Nitrites and nitrates
Sodium nitrate finds uses in artificial fertilisers, food preservatives, explosives (as do the potassium salts). Sodium nitrite is also used as a food preservative and in the manufacture of explosives and as a cleaner in dentistry. Nitrite has the additional property of causing vasodilation, which has led to its application in treating angina pectoris. This vasodilatory effect of alkyl nitrites (e.g. amyl, butyl and isobutyl nitrites) has led to abuse of these drugs for the ‘rush’ they provide and, reportedly, heightened sexual arousal, disinhibition and muscle relaxation. Ingestion of these substances, which are supplied to be inhaled, can be fatal. Organic nitrates (e.g. glyceryl trinitrate, isosorbide mononitrate and dinitrate) release nitrite ion when ingested and are also used as vasodilators. Symptoms of nitrate and/or nitrite poisoning include nausea, vomiting, diarrhoea, abdominal pain, confusion and coma. Nitrite in the body combines with haemoglobin to produce methaemoglobin which reduces the capacity of the blood to carry oxygen (methaemoglobinaemia). Unborn babies and those up to 3–4 months old are particularly susceptible to methaemoglobinaemia because they are deficient in methaemoglobin reductase, the enzyme that converts methaemoglobin to haemoglobin, and because their stomachs are less acidic than those of adults and hence are more likely to permit growth of nitrate-reducing bacteria. Thus, if nitrates are ingested by babies they are more likely to be converted to nitrites with resultant methaemoglobinaemia, which gives rise to the so-called ‘blue-baby’ syndrome because the skin appears blue. To minimise problems arising from high nitrate levels, levels in water in most developed countries are closely controlled, with maximum levels imposed. Public water supplies are treated to reduce nitrate levels but this may not be the case for water drawn from wells or other untreated sources. Methaemoglobin measurement is the most useful test in the diagnosis and clinical management of poisoning cases. Methaemoglobin causes blood to turn brown. This can be exploited in colour tests where the addition of two drops of potassium cyanide solution to 1 mL of blood gives an immediate colour change from brown to red in the presence of methaemoglobin.
Several other medical conditions can give rise to contraindications for nitrate and nitrite intake. These include persons with congenital NADPH-methaemoglobin reductase deficiency andthosewithglucose-6-phosphatedehydrogenase deficiency. Plasma nitrite and nitrate concentrations in unexposed subjects are about 0.2 mg/L and 1.2 mg/L, respectively. In a fatal suicide, nitrite concentrations were 0.5 mg/L in blood. Survival has been reported in two men who accidentally ingested about 1 g of sodium nitrite.
