Isoenzymes of phosphogluconate dehydrogenase (decarboxylating) from suspension-cultured Catharanthus roseus cells

Abstract

Two isoenzymes of phosphogluconate dehydrogenase (PGD; EC 1.1.1.44) were found in suspensioncultured Catharanthus roseus cells. They are located in the cytosol and the plastids (amyloplasts). These two isoenzymes were separated by fractionation on an anion-exchange column by HPLC. The two isoenzymes had similar kinetic properties and the same effectors. The K m values for 6-phosphogluconate were 15 and 18 μM and K m values for NADP + were 8 and 11 μM for the cytosolic and plastid isoenzymes, respectively. NADPH was a potent inhibitor of both isoenzymes. ATP and 5-phosphoribosyl-1-pyrophosphate also inhibited the activities of these isoenzymes. The maximum level of PGD was observed in cells at the lag phase of growth. The activity of cytosolic PGD relative to that of the plastid PGD was also high in these cells. In contrast, the activity of the plastid isoenzyme of PGD was ca two-fold higher than that of the cytosolic isoenzyme in the cells at the early stationary phase of growth. The role and regulation of these isoenzymes of PGD are discussed.