Biological nitrogen oxides

Health nd SMety Factors. The lowest pubhshed human oral toxic dose is 430 mg/kg, causing nervous system disturbances and gastrointestinal symptoms. The LD q (rat, oral) is 750 mg/kg (183). Thiocyanates are destroyed readily by soil bacteria and by biological treatment systems in which the organisms become acclimatized to thiocyanate. Pyrolysis products and combustion products can include toxic hydrogen cyanide, hydrogen sulfide, sulfur oxides, and nitrogen oxides. 

Nitrogen oxides are formed at various stages of the biological denitrification process. This process starts with nitrate as the nitrate is reduced through various steps, NO2, NO, N2O, and N2 can be formed and, depending on the conditions, released to the atmosphere. 

Nitrogen oxide (NO) has been found to be a key component in many biological processes. It also can react with oxygen to give die brown gas N02. When one mole of NO reacts with oxygen, 57.0 kj ofheat is evolved. 

The stratosphere contains, however, only small amounts--a few tenths of a ppb-of chlorine free radicals of natural origin. They are produced by the decomposition of methyl chloride, CH3Q. The nitrogen oxides (NO and NO2) are more abundant and are produced in the stratosphere by the decomposition of nitrous oxide, N2O. Both CH3CI and N2O are of biological origin these compounds, released at the Earth s surface, are sufficiently stable to reach the stratosphere in significant amounts. 

Nitrification. Nitrification refers to biologically mediated oxidation of reduced nitrogen to higher oxidation states ... 

Nitrogen oxide (NO) is an example of heteronuclear diatomic molecules, those composed of different atoms. This interesting molecule has been in the news several times in recent years, because of important discoveries about the role of NO as a biological messenger, as we describe in our introduction to Chapter 21. 

Bioprocesses for the removal of nitrogen oxides from polluted air are an interesting alternative [58], but current reaction rates are still too low for large-scale applications. Advanced biological processes for the removal of NO from flue gases are based on the catalytic activity of either eukaryotes or prokaryotes, e.g. nitrification, denitrification, the use of microalgae and a combined physicochemical and biological process (BioDeNO ). 

The chemistry of inorganic nitrogen compounds is very complicated, and therefore, it is difficult to prove which of these compounds is of a real importance in biological systems. In addition to NO and peroxynitrite, the formation of N02, N203, and NO- might be of importance in biological systems. Some reactions of nitrogen oxide species are cited below. 

In denitrification, part of the biological nitrogen cycle, nitrate in the soil is converted via four enzymatic reactions stepwise to nitrite, nitric oxide and nitrous oxide to finally yield gaseous nitrogen. 

A variety of nitrogen oxides (NO ) such as nitric oxide (NO) and nitrogen dioxide (NO2) as well as nitrous oxide (N2O) are present in the atmosphere. The sources of these oxides are biological actions and organic decomposition in the soil and in the ocean... 

Figure 4.1 shows that NOs" is the stable form of nitrogen over the usual range of pe + pH in aerobic environments. The fact that most of the N2 in the atmosphere has not been converted to NO3 therefore indicates that the biological mediation of this conversion in both directions is inefficient. Hence NO3 reduction to N2 occurs by indirect mechanisms involving intermediaries. Dissimilatory reduction of N03 (i.e. where the nitrogen oxide serves as an electron acceptor for the cell s metabolism but the N reduced is not used by the microbes involved) potentially occurs by two processes denitrification. 

AltshuUer. A. P.. D. L. Klosterman, P. W. Leach. I. J. Hindawi, and J. E. Sigsby. Jr. Products and biological effects from irradiation of nitrogen oxides with hydrocarbons or aldehydes under dynamic conditions. Int. J. Air Water Pollut. 10 81-96, 1966. 

National Research Council. Committee on Medical and Biologic Effects of Environmental Pollutants. Nitrogen Oxides. Washington, D.C. National Academy of Sciences, (in press)... 

Comparison of the observed pseudo-first-order decay of biological activity with a half-life of 30 sec at normal oxygen tensions versus decomposition via nitrogen dioxide by pseudo-second-order kinetics predicted by Reaction 4. The loss of nitric oxide through formation of nitrogen oxide is twice as fast as calculated by Reaction 4 because each nitrogen dioxide formed rapidly attacks a second nitric oxide to form nitrite. 

The chemistry of the nitrogen oxides dates back to the days of the Reverend Joseph Priestley, who used the reaction of nitric oxide to measure the concentration of oxygen in air. As a consequence, many of the recommended lUPAC names for nitrogen species have common names. As a general rule, common names are used when they have been widely utilized in the biological literature and lUPAC names for less well-known chemical species. Table 3 should help facilitate translation among the different names. 

Although much of the biological literature focuses on nitrosating reactions of nitric oxide, chemically nitric oxide is a moderate one-electron oxidant, making formation of nitroxyl anion feasible under physiological conditions. The reduction potential to reduce nitric oxide to nitroxyl anion is +0.39 V, whereas it requires +1.2 V to oxidize nitric oxide to nitrosonium ion. Nitrosating reactions of nitric oxide are often mediated by conversion of nitric oxide to another nitrogen oxide species or by direct reaction with transition metals (Wade and Castro, 1990). 

The study of the biosynthesis of inorganic nitrogen oxides has a long and distinguished history, principally in the field of microbial and plant nitrogen metabolism (Zumft, 1993). Relatively recently, it has been found that NO, in addition to other species such as nitrite, nitrate, and NjO, is a true intermediate in the biological nitrogen cycle, as described in detail in Chapter 9. 

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