Toxicity of N-nitrosodimethylamine, N-nitrosomethylbenzylamine, and 1,2-dimethylhydrazine in isolated rat hepatocytes

N-Nitrosodimethylamine and 1,2-dimethylhydrazine were shown to injure lethally primary monolayer cultures of rat hepatocytes only after incubation periods in excess of 24 hr. The toxic action of these agents, therefore, mimics the time dependency of their hepatoxicity in vivo. The viability of hepatocytes treated with N-nitrosomethylbenzylamine was not different from controls at times up to 54 hr following treatment, a result which is also consistent with the inability of this compound to produce hepatotoxicity in vivo.

Activation of the c-Ha-ras-1 proto-oncogene by methylation in vitro with alpha-acetoxy-N-nitrosodimethylamine

alpha-Acetoxy-N-nitrosodimethylamine, an activated derivative of the carcinogen N-nitrosodimethylamine, methylated in vitro a plasmid containing the human c-Ha-ras-1 proto-oncogene, resulting in the generation of a transforming oncogene, assayed by transfection into NIH 3T3 cells. The resulting transformed cells were tumorigenic and metastatic in immune-deprived mice. Further transfection using tumor DNA led to the formation of three secondary NIH 3T3 transformants. DNA from these secondary transformants contained human ras gene sequences. Two of the three secondary transformants contained G----A mutations at guanine 35 in codon 12, and the third secondary transformant retained the wild-type sequence at codons 12, and 61. For the latter, the activating mutation was not determined. These results demonstrate that a simple methylating agent can activate a normal human ras proto-oncogene to a transforming oncogene.

Methylation of DNA by three N-nitroso compounds: evidence for sequence specific methylation by a common intermediate

Methylation in vitro of calf thymus DNA, a supercoiled plasmid, poly(dG).poly(dC), and poly(dGdC).poly(dGdC) by N-nitroso(acetoxy-methyl)methylamine and N-nitroso(acetoxybenzyl)methylamine in the presence of esterase, and by N-nitrosomethylurea was investigated. Although there were differences in the amounts of 7-methylguanine and O6-methylguanine formed in the various DNA substrates, the methylation pattern was the same for each of these methylating agents. The three compounds reacted identically when methylation of a portion of a 345 bp restriction fragment of the plasmid pBR322 was examined at nucleotide resolution by a sequencing assay. They also showed a tendency to react preferentially with particular guanines. These data suggest that the three N-nitroso compounds methylate DNA via a common intermediate such as the methyl diazonium ion, which exhibits some sequence specificity.

Alterations in pancreatic islet function produced by carcinogenic nitrosamines in the Syrian hamster

Exposure of hamsters to 5 daily doses of 20 mg/kg N-nitrosobis(2-oxopropyl)amine (BOP) or 76 mg/kg N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP), resulted in reduced insulin secretion in freshly isolated pancreatic islets. These treatments also reduced plasma insulin and glucose levels, and were hepatotoxic. The inhibition of insulin secretion, however, was transient. Islets isolated from treated hamsters that were then placed in culture secreted elevated levels of insulin for many months. When cultured islets were directly exposed to the nitrosamines for 3 days, there was also a transient reduction of insulin secretion that was subsequently normalized after removal of the nitrosamine from the medium. These results show that BOP and HPOP modify beta-cell function both directly, and possibly indirectly, via damage to the liver. Furthermore, the lack of immediate inhibition of insulin secretion when islets were incubated in the presence of BOP or HPOP as well as glucose, suggests that the nitrosamines do not bind to the glucose receptor.

Interaction of alpha-acetoxy-N-nitrosopyrrolidine with DNA assessed by loss of restriction endonuclease recognition sites and formation of apurinic/apyrimidinic sites

alpha-Acetoxy-N-nitrosopyrrolidine (AcO-NPYR), in the presence of esterase, was reacted with the plasmid pBR322. The AcO-NPYR-treated plasmid was nicked by reaction with spermidine, suggesting the presence of apurinic sites. The treated plasmid also blocked the action of the restriction endonuclease BglI and, to a lesser extent, DraI. The recognition sequences of these restriction enzymes suggest that AcO-NPYR interacts preferentially with guanine and/or cytosine bases in DNA.

Streptozotocin toxicity to cultured pancreatic islets of the Syrian hamster

Pancreatic islets of the Syrian golden hamster were maintained in culture for extended periods of time. Toxicity of streptozotocin in these cultures was evaluated by measurement of insulin secretion. Exposure of islets to 1 or 2 mM streptozotocin immediately following isolation resulted in a permanent and dose-related inhibition of insulin secretion. This was accompanied by islet disruption as observed by phase-contrast microscopy. Culture of islets for 24 hours before streptozotocin exposure afforded protection from toxicity. For example, exposure of freshly isolated islets to 2 mM streptozotocin resulted in complete destruction of beta cells, whereas islets similarly exposed after a 24 hr culture period continued to secrete insulin for many months. Islets maintained in culture for one week before exposure to 0.1-0.5 mM streptozotocin, however, became more sensitive than freshly isolated islets. Repeated weekly exposure of cultured islets to a "non-toxic" concentration (0.1 mM) resulted in sustained suppression of insulin secretion after 11 weeks.

Alkylation of individual genes in rat liver by the carcinogen N-nitrosodimethylamine

A method for quantifying alkylated bases in defined gene sequences of rat liver following carcinogen treatment is described. After restriction, the genomic DNA is heated to 65 degrees at pH 8.0 to cause depurination of 7-alkylguanine and 3-alkyladenine residues. Reaction with spermidine then generates strand breaks. After gel electrophoresis and Southern transfer, sequences of interest are visualized using specific probes. The presence of strand breaks in restriction fragments reduces their intensity compared to unmethylated fragments. The method was applied to show that the transcriptionally active albumin gene is damaged by the hepatocarcinogen N-nitrosodimethylamine to a much greater extent than the untranscribed IgE gene in rat liver.

Base-catalyzed decomposition of N-nitrosobis(2-oxopropyl)amine

N-Nitrosobis(2-oxopropyl)amine, a potent carcinogen for the pancreas in Syrian hamsters, undergoes a facile, base-catalyzed, intramolecular aldol condensation to yield N-nitroso-5-hydroxy-5-methyl-3-piperidone. This cyclic nitrosamine then decomposes to yield 3-hydroxy-5-methylpyridine.

Micronucleus formation induced in rat liver and esophagus by nitrosamines

The rat hepatocarcinogen nitrosodimethylamine and the esophageal carcinogens nitrosomethylbenzylamine and nitrosomethylamylamine were shown to produce chromosomal damage, as manifested by micronucleus formation, in their target tissues. There was cross-reactivity in the two tissues, however, at high dose levels. Nitrosodiethylamine, which produces tumors in both the liver and esophagus in the rat, also produced micronuclei in both tissues.

Analysis of the immunoadjuvant octadecyl tyrosine hydrochloride

A simple, rapid, high performance liquid chromatographic (HPLC) method for the analysis of the immunoadjuvant octadecyl tyrosine hydrochloride is described. The HPLC procedure can be applied to the direct determination of amino acid reactants present as contaminants in the adjuvant (tyrosine, ethyl tyrosine) and from this information the content of octadecanol reactant can be estimated. Further, these same determinations provide a means of monitoring immunoadjuvant stability in any vaccine preparation.

Alcohol consumption does not lead to urinary excretion of N-nitrosodimethylamine in the fasting human

To test the hypothesis that N-nitrosodimethylamine (NDMA) and possibly other nitrosamines are synthesized naturally in the gastrointestinal tract, the urinary excretion of nitrosamines has been measured in people whose first pass clearance and metabolism of nitrosamines had been inhibited by administration of ethanol. Five fasted volunteers consumed 350 mg of nitrate in beet juice followed, 1 h later, by a volume of 10% alcohol in carbonated water sufficient to raise their blood alcohol concentrations to 50-80 mg/100 ml. This alcohol concentration was then maintained over a 6 h period. No NDMA or any other volatile nitrosamines were excreted in urine during this 6 h period or during the subsequent 12 h (detection limit = 0.01 mumol). These results suggest that less than 0.5 mumol NDMA, formed by endogenous nitrosation in the fasting human, is likely to be present in the stomach at any time.

Cellular toxicity of pancreatic carcinogens

Azaserine (CAS: 115-02-6), streptozocin (CAS: 18883-66-4; streptozotocin), and N-nitrosobis(2-oxopropyl)amine [(BOP) CAS: 60599-38-4] produce different types of pancreatic tumors in rodents. We have investigated the toxic effects of these compounds on pancreatic tissues from Wistar rats and Syrian hamsters. Inhibition of protein synthesis was used as a measure of toxicity. Pancreatic islets and acinar cells from rat and hamster were labeled with [3H]leucine for 60 minutes in vitro in the presence of the various carcinogens. Azaserine, which produces acinar cell tumors in the rat, inhibited synthesis by all tissues; rat acinar cells, however, were most sensitive. Glutamine, but not serine, provided some protection against azaserine toxicity. Streptozocin inhibited synthesis by islets of both species and acinar cells from hamster; islets were the most sensitive. BOP and N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine, which induce ductal tumors in the hamster, had no effect on any of the tissues examined. These results indicate that the specificities of the cellular toxicities of the pancreatic carcinogens parallel, to some degree, their tumorigenic effects.

Formation of O6-methylguanine in rat liver DNA by nitrosamines does not predict initiation of preneoplastic foci

Nitrosodimethylamine (NDMA) at 26 mumol/kg, and nitrosomethylbenzylamine (NMBzA) at 33.5 mumol/kg were equally potent in producing 7-methylguanine and O6-methylguanine in rat liver DNA. These doses were used to compare the abilities of the two nitrosamines to initiate production of putative preneoplastic foci in rat liver. Whereas NMBzA resulted in no increase above the background level of foci (0.9 +/- 0.1 foci/cm2), NDMA produced 7.5 +/- 0.6 foci/cm2. Co-administration of NDMA and NMBzA produced no more foci than NDMA alone, even though the combined effect of the two nitrosamines on DNA methylation was additive. The results suggest that methylation of hepatic DNA by nitrosamines does not predict initiation of preneoplastic foci.

Mechanism of DNA methylation by N-nitroso(2-oxopropyl)propylamine

Using an in vitro assay system for measuring 7-methylguanine formation in DNA, it was demonstrated that N-nitroso(2-oxopropyl)propylamine (NOPPA) is converted into a methylating agent by a microsomal, cytochrome P-450 dependent mixed function oxidase from rat liver. Formation of propionaldehyde, but not formaldehyde, in this assay system, indicated that a single alpha-oxidation reaction on the propyl side chain of NOPPA leads to the formation of a methylating agent. A mechanism for this reaction is proposed.

Monolayer culture of pancreatic islets from the Syrian hamster

Pancreatic islets of Langerhans of the Syrian hamster were maintained in culture for as long as 43 wk. Islets were prepared by collagenase/hyaluronidase digestion of minced pancreas. The islets quickly attached to the plastic culture flasks and lost their spherical form as they flattened out to form circular monolayers. Few fibroblastoid cells were observed. As outward migration continued, the islets became vacuolated with the ultimate formation of monolayer rings. Throughout the culture period the beta cells continued to synthesize and secrete insulin. Furthermore, the cells maintained a responsiveness to glucose stimulation with increased rates of hormone secretion in the presence of elevated concentrations of the sugar. These studies demonstrate the suitability of Syrian hamster islets for studies involving long-term culture.

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.

N-nitrosation of proline in smokers and nonsmokers

Endogenous nitrosation of proline was investigated in smokers and nonsmokers. Volunteers consumed a volume of beet juice equivalent to 325 mg nitrate, and 1 hour later they consumed 500 mg proline. In separate experiments volunteers ingested proline alone. Twenty-four-hour urine samples were collected and analyzed for N-nitrosoproline. When proline alone was ingested, there was no significant difference in urinary nitrosoproline excretion between smokers and nonsmokers. When beet juice and proline were consumed, however, smokers produced approximately 2.5 times as much N-nitrosoproline as nonsmokers. Salivary thiocyanate levels were approximately 3.2 times higher in smokers compared to those in nonsmokers. Salivary nitrite levels of smokers and nonsmokers, either before or after beet juice consumption, were not different. Salivary nitrate concentrations, however, were higher in nonsmokers than in smokers after beet juice consumption but not before. Our results suggest that the higher level of salivary thiocyanate in smokers is responsible for the increased rate of endogenous nitrosation of proline in this group compared to the rate in nonsmokers. Nitrosating agents in cigarette smoke do not appear to play a significant role.

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 .

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.

Studies on the metabolic activation of beta-keto nitrosamines: mechanisms of DNA methylation by N-(2-oxopropyl)-N-nitrosourea and N-nitroso-N-acetoxymethyl-N-2-oxopropylamine

At pH 7.35, N-(2-oxopropyl)-N-nitrosourea (OPNU) reacted with calf thymus DNA to yield O6-methylguanine, 7-methylguanine and 3-methyladenine. Kinetic measurements of the base catalyzed decomposition of OPNU and the extent of methylation of DNA by OPNU suggested that methylnitrosourea is not formed as an intermediate product. Diazoacetone, acetic acid and methanol were identified as products of decomposition of OPNU at pH 7.35. Reaction of OPNU with N-methylmaleimide yielded the product resulting from 1,3-dipolar cycloaddition of diazomethane. Hydrolysis of N-nitroso-N-acetoxymethyl-N-2-oxopropylamine (NAMOPA) in the presence of hog liver esterase also produced diazoacetone, acetic acid and methanol. Enzymatic hydrolysis of NAMOPA in the presence of DNA produced O6-methylguanine, 7-methylguanine and 3-methyladenine. These results suggest that OPNU undergoes base-catalyzed decomposition and NAMOPA undergoes enzymatic hydrolysis to yield the same intermediate, 2-oxopropyldiazotate. This diazotate then reacts either by protonation followed by loss of water to form diazoacetone, or by internal nucleophilic attack by the diazotate oxygen on the carbonyl carbon to form an oxadiazoline intermediate which then collapses to form acetate and the methylating agent diazomethane. These reaction schemes are used to suggest the mechanism by which N-nitroso-2-oxopropylpropylamine methylates hepatic DNA in vivo.

Oxidation of ammonia and hydroxylamine to nitrate in the rat and in vitro

We have demonstrated that ammonia is oxidized to nitrate in the rat. Male Sprague-Dawley rats gavaged with 1000 mumol N-15-ammonium chloride each day for 5 days were found to excrete low, but significant amounts of excess N-15-nitrate in their urines on the five days of treatment and on the five subsequent days. We recovered a total of 0.28 +/- 0.03 mumol excess N-15-nitrate (mean +/- SE) per rat, which indicates that ammonia is converted to nitrate in a yield of approximately 0.0080%. The oxidation of N-15-labeled glycine and L-glutamic acid to N-15-nitrate could not be detected. N-15-Hydroxylamine was oxidized in the rat to N-15-nitrate in a yield of 4.7%, which shows that hydroxylamine is a possible intermediate in the ammonia oxidation process. Injection of rats with Arochlor 1254, an inducer of several isozymes of cytochrome P-450, did not significantly affect the rate of endogeneous nitrate synthesis. Carbon tetrachloride, which causes hepatic lipid peroxidation, produced a small but significant increase in nitrate synthesis. We confirmed the observation that a bacterial endotoxin can greatly stimulate nitrate synthesis, and we showed that concurrent treatment with superoxide dismutase does not modify the effect of the endotoxin. An in vitro chemical model system was used to demonstrate that oxidation of ammonia to nitrate by the hydroxyl radical at physiological pH is chemically feasible. Our results are consistent with the hypothesis that ammonia is oxidized to nitrate in vivo by a non-enzymatic process which involves active oxygen species such as the hydroxyl radical. We estimate that a 215 g rat produces 3.0 mumol of nitrate per day via ammonia oxidation.

Enzymatic reduction of beta-ketonitrosamines

The reduction of N-nitrosobis(2-oxopropyl)amine (BOP) and N-nitroso(2-oxopropyl)propylamine (NOPPA) by hepatic and pancreatic cytosol and microsomes from Syrian golden hamsters and Sprague-Dawley rats has been examined. All hepatic fractions reduced both substrates, although the activity depended on the fraction tested and the cofactor employed (NADH or NADPH). Generally, hamster hepatic fractions contained higher activity than the rat hepatic fractions and BOP was a better substrate than NOPPA. Of the pancreatic fractions, only cytosol exhibited reductase activity. The hamster cytosol was able to utilise both cofactors, but the rat fraction exhibited activity only when NADPH was present. BOP was the better substrate for the pancreatic enzymes and in the presence of NADPH, the rat and hamster activities were about equal. These results suggest that the pancreatic reduction of BOP to HPOP is unlikely to be a significant factor in the species-specific induction of pancreatic cancer by BOP.

Nitrate formation in rats exposed to nitrogen dioxide

Sprague-Dawley rats exposed to atmospheres containing low levels of nitrogen dioxide (NO2) for 24 hr had increased levels of nitrate in their urine on the day of exposure and on the 3 subsequent days. The total increase in urinary nitrate was linearly related to the nitrogen dioxide concentration administered. We recovered in urine 8.4 +/- 1.1 mumol nitrate/ppm NO2/24-hr exposure (slope +/- 95% confidence limits) for 185-g rats. Both the linearity and magnitude of this effect imply that reaction with respiratory tract water is not a major pathway of NO2 absorption in the lung. Instead, our observations support the hypothesis that the major interaction of NO2 in the lung is with readily oxidizable tissue components to form nitrite. We estimate that 9.6 mumol of nitrite is formed in the respiratory tract of the rat per ppm NO2 per 24-hr exposure. We also estimate that humans breathing air containing 0.1 ppm NO2 have about 3.6 mg of nitrite formed in their respiratory tract per day.