Nitrate-reducing system

Guerrero, M.G., Vega, J.M. Losada, M. (1981). The assimilatory nitrate-reducing system and its regulation. Annual Review of Plant Physiology 32, 169-201. 

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. 

Nitrates (e.g., ISDN) and hydralazine were combined originally in the treatment of HF because of their complementary hemodynamic actions. Nitrates are primarily venodilators, producing reductions in preload. Hydralazine is a direct vasodilator that acts predominantly on arterial smooth muscle to reduce systemic vascular resistance (SVR) and increase stroke volume and cardiac output. Evidence also suggests that the combination may provide additional benefits by interfering with the biochemical processes associated with HF progression. 

Pharmacology The principal pharmacological action of nitrates is relaxation of the vascular smooth muscle and consequent dilation of peripheral arteries and especially the veins. Dilation of the veins promotes peripheral pooling of blood and decreases venous return to the heart, thereby reducing left ventricular end-diastolic pressure and pulmonary capillary wedge pressure (preload). Arteriolar relaxation reduces systemic vascular resistance, systolic arterial pressure, and mean arterial pressure (afterload). Dilation of the coronary arteries also occurs. The relative importance of preload reduction, afterload reduction, and coronary dilation remains undefined. 

Arcangeli,J. P. Arvin, E. (1994). Biodegradation of BTEX compounds in a biofilm system under nitrate-reducing conditions. In Hydrocarbon Bioremediation, ed. R. E. Hinchee, B. C. Alleman, R. E. Hoeppel R. N. Miller, pp. 374—82. Boca Raton, FL Lewis Publishers. 

Figure 18-21 Organization of the nitrate reduction system in the outer membrane of the bacterium Paracoccus dinitrifi-cans as outlined by Blaut and Gottschalk.315 The equations are not balanced as shown but will be balanced if two NO-, ions are reduced to N2 by five molecules of NADH (see also Eq. 18-28). Although this will also require seven protons, about 20 additional H+ will be pumped to provide for ATP synthesis.

Dilation of venous blood vessels leads to a decrease in cardiac preload by increasing venous capacitance arterial dilators reduce systemic arteriolar resistance and decrease afterload. Nitrates (see p. 175) are commonly employed venous dilators for patients with congestive heart failure. If the patient is intolerant of ACE inhibitors, the combination of hydralazine and isosorbide dinitrate is most commonly used. Amlodipine and felodipine (see p. 188) have less negative inotropic effect than other calcium channel blockers, and seem to decrease sympathetic nervous activity. 

Nitrite reduction in assimilatory nitrate-reducing Neurospora crassa, Torulopsis nitratophila, Azotobacter vinelandii, and Azotobacter chro-ococcum appears to be catalyzed by enzyme systems which require flavin and metals. The enzyme from N. crassa has been partially purified, and its molecular weight has been estimated to be 300,000 (344, 346, 351, 367). The enzyme reduces both nitrite and hydroxylamine to ammonia and utilizes NADH or NADPH as electron donor. It is reported to be a FAD-dependent enzyme and to contain iron, copper, and active thiol (346, 367). Three moles of NADH are oxidized per mole of nitrite reduced to ammonia. It has been suggested that the reduction of nitrite occurs in three steps, each involving two electrons. Thus, hyponitrite and hydroxylamine have been proposed as successive intermediates in the re-... 

Molecular mechanisms of nitrate accumulation depend not only on the nitrate reductase system, but also on the ability of roots to take from the soil, nitrate or ammonium ions, and on the plant s capacity for their conversion by assimilation processes to higher products. Besides this, the assimilation depends on the ability of a given genotype to transport substances necessary for the synthesis. It was shown that genotype differences of the nitrate reductase level do not depend on the nitrate content in tissues [25]. Nitrates are accumulated in plant organisms at high concentrations when aU the nitrogen accepted cannot be utilized for the production of amino acids and for subsequent protein synthesis [26]. This occurs when the plant, in the course of its metabolism, is unable to reduce the accepted nitrates into the assimilable ammonia form. 

Many facultative heterotrophic bacteria (e.g.. Pseudomonas, Micrococcus, Denitrobacillus, Spirillium, Achromo-bacter, etc.) are capable of denitrification. Denitrification takes place under anaerobic conditions, a precondition for the formation of a nitrate-reducing enzyme system. 

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