Nitrate reductase-dependent production

H. Yamasaki and Y. Sakihama, Simultaneous production of nitric oxide and peroxynitrite by plant nitrate reductase in vitro evidence for the NR-dependent formation of active nitrogen species. FEBS Lett. 468, 89-92 (2000). 

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. 

The production of an active nitrate reductase in response to nitrate is dependent upon a supply of molybdate. Molybdate-deficient fungi (Dantzig etal., 1978), higher plants (Notton et o/., 1974), algae (Vega et a/., 1971), and bacteria (Lester and DeMoss, 1971 Enoch and Lester, 1972) produce aber-... 

Vennesland s group has concluded that the inactivation in vivo after the cells have been treated with ammonia involves the formation of a firmly bound complex of reduced enzyme and cyanide. They have speculated that the product of ammonium assimilation which inhibits nitrate reductase is cyanide. In this respect the COj requirement for inactivation by ammonia in the light could be relevant. On the other hand, COg could potentiate the stimulating effect of ammonia on the photosynthetic non-cyclic electron flow.< If this interpretation would be correct, the activation of the ADP-dependent pyruvate kinase reaction by ammonia discovered by Bassham in Chlorella might be better explained by an increase in the cellular ADP level induced by the uncoupling effect of ammonia on photophosphorylation than by a direct activation of the kinase by the ammonium cation, as it has been postulated. 

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