N-nitrosamino Acids Research Articles

Relevance of N-nitrosamines to esophageal cancer in China.

Studies on the relevance of the N-nitrosamines to esophageal cancer in China are reviewed. Esophageal cancer is a complex and multifactorial problem. Although a causal association between nitrosamines exposure and esophageal cancer in China has not yet been rigorously established, exposure of Lin-Xian subjects to nitrosamines either directly or as a result of their in vivo formation has been detected in our study. Several N-nitrosamines (NDMA, NDEA, NMBzA, NPyr, NPip, and NSAR) in gastric juice collected from Lin-Xian inhabitants have been detected. A correlation was found between the lesions of esophageal epithelium and the amount of nitrosamines present. In addition, the amounts of N-nitrosamino acids (N-nitrosoproline, N-nitrosothiazolidine 4-carboxylic acid, NSAR, and nitrates) excreted in 24-hr urine of subjects in Lin-Xian were significantly higher than those in Fan-Xian, indicating a higher exposure to N-nitroso compound and their precursors of the inhabitants in the high-risk area. The effect of nitrosamines on human esophagus has been investigated at the cellular levels. The amounts of O6-MedG in DNA of esophageal or stomach mucosa of patients from Lin-Xian were higher than that from Europe (Lyon and Essen). The presence of O6-MedG in the human fetal esophagus cultured with NMBzA was also detected. These findings indicate that the elevated levels of O6-MedG in esophageal DNA could be the result of a recent exposure to N-nitroso compounds or a genetically determined reduced cellular capacity for repair of O6-MedG from DNA. The hyperplasia was induced in the esophagus of human fetus that cultured with NMBzA for 2 weeks to 2 months. The intervention studies of esophageal cancer in Lin-Xian have been pursued. Intake of moderate doses of ascorbic acids by Lin-Xian subjects effectively reduced the urinary levels of N-nitrosamino acids to those found in un-dosed subjects in the low-risk area. If N-nitroso compounds are formed in vivo and are among the causative factors of esophageal cancer in Lin-Xian, ascorbic acid appears to be effective in lowering the body burden of these carcinogenic compounds. Thus, the plan of chemoprevention is carried out in Lin-Xian.

Identification and occurrence of two new N-nitrosamino acids in tobacco products: 3-( N-nitroso- N-methylamino) propionic acid and 4-( N-nitroso- N-methylamino) butyric acid

Two new N-nitrosamino acids, 3-( N-nitroso- N-methylamino)propionic acid (CAS: 10478-42-9) and 4-( N-nitroso- N-methylamino)butyric acid (CAS: 61445-55-4) were isolated and identified for the first time in various types of tobacco, including snuff, chewing and pipe tobacco, cigars and cigarettes. Their levels ranged from 0.15 to 7.4 and 0 to 2.2 mg/kg of dry weight tobacco, respectively. For comparison, amounts of other N-nitrosamino acids like N-nitrosoproline (NPRO) and tobacco-specific-nitrosamines (TSNA) were determined in the same samples. The levels of N-nitrosamino acids were highly correlated with the levels of TSNA.

Tobacco-specific and betel nut-specific N-nitroso compounds: occurrence in saliva and urine of betel quid chewers and formation in vitro by nitrosation of betel quid.

In order to evaluate exposure of betel quid chewers to N-nitroso compounds, saliva and urine samples were collected from chewers of betel quid with or without tobacco, from tobacco chewers, from cigarette smokers and from people with no such habit, and were analysed for the presence of N-nitrosamines by gas chromatography coupled with Thermal Energy Analyzer and alkaloids derived from betel nut and tobacco by capillary gas chromatography fitted with nitrogen-phosphorous selective detector. The levels of the betel nut-specific nitrosamines, N-nitrosoguvacoline and N-nitrososoguvacine (the latter being detected for the first time in saliva), ranged from 0 to 7.1 and 0 to 30.4 ng/ml, respectively. High levels of tobacco-specific nitrosamines were detected in the saliva of chewers of betel quid with tobacco and in that of chewers of tobacco, ranging from 1.6 to 59.7 (N'-nitrosonornicotine), 1.0 to 51.7 (N'-nitrosoanatabine) and 0 to 2.3 [4-(methyl-nitrosamino)-1-(3-pyridyl)-1-butanone] ng/ml. Urinary concentrations of certain N-nitrosamino acids, including N-nitrosoproline, were determined as a possible index of exposure to nitroso compounds and their precursors in the study groups: no clear difference was observed. The betel nut-specific alkaloid, arecoline, was present at high levels in the saliva of betel quid chewers with or without tobacco. Nicotine and cotinine were also detected in saliva and urine of chewers of tobacco and of betel quid with tobacco. In order to assess whether N-nitroso compounds are formed in vivo in the oral cavity during chewing or in the stomach after swallowing the quids, the levels of N-nitroso compounds in betel quid extracts were determined before and after nitrosation at pH 7.4 and 2.1. The results indicate that N-nitroso compounds could easily be formed in vivo. The possible role of N-nitroso compounds in the causation of cancer of the upper alimentary tract in betel quid chewers is discussed.

Occurrence in human urine of new sulphur-containing N-nitrosamino acids N-nitrosothiazolidine 4-carboxylic acid and its 2-methyl derivative, and their formation.

To quantitate endogenous nitrosation reactions in man, the quantity of N-nitrosoproline (NPRO) excreted in the urine after ingestion of proline and/or nitrate was estimated. When this monitoring method (NPRO test) was applied in clinical and field studies, several hitherto unidentified N-nitroso compounds were frequently detected. These were recently identified as sulphur-containing N-nitrosamino acids, N-nitrosothiazolidine 4-carboxylic acid (NTCA), and trans- and cis-isomers of N-nitroso-2-methylthiazolidine 4-carboxylic acid (NMTCA). NTCA and NMTCA were readily formed in vitro following nitrosation at acidic pH of the respective precursor, thiazolidine 4-carboxylic acid (TCA) or of 2-methylthiazolidine 4-carboxylic acid (MTCA). As the latter compounds can be formed by reaction of L-cysteine with formaldehyde or acetaldehyde, respectively, NTCA and NMTCA were also formed by reacting L-cysteine with the respective aldehyde and with nitrite at optimal pH (2.5 for NTCA and 4.5 for NMTCA). Up to 95% of NTCA and NMTCA given orally to fasted rats was recovered as such in urine and faeces within 2 days. Administration of TCA or MTCA, together with nitrite increased the urinary excretion of NTCA and NMTCA, as did co-administration of L-cysteine, nitrite, and the respective aldehyde. NTCA and NMTCA were also detected in the 24-h urine of human volunteers, and smokers tended to excrete higher levels than nonsmokers. Daily excretion levels varied, however, and a diet supplemented with ascorbic acid significantly decreased the total amount of nitrosamino acids. NTCA and NMTCA may occur in human urine as a result of (i) intake of preformed N-nitroso compounds; (ii) intake of thiazolidine 4-carboxylic acid or its 2-methyl derivative and subsequent nitrosation in vivo; (iii) endogenous two-step synthesis by the reaction of L-cysteine with the respective aldehyde and a nitrosating agent. Thus, measurement of NTCA and NMTCA together with NPRO in urine may provide an index for the exposure of human subjects to nitrosamines or their precursors, i.e., nitrosating agents, certain aldehydes, or aldehyde-generating compounds. Our data demonstrate unequivocally that N-nitroso compounds are formed in the human body, as suggested previously by Druckrey. Their relevance to human cancer at specific sites should now be investigated.

Further validation of the SOS chromotest as a primary screening tool for genotoxins

A high gastric cancer mortality in Fujian province (Peoples Republic of China) has been associated with the consumption of certain salted fermented fish products such as fish sauce (FS). We have investigated the levels and nature of N-nitroso compounds (NOC) and genotoxins present, before and after nitrosation, in 49 FS samples collected from villages in this high-risk area, pooled into six samples. The concentrations of total NOC before nitrosation ranged from 0.2 to 16 μmoles/1, and after nitrosation at pH 2 and pH 7, they rose by up to 4800- and 100-fold, respectively. In nitrosated samples, 40–50% of total NOC was not extractable into organic solvents; volatile N nitrosamines accounted for 1–2% and N-nitrosamino acids for 8–16% of total NOC.None of the FS samples exhibited genotoxic activity, but after nitrosation all were weakly active in the SOS chromotest. The highest SOS-inducing potency was observed with nitrosated ethyl acetate extracts of most samples. The formation of methylating agents was measured by incubation of nitrosated FS with DNA and subsequent analysis of 7-methylguanine adduct. 2 of the 6 nitrosated FS samples caused a slight increase in DNA methylation. 1 pooled home-made FS sample (the only one tested) contained tumour promoter-like substances, as measured by expression of certain EBV genes in Raji cells.HPLC fractionation of ethyl acetate extracts of FS samples allowed identification of three UV-absorbing peaks that, upon nitrosation, produced direct-acting genotoxins. This genotoxicity was partly ascribed to the formation of nitrite-derived arene diazonium cations that were characterized by a coupling reaction with N-ethyl-1-naphthylamine and thin-layer chromatography.The presence of nitrite-dependent genotoxins and tumour promoter-like substances supports the hypothesis that FS consumption is a risk factor for gastric cancer in certain areas of China.

Monitoring N-nitrosamino acids excreted in the urine and feces of rats as an index for endogenous nitrosation.

A simple and sensitive method for the quantitative estimation of endogenous N-nitrosation in rats has been developed. This approach is based on the findings that N-nitrosamino acids (e.g., nitrosoproline (NPRO), nitrosohydroxyproline (NHPRO) and nitrososarcosine (NSAR)) when administered orally to rats, are excreted unchanged almost quantitatively (88-96% of the dose) in the urine and feces. After sequential administration of a nitrosatable amino acid and sodium nitrite the nitrosamino acid excreted in the urine and feces was analyzed. The amount of NPRO excreted in the urine of rats was proportional to the dose of proline and to the square of the nitrite dose. Co-administration of ascorbic acid and alpha-tocopherol together with the precursors decreased the urinary NPRO whereas thiocyanate increased the yield. After feeding an amino acid precursor and nitrite, the yield of nitrosamino acids formed in vivo and excreted in the urine increased in the order: NPRO less than NSAR less than NHPRO. The same order was seen when the nitrosation rates of the amino acids in vitro were compared. Thus N-nitrosation in vivo in rats occurs via a similar mechanism as observed in vitro. Monitoring of N-nitrosamino acids excreted in the urine and feces thus appears to provide a valuable index for endogenous N-nitrosation.

Chiroptical properties of n-nitrosopyrrolidines and n-nitrosamino acids : Implications for the nitrosamine sector rule

CD data for a variety of N-nitrosamino acids and N-nitrosopyrrolidines are presented. The effects of nitrosamino group conformation, pyrrolidine ring geometry, different perturbing substituents, and especially intramolecular H-bonding upon the n → π* CD band are discussed. Stereochemical conclusions can be made with confidence in many cases, although no sector diagram, as yet published, successfully correlates all the available chiroptical data in this series of compounds. However, a negative CD band due to the π → π* transition was observed tor all N-nitrosamines having the L-proline configuration at C-2, regardless of nitroso group conformation; it is suggested that this band be used whenever possible for stereochemical correlations.

Detection and determination of N-Nitrosamino acids by thin/3-layer chromatography using fluorescamine

A novel procedure is described for the detection and determination of N-Nitrosamino acids (NAAs) on activated silica gel thin-layer chromatographic plates. N-Nitrososarcosine, N-nitrosoproline, and N-nitroso-4-hydroxyproline could be detected as fluorophors at the 200-pmole level (20–30 ng) after being irradiated with ultraviolet light and sprayed with fluorescamine reagent. Spectrophotometric determination of the relative fluorescence of 0.4–40 nmoles of NAAs gave rise to similar calibration curves when plotted on a log—log scale. An application of this method to the detection o NAAs in uncooked bacon is described.

Occurrence of N-nitrosamino acids in cured meat products and their effect on formation of N-nitrosamines during heating

A method for the isolation and detection by gas chromatography of nitrosamino acids in foods has been developed: of 30 cured meat products examined, only two contained more than 100 ppb (b = 10 9) of a nitrosamino acid. The heat-induced decarboxylation of N-nitrososarcosine, N-nitrosoproline and N-nitroso-4-hydroxyproline was investigated. In silicone oil at 230°C, N-nitrosodimethylamine was formed at 90% of theoretical yield and N-nitrosopyrrolidine and N-nitroso-3-hydroxypyrrolidine were formed at 11 and 9% of theoretical yield, respectively. Frying ham in an open pan gave less than 3% of the theoretical yield for all nitrosamines. During the frying process, about 80% of the N-nitrosodimethylamine, 50% of the N-nitrosopyrrolidine and 5% of the N-nitroso-3-hydroxypyrrolidine formed were volatilized in the fumes. The data obtained clearly indicate that the decarboxylation of nitrosamino acids is not the essential pathway of nitrosamine formation during the frying of cured meat products.

A new method for nitrosation of proline and related sec- -amino acids to N-nitrosamino acids with possible oncogenic activity.

A new method for nitrosation of proline and related sec- -amino acids to N-nitrosamino acids with possible oncogenic activity.