Studies on the distribution and metabolism of N-[14C]nitrosopyrrolidine in mice

Risk assessment of N-nitrosamines in food

EFSA was asked for a scientific opinion on the risks to public health related to the presence of N-nitrosamines (N-NAs) in food. The risk assessment was confined to those 10 carcinogenic N-NAs occurring in food (TCNAs), i.e. NDMA, NMEA, NDEA, NDPA, NDBA, NMA, NSAR, NMOR, NPIP and NPYR. N-NAs are genotoxic and induce liver tumours in rodents. The in vivo data available to derive potency factors are limited, and therefore, equal potency of TCNAs was assumed. The lower confidence limit of the benchmark dose at 10% (BMDL10) was 10 μg/kg body weight (bw) per day, derived from the incidence of rat liver tumours (benign and malignant) induced by NDEA and used in a margin of exposure (MOE) approach. Analytical results on the occurrence of N-NAs were extracted from the EFSA occurrence database (n = 2,817) and the literature (n = 4,003). Occurrence data were available for five food categories across TCNAs. Dietary exposure was assessed for two scenarios, excluding (scenario 1) and including (scenario 2) cooked unprocessed meat and fish. TCNAs exposure ranged from 0 to 208.9 ng/kg bw per day across surveys, age groups and scenarios. 'Meat and meat products' is the main food category contributing to TCNA exposure. MOEs ranged from 3,337 to 48 at the P95 exposure excluding some infant surveys with P95 exposure equal to zero. Two major uncertainties were (i) the high number of left censored data and (ii) the lack of data on important food categories. The CONTAM Panel concluded that the MOE for TCNAs at the P95 exposure is highly likely (98-100% certain) to be less than 10,000 for all age groups, which raises a health concern.

Metabolism-dependent activation and toxicity of chemicals in nasal glands

The mucosae of the nasal passages contain a large amount of glands which express secretory proteins as well as phase I and phase II biotransformation enzymes. In this review the metabolic activation, covalent binding and toxicity of chemicals in the Bowman's glands in the olfactory mucosa, in the sero-mucous glands in the nasal septum and in the lateral nasal glands and maxillary glands around the maxillary sinuses are discussed. Light microscopic autoradiographic studies have demonstrated a selective covalent binding of nasal toxicants and carcinogens such as halogenated hydrocarbons and N-nitrosamines, especially in the Bowman's glands following a single systemic exposure, suggesting a high rate of metabolic activation of chemicals in these glands. Special attention is put on the herbicide dichlobenil which induces necrosis in the olfactory mucosa following a cytochrome-P450-mediated metabolic activation and covalent binding in the Bowman's glands.

Nasal cavity enzymes involved in xenobiotic metabolism: effects on the toxicity of inhalants

A decade ago, the ability of nasal tissues to metabolize inhalants was only dimly suspected. Since then, the metabolic capacities of nasal cavity tissues has been extensively investigated in mammals, including man. Aldehyde dehydrogenases, cytochrome P-450-dependent monooxygenases, rhodanese, glutathione transferases, epoxide hydrolases, flavin-containing monooxygenases, and carboxyl esterases have all been reported to occur in substantial amounts in the nasal cavity. The contributions of these enzyme activities to the induction of toxic effects from inhalants such as benzo-a-pyrene, acetaminophen, formaldehyde, cocaine, dimethylnitrosamine, ferrocene, and 3-trifluoromethylpyridine have been the subject of dozens of reports. In addition, the influence of these enzyme activities on olfaction and their contribution to vapor uptake is beginning to receive attention from the research community. Research in the next decade promises to provide answers to the many still unanswered questions posed by the presence of the substantial xenobiotic metabolizing capacity of the nasal cavity.

Effects of N-nitrosopyrrolidine and N-nitrosoproline on the incorporation of 3H-thymidine into the DNA of various organs of the mouse: tissue specificity and effects of ethanol consumption

The effects of the carcinogenic N-nitrosamine N-nitrosopyrrolidine (NPYR) and the non-carcinogenic N-nitrosamino acid N-nitrosoproline (NPRO) on 3H-thymidine incorporation into DNA were evaluated in various organs of male C57BL mice. The N-nitroso compounds were given intraperitoneally 24 hrs before sacrifice in equimolar amounts (148 mumol/kg b.wt.). Two hours before the mice were killed, they were given an intraperitoneal injection of 3H-thymidine. NPYR, but not NPRO, induced a tissue-specific inhibition of 3H-thymidine incorporation into DNA. The inhibition occurred only in organs reported to be involved in the biotransformation of NPYR, i.e., the liver, lung, and nasal mucosa. Daily oral consumption of ethanol (1.8 ml 30% ethanol for 30 days) had no effects in itself on 3H-thymidine incorporation, but resulted in an enhancement of the inhibitory action of NPYR in the lung and nasal mucosa.

Application of whole-body autoradiography in toxicology

Whole-body autoradiography enables the drugs and toxicants to be distributed throughout the animal. Good results are obtained with this technique. However, certain artifacts can occur that could lead to misinterpretation, and these must be known. These artifacts are described. From the metabolic point of view, autoradiography provides data on the distribution kinetics of a compound and the elimination of radioactivity in various organs. These data are a guide for quantitative research into the metabolism of a compound. From the toxicological point of view, it must be admitted that the main purpose of this technique is to reveal the sites of retention of radioactivity. Such specific organ retention could be the consequence of the activation of a minor metabolite into a very reactive compound. If this is so, it is a specific organ effect which could not be studied by other techniques and could lead the way to a more specific organ effect which could not be studied by other techniques and could lead the way to a more appropriate line of research in the study of chronic toxicity. However, it must be recalled that the fact that a compound is retained by a specific organ does not always mean that the compound exerts a toxic effect upon the said organ. With this technique, distribution study can be performed on pregnant animals, and it provides us with more data concerning the transplacental passage of radioactive metabolites. All these aspects of the technique clearly indicate that whole-body autoradiography should be insisted upon during the early stages of development of new molecules. Successive experiments could then lead to selecting the best experimental conditions for metabolic pharmacokinetics and studies in toxicology.

Application and results of whole-body autoradiography in distribution studies of organic solvents

With the growing concern for the health hazards of occupational exposure to toxic substances attention has been focused on the organic solvents, which are associated with both deleterious nervous system effects and specific tissue injuries. Relatively little is known about the distribution of organic solvents and their metabolites in the living organism. Knowledge of the specific tissue localizations and retention of solvents and solvent metabolites is of great value in revealing and understanding the sites and mechanisms of organic solvent toxicity. Whole-body autoradiography has been modified and applied to distribution studies of benzene, toluene, m-xylene, styrene, methylene chloride, chloroform, carbon tetrachloride, trichloroethylene and carbon disulfide. The high volatility of these substances has led to the development of cryo-techniques. Whole-body autoradiographic techniques applicable to the study of volatile substances are reviewed. The localizations of nonvolatile solvent metabolites and firmly bound metabolites have also been examined. The obtained results are discussed in relation to toxic effects and evaluated by comparison with other techniques used in distribution studies of organic solvents and their metabolites.

The metabolism of nitrosopyrrolidine by hepatocytes from Fischer rats

Nitrosopyrrolidine (NO-PYR), an hepatocellular carcinogen, is rapidly metabolized to CO2 by hepatocytes freshly isolated from the livers of male Fischer rats. Using CO2 evolution as a measure of NO-PYR metabolism, we observed two kinetic constants; a high affinity component (Km = 0.11 mM), and a lower affinity component (K m = 3.2 mM). The high affinity component has similar kinetic constants to those observed for in vitro reactions with microsomes plus cytosol (Km = 0.36 mM). Therefore, it is probable that the microsomal reaction is the limiting factor in the metabolism of NO-PYR in hepatocytes. NO-PYR may be metabolized to CO2 through normal anaplerotic sequences. Some metabolites of NO-PYR which have been tentatively identified are gamma-hydroxybutyrate, succinic semialdehyde, 3,4-dihydroxybutyric acid lactone, lactate, acetate, pyruvate, glyoxylate, gamma-aminobutyrate and alanine. 2-Hydroxytetrahydrofuran (2-hydroxy-THF). a product of alpha-hydroxylation was detected at low levels in only one of four reactions. 3-Hydroxy-NO-PYR is present but represents only a small percentage of the total metabolism and is probably of little significance in the overall catabolism of NO-PYR in hepatocytes.

Extrahepatic sites of metabolism of N-nitrosopyrrolidine in mice and rats

1. By the use of whole-body autoradiography, the localization of N-nitroso[14C]pyrrolidine and its metabolites was demonstrated in the tissues of C57B1-mice and Sprague-Dawley rats. 2. On the basis of the autoradiographic data, tissues were selected and tested for their ability to metabolize N-nitroso[14C]pyrrolidinie to 14CO2 and tissue-bound metabolites. 3. The results indicated that only few tissues had a marked ability to degrade N-nitrosopyrrolidine. These were the liver and the nasal mucosa in both species, the tracheobronchial mucosa in mice, and to a lower extent the kidneys in both species.

Sites of metabolism of N-nitrosodiethylamine in mice

Low-temperature whole-body autoradiography and autoradiography with heated sections in C57Bl mice injected with N-[14C]nitrosodiethylamine showed a homogenously distributed volatile radioactivity in most tissues--indicating an ability of the non-metabolized substance to freely pass the biological membranes and distribute evenly in the intra- and extracellular tissue-water. A high level of non-volatile metabolites was found in several tissues: the nasal and tracheal mucosa, the mucosa of the bronchial tree, the salivary glands, the liver, the mucosa of the oesophagus and the tongue, and the lacrimal glands. Studies in vitro indicated that these tissues had a capacity to degrade N-[14C]nitrosodiethylamine (14CO2-production and incorporation of radioactivity in the acid-insoluble material of the tissues were used as indices of the metabolism), whereas several other tissues, which did not accumulate metabolites at short survival intervals in vivo, were devoid of significant metabolic capacity. The relationship between metabolic ability and carcinogenic response of the tissues for N-nitrosodiethylamine is discussed on basis of the obtained results.

Autoradiographic observations on the distribution and metabolism of N'-/14C/nitrosonornicotine in mice

The fate of the weak base N'-nitrosonornicotine (2'-14C-labeled) has been studied in mice. Whole-body autoradiography showed that the radioactivity was rapidly distributed throughout the body, which probably reflects an ability of the non-protonated compound to freely pass biological membranes and distribute evenly in the intra- and extracellular tissue water. Metabolites which were firmly bound to tissue macromolecules - retained throughout the observational period (24 h) - were present in the tracheobronchial and nasal mucosa, the liver, the submaxillary and sublingual salivary glands and the esophagus. Radioactivity in the lacrimal gland, the gastrointestinal contents, the urinary bladder, and gallbladder seems to be related to excretion of the substance and/or its metabolites. A binding to the melanin in the eye and hair was observed in vivo and in vitro. Experiments with mice pretreated with diethyldithiocarbamate and nialamide showed that these substances partially inhibited the metabolism of N'-/14C/nitrosonornicotine. CO2 was not recovered in the breath during the 4 h following the administration of N'-/14C/nitrosonornicotine.

Studies on the tissue-disposition and fate of N-[14C]ethyl-N-nitrosourea in mice

The tissue-disposition and fate of N-[14C]ethyl-N-nitrosourea has been studied in mice. A large part of the injected N-[14C]ethyl-N-nitrosourea radioactivity was found to be exhaled as 14CO2. Whole-body autoradiography showed evenly distributed radioactivity in most tissues shortly after the administration of N-[14C]ethyl-N-nitrosourea which probably is due to the homogeneously distributed substance and the non-enzymatically formed ethyl-carbonium ions which have reacted with the tissues. The blood-brain barrier seemed to have a capacity to partially prevent the uptake of the substance in the central nervous system. A high radioactivity was observed in the liver, which may imply that N-[14C]ethyl-N-nitrosourea is enzymatically decomposed in this tissue. An observed labelling of kidneys may be connected with urinary excretion of radioactivity. The radioactivity in the liver and kidney decreased at later survival intervals and a distribution pattern appeared, which was characterized by a labelling of tissues with a high protein or steroid synthesis and of fat containing tissues. The distribution pattern corresponded to the one seen after the administration of [14C]acetaldehyde and is probably due to normal biosynthetic incorporation of radioactivity in the 2-carbon pool. Pretreatments with pyrazole, nialamide and diethyldithiocarbamate caused a marked inhibition of the exhalation of 14CO2 and of the incorporation of radioactivity in the liver. This effect may be directed towards the decomposition of N-[14C]ethyl-N-nitrosourea itself, but an effect on the metabolism of formed 2-carbon fragments is also possible. The incorporation of radioactivity in other tissues was not influenced by the pretreatments.