Responses to nitrogen sources and regulatory properties of root phosphoenolpyruvate carboxylase

Abstract

Time course of changes in extractable root phosphoenolpyruvate carboxylase (PEP C) activity was investigated in wheat, barley, and tomato plants fed with different nitrogen sources. Ammonium-fed plants exhibited a 2–2.5-fold higher PEPC activity than nitrate-fed plants at 7 d after the onset of nitrogen supply. Western blot analysis revealed that the amounts of PEPC subunit proteins increased gradually as reflected in the extractable PEPC activity. These results suggest that the increase in PEPC activity may be due to de novo protein synthesis. PEPC was SO-fold purified from tomato roots after several chromatographic steps. Metabolite effects on the partially purified enzyme were also investigated under optimal or suboptimal conditions in terms of pH and concentrations of phosphoenolpyruvate. Organic acids and acidic amino acids inhibited the enzyme activity, while hexose phosphates stimulated it. Glutamine and asparagine produced in the course of ammonium assimilation hardly affected the activity. © 1997 Taylor & Francis Group, LLC.