Modulation of Nitrate Reduction - Environmental and Internal Factors Involved

The major purpose of the cytosolic enzyme nitrate reductase (NR, EC 1.6.6.1) is to catalyse the reduction of nitrate to nitrite at the expense of NAD(P)H. In addition to that, however, NR also catalyses, though with much lower capacity, two side reactions: 1) the reduction of nitrite to nitric oxide (NO) and 2) the reduction of molecular oxygen to superoxide (Ruoff and Lillo, 1990, Barber and Kay 1996), which can both react chemically to give the highly toxic compound peroxynitrite (Figure 1, Yamasaki and Sakihama 2000). The production of such potentially toxic products (of which NO may also act as a signalling molecule in plants) is probably one reason why NR expression and activity are so tightly controlled.

For regulation of NR expression and activity by light see chapter 6, Lillo (1994) and Lillo and Appenroth (2001). In leaves, NR becomes more active upon illumination within minutes, and is as rapidly inactivated in the dark. Based on these short response times it was suggested that NR activity would not be modulated via changes in transcription or translation, but via modulation of the catalytic activity of the existing NR protein. More recently, in plants expressing NR under the control of the constitutive 35S-promoter, rapid light/dark changes in extractable NR activity from leaves were still observed, although mRNA levels expectedly did not vary in dark/light (Vincentz et al. 1993). Such observations strongly confirm that the existing NR molecule undergoes a reversible modulation in catalytic activity. In the past decade, the mechanism of the post-translational modulation has been investigated in detail.