Molecular Cloning and Characterization of Cytosolic Isoform of Ribulose-5-Phosphate 3-Epimerase from Rice

Abstract

Although the oxidative pentose phosphate pathway (OPPP) in plants partially contributes to respiration, it plays a much more important role as a source of NADPH for the reduction of metabolites in various biosynthetic pathways. In animal and yeast cells, OPPP is present in the cytosol and, therefore, it was generally accepted that the cytosol of plant cells also contains this pathway (1). After a CO2 release from [1-14C]glucose by isolated chloroplasts was demonstrated it was believed that two sets of isoenzymes of this path­way occur in chloroplasts and in cytosol (2,3). However, the quantitative distribution of OPPP enzymes among the cell compartments in green and non-green plant cell is still unclear. There remains no doubt about the existence of cytosolic and plastidic isoforms of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in leaf mesophyll and non-green plant cells (4). In contrast, cell fractionation experiments combined with chromatographic separation of enzyme isoforms did not provide evidence for the occurrence of cytoplasmic OPPP enzymes catalyzing the regenerative sequence of the pathway including transketolase, ribose-5-phosphate isomerase, and ribulose-5-phosphate 3-epimerase (RPE) (5). This led to the conclusion that plastids contain a complete OPPP, whereas an incomplete pathway is operating in the cytoplasm of leaf mesophyll cells.