Serine metabolism in legume nodules: Purification and properties of phosphoserine aminotransferase [ureides]

Enzymes involved in serine metabolism were compared in the plant cytosol fraction of root nodules from soybean [Glycine max (L.) Merr., cv. Amsoy] and lupin [Lupinus angustifolius L., cv. Uniharvest]. Soybean nodules are ureide exporters and have a serine requirement in the synthesis of these compounds, whereas lupin nodules are amide transporters and do not have a serine requirement in asparagine synthesis. Specific activities were measured for phosphoserine aminotransferase (PSAT; EC 2.6.1.52) and phosphoglycerate dehydrogenase (EC 1.1.1.95), as representative activities of the ‘phosphorylated’ pathway, and for serine‐pyruvate aminotransferase (EC 2.6.1.51), and glycerate dehydrogenase (EC 1.1.1.29), as representative activities of the ‘non‐phosphorylated’ pathway. Phosphorylated pathway enzyme levels were higher in soybean nodules. During nodule development nonphosphorylated pathway activities fell in soybean nodules, whereas in lupin nodules the specific activities of these enzymes increased. These data are consistent with roles for the phosphorylated and non‐phosphorylated pathways in serine anabolism and catabolism, respectively. PSAT was purified 250‐fold from soybean nodules. The enzyme showed linear reaction kinetics and was unaffected by serine. It had a molecular weight of 85 kDa, high specificity for its substrates and an optimum pH between pH 7.5 and 9. These results are discussed in relation to regulation of carbon supply to purine biosynthesis in ureide biogenesis in soybean root nodules.