Involvement of magnesium and ATP in the regulation of nitrate reductase activity in Sinapis alba

Abstract

The activity of NADH: nitrate reductase (NR, EC 1.6.6.1) was extracted from mustard ( Sinapis alba) cotyledons and leaves. When seedlings/plants were grown following light/dark cycles in the presence of nitrate, an increase in the activity of NR was observed during light periods compared to dark periods. An inhibition in NR activity was observed after treatment with Mg 2+ and ATP in extracts from both cotyledons and leaves prepared from tissue harvested during light periods. No apparent inhibition was observed in tissue extracts harvested during dark periods. Two isoforms of mustard NR (NR 1 and NR 2) were separated by the use of anion exchange chromatography. The results indicate that NR 2 (the more negatively charged isoform) corresponds to a light-inducible form, which was selectively and strongly inhibited by Mg 2+/ATP. The results presented here indicate that the modulation of NR activity by light can be attributed, at least in part, to a protein reversible phosphorylation/dephosphorylation mechanism on NR 2.