A gas chromatography-mass spectrometry method for the quantitation of N-nitrosoproline and N-acetyl-S-allylcysteine in human urine: Application to a study of the effects of garlic consumption on nitrosation

Biomarkers in urine can provide useful information about the bioactivation of chemical carcinogens and can be used to investigate the chemoprotective properties of dietary nutrients. N-Nitrosoproline (NPRO) excretion has been used as an index for endogenous nitrosation. In vitro and animal studies have reported that compounds in garlic may suppress nitrosation and inhibit carcinogenesis. We present a new method for extraction and sensitive detection of both NPRO and N-acetyl-S-allylcysteine from urine. The latter is a metabolite of S-allylcysteine, which is found in garlic. Urine was acidified and the organic acids were extracted by reversed-phase extraction (RP-SPE) and use of a polymeric weak anion exchange (WAX-SPE) resin. NPRO was quantified by isotope dilution gas chromatography-mass spectrometry (GC-MS) using [¹³C₅]NPRO and N-nitrosopipecolinic acid (NPIC) as internal standards. This method was used to analyze urine samples from a study that was designed to test whether garlic supplementation inhibits NPRO synthesis. Using this method, 2.4 to 46.0 ng NPRO/ml urine was detected. The method is straightforward and reliable, and it can be performed with readily available GC-MS instruments. N-Acetyl-S-allylcysteine was quantified in the same fraction and detectable at levels of 4.1 to 176.4 ng/ml urine. The results suggest that 3 to 5 g of garlic supplements inhibited NPRO synthesis to an extent similar to a 0.5-g dose of ascorbic acid or a commercial supplement of aged garlic extract. Urinary NPRO concentration was inversely associated with the N-acetyl-S-allylcysteine concentration. It is possible that allyl sulfur compounds found in garlic may inhibit nitrosation in humans.     

Effect of ascorbic acid and green tea on endogenous formation of N-nitrosodimethylamine and N-nitrosopiperidine in humans.

Many constituents present in the human diet may inhibit endogenous formation of N-nitroso compounds (NOC). Studies with human volunteers showed inhibiting effects of intake of ascorbic acid and green tea consumption on nitrosation using the N-nitrosoproline test. The aim of the present study was to evaluate the effects of ascorbic acid and green tea on urinary excretion of carcinogenic N-nitrosodimethylamine (NDMA) and N-nitrosopiperidine (NPIP) in humans. Twenty-five healthy female volunteers consumed a fish meal rich in amines as nitrosatable precursors in combination with intake of nitrate-containing drinking water at the Acceptable Daily Intake level during 7 consecutive days. During 1 week before and after nitrate intake a diet low in nitrate was consumed. Using the same protocol, the effect of two different doses of ascorbic acid (250 mg and 1 g/day) and two different doses of green tea (2 g and 4 g/day) on formation of NDMA and NPIP was studied. Mean nitrate excretion in urine significantly increased from control (76+/-24) to 167+/-25 mg/24 h. Intake of nitrate and fish resulted in a significant increase in mean urinary excretion of NDMA compared with the control weeks: 871+/-430 and 640+/-277 ng/24 h during days 1-3 and 4-7, respectively, compared with 385+/-196 ng/24 h (p<0.0002). Excretion of NPIP in urine was not related to nitrate intake and composition of the diet. Intake of 250 mg and 1 g of ascorbic acid per day resulted in a significant decrease in urinary NDMA excretion during days 4-7 (p=0.0001), but not during days 1-3. Also, consumption of four cups of green tea per day (2 g) significantly decreased excretion of NDMA during days 4-7 (p=0.0035), but not during days 1-3. Surprisingly, consumption of eight cups of green tea per day (4 g) significantly increased NDMA excretion during days 4-7 (p=0.0001), again not during days 1-3. This increase is probably a result of catalytic effects of tea polyphenols on nitrosation, or of another, yet unknown, mechanism. These results suggest that intake of ascorbic acid and moderate consumption of green tea can reduce endogenous NDMA formation.     

Levels of direct-acting mutagens, total N-nitroso compounds in nitrosated fermented fish products, consumed in a high-risk area for gastric cancer in southern China.

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 mumoles/l, 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.     

Genotoxic and carcinogenic risk to humans of drug-nitrite interaction products

The large majority of N-nitroso compounds (NOC) have been found to produce genotoxic effects and to cause tumor development in laboratory animals; four NOC have been classified by the International Agency for Research on Cancer (IARC) as probably and another 15 as possibly carcinogenic to humans. A considerable fraction of drugs are theoretically nitrosatable due to the presence of amine, amide or other groups which by reacting with nitrite in the gastric environment, or even in other sites, can give rise to the formation of NOC, and in some cases other reactive species. This review provides a synthesis of information on the chemistry of NOC formation, the carcinogenic activity of NOC in animals and humans and the inhibitors of nitrosation reactions. It contains information on the drugs which have been tested for the formation of NOC by reaction with nitrite and the genotoxic-carcinogenic effects of their nitrosation products. In an extensive search we have found that 182 drugs, representing a wide variety of chemical structures and therapeutic activities, were examined in various experimental conditions for their ability to react with nitrite, and 173 (95%) of them were found to form NOC or other reactive species. Moreover, 136 drugs were examined in short-term genotoxicity tests and/or in long-term carcinogenesis assays, either in combination with nitrite or using their nitrosation product, in order to establish whether they produce genotoxic and carcinogenic effects; 112 (82.4%) of them have been found to give at least one positive response. The problem of endogenous drug nitrosation is largely unrecognized. Only a small fraction of theoretically nitrosatable drugs have been examined for the possible formation of genotoxic-carcinogenic NOC, guidelines for genotoxicity testing of pharmaceuticals do not indicate the need of performing the appropriate tests, and patients are not informed that the drug-nitrite interaction and the consequent risk can be reduced to a large extent by consuming the nitrosatable drug with ascorbic acid.     

Evidence for metabolism of N-nitrosoproline

The metabolism of nitrosoproline (NPRO) was re-investigated in uni- and bilaterally nephrectomized rats that have reduced or absent ability to excrete urine. About 1% of the administered radioactivity from L-[U-14C]-NPRO appeared as 14CO2 in the expired air and the production of 14CO2 was time-dependent over a period of 23 h. As compared with sham-operation, uni- or bilateral nephrectomy did not significantly increase the amount of NPRO metabolism, though urinary excretion of radioactivity was decreased in the unilaterally nephrectomized animals. In microsome-mediated and in vitro enzyme-free (Udenfriend-hydroxylating) systems covalent binding of [2,3,4,5-3H]NPRO to exogenous calf thymus DNA was demonstrated. The above findings confirm that in vivo metabolism of NPRO is possible, albeit, to a very small extent.     

Determination of methyl-, 2-hydroxyethyl- and 2-cyanoethylmercapturic acids as biomarkers of exposure to alkylating agents in cigarette smoke

Alkylating agents occur in the environment and are formed endogenously. Tobacco smoke contains a variety of alkylating agents or precursors including, among others, N-nitrosodimethylamine (NDMA), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), acrylonitrile and ethylene oxide. We developed and validated a method for the simultaneous determination of methylmercapturic acid (MMA, biomarker for methylating agents such as NDMA and NNK), 2-hydroxyethylmercapturic acid (HEMA, biomarker for ethylene oxide) and 2-cyanoethylmercapturic acid (CEMA, biomarker for acrylonitrile) in human urine using deuterated internal standards of each compound. The method involves liquid/liquid extraction of the urine sample, solid phase extraction on anion exchange cartridges, derivatization with pentafluorobenzyl bromide (PFBBr), liquid/liquid extraction of the reaction mixture and LC–MS/MS analysis with positive electrospray ionization. The method was linear in the ranges of 5.00–600, 1.00–50.0 and 1.50–900 ng/ml for MMA, HEMA and CEMA, respectively. The method was applied to two clinical studies in adult smokers of conventional cigarettes who either continued smoking conventional cigarettes, were switched to test cigarettes consisting of either an electrically heated cigarette smoking system (EHCSS) or having a highly activated carbon granule filter that were shown to have reduced exposure to specific smoke constituents, or stopped smoking. Urinary excretion of MMA was found to be unaffected by switching to the test cigarettes or stop smoking. Urinary HEMA excretion decreased by 46 to 54% after switching to test cigarettes and by approximately 74% when stopping smoking. Urinary CEMA excretion decreased by 74–77% when switching to test cigarettes and by approximately 90% when stopping smoking. This validated method for urinary alkylmercapturic acids is suitable to distinguish differences in exposure not only between smokers and nonsmokers but also between smoking of conventional and the two test cigarettes investigated in this study.     

The mutagenicity analysis of imidapril hydrochloride and its degradant,diketopiperazine derivative,nitrosation mixtures by in vitro Ames test with two strains of Salmonella typhimurium

AimThe evaluation of mutagenic properties of imidapril hydrochloride (IMD) and its degradation impurity, diketopiperazine derivative (DKP), nitrosation mixtures was conducted in order to analyze the carcinogenic risk of IMD long-term treatment in patients. In this study an in vitro Ames test with Salmonella enterica serovar Typhimurium TA 98 and TA 100 strains was used.BackgroundIMD and DKP contain nitrogen atoms, which makes them theoretically vulnerable to in vivo nitrosation with the production of N-nitroso compounds (NOC). NOC, in turn, are known animal mutagens indicating that their endogenous production from nitrosable drugs constitutes a carcinogenic hazard.Materials and methodsPure IMD sample was exposed to forced degradation conditions of increased temperature and dry air in order to achieve a DKP sample. Both samples were then treated with a nitrosating agent and the obtained nitrosation mixtures were subjected to mutagenicity analysis by the Ames test with S. typhimurium TA 98 and TA 100 strains in the presence and absence of metabolic activation system (S9 mix) using a commercial Ames MPF 98/100 microplate format mutagenicity assay kit.ResultsNone of the six concentrations of the investigated nitrosation mixtures exhibited any mutagenic potential in both S. typhimurium strains. The addition of S9 mix did not alter the non-mutagenic properties of the studied compounds.ConclusionsThe nitrite treatment of both studied compounds has no impact on their mutagenic properties under the conditions of the present studies. Hence, IMD and DKP nitrosation mixtures are classified as non-mutagens in this test.     

Cancer-preventive isothiocyanates: measurement of human exposure and mechanism of action

Numerous studies in rodents have documented the cancer-preventive activity of a significant number of isothiocyanates (ITCs), the majority of which occur in plants, especially in cruciferous vegetables. Dietary ITCs may play an important role in the prevention of human cancers. Several recent epidemiological studies have already shown that dietary consumption of ITCs inversely correlates with the risk of developing lung, breast and colon cancers. ITCs are principally metabolized through the mercapturic acid pathway in vivo, giving rise to N-acetylcysteine conjugates, which are excreted in the urine. Analytical methods have been developed to allow detection of ITCs and their metabolites formed in the mercapturic acid pathway. Studies show that total urinary level of ITC equivalent is an excellent biomarker of human exposure to ITCs. Moreover, these methods also have made it possible to learn the bioavailability of ITCs from cruciferous vegetables. ITCs possess multiple anticarcinogenic mechanisms, including inhibition of carcinogen-activating enzymes, induction of carcinogen-detoxifying enzymes, increase of apoptosis, arrest of cell cycle progression, as well as several other mechanisms that are not yet fully described. These mechanisms, which are discussed in detail in this review, illustrate the remarkable ability of ITCs to inhibit cancer development-effective against both developing and developed cancer cells.     

Diet and colorectal cancer prevention

The majority of cancers are sporadic and epidemiological estimates suggest that up to 80% of colorectal cancer is attributable to diet. Epidemiologically, cross-sectional comparisons, case-control studies and trends in food intake show high rates of colorectal cancer in populations consuming diets high in meat and fat, and low in starch, NSP (non-starch polysaccharides, fibre) and vegetables. In general, prospective studies tend to support these findings although estimates of relative risk are not high. Existing prospective studies have however used crude indices of diet subject to substantial measurement error, and interactions with genetic polymorphisms in, for example, phase-I and -II enzymes have been studied only rarely. The association between meat consumption and colorectal cancer is usually attributed to the formation of heterocyclic amines in meat when it is cooked. In addition, in humans high-meat diets increase the level of nitrosatable material entering the colon so that faecal N-nitroso compounds (NOCs) increase in a dose-responsive manner following endogenous synthesis in the colon. Some of the mutations and guanine adducts accumulated during colorectal cancer progression are characteristic of alkylative damage, which would be compatible with NOC exposure. To date, NSP, resistant starch and vegetables have not reduced faecal NOC levels.     

Nutrition,aging and cancer: lessons from dietary intervention studies

There is convincing epidemiological and clinical evidence that, independent of aging, lifestyle and, notably, nutrition are associated with development or progression of major human cancers, including breast, prostate, colorectal tumors, and an increasingly large collection of diet-related cancers. Mechanisms underlying this association are mostly related to the distinct epigenetic effects of different dietary patterns. In this context, Mediterranean diet has been reported to significantly reduce mortality rates for various chronic illnesses, including cardiovascular diseases, neurodegenerative diseases and cancer. Although many observational studies have supported this evidence, dietary intervention studies using a Mediterranean dietary pattern or its selected food components are still limited and affected by a rather large variability in characteristics of study subjects, type and length of intervention, selected end-points and statistical analysis. Here we review data of two of our intervention studies, the MeDiet study and the DiMeSa project, aimed at assessing the effects of traditional Mediterranean diet and/or its component(s) on a large panel of both plasma and urine biomarkers. Both published and unpublished results are presented and discussed.     

Assessment of dietary exposure related to dietary GI and fibre intake in a nutritional metabolomic study of human urine

There is a need for a tool to assess dietary intake related to the habitual dietary glycaemic index (GI) and fibre in groups with large numbers of individuals. Novel metabolite-profiling techniques may be a useful approach when applied to human urine. In a long-term, controlled dietary intervention study, metabolomics were applied to assess dietary patterns. A targeted approach was used to evaluate the effects on urinary C-peptide excretion caused by the dietary treatments. Seventy-seven overweight subjects followed an 8-week low-calorie diet (LCD) and were then randomly assigned to a high-GI or low-GI diet for 6 month during which they completed 24-h urine collections at baseline (prior to the 8-week LCD) and after randomisation to the dietary intervention, at month 1, 3 and 6, respectively. Metabolite profiling in 24-h urine was performed by ¹H NMR and chemometrics. Partial least squares (PLS) analysis indicated that urinary formate could discriminate between high-GI and low-GI diets (correlation coefficient r = 0.82), and this finding was confirmed statistically (P = 0.01). PLS analysis also indicated that urinary hippurate could be associated with fibre intake, but this finding was not confirmed statistically. No associations between GI and urinary C-peptide were found. Our results emphasise that application of metabolomics is useful in the assessment of dietary exposure related to dietary GI and fibre seen at group level in a nutritional metabolomic study of human urine. As our design allowed for large variations in individually selected food items, biomarkers identified at group level may be interpreted as more general and robust markers, largely not confounded with markers from single dietary factors.     

The Influence of Dietary Protein Intake on Mammalian Tryptophan and Phenolic Metabolites

Although there has been increasing interest in the use of high protein diets, little is known about dietary protein related changes in the mammalian metabolome. We investigated the influence of protein intake on selected tryptophan and phenolic compounds, derived from both endogenous and colonic microbial metabolism. Furthermore, potential inter-species metabolic differences were studied. For this purpose, 29 healthy subjects were allocated to a high (n = 14) or low protein diet (n = 15) for 2 weeks. In addition, 20 wild-type FVB mice were randomized to a high protein or control diet for 21 days. Plasma and urine samples were analyzed with liquid chromatography–mass spectrometry for measurement of tryptophan and phenolic metabolites. In human subjects, we observed significant changes in plasma level and urinary excretion of indoxyl sulfate (P 0.004 and P 0.001), and in urinary excretion of indoxyl glucuronide (P 0.01), kynurenic acid (P 0.006) and quinolinic acid (P 0.02). In mice, significant differences were noted in plasma tryptophan (P 0.03), indole-3-acetic acid (P 0.02), p-cresyl glucuronide (P 0.03), phenyl sulfate (P 0.004) and phenylacetic acid (P 0.01). Thus, dietary protein intake affects plasma levels and generation of various mammalian metabolites, suggesting an influence on both endogenous and colonic microbial metabolism. Metabolite changes are dissimilar between human subjects and mice, pointing to inter-species metabolic differences with respect to protein intake.     

Effects of dietary tin and aluminum on selenium utilization by adult males

The main purpose of these studies was to determine whether the amounts of tin and aluminum that can enter foods during processing and storage are sufficient to affect the utilization of selenium by human subjects. Two 40-day balance studies were conducted. The eight adult males who participated in the first study lost significantly more selenium in their feces when fed a test diet containing 50 mg tin daily than when fed the control diet containing 0.1 mg tin daily. During the first study subjects tended to excrete less selenium in the urine when fed the test diet rather than the control diet. In the second study, the dietary treatments (5 and 125 mg aluminum daily) had no effect on the excretion and apparent retention of selenium by eight adult males.     

Generation of an endogenous DNA-methylating agent by nitrosation in Escherichia coli.

Escherichia coli ada ogt mutants, which are totally deficient in O6-methylguanine-DNA methyltransferases, have an increased spontaneous mutation rate. This phenotype is particularly evident in starving cells and suggests the generation of an endogenous DNA alkylating agent under this growth condition. We have found that in wild-type cells, the level of the inducible Ada protein is 20-fold higher in stationary-phase and starving cells than in rapidly growing cells, thus enhancing the defense of these cells against DNA damage. The increased level of Ada in stationary cells is dependent on RpoS, a stationary-phase-specific sigma subunit of RNA polymerase. We have also identified a potential source of the mutagenic agent. Nitrosation of amides and related compounds can generate directly acting methylating agents and can be catalyzed by bacteria] enzymes. E. coli moa mutants, which are defective in the synthesis of a molybdopterin cofactor required by several reductases, are deficient in nitrosation activity. It is reported here that a moa mutant shows reduced generation of a mutagenic methylating agent from methylamine (or methylurea) and nitrite added to agar plates. Moreover, a moa mutation eliminates much of the spontaneous mutagenesis in ada ogt mutants. These observations indicate that the major endogenous mutagen is not S-adenosylmethionine but arises by bacterially catalyzed nitrosation.     

Renal responsiveness to aldosterone during exposure to head-down tilt bedrest

The kidneys represent a fundamental organ system responsible in part for the control of vascular volume. A 10% to 20% reduction in plasma volume is one of the fundamental adaptations during exposure to low gravity environments such as bedrest and space flight. Bedrest-induced hypovolemia has been associated with acute diuresis and natriuresis. Elevated baseline plasma renin activity and aldosterone levels have been observed in human subjects following exposure to head-down tilt and spaceflight without alterations in renal sodium excretion. Further, attempts to restore plasma volume with isotonic fluid drinking or infusion in human subjects exposed to head-down bedrest have failed. One explanation for these observations is that renal distal tubular cells may become less sensitive to aldosterone following exposure to head-down tilt, with a subsequent reduction in renal capacity for sodium retention. We hypothesized that elevated sodium and water excretion observed during prolonged exposure to bedrest and the subsequent inability to restore body fluids by drinking might be reflected, at least in part, by reduced renal tubular responsiveness to aldosterone. If renal tubular responsiveness to aldosterone were reduced with confinement to bedrest, then we would expect measures of renal sodium retention to be reduced when a bolus of aldosterone was administered in head-down tilt (HDT) bedrest compared to a control experimental condition. In order to test this hypothesis, we conducted an investigation in which we administered an acute bolus of aldosterone (stimulus) and measured responses in renal functions that included renal clearances of sodium and free water, sodium/potassium ratio in urine, urine sodium concentration, and total and fractional renal sodium excretion.     

DNA adduct formation from tobacco-specific N-nitrosamines

Tobacco-specific N-nitrosamines are a group of carcinogens derived from the tobacco alkaloids. They are likely causative factors for cancers of the lung, esophagus, pancreas, and oral cavity in people who use tobacco products. The most carcinogenic tobacco-specific nitrosamines in laboratory animals are 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), and N'-nitrosonornicotine (NNN). DNA adduct formation from NNK and NNN has been studied extensively and is reviewed here. NNK is metabolically activated by cytochromes P450 to intermediates which methylate and pyridyloxobutylate DNA. The resulting adducts have been detected in cells and tissues susceptible to NNK carcinogenesis in rodents. The methylation and pyridyloxobutylation pathways are both important in carcinogenesis by NNK. NNK also induces single strand breaks and increases levels of 8-oxodeoxyguanosine in DNA of treated animals. NNAL, which like NNK is a potent pulmonary carcinogen, is also metabolically activated to methylating and pyridyloxobutylating intermediates. NNN pyridyloxobutylates DNA in its rat target tissues, esophagus and nasal mucosa. Methyl and pyridyloxobutyl DNA adducts are detected in human tissues. The methyl adducts most likely result in part from exposure of smokers to NNK, but these adducts are also detected in non-smokers. Some of the methyl adducts detected in non-smokers may be due to environmental tobacco smoke exposure. There are also potential dietary and endogenous sources of these adducts. Pyridyloxobutyl DNA adducts in human tissues result mainly from exposure to tobacco-specific N-nitrosamines. In laboratory animals, DNA adduct formation and carcinogenicity of tobacco-specific N-nitrosamines are closely correlated in many instances, and it is likely that similar relationships will hold in humans.     

Dietary phenolic acids and ascorbic acid: Influence on acid-catalyzed nitrosative chemistry in the presence and absence of lipids

Acid-catalyzed nitrosation and production of potentially carcinogenic nitrosative species is focused at the gastroesophageal junction, where salivary nitrite, derived from dietary nitrate, encounters the gastric juice. Ascorbic acid provides protection by converting nitrosative species to nitric oxide (NO). However, NO may diffuse into adjacent lipid, where it reacts with O₂ to re-form nitrosative species and N-nitrosocompounds (NOC). In this way, ascorbic acid promotes acid nitrosation. Using a novel benchtop model representing the gastroesophageal junction, this study aimed to clarify the action of a range of water-soluble antioxidants on the nitrosative mechanisms in the presence or absence of lipids. Caffeic, ferulic, gallic, or chlorogenic and ascorbic acids were added individually to simulated gastric juice containing secondary amines, with or without lipid. NO and O₂ levels were monitored by electrochemical detection. NOC were measured in both aqueous and lipid phases by gas chromatography–tandem mass spectrometry. In the absence of lipids, all antioxidants tested inhibited nitrosation, ranging from 35.9 ± 7.4% with gallic acid to 93 ± 0.6% with ferulic acid. In the presence of lipids, the impact of each antioxidant on nitrosation was inversely correlated with the levels of NO they generated (R² = 0.95, p < 0.01): gallic, chlorogenic, and ascorbic acid promoted nitrosation, whereas ferulic and caffeic acids markedly inhibited nitrosation.     

The effect of a 2-h exposure to cigarette smoke on the metabolic activation of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone in A/J mice.

4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific nitrosamine, induces lung adenomas in A/J mice following a single intraperitoneal (i.p.) injection. However, inhalation of mainstream cigarette smoke does not induce or promote NNK-induced lung tumors in this mouse strain purported to be sensitive to chemically-induced lung tumorigenesis. The critical events for NNK-induced lung tumorigenesis in A/J mice is thought to involve O(6)-methylguanine (O(6)MeG) adduct formation, GC-->AT transitional mispairing, and activation of the K-ras proto-oncogene. The objective of this study was to test the hypothesis that a smoke-induced shift in NNK metabolism led to the observed decrease in O(6)MeG adducts in the lung and liver of A/J mice co-administered NNK with a concomitant 2-h exposure to cigarette smoke as observed in previous studies. Following 2 h nose-only exposure to mainstream cigarette smoke (600 mg total suspended particulates/m(3) of air), mice (n=12) were administered 7.5 micromol NNK (10 microCi [5-3H]NNK) by i.p. injection. A control group of 12 mice was sham-exposed to HEPA-filtered air for 2 h prior to i.p. administration of 7.5 micromol NNK (10 microCi [5-3H]NNK). Exposure to mainstream cigarette smoke had no effect on total excretion of NNK metabolites in 24 h urine; however, the metabolite pattern was significantly changed. Mice exposed to mainstream cigarette smoke excreted 25% more 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) than control mice, a statistically significant increase (P<0.0001). Cigarette smoke exposure significantly reduced alpha-hydroxylation of NNK to potential methylating species; this is based on the 15% reduction in excretion of the 4-(3-pyridyl)-4-hydroxybutanoic acid and 42% reduction in excretion of 4-(3-pyridyl)-4-oxobutanoic acid versus control. Detoxication of NNK and NNAL by pyridine-N-oxidation, and glucuronidation of NNAL were not significantly different in the two groups of mice. The observed reduction in alpha-hydroxylation of NNK to potential methylating species in mainstream cigarette smoke-exposed A/J mice provides further mechanistic support for earlier studies demonstrating that concurrent inhalation of mainstream cigarette smoke results in a significant reduction of NNK-induced O(6)MeG adduct formation in lung and liver of A/J mice compared to mice treated only with NNK.     

Protein absorption and energy digestibility at high altitude.

To test the hypothesis that malabsorption of dietary protein is partly responsible for the weight loss observed during prolonged altitude exposure, six healthy male subjects [31.8 +/- 4.5 (SD) yr] received 15N-labeled soya protein by mouth and [15N]glycine intravenously at 122 and 5,000 m. From the subsequent 4-day total urine and fecal pools, the different fractions of the administered 15N were determined by mass spectrometry. Weight and skinfold thickness were measured at the beginning and end of the altitude exposure. In addition, the overall digestible energy of the diet at altitude was assessed by a 3-day diet control and adiabatic bomb calorimetric assessment of the energy content of the corresponding fecal pool. The average decrease of the subjects' weight during altitude exposure was 3%. Loss of fat mass at altitude estimated from the skinfold measurements was 9%. Protein absorption, calculated as 100--[fecal excretion of 15N after ingestion of 15N soya protein (% of dose given)--fecal excretion of 15N after injection of 15N glycine (% of dose given)], was not significantly impaired at altitude compared with sea level (96 vs. 97%, respectively), and overall digestible energy at altitude, calculated as 100--percent undigested gross energy in the feces, amounted to 96%. It is concluded that, at least up to an altitude of 5,000 m, malabsorption does not play a role in altitude-related weight loss.     

DNA adducts in humans after exposure to methylating agents

Human exposure to methylating agents appears to be widespread, as indicated by the frequent occurrence of methylated DNA adducts in human DNA. The high incidence of methylated DNA adducts even in humans thought not to have suffered extensive exposure to environmental methylating agents implies that chemicals of endogenous origin, probably N-nitroso compounds such as the strongly carcinogenic N-nitrosodimethylamine (NDMA), may be primarily responsible for their formation and raises the question of the carcinogenic risks associated with such exposure. In addition to accumulation of DNA damage, other factors (such as induced cell proliferation) appear to be important in determining the probability of induction of mutation or cancer by NDMA, implying that high to low dose risk extrapolations should not be based on the assumption of dose- or even adduct-linearity. Comparative studies of the accumulation and repair of methylated adducts in humans and animals treated with methylating cytostatic drugs do not reveal significant species differences. Based on this and the dosimetry of adduct accumulation in rats chronically exposed to very low doses of NDMA, it is suggested that the exposure needed to account for the levels of adducts found in human DNA may be of the order of hundreds of micrograms NDMA (or equivalent) per day, a level of exposure which may well represent a significant carcinogenic hazard for man.