Oxidation of Inorganic Nitrogen Compounds

Nitrogen plays an integral part in life processes of a biological cell since the synthesis of cellular proteins, amino acids, purines, pyrimidines, nucleic acids, and enzymes is dependent on this essential element. The biological conversion of the oxidized inorganic nitrogen compounds to the ammonia or amino level appears to be an essential metabolic aspect of almost all plants and many microogranisms. They play a vital role in providing nitrogen to many forms of life, including some particular microorganisms and virtually all animals as well as man ; the latter systems lack the ability to reduce more oxidized states of nitrogen and, therefore, they fulfill their obligate nitrogen requirements from the exogenous supply of organic nitrogen and ammonia. It is obvious , therefore, that the inorganic nitrogen is the ultimate nitrogen source for all forms of life on this earth. In nature the nitrogen atom exists in the following oxidation states: N205 or HNOa (+5), N02 (+4), N20a or HN02 (+3), NO or H2N20a (+2), the (+1) oxidation states as represented by N20, HNO, H2N202 and NOzNH2; Nz(O)NHzOH (-1), NHzNH2 ( 2), and finally the most reduced state of nitrogen, NHa (-3). Each of these compounds has been implicated in the inorganic nitrogen metabolism of either intact organisms or their cell-free preparation (51). Although the (+6) oxidation state as represented by NOa, a short-lived compound, has been reported (74), its metabolic role if any is unknown at present. The biological oxidation of reduced inorganic nitrogen compounds is catalyzed predominantly by the bacterial genera Nitrosomonas and Nitro­ bacter, which oxidize ammonia to nitrite and nitrite to nitrate respectively, a process commonly known as nitrification. These bacteria are obligate chemoautotrophs which derive their energy from the respective primary inorganic oxidations that are coupled to the cellular biosynthetic reactions involving the reduction of carbon dioxide by mechanisms very similar to those of the carbon reduction cycle in photosynthetic organisms. The basic mechanisms of the respiratory and intermediary metabolism of the nitrifying bacteria have been reviewed previously by Lees (44, 45) and by Aleem & Nason (13), and very recently in excellent review articles by Peck (55) and by Wallace & Nicholas (73).