Drinking water that contains high levels of nitrates can be toxic to humans. Nitrate present in the environment comes from a variety of sources: 25% is derived from the atmosphere, while the rest comes from the geology of the area and human activities such as fertilizer use and ejection of sewage and industrial waste into water sources (Gupta, Gupta, Chhabra, Eskiocak, Gupta, and Gupta, 2008). The nitrates present in humans result from consumption of meat, nitrate-rich vegetables and fruits, and water, but nitrate may also be produced by endogenous pathways (Gupta, et.al., 2008). Excessive intake of nitrate is toxic to humans and may result in any number of health issues, including cancer, chronic diarrhea, detrimental changes in the respiratory and cardiovascular systems, and other effects (Gupta, et.al., 2008).
Consumption of excessive amounts of nitrates can also result in a condition known as methemoglobinemia, which starves the body’s tissues of oxygen. As the body processes nitrate (NO3-), it is reduced to nitrite (NO2-) (Gupta, Gupta, Seth, Gupta, Bassin, and Gupta, 1999). Therefore nitrate’s toxic effect depends on the amount present in potable water and on the reducing conditions present in an individual’s body (Gupta, et.al., 1999). The reduction of nitrate is facilitated by bacteria that require a high stomach pH (pH greater than 4) in order to grow (Gupta, et.al., 2008). Nitrate is reduced to nitrite in the oral cavity and intestinal tract, and upon reentering the bloodstream is converted back to nitrate (Gupta, et.al., 2008; Gupta et.al., 1999). The process of converting nitrite in the blood back to nitrate directly oxidizes the ferrous ion (Fe2+) of hemoglobin to a ferric ion (Fe3+) to create methemoglobin (Gupta, et.al., 1999). The image below shows the structure of hemoglobin, with the red and blue areas representing the different subunits of the protein. The green structures represent heme groups, which contain the iron involved in the process described above. The normal function of hemoglobin is to carry oxygen to various tissues throughout the body (Dugdale, 2010).
The structure below is the most common type of heme group.
Methemoglobin has the same structure as that of hemoglobin. The only exception is that the iron exists in a different oxidation state. Upon oxidation to methemoglobin, the body naturally restores the hemoglobin through a process involving cytochrome b5, an electron transport protein, as shown in the reaction below where Hb3+ is methemoglobin and Hb2+ is hemoglobin (Gupta, et.al., 1999).
The enzyme cytochrome-b5 reductase then restores the oxidized cytochrome b5 to its original state (Gupta, et.al. 1999).
In summary, ingested nitrate is converted to nitrite, which is then converted back to nitrate and in the process oxidizes hemoglobin to methemoglobin. The methemoglobin is restored to hemoglobin by the protein cytochrome b5. Finally the cytochrome-b5 reductase enzyme returns the now oxidized cytochrome b5 to its previously reduced state.
In cases where nitrate levels are too excessive or the cytochrome-b5 reductase system is weakened, the cytochrome-b5 reductase enzyme reserves become exhausted and methemoglobin accumulates in appreciable levels in the blood (Gupta, et.al., 2008). The presence of high concentrations of methemoglobin in the blood can prevent oxygen from being delivered properly to tissues. When more than 10% of hemoglobin is present as methemoglobin, blue discoloration of the skin is evident, and if greater than 60% of hemoglobin has been oxidized, death results (Gupta, et.al., 1999).
Gupta, et.al. reported that the cytochrome-b5 reductase system is able to accommodate and reduce amounts of methemoglobin in the blood until the levels of nitrate in drinking water reach about 95 mg/L (1999). The activity of cytochrome-b5 reductase then declines until it returns to normal levels at around 200 mg/L of nitrate in drinking water (Gupta, et.al., 1999). This decline in cytochrome-b5 reductase activity is correlated with an increase in nitrate concentration in the water and methemoglobin concentration in the blood (Gupta, et.al., 1999). The enzyme’s ability to compensate for increasing levels of nitrate worked best for children age one to eighteen (Gupta, et.al., 1999). Infants and adults in this study had poor cytochrome-b5 reductase response to increasing levels of nitrate and methemoglobin, which may be due to the incomplete development of the reductase system in infants or a saturation of the system in adults (Gupta, et.al., 1999). Infants are also more likely to be affected by nitrate due to a higher stomach pH, which encourages growth of nitrate-reducing bacteria (Gupta, et.al., 2008).
The World Health organization sets the maximum limit of nitrate ion in drinking water at 50 mg/L (Gupta, et.al., 1999). The Bureau of Indian Standards sets the limit slightly lower at 45 mg/L (Gupta, et.al., 1999). Despite the intentions of these regulatory institutions, people in India are still frequently exposed to levels of up to 500 mg/L of nitrate in their drinking water (Gupta, et.al., 1999). The populations most affected by methemoglobinemia are infants and people older than 45 years (Gupta, et.al., 2008).
Cleaning up the extremely high levels of nitrates present in drinking water is expensive and not cost-efficient (Gupta, et.al., 2008). Since this is the case, governments need to focus on solutions that will decrease anthropogenic contributions to natural nitrate levels in water. The use of nitrogen containing fertilizers should be reduced to keep too many nitrates from washing into drinking water sources (Gupta, et.al., 2008). Individuals should also limit the use of antacids, as this will raise stomach pH and encourage the growth of nitrate-reducing bacteria (Gupta, et.al., 2008). Governments must also focus on educating people on the health effects that can result from excessive nitrate consumption (Gupta, et.al., 2008). Gupta, et.al. recommend that additional research needs to be done to better understand the toxicity of nitrate compounds and what levels of nitrate are safe for drinking water (2008).
References:
Gupta, S.K., Gupta, R.C., Chhabra, S.K., Eskiocak, S., Gupta, A.B., and Gupta, R. (2008). Health Issues Related to N Pollution in Water and Air. Current Science. 94(11): 1469-1477).
Gupta, S.K., Gupta, R.C., Seth, A.K., Gupta, A.B., Bassin, J.K., and Gupta, A. (1999). Adaptation of Cytochrome-b5 Reductase Activity and Methaemoglobinaemia in Areas with a High Nitrate Concentration in Drinking-water. Bull. WHO. 77(9): 749-753.
Dugdale, D.C. (2010). Hemoglobin. Medline Plus. http://www.nlm.nih.gov/medlineplus/ency/article/003645.htm
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Nitrate reductase (NADH) is an enzyme with system name nitrite:NAD+ oxidoreductase. This enzyme catalises the following chemical reaction:nitrite + NAD+ + H2O↔ nitrate + NADH + H+. Nitrate reductase us an iron-sulfur molybdenum flavoprotein. nitrate reductase
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