An examination of human blood for the presence of volatile nitrosamines

Commentary of the SKLM to the EFSA opinion on risk assessment of N-nitrosamines in food

Dietary exposure to N-nitrosamines has recently been assessed by the European Food Safety Authority (EFSA) to result in margins of exposure that are conceived to indicate concern with respect to human health risk. However, evidence from more than half a century of international research shows that N-nitroso compounds (NOC) can also be formed endogenously. In this commentary of the Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG), the complex metabolic and physiological biokinetics network of nitrate, nitrite and reactive nitrogen species is discussed with emphasis on its influence on endogenous NOC formation. Pioneering approaches to monitor endogenous NOC have been based on steady-state levels of N-nitrosodimethylamine (NDMA) in human blood and on DNA adduct levels in blood cells. Further NOC have not been considered yet to a comparable extent, although their generation from endogenous or exogenous precursors is to be expected. The evidence available to date indicates that endogenous NDMA exposure could exceed dietary exposure by about 2-3 orders of magnitude. These findings require consolidation by refined toxicokinetics and DNA adduct monitoring data to achieve a credible and comprehensive human health risk assessment.

Risk characterization of N-nitrosodimethylamine in pharmaceuticals

Since 2018, N-nitrosodimethylamine (NDMA) has been a reported contaminant in numerous pharmaceutical products. To guide the pharmaceutical industry, FDA identified an acceptable intake (AI) of 96 ng/day NDMA. The approach assumed a linear extrapolation from the Carcinogenic Potency Database (CPDB) harmonic-mean TD50 identified in chronic studies in rats. Although NDMA has been thought to act as a mutagenic carcinogen in experimental animals, it has not been classified as a known human carcinogen by any regulatory agency. Humans are exposed to high daily exogenous and endogenous doses of NDMA. Due to the likelihood of a threshold dose for NDMA-related tumors in animals, we believe that there is ample scientific basis to utilize the threshold-based benchmark dose or point-of-departure (POD) approach when estimating a Permissible Daily Exposure limit (PDE) for NDMA. We estimated that 29,000 ng/kg/day was an appropriate POD for calculating a PDE. Assuming an average bodyweight of 50 kg, we expect that human exposures to NDMA at doses below 5800 ng/day in pharmaceuticals would not result in an increased risk of liver cancer, and that there is little, if any, risk for any other type of cancer, when accounting for the mode-of-action in humans.

The role of endogenous versus exogenous sources in the exposome of putative genotoxins and consequences for risk assessment

The “totality” of the human exposure is conceived to encompass life-associated endogenous and exogenous aggregate exposures. Process-related contaminants (PRCs) are not only formed in foods by heat processing, but also occur endogenously in the organism as physiological components of energy metabolism, potentially also generated by the human microbiome. To arrive at a comprehensive risk assessment, it is necessary to understand the contribution of in vivo background occurrence as compared to the ingestion from exogenous sources. Hence, this review provides an overview of the knowledge on the contribution of endogenous exposure to the overall exposure to putative genotoxic food contaminants, namely ethanol, acetaldehyde, formaldehyde, acrylamide, acrolein, α,β-unsaturated alkenals, glycation compounds, N-nitroso compounds, ethylene oxide, furans, 2- and 3-MCPD, and glycidyl esters. The evidence discussed herein allows to conclude that endogenous formation of some contaminants appears to contribute substantially to the exposome. This is of critical importance for risk assessment in the cases where endogenous exposure is suspected to outweigh the exogenous one (e.g. formaldehyde and acrolein).

Food intake biomarkers for green leafy vegetables, bulb vegetables, and stem vegetables: a review

BACKGROUND

Numerous studies acknowledged the importance of an adequate vegetable consumption for human health. However, current methods to estimate vegetable intake are often prone to measurement errors due to self-reporting and/or insufficient detail. More objective intake biomarkers for vegetables, using biological specimens, are preferred. The only concentration biomarkers currently available are blood carotenoids and vitamin C, covering total fruit and vegetable intake. Identification of biomarkers for specific vegetables is needed for a better understanding of their relative importance for human health. Within the FoodBAll Project under the Joint Programming Initiative "A Healthy Diet for a Healthy Life", an ambitious action was undertaken to identify candidate intake biomarkers for all major food groups consumed in Europe by systematically reviewing the existent literature. This study describes the review on candidate biomarkers of food intake (BFIs) for leafy, bulb, and stem vegetables, which was conducted within PubMed, Scopus and Web of Science for studies published through March 2019.

RESULTS

In total, 65 full-text articles were assessed for eligibility for leafy vegetables, and 6 full-text articles were screened for bulb and stem vegetables. Putative BFIs were identified for spinach, lettuce, endive, asparagus, artichoke, and celery, but not for rocket salad. However, after critical evaluation through a validation scheme developed by the FoodBAll consortium, none of the putative biomarkers appeared to be a promising BFI. The food chemistry data indicate that some candidate BFIs may be revealed by further studies.

CONCLUSION

Future randomized controlled feeding studies combined with observational studies, applying a non-targeted metabolomics approach, are needed in order to identify valuable BFIs for the intake of leafy, bulb, and stem vegetables.

Drinking water as a proportion of total human exposure to volatile N-nitrosamines

Some volatile N-nitrosamines, primarily N-nitrosodimethylamine (NDMA), are recognized as products of drinking water treatment at ng/L levels and as known carcinogens. The U.S. EPA has identified the N-nitrosamines as contaminants being considered for regulation as a group under the Safe Drinking Water Act. Nitrosamines are common dietary components, and a major database (over 18,000 drinking water samples) has recently been created under the Unregulated Contaminant Monitoring Rule. A Monte Carlo modeling analysis in 2007 found that drinking water contributed less than 2.8% of ingested NDMA and less than 0.02% of total NDMA exposure when estimated endogenous formation was considered. Our analysis, based upon human blood concentrations, indicates that endogenous NDMA production is larger than expected. The blood-based estimates are within the range that would be calculated from estimates based on daily urinary NDMA excretion and an estimate based on methylated guanine in DNA of lymphocytes from human volunteers. Our analysis of ingested NDMA from food and water based on Monte Carlo modeling with more complete data input shows that drinking water contributes a mean proportion of the lifetime average daily NDMA dose ranging from between 0.0002% and 0.001% for surface water systems using free chlorine or between 0.001% and 0.01% for surface water systems using chloramines. The proportions of average daily dose are higher for infants (zero to six months) than other age cohorts, with the highest mean up to 0.09% (upper 95th percentile of 0.3%).

Effect of ethanol on nitrite oxidation in the perfused rat liver

The effect of ethanol on nitrite oxidation was investigated in the perfused rat liver. Real-time spectral changes in catalase were obtained using a reflectance scanning spectrophotometer in rat liver perfused with Krebs-Henseleit bicarbonate buffer in a non-recirculating system. The nitrite oxidation rate and nitrate production rate were calculated from the differences in concentrations between the influx and efflux perfusates and from the flow rate/g liver weight. Nitrite infusion caused an increase in absorbance difference delta A (640-660 nm), indicating decomposition of catalase compound I to the free form. Administration of ethanol during the nitrite infusion caused a further increase in delta A (640-660 nm) and significant decreases in both the nitrite oxidation rate and the nitrate production rate. Both the nitrite oxidation rate and nitrate production rate decreased, depending on the concentration of ethanol administered. At 10 mM ethanol, they reached about half the rates before the ethanol infusion. In conclusion, ethanol inhibits nitrite oxidation by catalase in the perfused rat liver at relatively low concentrations that can be realized in blood by daily alcohol consumption.

Clearance of N-nitrosodimethylamine and N-nitrosodiethylamine by the perfused rat liver. Relationship to the Km and Vmax for nitrosamine metabolism

The first-pass clearance of dietary N-nitrosodimethylamine (NDMA) by the liver is the most important factor in the pharmacokinetics of this carcinogen in the rat, but is less important in the pharmacokinetics of N-nitrosodiethylamine (NDEA). The reason for the difference in clearance of these two nitrosamines is not known. These experiments were carried out to see whether the general characteristics of the clearance of these two carcinogens in vivo could be reproduced in the perfused liver, and whether the clearance could be correlated with the Michaelis-Menten parameters Km and Vmax for their metabolism. If this could be done one would be able to predict the possible extent of first-pass clearance of nitrosamines in man from measurement of Km and Vmax for nitrosamine metabolism by the human liver. The Km (22 microM) and Vmax (10.2 and 13.4 nmol/g liver/min) for the metabolism of NDMA by slices from two human livers, the inhibition of that metabolism by ethanol (Ki 0.5 microM), and the rate of N-7 methylation of DNA when slices are incubated with NDMA, were measured. These results are similar to those reported previously with rat liver. The Km (27 microM) for the metabolism of NDEA by rat liver slices and the inhibition of that metabolism by ethanol (Ki 1 microM) were estimated from the rate of ethylation of the DNA of the slices. The clearance of both these nitrosamines by the perfused rat liver was measured, and the results appeared to parallel those in vivo with a striking difference between the clearance of NDMA and NDEA. The maximal rate of clearance of NDMA was 11.2 nmol/g liver/min and of NDEA 8.9 nmol/g liver/min, similar to the Vmax for metabolism of NDMA by liver slices and to the estimated maximal rate of liver metabolism of both nitrosamines in the living rat. However, although the Km for metabolism of these two nitrosamines by liver slices is similar (about 25 microM), the logarithmic mean sinusoidal concentration [see Bass and Keiding, Biochem Pharmacol 37: 1425-1431, 1988] giving half maximal clearance during perfusion (the equivalent to Km) was 2.3 microM for NDMA and 10.6 microM for NDEA. The almost 5-fold difference between these two values is the basis for the difference between the clearance of the two nitrosamines.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Toxicokinetics of N-nitrosodimethylamine in the Syrian golden hamster

The single-dose toxicokinetics of N-nitrosodimethylamine (NDMA) has been characterized in 8-week-old male Syrian golden hamsters by analysis using high performance liquid chromatography of serial blood samples. An i.v. bolus dose of 4.2 mumols/kg [14C]NDMA revealed biphasic first-order elimination with a terminal half-life of 8.7 +/- 1.0 min (mean +/- SE) for unchanged NDMA and 31.5 +/- 5.5 min for total radioactivity, and evidence for conversion to polar metabolites was seen in the chromatographic assays. The systemic blood clearance and apparent steady-state volume of distribution for unchanged NDMA were 51.2 +/- 3.0 ml/min/kg and 582 +/- 60 ml/kg, respectively. No unchanged NDMA was detected in the urine following an i.v. bolus dose of 15 mumols/kg [14C]NDMA, but 31% of the total radioactivity was eliminated by that route. A dose of 38 mumols/kg given by gavage indicated a systemic bioavailability of 11 +/- 4% for unchanged NDMA. Reversible binding of NDMA to hamster plasma proteins was found to be negligible. Estimation of the intrinsic hepatic clearance (ClI) in the hamster produced a value of 648 ml/min/kg, which is greater than that previously obtained for the rat, and indicates that the metabolic capacity of the hamster liver is greater than that of the rat. These results suggest that this difference in ClI may play a role in the previously reported (Lijinsky et al. 1987) switch in organotropism from almost exclusivity for liver tumors in hamsters dosed by gavage to additional high incidences of lung and kidney tumors in the rat.

N-nitrosamines in the stomach with special reference to in vitro formation, and kinetics after intragastric or intravenous administration in rats

To study the implications of nitrosation in the stomach, the formation of N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) from nitrite and secondary amines was examined in vitro under conditions simulating gastric juice. Kinetics of NDMA were investigated after intragastric or intravenous administration of 0.2 mg/kg of NDMA in rats. NDMA and NDEA were measured using combined gas chromatography and thermal energy analyzer. Nitrite levels in human gastric juice were less than 10 micrograms/ml. Optimal pH for nitrosation was between 2.0 to 3.5. Nitrosamine formation reached maximum concentration at 3 to 6 hours. The maximum ratios of nitrosation were 0.15 and 0.11% in NDMA/nitrite and NDEA/nitrite, respectively. In the kinetic study, the highest blood levels of NDMA were observed at 5 min, reaching 174 +/- 40 and 374 +/- 40 ng/ml after administration into the stomach and duodenum, respectively. Then they decreased exponentially and were not detectable after 4 hours. Tissue levels of NDMA in the liver, spleen, kidney, lung and brain showed 70% of the blood levels. Urinary excretion of intravenously administered NDMA during the first 4 hours was less than 0.2%. These results supported the hypothesis that nitrosation occurred in gastric juice under optimal conditions, and indicated that nitrite levels were the limiting factor for nitrosation. Quick disappearance from the gastrointestinal tract suggested that the pathologic implication of nitrosamines formed in the stomach could be important for other organs.

An investigation of the endogenous formation of apparent total N-nitroso compounds in conventional microflora and germ-free rats

The endogenous formation of apparent total N-nitroso compounds (ATNC) has been investigated in germ-free (GF) and conventional (CV) microflora rats as a function of the drinking-water nitrate concentration. ATNC levels were below the 40 micrograms (N-NO)/kg detection limit in the blood, liver, kidney, spleen and small intestine of all CV and GF rats. For the CV rats ATNC were detected in concentrations of up to 370 micrograms (N-NO)/kg in the large intestine and up to 50 micrograms (N-NO)/kg in the stomach and there was a significant positive correlation between ATNC formation and the drinking-water nitrate level. Comparison of these results with those from GF rats showed that the ATNC in the stomach and large intestine of the CV animals were formed by microbial action, most probably involving bacterial nitrate-reductase activity.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 15/6. Effect of ethanol on nitrosamine metabolism and distribution. Implications for the role of nitrosamines in human cancer and for the influence of alcohol consumption on cancer incidence

Alcohol consumption is associated with an increase in the incidence of cancers of several sites, including oesophagus, larynx and mouth. The mechanism of the induction of cancer by alcohol is not clear. Humans are exposed to a variety of carcinogenic N-nitroso compounds. Ethanol changes the pharmacokinetics of nitrosamines in rats particularly by decreasing the ability of the liver to metabolize them. A hypothesis is put forward that the influence of alcohol on human cancer is mediated by its effect on the metabolism and distribution of nitrosamines from the diet, from tobacco smoke and from endogenous synthesis.

Analysis of human blood for volatile N-nitrosamines by gas chromatography-chemiluminescence detection

A method was developed to separate and measure trace levels of volatile N-nitrosamines (NAs) in human blood that either eliminated or accounted for in vitro artifactual formation of N-nitrosodimethylamine (NDMA) through the use of water blanks, added inhibitor (ascorbic acid) and added morpholine. The absolute minimum detectable limit was 8 pg; minimum level of reliable measurement was 0.05 microgram/kg for a 20-g blood specimen. Recovery of NDMA from blood was 93 +/- 5%. Coefficient of variation was 25%. Bloods from 242 people were analyzed for volatile NAs. NDMA was the only NA found. Positive specimens were presumptively confirmed by their non-detection after ultraviolet photolysis and/or mass spectrometry. This paper presents additional evidence that in vivo NA formation occurs.

Survey of human milk for volatile N-nitrosamines and the influence of diet on their formation

Human milk was analysed for volatile N-nitrosamines and the influence of diet on their possible presence and formation was assessed. 175 samples were obtained from 16 different nursing women. Analysis of samples collected at random, to ascertain baseline levels, indicated that 76.5% of the human milk samples contained less than 0.2 ppb N-nitrosodimethylamine (minimum level of reliable measurement). No other volatile nitrosamines were detected. Expressed milk was collected from volunteers over a 6-hr period after eating meals which included bacon (a source of performed nitrosamines) and at times a vegetable high in nitrate. Eating a meal containing bacon did not result in increased nitrosamine levels in milk. In certain individuals, eating a meal of bacon and a vegetable high in nitrate occasionally resulted in higher levels of N-nitrosodimethylamine in their milk.