Nitrite

About: Nitrite is a research topic. Over the lifetime, 15425 publications have been published within this topic receiving 484581 citations.

Posttranslational Regulation of Nitrate Reductase in Higher Plants.

Abstract: In many plants, nitrogen represents 2 to 6% of dry matter, most of it being present in the form of amino acids, proteins, or nucleic acids. In these organic compounds, nitrogen exists in its most reduced state (oxidation state -3). It is taken up from the soil primarily in the form of nitrate (oxidation state +5). Thus, nitrate has to be reduced by plants at the expense of reductants such as NADH or NADPH, requiring 8 mol of electrons [or 4 mol of NAD(P)H] per mol of nitrate. Reduction is a two-step mechanism. The first step, reduction of nitrate to nitrite (+3 oxidation state), is catalyzed by assimilatory NR, which is an NAD(P)H-dependent cytosolic enzyme. Nitrite is further reduced to ammonia (-3 oxidation state) in the plastids of leaves or roots by NiR. There are at least two important reasons why plants must exert control on the velocity of nitrate reduction. First, it is an energy-consuming process. As shown above, 8 electrons are required to reduce one nitrate to ammonium, but only 4 electrons are needed to reduce CO2 to the carbohydrate level. Accordingly, a C/N ratio of 10 in the plant biomass (a value found in many herbaceous plants) indicates that about 20% of the photosynthetically produced electrons are consumed for nitrate reduction. Plants have to avoid 'luxury' consumption of nitrate and energy. Second, and perhaps more important, the primary product of nitrate reduction, nitrite (NO2-), is cytotoxic and regarded to be mutagenic as a result of the ability to diazotize amino groups. HNO2 is also a weak acid that, in its undissociated form, can easily penetrate biomembranes, thereby leading to acidification of cells or subcellular compartments. Thus, it makes sense if nitrate reduction is controlled in such a way that it does not exceed nitrite and ammonium consumption. In fact, even under rapidly fluctuating environmental conditions, nitrite levels in plant tissues remain low (below 0.1 mM). Apparently, the overall rates of nitrate and nitrite reduction always match the availability of energy and of carbohydrate, perhaps with the exception of some extreme conditions to be discussed below. Synchronization of nitrate reduction and carbon metabolism may occur at the level of transcription or translation of participating enzymes. The expression of NR genes at the transcription level is highly affected by nitrate, but also by light, plant hormones, and other factors (for review, see Solomonson and Barber, 1990; Lillo, 1994). The enzyme protein is rather short-lived, being degraded with a half-time of a few hours (Li and Oaks, 1993). This high turnover rate permits control of nitrate reduction, e.g. in response to nitrate availability (Li and Oaks, 1993). However, over the years, a number of situations have been described in which the level of extractable NRA did not match NR protein or the rate of nitrate reduction in vivo, indicating that yet other regulatory mechanisms might exist that modulate the catalytic activity of the protein (Lillo, 1994, and refs. cited therein). Such a newly discovered type of NR modulation, a reversible protein phosphorylation, will be described below.

Bacterial transformations of inorganic nitrogen in the oxygen-deficient waters of the Eastern Tropical South Pacific Ocean

Abstract: Rates of transformations of inorganic nitrogen were measured in the low oxygen, subsurface waters (50–450 m) of the Eastern Tropical South Pacific during February 1985, using 15N tracer techniques. Oxygen concentrations over the entire region were in a range (O2 Measured rates of nitrate reduction and estimated rates of denitrification were sufficient to respire nearly all of the surface primary production that might be transported into the oxygen deficient zone. These results imply that the supply of labile organic material, especially from the surface, was more important than oxygen concentration in modulating the rates of nitrogen transformations within the low oxygen water mass of the Eastern Tropical South Pacific. The pattern of nitrite oxidation and nitrite reduction activities in the oxygen minimum zone supports the hypothesis ( Anderson et al., 1982, Deep-Sea Research, 29, 1113–1140) that nitrite, produced from nitrate reduction, can be recycled by oxidation at the interface between low and high oxygen waters. Rates for denitrification, estimated from nitrate reduction rates, were in harmony with previous estimates based on electron transport system (ETS) measurements and analysis of the nitrate deficit and water residence times. Assimilation rates of NH4+ were substantial, providing evidence for heterotrophic bacterial growth in low oxygen waters. Ambient concentrations of ammonium were maintained at low values primarily by assimilation; ammonium oxidation was an important mechanism at the surface boundary of the low oxygen zone.

Ammonium and nitrite oxidation at nanomolar oxygen concentrations in oxygen minimum zone waters

Abstract: A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O2) sensitivity of these processes down to the O2 concentrations present in the OMZ core (<10 nmol⋅L−1) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O2 levels between 5 nmol⋅L−1 and 20 μmol⋅L−1. Rates of both processes were detectable in the low nanomolar range (5–33 nmol⋅L−1 O2), but demonstrated a strong dependence on O2 concentrations with apparent half-saturation constants (Kms) of 333 ± 130 nmol⋅L−1 O2 for ammonium oxidation and 778 ± 168 nmol⋅L−1 O2 for nitrite oxidation assuming one-component Michaelis–Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis–Menten model, indicating a high-affinity component with a Km of just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O2 affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O2 is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.