Nitrate reduction, inhibition

Biodegradation of fuel oils in sediments is inhibited under anaerobic conditions (Bartha and Atlas 1977). Under anaerobic conditions, some soil microorganisms are capable of nitrate reduction using fuel oils as the carbon source, although nitrite may be an unwanted by-product. However, the addition of a small amount of oxygen (0.2 volume percent oxygen) to the medium can accelerate the degradation of the oil without the concomitant production of nitrite (Riss and Schweisfurth 1987). 

When soybean leaves and pine needles were exposed to ozone, there was an initial decrease in the levels of soluble sugars followed by a subsequent increase. Ozone exposure also caused a decrease in the activity of the glycolytic pathway and the decrease in the activity was reflected in a lowered rate of nitrate reduction. Amino acids and protein also accumulated in soybean leaves following exposure. Ozone increased the activities of enzymes involved in phenol metabolism (phenylalanine ammonia lyase and polyphenoloxidase). There was also an increase in the levels of total phenols. Leachates from fescue leaves exposed to ozone inhibited nodulation. 

It was found that underpotentially deposited germanium strongly enhances the reduction rate of nitrate. The reduction of nitrite is enhanced to a lesser extent, whereas germanium is inactive for NO and hydroxylamine reduction. It is of interest that the well-known inhibition of the nitrate reduction at low potentials was absent for germanium-modified electrodes. 

The nitrite formed is either excreted directly or reduced by non-ATP-yielding reactions to ammonia. The enzyme machinery for both processes, nitrate/nitrite respiration and denitrification, is formed only under anaerobic conditions or conditions of low oxygen tension. In fact, the activities of the enzymes involved in dissimila-tory nitrate reduction are strongly inhibited by oxygen. Thus, denitrification and nitrate/nitrite respiration take place only when oxygen is absent or available in insufficient amounts. 

Reduction of Cu and Fe organic complexes is saturable, and is inhibited by cell-impermeable probes, including polyclonal antibodies raised to Chlorella nitrate reductase (Jones and Morel, 1987). The model proposed suggests that a fraction of a membrane-bound nitrate reductase (the so-called diaphorase component that catalyzes NADH reduction), spans the cell membrane (Fig. 4). Organic metal complexes intercept electrons (destined for nitrate reduction intracellularly) at the outer cell surface. 

A wide range of techniques are used to determine catabolic nitrate reduction rates. These include mass balance methods using input-outputs, acetylene inhibition techniques, dinitrogen production rates, nitrate consumption rates, nitrate pore water profiles, and stable isotope tracer techniques. The limitations and advantages of these methods are discussed by Seitzinger (1988) and Herbert (1999). 

Dissimilatory nitrate reductases (Pichinoty type A) in membrane fractions from bacteria have been shown capable of utilizing a variety of respiratory Intermediates and reduced pyridine nucleotides for nitrate reduction (Cole and Wimpeny, 1968 Knook et ai, 1973 Burke and Lascelles, 1975 Enoch and Lester, 1975). Reduction of nitrate by the membrane fractions, when respiratory substrates or pyridine nucleotides serve as reduc-tant, is generally inhibited by azide, cyanide and p-chloromercuribenzoate. Nitrate reduction, mediated by respiratory substrates, could be inhibited by n-heptylhydroxyquinoline-N oxide (HONO) or dicoumoral (Ruiz-Herrera and DeMoss, 1%9 Knook et al., 1973 Burke and Lascelles, 1975). However, in Micrococcus denitrificans (Lam and Nicholas, 1969) and in Bacillus stearothermophilus (Downey, 1%6) nitrate reduction is not inhibited by... 

Dissimilatory nitrate reduction by particulate preparations from bacteria can utilize reduced viologen dyes as electron donors. With this reductant, nitrate reduction is insensitive to inhibition byp-chloromercuribenzoate and HONO, but is prevented by cyanide and azide (Burke and Lascelles, 1975 Ruiz-Herrera and DeMoss, 1969). Respiratory intermediates or reduced pyridine nucleotides cannot serve as electron donors for the nitrate reductase solubilized from membrane complexes and the assay must be performed with reduced viologen dyes. 

Electrons are transferred by the respiratory electron chain to a cytochrome b which then donates electrons to the Mo-protein (nitrate reductase). By using reduced viologens it should be possible to determine the kinetics of nitrate reduction by the nitrate reductase however, no information on this aspect is currently available. The sensitivity of nitrate reduction to inhibition by cyanide is considered to be due to association of the inhibitor with molybdenum residues in the nitrate reductase (Enoch and Lester, 1975). Cytochrome b involved in nitrate reduction does not bind cyanide. 

Pyridine nucleotide mediated nitrate reductase and NADPH cytochrome c reductase activities in the purified preparations are inhibited by p-hydroxymercuribenzoate (Garrett and Nason, 1969 McDonald and Coddington, 1974 Guerrero and Gutierrez, 1977). This inhibition could be overcome with cysteine or dithiothreitol. Cyanide and azide inhibit pyridine nucleotide and reduced viologen dye mediated nitrate reduction but do not affect NADPH cytochrome c reductase activity. 

Nitrate reduction mediated by either reduced pyridine nucleotides or reduced viologen dyes is inhibited by cyanide and azide however, pyridine nucleotide mediated reduction of cytochrome c is insensitive to these inhibitors (Wray and Filner, 1970 Wallace and Johnson, 1978). Nitrate reduction mediated by reduced pyridine nucleotides and cytochrome c reduction is prevented by p-chloromercuribenzoate and is relatively heat sensitive in contrast reduced viologen or FMNH2 stimulated reduction is comparatively insensitive to sulfhydryl inhibitors and is more tolerant to heat (Schrader et al., 1968). 

Sulfhydryl reagents and heat will inhibit nitrate and cytochrome c reduction, but do not interfere with nitrate reduction mediated by FMNH2, etc. Azide and cyanide inhibit nitrate reduction but do not interfere with the NAD(P)H-mediated cytochrome c reduction. From work with barley (Wray and Filner, 1970) and spinach (Rucklidge et al., 1976) it can be inferred that NADH nitrate reductase is composed of a NADH-cytochrome c dehydrogenase and a Mo-protein. Nitrate induces the apoprotein (dehydrogenase) in molybdenum-deficient spinach leaves. This apoprotein mixed with an acid-dissociated product of spinach nitrate reductase can form an active nitrate reductase. 

Indications that the mitochondria regulate nitrate reduction in a direct manner are obtained from the observations that oxygen inhibits nitrite accumulation (Ferrari and Varner, 1970 Radin, 1973) and that uncouplers of the electron transport system permit nitrite accumulation under aerobic conditions (Ferrari and Varner, 1970). Canvin and Atkins (1974) and Atkins and Canvin (1975) reported that excised leaves under a dark aerobic gaseous environment do not assimilate into amino acids and that vac-... 

Other metabolic processes such as PEP-carboxylation in the cytosol appeared to be as insensitive to dehydration as photosynthesis (11). It is therefore difficult to interprete reports pointing to a rather high sensivity to water stress of nitrate reduction or nitrate reductase activity (NRA) in leaves. E.g.,it was shown that a 35% water deficit caused a more than 80% inhibition of NRA in cucumber leaves (5, and literature). Such a water deficit has practically no effect on photosynthesis at high external CO, (compare Fig.l). Therefore it seemed possible that the inhibition of the nitrate reducing system by water stress was a consequence of decreased photosynthesis rates, and not an independent event. This is in fact suggested by the following observations. 

When spinach leaves were slightly wilted to a water deficit of about 10%, nitrate reduction in air ceased (Fig.4), and so did malate accumulation (not shown). However, when the external COp concentration was drastically increased to 15%, nitrate reduction was restored, as was photosynthesis (compare Fig.l and 4). Thus, the inhibition of nitrate reduction by mild water stress in air appeared to be a consequence of stomatal closure, just like the inhibition of photosynthesis. 

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To induce this reaction, the kinetic inhibition of the reaction must be overcome by applying an overpotential, which must be minimized. This reaction, in which electrons are transferred across the metal-solution interface with a resulting nitrate reduction, is called a faradaic process. Also, the complexity of the interfacial system is such that other phenomena do occur that can affect the electrode behavior. These processes include adsorption, desorption, and charging of the interface as a result of changing electrode potential these are called non-faradaic processes. Both the efficiency and the selectivity of nitrate electroreduction strongly depend on several parameters such as the electrode composition, physicochemical properties of the electrolyte (pH, coexisting species, temperature, etc.) and the applied potential. 

Nitrate reduction. According to Kaspar (1982), P. acidipropionici, P. freudenreichii, P. jensenii, P. shermanii and P. thoenii can reduce nitrate to nitrite and further to N2O. Formation of N2O from nitrite in prokaryotes may represent a mechanism of detoxification rather than transformation of energy. N2O was not further reduced oxygen inhibited nitrate reduction by P. acidipropionici and P. thoenii. The enzymes of nitrate and nitrite reduction were either constitutive or derepressed in anaerobiosis only P. pentosaceum contained a constitutive nitrate reductase. Nitrate stimulated the synthesis of nitrate reductase in P. acidipropionici, specific growth rates and biomass yields were increased by the addition of nitrate. Nitrite at a concentration of 10 mM was not inhibitory. 

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