Toxicology of nitrates and nitrites

Silver Nanoparticles-Chitosan Composite Embedded Graphite Screen-Printed Electrodes as a Novel Electrochemical Platform in the Measurement of Trace Level Nitrite: Application to Milk Powder Samples

Background:

Nitrites can exert acute toxic effects in humans. It is widely used as a preservative in dairy and meat products. The nitrites form N-nitrosamines, which are potential carcinogens and cause detrimental health effects. Herein we report a disposable graphite screen-printed sensor developed using silver metal nano particle embedded chitosan composite in the quantification of nitrite at trace level.

Methods:

Conventional methods possess various limitations. Electrochemical methods provide an ideal platform for trace nitrite analysis. The prepared composite has been characterized by UV-Visible spectrometry, SEM, EDS and XRD techniques. The proposed sensor has been fabricated by using graphite screen-printed electrodes through drop coating of the composite material. The redox behavior and its application of the fabricated electrode have been studied using cyclic and anodic stripping voltammetric methods.

Results:

Graphite screen-printed electrodes after modification have been used to identify the electrocatalytic behavior of nitrite oxidation in an aqueous medium. All the parameters influencing the analytical signal have been optimized and incorporated in the recommended procedure. The proposed sensor has been used to measure the nitrite levels from commercially available milk powder samples and the results have been compared with the standard protocol. The results of the proposed sensor are in good agreement with the standard protocol.

Conclusion:

Ag metal nanoparticles have been embedded in chitosan matrix and used as a composite material in the chemical modification of graphite screen-printed electrodes. GSPEs are easy to fabricate. They provide wide linear working range i.e. 30 - 1140 µM of nitrite. The sensor is highly stable, reproducible and provides a very low detection limit of 1.84 µM. The method has been applied to measure trace level nitrite from milk powder samples.

Profound differences between humans and rodents in the ability to concentrate salivary nitrate: Implications for translational research

In humans dietary circulating nitrate accumulates rapidly in saliva through active transport in the salivary glands. By this mechanism resulting salivary nitrate concentrations are 10–20 times higher than in plasma. In the oral cavity nitrate is reduced by commensal bacteria to nitrite, which is subsequently swallowed and further metabolized to nitric oxide (NO) and other bioactive nitrogen oxides in blood and tissues. This entero-salivary circulation of nitrate is central in the various NO-like effects observed after ingestion of inorganic nitrate. The very same system has also been the focus of toxicologists studying potential carcinogenic effects of nitrite-dependent nitrosamine formation. Whether active transport of nitrate and accumulation in saliva occurs also in rodents is not entirely clear. Here we measured salivary and plasma levels of nitrate and nitrite in humans, rats and mice after administration of a standardized dose of nitrate.

After oral (humans) or intraperitoneal (rodents) sodium nitrate administration (0.1 mmol/kg), plasma nitrate levels increased markedly reaching ~300 µM in all three species. In humans ingestion of nitrate was followed by a rapid increase in salivary nitrate to >6000 µM, ie 20 times higher than those found in plasma. In contrast, in rats and mice salivary nitrate concentrations never exceeded the levels in plasma. Nitrite levels in saliva and plasma followed a similar pattern, ie marked increases in humans but modest elevations in rodents. In mice there was also no accumulation of nitrate in the salivary glands as measured directly in whole glands obtained after acute administration of nitrate.

This study suggests that in contrast to humans, rats and mice do not actively concentrate circulating nitrate in saliva. These apparent species differences should be taken into consideration when studying the nitrate-nitrite-nitric oxide pathway in rodents, when calculating doses, exploring physiological, therapeutic and toxicological effects and comparing with human data.

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In humans, dietary nitrate is effectively concentrated in saliva through active transport in the salivary glands.

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In humans salivary nitrate levels are10–20 times higher than in plasma.

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In contrast to humans, rats and mice do not actively concentrate nitrate in saliva.

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These species differences have implcations for translational research.

A survey of nitrate and nitrite concentrations in conventional and organic-labeled raw vegetables at retail

A national survey of the nitrate ( NO3(-)) and nitrite ( NO2(-)) concentrations in raw and highly consumed vegetables available at retail in the United States was conducted. A total of 194 samples of fresh broccoli, cabbage, celery, lettuce, and spinach categorized as conventional or organic by label were collected from 5 major cities in different geographic regions of the United States and analyzed to determine NO3(-) and NO2(-) concentrations. There were no differences in the mean NO2(-) values of conventional compared with organic vegetables taken from the 5 metropolitan areas. However, significant differences in mean pairwise comparisons between some conventional and organic vegetables for NO3(-) content were observed. The mean NO2(-) concentration of both conventional and organic vegetables ranged between 0.1 and 1.2 mg/kg of fresh weight (FW) with the exception of conventional spinach that contained 8.0 mg/kg FW. Mean NO3(-) contents of conventional broccoli, cabbage, celery, lettuce, and spinach were 394, 418, 1496, 851, and 2797 mg/kg FW, respectively, while their organic-labeled counterparts averaged 204, 552, 912, 844, and 1318 mg/kg FW. In most cases, organic vegetables were numerically lower in NO3(-) content than their conventional counterparts. Based on survey results, the finding that low NO3(-) levels were observed in some organic vegetables in different cities may warrant further study to determine if true differences exist, due to production practices, seasonal differences, and the magnitudes of those differences. Furthermore, the geographic differences in NO3(-) content of vegetables may flaw estimates of daily NO2(-) and NO3(-) exposure.

Methemoglobinemia induced by vegetable intake in infants in northern Spain

BACKGROUND AND OBJECTIVE

Acquired methemoglobinemia (MHb) induced in infants by intake of vegetables is a condition uncommonly reported in the literature. The purpose of the present study was to study new vegetables involved and other epidemiological risk factors.

METHODS

Seventy-eight cases of diet-induced MHb seen in Pamplona from 1987 to 2010 are reported. Infant characteristics were collected, and a case-control study was conducted using as controls 78 age- and sex-matched infants selected at the same geographic area. Bivariate logistic regression analyses were performed to detect factors involved in MHb occurrence. Nitrate levels were tested in natural vegetables used to prepare purées.

RESULTS

A clear relation was found between MHb and use of borage (Borago officinalis) (OR 5.2; 95% CI 1.1-24.6) and maybe chard (Beta vulgaris var cicla) (OR 2.0; 95% CI 0.4-8.7), time from preparation to use (OR 17.4, 95% CI 3.5-86.3 if the purée had been prepared 24-48 hours before and OR 24.9, 95% CI 3.3-187.6 if prepared >48 hours before), and breast-feeding (OR 10.4; 95% CI 1.9-57.2). Tests confirmed that vegetables with the highest nitrate levels were borage (n = 15), with mean nitrate levels of 3968 mg/kg, and chard (n = 17), with mean levels of 2811 mg/kg.

CONCLUSIONS

The main associated factors were shown to be time from purée preparation to use (>24 hours), use of certain vegetables (borage and chard), and breast-feeding. Nitrate levels in both vegetables implicated as etiological factors in acquired MHb are high.

The NII2 gene of Hansenula polymorpha is involved in nitrite assimilation

To establish a basis for genetic and molecular studies of nitrite assimilation in the methylotrophic yeast Hansenula polymorpha, we isolated and characterised six nitrite-negative mutants still capable of growing on nitrate. Gene isolation work yielded the NII2 gene, encoding a membrane protein homologous to the Saccharomyces cerevisiae Pho86p. Sequence analysis revealed an ORF of 860 bp encoding a 286-amino-acid protein with a predicted molecular mass of 32.8 kDa. This protein is shorter than its S. cerevisiae homologue, and is predicted to lack an ER-retention signal. Cell suspension work revealed that the null mutant is unable to take up nitrite from the medium.

Protective effect of dietary nitrate on experimental gastritis in rats

Nitrates have long been considered as harmful dietary components and judged responsible for deleterious effects on human health, leading to stringent regulations concerning their levels in food and water. However, recent studies demonstrate that dietary nitrate may have a major role in human health as a non-immune mechanism for host defence, through its metabolism to NO in the stomach. NO is a versatile molecule and although evidence exists showing that administration of low doses of exogenous NO protects against gastrointestinal inflammation, higher NO doses have been shown to exacerbate injury. So, the effect of an ingestion of nitrates in doses corresponding to a normal diet in human consumers on an experimental gastritis induced by iodoacetamide in rats was investigated. During gastritis one of the following compounds was given orally: water; KNO3; the NO donor sodium nitroprusside; the NO scavenger haemoglobin given with either water or KNO3. N(G)-nitro-l-arginine methyl ester (l-NAME), a non-specific NO synthase inhibitor, was administered with either water, iodoacetamide alone, or combined with KNO3. After killing, the stomach was resected and microscopic damage scores, myeloperoxidase and NO synthase activities were determined. Iodoacetamide-induced gastritis was significantly reduced by KNO3 administration, an effect which was reproduced by sodium nitroprusside and reversed by haemoglobin. l-NAME induced gastric mucosal damage in itself, and KNO3 did not prevent the gastritis induced by iodoacetamide associated with l-NAME. In conclusion, dietary nitrate exerts a protective effect against an experimental gastritis in rats by releasing NO in the stomach but such an effect requires the production of endogenous NO.

Exposure methodology and findings for dietary nitrate exposures in children of Transylvania, Romania

AIMS

The primary focus of this exposure assessment work involved developing an exposure model and determining a numerical point estimate of the amount of biologically relevant nitrate/nitrite exposure that occurred for each child in the study. This assessment was done in support of two epidemiological studies. The first study was an epidemiological cohort study (three cohorts based on nitrate/nitrite exposure) that explored the relationship between high nitrate/nitrite exposure and neuropsychological development. The second study was a nested case/control study (cases of methemoglobinemia versus disease-free controls) that sought to explore the relationship between MHG and various risk factors for the disease.

METHODS

This work uses both dietary survey and environmental sampling and modeling in order to develop two point estimates of nitrate exposure in milligrams per kilogram per day of nitrite (the biologically active form of the hemoglobin-oxidizing agent) for the first 6 months of the child's life (2-months-of-age and 6-months-of-age point estimates). Methodologies included proxy interviews of primary caregivers, review of existing medical and environmental sampling and analysis.

RESULTS

Exposure to nitrate--nitrogen (with final calculations converted to the biologically active form of the toxin, nitrite) was categorized as high, medium, and low as determined from the distribution of the data derived from final exposure calculations at both the 2-months-of-age point estimate and at the 6-months-of-age point estimate. These tertiles correspond to greater-than-or-equal1.5 mg/kg/day nitrite-nitrogen for high-exposure individuals, <1.5-> or = 0.1 mg/kg/day for medium-exposure individuals, and <0.1 mg/kg/day for low-exposure individuals. Analyses illustrate that over 90% of the nitrate exposure occurred through the consumption of liquids (water) at the 2-months-of-age point estimate while at the 6-months-of-age point estimate, a 10-fold change in the amount of solid consumables occurred.

CONCLUSIONS

Final exposure calculations were well differentiated into three tertiles based on a point estimate of average daily intake of nitrite in milligrams per kilogram body weight per day at roughly 2 and 6 months of age. These categories of exposure, based on the exposure model point estimate, correspond well with the exposure estimates as estimated only on the basis of cohort status and their corresponding nitrate/nitrite well water levels. Comparisons of these two sets of data illustrate that following the MHG incident, Cohort II shifted places with Cohort I to become the high-exposure cohort. Further, the predictive ability of the exposure assessment in regard to the outcome of MHG was estimated using a Likelihood Ratio and Pearson's Crosstab analysis. This was performed on the 2-month-of-age point estimate. Likelihood Ratio and Pearson's chi-square were 39.40 and 33.74, respectively, with a probability of achieving these fits by chance alone of <0.0001. This indicates clearly that the children who experienced MHG were also the children at the 2-month-of-age point estimate who had received the highest exposure to nitrate/nitrite through their diet.

Nitrite poisoning in cats and dogs fed a commercial pet food

The death of three cats, from two separate households, was linked to toxic concentrations of sodium nitrite used as a preservative in a commercial pet food. Post-mortem examination of the three cats revealed a brownish discoloration of the blood suggestive of methaemoglobinaemia. Analysis of two samples of the pet food fed to the cats revealed a mean concentration of 2850 mg of total nitrite per kg of food (as fed). In a further incident, ataxia and weakness was noticed in two of four dogs after they were fed the same brand of pet food. One dog was successfully treated for methaemoglobinaemia with intravenous methylene blue.

A survey of nitrate contents in Indonesian milk by enzymic analysis

In this paper, a rapid and simple enzymic method is described for the determination of nitrate in 32 fresh and five dry Indonesian milk samples, deproteinized by Carrez reagents. Interference from albumin, casein, lactose and chloride ions was controlled. The calibration graph was linear over the range l-12.5 micrograms/ml NO3-; r = 0.9998. The limits of detection and quantification were found to be 0.45 micrograms/ml NO3- and 1 microgram/ml NO3- respectively. Standard nitrate solutions (10 micrograms/ml NO3-) were used to evaluate the precision. The results showed an average of 10.1 micrograms/ml, a standard deviation of 0.3 and a relative standard deviation of 3.4%. Adequate agreement was found between results obtained by the enzymic method and those of the French official reduction/photometric reference method (AFNor). Good recoveries (100% +/- 5%) were found for nitrate added to milk. The nitrate levels were in the range 1-2.6 mg/kg NO3- for fresh milk and 1.1-18 mg/kg NO3- for dry milk. All the results are in good agreement with those previously published for UK and American milk.

Effect of graded levels of dietary nitrite on pulmonary hypertension in broiler chickens and dilatory cardiomyopathy in turkey poults

In two separate trials, day-old broiler chicks and turkey poults were fed graded levels of dietary sodium nitrite in order to investigate the possible role of nitrite-induced methaemoglobinaemia on the incidence of chicken pulmonary hypertension and spontaneous turkey cardiomyopathy (STC). A transient low level of methaemoglobinaemia was observed after 7 days of exposure to dietary nitrite, but the levels returned to normal afterwards in spite of the presence of nitrite in the diet throughout the experiment. No effect on pulmonary hypertension, as measured by the right ventricle weight to total ventricle weight ratio, was observed in chickens exposed to dietary nitrite. The group of turkey poults receiving 1200 mg/kg nitrite in the diet had a higher incidence of STC than controls (20 v. 5%), although the difference was not statistically significant. The levels of nitrite-induced methaemoglobinaemia were probably too low to be considered as the aetiology of increased STC incidence.