RAPID MODULATION OF SPINACH LEAF NITRATE REDUCTASE-ACTIVITY BY PHOTOSYNTHESIS .1. MODULATION INVIVO BY CO2 AVAILABILITY

It has been shown recently that in spinach leaves (Spinacia oleracea) net photosynthesis and nitrate reduction are closely linked: when net photosynthesis was low because of stomatal closure, rates of nitrate reduction decreased (WM Kaiser, J Forster [1989] Plant Physiol 91: 970-974). Here we present evidence that photosynthesis regulates nitrate reduction by modulating nitrate reductase activity (NRA, EC 1.6.6.1). When spinach leaves were exposed to low CO2 in the light, extractable NRA declined rapidly with a half-time of 15 minutes. The inhibition was rapidly reversed when leaves were brought back to air. NRA was also inhibited when leaves were wilted in air; this inhibition was due to decreased CO2 supply as a consequence of stomatal closure. The modulation of NRA was stable in vitro. It was not reversed by gel filtration. In contrast, the in vitro inhibition of nitrate reductase (NR) by classical inhibitors such as cyanide, hydroxylamine, or NADH disappeared after removal of free inhibitors by gel filtration. The negative modulation of NRA in -CO2-treated leaves became manifest as a decrease in total enzyme activity only in the presence of free Mg2+ or Ca2+. Mg2+ concentrations required for observing half-maximal inhibition were about 1 millimolar. In the presence of EDTA, the enzyme activity was always high and rather independent of the activation status of the enzyme. NRA was also independent of the pH in the range from pH 7 to pH 8, at saturating substrate and Mg2+ concentrations. The apparent substrate affinities of NR were hardly affected by the in vivo modulation of NR. Only V(max) changed.