Abstract

VIEGAS, Ricardo Almeida  and  SILVEIRA, Joaquim Albenísio Gomes. Activation of nitrate reductase of cashew leaf by exogenous nitrite . Braz. J. Plant Physiol. [online]. 2002, vol.14, n.1, pp. 39-44. ISSN 1677-0420.  http://dx.doi.org/10.1590/S1677-04202002000100005.

Nitrate reductase (NR) plays a central role in plant primary metabolism and exhibits complex regulation mechanisms for its catalytic activity. There is controversy in the literature concerning the possible direct effect of NO₂^- on the stimulation and/or activation of NR activity. The influence of NO₂^- was studied on the NR activity in the leaves of 30-day-old cashew (Anacardium occidentale L.) seedlings. Addition of NO₂^- to the reaction mixture containing leaf enzymatic extract resulted in a remarkable increase in NR activity. A trace amount (5 mmol.L^-1) of NO₂^- was required to achieve full NR activity. The in vitro NR-activity showed a steady time-dependent increase when incubated in the presence of only NO₃^- + NO₂^-_. In contrast, in vitro NR activity was practically absent in a NO₂^- -free reaction medium, even in the presence of a saturating NO₃^- concentration. No oxidation of NO₂^- to NO₃^- was detected during the experiment. Although NO₂^- clearly activated the in vitro NR activity, it had no effect on the in vivo leaf-NR activity determined under absence of oxygen. NADH at concentrations greater than 0.5 mmol.L^-1 decreased the rates of in vitro NR activity. These results indicated, at least partially, a strong cashew leaf NO₂^- dependency of NR activation and/or activity. Finally, based on these results, it is suggested that this singular NR activity model induced by exogenous NO₂^- in cashew leaf extracts is caused by changes in the NR activation state by NO₂^- itself.

Keywords : Anacardium occidentale; enzymes; NADH; nitrate metabolism; nitrate reductase.