, thereby confirming tissue alkylation by reactive metabolites under the present experimental conditions, only carbon tetrachloride gave significant covalent binding to the alkyl residues of hepatic lipids (4.34 nmoles/mg). Thus, although these data further support the already well-documented role of a free radical in the reaction of carbon tetrachloride with target tissue molecules, none of the other hepatotoxins gave similar indications. Dimethylnitrosamine did give significant covalent binding to lipids, but the removal of the binding by transesterification indicates that the binding apparently resulted from electrophilic attack on nucleophilic centers present in phospholipids rather than from radical attack on electroneutral alkyl residues of the lipids.

Induction and suppression of N-nitrosomethylbenzylamine activation by microsomes from rat liver and esophagus

Male Sprague-Dawley rats were pretreated with various chemicals to determine their effects on the microsomal activation of the esophageal carcinogen N-nitrosomethylbenzylamine [( NMBzA ) CAS: 937-40-6; N-methyl-N- nitrosobenzylamine ] in the rat esophagus and, for comparative purposes, in the rat liver. When rats were pretreated with NMBzA , little change in hepatic NMBzA - debenzylase activity was observed. In contrast, NMBzA metabolism in the esophagus was significantly (60-65%) reduced. Similarly, pretreatment of rats with disulfiram [CAS: 97-77-8; bis( diethylthiocarbamoyl )disulfide] caused a 40% decrease in esophageal metabolism, but it had no significant effect in the liver. Pretreatments with the methylenedioxybenzenes safrole [CAS: 94-59-7; 4-allyl-1,2-(methylenedioxy)benzene], isosafrole [CAS: 120-58-1; 1,2-(methylenedioxy)-4-propenylbenzene], and dihydrosafrole (CAS: 94-58-6; 1,2-(methylenedioxy)-4- propylbenzene ) caused a marked induction (twofold to fivefold) of the hepatic metabolism of NMBzA , but again esophageal metabolism was suppressed. The results indicate that esophageal metabolism of NMBzA is either unchanged or suppressed by the various chemical pretreatments, but hepatic metabolism of the nitrosamine is induced by the methylenedioxybenzenes .

Dimethylnitramine metabolism in vitro by NZR rat liver slices

The net metabolism of dimethylnitramine (DMNO) was studied in NZR rat liver slices in tissue culture medium (Dulbecco's MEM). In rats, mice and fish, liver is the principal target organ for DMNO carcinogenesis. Destruction of DMNO in vitro with oxygenated medium was linear with amount of tissue (0.3-3.0 g liver), and with substrate concentration (0.14-4.44 mM). Substrate destruction (initially 0.2 mM DMNO) was linear for 60 min (average rate 0.9 +/- 0.1 microgram DMNO/g liver/min) and then slowed to become linear again at about half the initial rate from 90 min to longer than 5 h. In anoxic (N2) conditions DMNO metabolism slowed or stopped completely after 70 min. Metabolism of dimethylnitrosamine (DMN) was studied in the same preparation. DMN destruction rates were initially about 50% higher than DMNO, but were equal at longer incubation times. Simultaneous metabolism of DMNO and DMN by the same tissue slices showed DMNO rates unaltered in the presence of equimolar DMN (0.24 mM), but DMN rates were 20-40% depressed. No evidence was found for the oxidation of DMN to form DMNO, or for reduction of DMNO to DMN.

Metabolism of 15N-labelled N-nitrosodimethylamine and N-nitroso-N-methylaniline by isolated rat hepatocytes

The N-demethylation of 15N-labeled N-nitrosodimethylamine (DMN) and N-nitroso-N-methylaniline (NMA) by isolated rat hepatic cells has been investigated. The values obtained in this system for molecular nitrogen formed during metabolism, compared with substrate consumed, were DMN 47%, NMA 23%, and N-nitroso-N-methylurea (NMU) 105%. The results for DMN are roughly halfway between those previously determined with rat liver S-9 fraction in vitro (33%) and in vivo (67%). For NMA, the hepatocyte data are closer to those obtained from S-9 in vitro (19%), rather than the in vivo (52%). No mixed nitrogen ( 15N14N ) or labeled nitrogen oxides were found.

Potentiation of the hepatotoxicity of N-nitrosodimethylamine by fasting, diabetes, acetone, and isopropanol

Previous studies indicate that pretreatment with acetone or isopropanol, fasting, and streptozotocin-induced diabetes enhance hepatic microsomal nitroso-dimethylamine (NDMA) demethylase in rats. This study demonstrates that these same treatments also potentiate the hepatotoxicity of NDMA as indicated by plasma glutamic pyruvate transaminase (GPT) levels and histologic data. Pretreatment with acetone or isopropanol (2.5 ml/kg) and 2 days of fasting caused a 2-fold potentiation of NDMA-induced plasma GPT elevation, whereas streptozotocin-induced diabetes caused a 4.6-fold potentiation. The centrilobular necrosis produced by NDMA was more severe after pretreatment with the inducers. NDMA treatment also decreased hepatic microsomal demethylase activity. These results lend support to the concept that a NDMA demethylase is responsible for the activation of NDMA in vivo to a toxic intermediate, and induction of this enzyme activity potentiates NDMA hepatotoxicity.

Effects of N-nitrosodimethylamine on humoral immunity

Female B6C3F1 mice were given i.p. injections with 1.5, 3.0 and 5.0 mg/kg N-nitrosodimethylamine (DMN) daily for 14 days and evaluated on day 15. The day 4 immunoglobulin M (peak day) antibody response to sheep red blood cells (sRBC) was inhibited by 20, 53 and 81%, respectively. The day 5 immunoglobulin G (peak day) antibody response to sRBC was only inhibited significantly (60%) at the highest dose. Recovery studies indicated that the IgM antibody response was still significantly inhibited (48%) 30 days after the completion of the exposure to 5 mg/kg of DMN. The peak response (day 3) to 100 micrograms of the B-cell mitogen, lipopolysaccharide, was inhibited by 15, 26 and 32%, respectively, indicating that a portion of the suppression of the antibody response by DMN may be due to an effect on the ability of the lymphocytes to proliferate. Concentrations of DMN up to 100 mM added directly to untreated spleen cell suspensions had no effect on the in vitro antibody responses to lipopolysaccharide and sRBC. Preincubating DMN (100 mM) with either phenobarbital-induced or 3-methylcholanthrene-induced liver proteins (postmitochondrial supernatant from a 9000 X g liver homogenate) was still ineffective. The activation of DMN by either preparation was verified by measuring formaldehyde production, which reflects demethylation. In contrast to the results with DMN added directly to untreated spleen cell suspensions, the most sensitive indicator of suppression by DMN was the in vitro antibody responses to lipopolysaccharide and sRBC by spleen cell suspensions from DMN-treated mice.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of ascorbic acid deficiency and excess on the metabolism and toxicity of N-nitrosodimethylamine and N-nitrosodiethylamine in the guinea pig

The influence of ascorbate deficiency and megadosage on the metabolism of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) was investigated in the guinea pig. After 21 days on a scorbutogenic diet, microsomal cytochrome P-450 and cytochrome b5 levels fell by 51 and 32%, respectively, while cytochrome c reductase activity remained constant. The activities of NDMA and NDEA dealkylase I were also depressed significantly. The Vmax of NDMA demethylase I and NDEA deethylase I was significantly depressed. Also, ascorbate deficiency significantly decreased the plasma clearance of both nitrosamines though the LD50 of neither were altered by ascorbate nutrition. Covalent binding of 14C from [14C]NDMA and [14C]NDEA to DNA obtained from liver slices was significantly lower in the deficient than in the control samples; megadosage appeared to have the opposite effect.

Formation of O6-methylguanine in regenerating rat liver by N-nitrosomethylbenzylamine is not sufficient for initiation of preneoplastic foci

The DNA methylating activities of N-nitrosodimethylamine (NDMA), an initiator of hepatic gamma-glutamyltranspeptidase-positive foci, and N-nitrosomethylbenzylamine (NMBzA), which does not initiate, were studied in regenerating rat liver. Equimolar doses of 14C-labelled NDMA and NMBzA (33.5 mumol/kg) were administered to male Sprague-Dawley rats 18 h after partial hepatectomy. NDMA and NMBzA both produced 7-methylguanine and O6-methylguanine. The results suggest that although the formation of O6-methylguanine may be necessary it is not sufficient for initiation of preneoplastic foci.

Metabolism of low concentrations of N-nitrosodimethylamine in isolated liver cells of the guinea pig

Freshly isolated liver cells of guinea pig were used to study the metabolism of NDMA in the concentration range 0.05 to 100 microM. Analysis was performed using the gas chromatograph-thermal energy analyzer nitrosamine detector method and with radiolabeled NDMA. At concentrations below 10 microM, NDMA was degraded by liver cells (10 mg of protein in 2.5 ml of medium) within 20 min (at 100 microM in 80 min). The majority of metabolized methyl groups were initially associated with volatile compounds and were subsequently integrated into nonvolatile, acid-soluble molecules (57%) or liberated as CO2 (14%). Less than 2% were bound to cellular macromolecules. Ethanol inhibited NDMA degradation competitively, with a Ki of 0.8 mM ethanol. It is concluded that low concentrations of NDMA are metabolized in liver cells, primarily by the high-affinity demethylase and that there are no additional catalytic activities with Km values below 5 microM. Most of the methyl groups, released during metabolism, enter the C1 pool.

Zinc deficiency and the development of esophageal and forestomach tumors in Sprague-Dawley rats fed precursors of N-nitroso-N-benzylmethylamine

Nine-week-old zinc-sufficient (100 mg zinc/kg feed) and zinc-deficient (7 mg zinc/kg feed) noninbred male Sprague-Dawley rats were given free access 5 days a week to deionized drinking water containing low (0.05%) or high (0.25%) quantities of benzylmethylamine (BMA) and concurrently 0.5% NaNO2. In contrast to the action of the preformed carcinogen N-nitroso-N-benzylmethylamine, which almost invariably produced esophageal tumors, oral administration of its precursors, BMA and NaNO2, resulted in forestomach tumors as well. In both the high- and low-BMA groups given precursors for 16 weeks, the incidence of papillomas in both the esophagus and forestomach was significantly higher in the zinc-deficient than in the zinc-sufficient rats, but zinc deficiency did not significantly increase the yield of forestomach carcinomas. However, when combined high BMA and NaNO2 administration was prolonged to 37 weeks, the yield of forestomach carcinomas was significantly greater in the zinc-deficient than in the zinc-sufficient animals. Because endogenous synthesis of N-nitrosamines from ingested precursors is an important source of human exposure to these carcinogenic compounds and because dietary zinc deficiency might be operating in some areas with a high incidence of esophageal cancer, our data are of more than routine significance.

Chemical interaction of disulfiram with nitrosodimethylamine after in vitro enzymatic activation

The in vitro reaction between disulfiram (DSF) and N-nitroso[14C]dimethylamine [( 14C]NDMA) was studied. Incubations of DSF with [14C]NDMA were carried out in the presence of rat liver microsomes, control 9000 g (S9) supernatant fraction and phenobarbital-induced S9 fraction. H.p.l.c. analysis and liquid scintillation measurement provided evidence for the formation of methyldiethyldithiocarbamate (MeDDTC) as a product of the reaction between diethyldithiocarbamate (DDTC), the main active metabolite of DSF and the 'methyl-cation' released by NDMA after enzymatic activation. The amount of MeDDTC found here was consistent with the rate of oxidation of NDMA to formaldehyde. Scintillation counting confirmed that other radioactive peaks, not due to MeDDTC, were unrelated to the methylation of L-cysteine by [14C]NDMA.

[Possibility of N-dimethylnitrosamine formation after parenteral administration of amidopyrine and nitrite to rats]

The intravenous or intraperitoneal injection of amidopyrine and sodium nitrite to rats induces liver necrosis caused by endogenic synthesis of carcinogenic N-nitrosodimethylamine (NDMA). Ascorbic acid, which inhibits this synthesis when injected intravenously, phenobarbital, which is an inductor of enzymatic decomposition when injected intraperitoneally, prevented the formation of necrosis.

Inhibition of N-nitrosodimethylamine metabolism by ethanol and other inhibitors in the isolated perfused rat liver

The effects of several potential inhibitors of N-nitrosodimethylamine (NDMA) metabolism have been studied in isolated perfused rat livers. Low concentrations of ethanol (less than 0.5 mM) were found to strongly inhibit hepatic metabolism of NDMA. This effect does not require the metabolism of ethanol. The metabolism and toxicity of NDMA may therefore be influenced by the intake of alcohol as well as by ethanol produced endogenously by the intestinal flora. Other compounds found to inhibit NDMA metabolism include n-propanol, tert-butanol, 4-methylpyrazole, amino-triazole and acetaldehyde.

Stability of various alpha-esters of 1-(N-methyl-N-nitrosamino)-methanol in vitro and in vivo

The hydrolytic activities of various rat tissue homogenates on the substrates 1-(N-methyl-N-nitrosamino)-methylacetate (NNMA), 1-(N-methyl-N-nitrosamino)-methylbutyrate (NNMB), 1-(N-methyl-N-nitrosamino)-methylbenzoate and 1-(N-methyl-N-nitrosamino)-methylpivaloate (NNMP) were investigated+. The hydrolytic enzymes of liver and serum showed similar activities toward all investigated compounds, while those of kidney, lung, heart, stomach and forestomach hydrolyzed the various esters differently. Differences in the in vivo elimination of NNMA, NNMB and NNMP from blood could not be found. After 20 s only 10% of the initial concentrations still occurred in blood after i.v. application.

Indole-3-carbinol protects against covalent binding of benzo[a]pyrene and N-nitrosodimethylamine metabolites to mouse liver macromolecules

Benzo[a]pyrene (BaP) and N-nitrosodimethylamine (NDMA) are carcinogens and indirect acting mutagens. A naturally occurring dietary indole, indole-3-carbinol (I-3-C), has been shown to decrease the incidence of aryl hydrocarbon induced neoplasia in experimental animals. We examined the relationship between the ability of I-3-C to alter the rate of carcinogen oxidation and its ability to decrease the rate of covalent binding of carcinogen metabolites to DNA and protein. We found that I-3-C inhibited the covalent binding of NDMA oxidation products to DNA in vitro in proportion to its ability to inhibit carcinogen metabolism. Pretreatment of mice by gavage with I-3-C resulted in no change in the rate of aryl hydrocarbon hydroxylase or NDMA demethylase in hepatic post-mitochondrial supernatant. However, this pretreatment resulted in a 60-90% decrease in the ability of carcinogen oxidative metabolites to bind covalently to DNA or protein in vitro. Similarly, in in vivo experiments, gavage with I-3-C, followed by gavage with BaP or NDMA, resulted in a 63-85% decrease in covalent binding to macromolecules, with no concomitant change in carcinogen metabolism. The results suggest that the in vivo administration of I-3-C may confer protection for hepatic macromolecules against covalent binding of the metabolites of these two indirect acting mutagens.