Assessment of the risk of formation of carcinogenic N-nitroso compounds from dietary precursors in the stomach

Geographic association between urinary excretion of N-nitroso compounds and oesophageal cancer mortality in China

Overnight urine samples were collected from approximately 60 male adults in each of 69 counties of China in 1989. Two specimens were collected from each subject--one after a loading dose of proline and ascorbic acid and another after a loading dose of proline only. Levels of N-nitrosamino acids and nitrate were measured in urine samples and correlated with cumulative mortality rates for subjects aged between 0 and 64 years in the 1970s. Oesophageal cancer mortality rates were positively and significantly associated with (i) urinary levels of excreted N-nitrosoproline (NPRO) (after proline and ascorbic acid loading or proline loading only), (ii) N-nitrososarcosine levels, and (iii) nitrosation potential (the decrease in the amount of urinary NPRO after adding ascorbic acid to the proline load). There were also positive correlations between the urinary level of NPRO or other N-nitrosamino acids and that of nitrate. The urinary excretion of nitrate was associated with consumption of various nitrate-rich vegetables. The results suggest that N-nitroso compounds (NOC) or other nitrite-derived carcinogens are implicated in the aetiology of oesophageal cancer in China.

Inhibition of nitrosation

Humans are exposed through ingestion or inhalation to preformed N-nitroso compounds (NOC) in the environment and through the endogenous nitrosation of amino precursors in the body. Activated macrophages and bacterial strains isolated from human infections can enzymatically produce nitrosating agents and NOC from precursors at neutral pH. As a consequence, endogenous nitrosation may occur at various sites of the body, such as the oral cavity, stomach, urinary bladder, and at other sites of infection or inflammation. Numerous substances to which humans are exposed have been identified and shown to inhibit formation of NOC. Such inhibitors include vitamins C and E, certain phenolic compounds, and complex mixtures such as fruit and vegetable juices or other plant extracts. Nitrosation inhibitors normally destroy the nitrosating agents and, thus, act as competitors for the amino compound that serves as substrate for the nitrosating species. Independently, epidemiological studies have already established that fresh fruits and vegetables that are sources of vitamin C, other vitamins, and polyphenols have a protective effect against cancers at various sites and in particular gastric cancer. This article briefly reviews (a) the chemistry of NOC formation and inhibition; (b) the studies in experimental animals that showed that inhibition of endogenous NOC synthesis leads to a reduction of toxic, mutagenic, and carcinogenic effects; (c) recent studies in humans where the degree of inhibition of endogenous NOC synthesis was directly quantified; and (d) the possible contribution of nitrosation inhibitors to human cancer prevention.

Inhibitory effect of diet related sulphydryl compounds on the formation of carcinogenic nitrosamines

N-Nitroso compounds (NOCs) are known to be strong carcinogens in various animals including primates (Preussman and Stewart, (1984) N-Nitroso Compounds). Human exposure to these compounds can be by ingestion or inhalation of preformed NOCs or by endogenous nitrosation from naturally occurring precursors (Bartsch and Montesano, Carcinogenesis, 5 (1984) 1381-1393; Tannebaum (1979) Naturally Occuring Carcinogens, Mutagens and Modulators of Carcinogenesis; Shephard et al., Food Chem. Toxicol., 25 (1987) 91-108). Several factors present in the diet can modify levels of endogenously formed nitrosamines by acting as catalysts or inhibitors. Compounds in the human diet that alter nitrosamine formation would thus play an important role in carcinogenesis study. Earlier researchers have reported the nitrite scavenging nature of sulphydryl compounds (Williams, Chem. Soc. Rev., 15 (1983) 171-196). We therefore studied the modifying effect of sulphydryl compounds viz., cysteine (CE), cystine (CI), glutathione (GU), cysteamine (CEA), cystamine (CEI), cysteic acid (CIA) and thioglycolic acid (TGA) on the nitrosation of model amines viz., pyrrolidine (PYR), piperidine (NPIP) and morpholine (NMOR). Many of these compounds are present in the food we consume. The present work also describes the inhibitory effect of onion and garlic juices on the nitrosation reactions. Both onion and garlic are known to contain sulphur compounds (Block, Sci. Am., 252 (1985) 114-119). Most of these compounds behave as antinitrosating agents and their inhibitory activity towards formation of carcinogenic nitrosamines, under different conditions is described.

Nitrosamine excretion in patients with continent ileal reservoirs for urinary diversion

Adenocarcinomas are a recognized complication following ureterosigmoidostomy for which the endogenous formation of N-nitroso compounds may be a risk factor. As an alternative means of urinary diversion, the continent ileal reservoir has recently been developed. Microbiological and chemical investigations on the urine of patients with an ileal reservoir showed the presence of bacteria, nitrate, nitrite and N-nitrosamines formed endogenously in the ileal pouch. The role of nitrosamines in carcinogenesis in these patients as a late stage complication resulting from the use of a continent ileal reservoir is discussed.

Review of the occurrence and formation of non-volatile N-nitroso compounds in foods

A review of the literature published prior to July 1991 covers the occurrence and formation of non-volatile N-nitrosamines occurring in foods and beverages. The presence of identified volatile and non-volatile N-nitrosamines accounts for less than 10% of the total apparent N-nitrosamine concentration. N-Nitrosoproline and N-nitrosothiazolidine-4-carboxylic acid are the most commonly identified non-volatile N-nitrosamines in the diet. Non-volatile N-nitrosamines account for 12 of the 21 currently identified N-nitroso compounds in foods and beverages.

Mean daily intake of primary and secondary amines from foods and beverages in West Germany in 1989-1990

264 samples of foods and beverages from the German market in 1989-1990 were analysed for primary and secondary amines. Amines were determined as benzenesulphonyl chloride derivatives by gas chromatography with chemiluminescense detection. The mean daily intake of primary amines was calculated to be 29 mg/day for women and 37 mg/day for men. For secondary amines, the mean daily intake was 6 mg/day for women and 8 mg/day for men.

Ethyl carbamate: analytical methodology, occurrence, formation, biological activity and risk assessment

Ethyl carbamate (EC) is a genotoxic compound in vitro and in vivo, it binds covalently to DNA and is an animal carcinogen. Today, EC is mainly found as a natural trace constituent in different alcoholic beverages and in fermented food items. Data on analytical methodology and the levels of EC in different food items are summarized and the daily burden of humans is estimated. Under normal dietary habits excluding alcoholic beverages, the unavoidable daily intake is 10-20 ng/kg b.w. On the basis of the evaluation of all toxicity data and its mode of action a conventional risk assessment of EC indicates that this level represents a negligible lifetime cancer risk (less than 0.0001%). Individual habits may greatly enhance the risk. Regular drinking of table wine (500 ml/day) would increase the risk up to 5 times, regular drinking of stone-fruit distillates (20-40 ml/day) would raise the calculated hypothetical tumor risk to near 0.01%. Human exposure to carcinogenic compounds should be as low as reasonably achievable. In order to take reliable measures to reduce EC levels in beverages and foods, it is crucial to know the mode of its formation. For its natural formation the presence of ethanol is absolutely required. In stone-fruit distillates hydrogen cyanide together with photochemically active substances are crucial to form EC. The main part of EC is formed after the distillation involving photochemical reactions. In wine (and probably bread) significant EC formation seems to depend on heat treatment. While in distillates hydrogen cyanide is the most important single precursor, in wine different carbamyl compounds, mainly urea, seem to be involved in EC formation. Despite this apparent difference a common EC formation pathway is discussed for all alcoholic beverages by assuming cyanic-/isocyanic acid as an important ultimate reactant with ethanol. Some ideas are presented as to the possible course of future work.

Is hypergastrinaemia dangerous for man?

Several gastric mucosal diseases and the response to powerful gastric secretory inhibitors are accompanied by hypergastrinaemia. When the gastric mucosa is functioning normally, hypergastrinaemia may be dangerous as a consequence of gastric hypersecretion. When the gastric mucosa is functionally abnormal, hypergastrinaemia produces no apparent adverse effects on health. However, the diseases giving rise to hypergastrinaemia are very dangerous because they are often the precursors of gastric cancer. In view of the probable mechanisms of gastric carcinogenesis, it is argued that therapeutic achlorhydria is also potentially dangerous irrespective of the absence or presence of 'hypergastrinaemia'.

Nitrates, nitrites and N-nitrosocompounds: a review of the occurrence in food and diet and the toxicological implications

Data on occurrence of nitrate, nitrite and N-nitrosocompounds in food and drinking water, and on total dietary intakes are reviewed. Metabolic, toxicological and epidemiological studies are surveyed and the implications with respect to safety evaluation are addressed. It is concluded that, on the basis of recent long-term animal studies and of clinical experience in man, the current Acceptable Daily Intake (ADI) allocated to nitrate by the Joint FAO/WHO Expert Committee on Food Additives of 0-5 mg/kg body weight/day (expressed as sodium nitrate) might be increased to 0-25 mg/kg body weight/day. Based on similar criteria, the ADI for nitrite would be 0-0.1 mg/kg body weight/day (expressed as sodium nitrite). In view of the known carcinogenicity of N-nitrosocompounds, exposure to these compounds in food should be minimized by appropriate technological means, such as lowering the nitrite concentration in preserved foods to the minimum required to ensure microbiological safety and use of inhibitors of nitrosation like alpha-tocopherol or ascorbic acid. Further work is needed to define the minimal levels of nitrite in foods needed to inhibit outgrowth of Clostridium botulinum and toxin production.

Endogenous genotoxic agents and processes as a basis of spontaneous carcinogenesis

A list of endogenous DNA-damaging agents and processes is given. Endogenous electrophiles are found with the cosubstrates of physiological transfer reactions (S-adenosylmethionine for methylation, ATP for phosphorylation, NAD+ for ADP-ribosylation, acetyl CoA for acetylation). Aldehyde groups (glyceraldehyde-3-phosphate, formaldehyde, open forms of reducing sugars, degradation products of peroxidation) or alkylating degradation products derived from endogenous nitroso compounds represent additional possibilities. Radical-forming reactions include leakage of the superoxide anion radical from terminal cytochromes and redox cycles, hydroxyl radical formation by the Fenton reaction from endogenous hydrogen peroxide, and the formation of lipid peroxides. Genetic instability by spontaneous deaminations and depurinations as well as replicative instability by tautomer errors and in the presence of mutagenic metal ions represent a third important class of endogenous genotoxic processes. The postulated endogenous genotoxicity could form the mechanistic basis for what is called 'spontaneous' tumor incidence and explain the possibility of an increased tumor incidence after treatment of animals with non-genotoxic compounds exhibiting tumor-promoting activity only. Individual differences are expected to be seen also with endogenous DNA damage. The presence of endogenous DNA damage implies that exogenous DNA-carcinogen adducts give rise to an incremental damage which is expected to be proportional to the carcinogen dose at lowest levels. An increased tumor risk due to exposure to exogenous genotoxic carcinogens could therefore be assessed in terms of the background DNA damage, for instance in multiples of the mean level or of the interindividual variability in a population.

Nitrosation of aspartic acid, aspartame, and glycine ethylester. Alkylation of 4-(p-nitrobenzyl)pyridine (NBP) in vitro and binding to DNA in the rat

In a colorimetric assay using 4-(p-nitrobenzyl)pyridine (NBP) as a nucleophilic scavenger of alkylating agents, the nitrosation and alkylation reactions were investigated for a number of amino acids and derivatives. The alkylating activity increased with the square of the nitrite concentration. The nitrosation rate constants for aspartic acid, aspartame, and glycine ethylester (= precursors C) were 0.08, 1.4 and less than or equal to 0.2, respectively, expressed in terms of the pH-dependent k2 rate constant of the equation dNOC/dt = k2.[C].[nitrite]2. The rates correlated inversely with the basicity of the amino group. The stability of the alkylating activity was astonishingly high, both in acid and at neutral pH. Half-lives of 500, 200, and 30 min were determined for aspartic acid (pH 3.5), aspartame (pH 2.5), and glycine ethylester (pH 2.5). Values of 60, 15, and 2 min, respectively, were found at pH 7. It is concluded that rearrangement of the primary N-nitroso product to the ultimate alkylating agent could be rate-limiting. The potential of nitrosated alpha-amino acids to bind to DNA in vivo was investigated by oral gavage of radiolabelled glycine ethylester to rats, followed immediately by sodium nitrite. DNA was isolated from stomach and liver and analysed for radioactivity and modified nucleotides. No indication of DNA adduct formation was obtained. Based on an estimation of the dose fraction converted from glycine ethylester to the nitroso product under the given experimental conditions, the maximum possible DNA-binding potency of nitroso glycine ethylester is about one order of magnitude below the methylating potency of N-nitrosomethylurea in rat stomach. The apparent discrepancy to the in vitro data could be due to efficient detoxification processes in mammalian cells.

Therapeutic achlorhydria and risk of gastric cancer

New, powerful gastric secretory inhibitors, such as omeprazole, produce gastric cancer in rats. The mechanism by which the drugs elicit gastric carcinogenesis is considered to depend on the production of therapeutic achlorhydria, with subsequent release in to the circulation of peptides (such as gastrin) which are trophic to the gastric mucosa. It has been argued that the drugs do not pose a carcinogenic risk to man because the neoplastic response to gastric inhibitors in rats is a reaction to a 'toxic' insult; or because rats and humans react differently to the drugs; or because the mechanisms of gastric carcinogenesis are different in the two species. In any case, since most of the powerful gastric secretory inhibitors produce carcinoid tumours in rats, and carcinoid tumours of the human stomach are rare and largely benign, there would be no risk even if the drugs did produce proliferative abnormalities of the human stomach. Not one of the above hypotheses has been confirmed or, indeed, even satisfactorily tested. The mechanisms of the drug-induced gastric carcinogenesis in rats has not been defined and consequently it is not even possible to attempt to guess the risk to man. Until information is available about the effects of the powerful gastric secretory inhibitors on the proliferative indices and patterns of the human gastric mucosa, the drugs must be categorized as too dangerous to use therapeutically, especially since the proposed therapeutic benefits are minimal.

Trapping of reactive intermediates from the nitrosation of primary amines by a new type of scavenger reagent

N-Methyl-N-nitrosoethanolamine has been used as a scavenger reagent for the in vitro detection of unstable alkylating intermediates. The nucleophilic hydroxy group of the scavenger reagent binds with electrophiles to yield chemically stable ethers, which are analysed using gas-liquid chromatography with the highly sensitive and specific chemiluminescence detector, the thermal energy analyser. In contrast to other test systems for the in vitro detection of alkylating species, non-physiological reaction conditions are avoided and direct conclusions on the structure of the reaction products can be drawn. This test system is shown to be effective for investigating the alkylating capacity of primary amines on nitrosation. From methylamine to butylamine the yield of alkylated scavenger is low, decreasing with elongation of the carbon chain. In contrast, nitrosation of aniline, benzylamine or phenylethylamine leads to much greater yields of alkylated scavenger. These compounds should therefore be included in risk assessment of endogenous nitrosation of primary amine precursors.

Formation of direct-acting genotoxic substances in nitrosated smoked fish and meat products: identification of simple phenolic precursors and phenyldiazonium ions as reactive products

Epidemiological studies have associated the consumption of smoked fish and meat products with an increased risk of stomach cancer. Therefore, the reaction of such smoked foods with nitrite under acidic conditions was investigated and was shown to produce potent direct-acting genotoxic substances as detected by the SOS Chromotest. Similar genotoxic activity was observed in nitrosated samples of wood-smoke condensates. Simple phenolic compounds such as phenol, 3-methoxycatechol, catechol and vanillin were identified as the precursors of the genotoxic substances. These phenolic compounds also exhibited direct-acting genotoxicity after nitrosation. The major genotoxic substances formed after nitrosation of phenol were isolated and identified as 4- and 2-hydroxyphenyldiazonium ions. Nitrosation of various wood-smoke condensates was found to generate the same type of diazonium compounds, which in part account for the genotoxicity of nitrosated smoked foods.

Inhibitors of endogenous nitrosation. Mechanisms and implications in human cancer prevention

Although the proof that N-nitroso compounds (NOC), a versatile class of carcinogens in animals, are also carcinogenic in man is lacking, humans are exposed through ingestion or inhalation to preformed NOC in the environment and through the endogenous nitrosation of amino precursors in the body. Activated macrophages can synthesize nitrate, nitrite and nitrosating agents that can form NOC. A number of bacterial strains isolated from human infections can produce NOC enzymatically from precursors at neutral pH. As a consequence endogenous nitrosation may occur at various sites of the body such as the oral cavity, stomach, urinary bladder, lungs, and at other sites of infection or inflammation. Since the demonstration by Mirvish et al. (1972) showing that ascorbate can reduce tumor formation in animals following feeding of nitrite plus amine, numerous substances to which humans are exposed have been identified and shown to inhibit formation of NOC in vitro, in animal models and in humans. Such inhibitors of nitrosation include vitamins C and E, phenolic compounds, and complex mixtures such as fruit and vegetable juices or other plant extracts. Nitrosation inhibitors normally destroy the nitrosating agents and thus act as competitors for the amino compound that serves as substrate for the nitrosating species. Independently, epidemiological studies have already established that fresh fruits and vegetables that are sources of vitamin C, other vitamins and polyphenols have a protective effect against cancers at various sites and in particular gastric cancer. Although the evidence that endogenously formed NOC are involved in human cancers is far from conclusive, it is suggestive and justifies preventive measures for reducing exposure to NOC. This article briefly reviews (i) the chemistry of NOC formation and inhibition, (ii) the studies in experimental animals which showed that inhibition of endogenous NOC synthesis leads to a reduction of toxic, mutagenic and carcinogenic effects, (iii) recent studies in humans where the degree of inhibition of endogenous NOC synthesis was directly quantified and lastly (iv) the contribution of nitrosation inhibitors to human cancer prevention.

Augmenting effect of a nonmutagenic fraction in soy sauce on mutagenicity of 3-diazotyramine produced in the nitrite-treated sauce

When 25 kinds of Japanese soy sauce at a concentration of 5% were incubated with 50mM sodium nitrite (pH 2) at 37 degrees for 1 hr, the reaction mixtures induced 34-834 (average 368 +/- 228) revertants per microliter of soy sauce equivalent in Salmonella typhimurium strain TA100 in the absence of S9 mix. The mutagen(s) formed was very unstable under natural daylight and a fluorescent lamp but quite stable under a yellow lamp as well as in the dark. In addition to the known precursors, i.e., tyramine and 1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid, 1-methyl-1,2,3,4-tetrahydro-beta-carboline, which caused weak mutagenesis, was found in the soy sauce. However, the sum of the activities of the three mutagen-precursors after nitrite treatment accounted for only a part of the mutagenicity of nitrite-treated soy sauce. There was in the soy sauce a factor which increased ninefold the mutagenicity of nitrite-treated tyramine, 3-diazotyramine. Therefore, tyramine was considered the principal precursor of the mutagen produced in the nitrite-treated soy sauce. These three precursors together with the mutagenicity augmentation accounted for all the mutagenicity of nitrite-treated sauce. The mutagenicity-augmenting factor in the soy sauce was nonmutagenic before and after nitrite treatment and was stable to heat and light irradiation.

Curcumins as inhibitors of nitrosation in vitro

The effects of turmeric extract and its pure yellow pigments curcumin I, II and III were tested on the nitrosation of methylurea by sodium nitrite at pH 3.6 and 30 degrees C. The nitrosomethylurea formed was monitored by checking the mutagenicity in S. typhimurium strains TA1535 and TA100 without metabolic activation. Turmeric extract as well as curcumins exhibit dose-dependent decreases of nitrosation. Curcumin III was the most effective nitrosation inhibitor among the compounds tested. The simultaneous treatment of inhibitor with nitrosation precursors was essential and pre- or post-treatment of inhibitor had no effect on the mutagenicity of nitrosomethylurea. The binding of nitrite with the inhibitors was studied at pH 3.6 and 30 degrees C. Curcumin I shows a dose-dependent depletion of nitrite ions thus making nitrite non-available for nitrosation. Curcumin I and III when tested also showed a time-dependent depletion of nitrite ions at pH 3.6 and 30 degrees C. Curcumin III has a higher affinity for nitrite ions than curcumin I.

The inhibitory effect of thioproline on carcinogenesis induced by N-benzylmethylamine and nitrite

Thioproline, which is readily nitrosated to form nitrosothioproline, is expected to act as a nitrite scavenger. The effect of thioproline as an inhibitor of the carcinogenesis induced by N-nitroso-N-benzylmethylamine precursors was examined. Two groups of male F-344 rats were given diet containing 0.25% N-benzylmethylamine (group I) or 0.25% N-benzylmethylamine plus thioproline (0.25% until wk 17 and then 0.5%; group II). Both groups were given drinking-water containing sodium nitrite (0.1% until wk 17 and then 0.2%). The experiment was continued for 717 days. Squamous cell carcinoma of the forestomach developed in six out of seven rats in group I and in significantly fewer, two out of nine rats, in group II. The degree of invasion by the tumours was also less in group II rats, given thioproline, than in group I. Thus thioproline suppressed carcinogenesis induced by N-benzylmethylamine and nitrite, possibly by inhibiting the in vivo nitrosation of N-benzylmethylamine.