ENZYMATIC EVIDENCE FOR A COMPLETE OXIDATIVE PENTOSE-PHOSPHATE PATHWAYIN CHLOROPLASTS AND AN INCOMPLETE PATHWAY IN THE CYTOSOL OF SPINACH LEAVES

The intracellular localization of transaldolase, transketolase, ribose -5-phosphate isomerase, and ribulose-5-phosphate epimerase was reexami ned in spinach (Spinacia oleracea L.) leaves. We found a highly predom inant if not exclusive localization of these enzyme activities in chlo roplasts isolated by isopyknic centrifugation in sucrose gradients. Gl ucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glu cose phosphate isomerase, and triose phosphate isomerase activity was present in the chloroplast fraction but showed additional activity in the cytosol (supernatant) fraction attributable to the cytosol-specifi c isoforms known to exist for these enzymes. Anion-exchange chromatogr aphy of proteins of crude extracts on diethylaminoethyl-fractogel reve aled only a single enzyme each for transaldolase, transketolase, ribos e-5-phosphate isomerase, and ribulose-5-phosphate epimerase. The data indicate that chloroplasts of spinach leaf cells possess the complete complement of enzymes of the oxidative pentose phosphate pathway (OPPP ), whereas the cytosol contains only the first two reactions, contrary to the widely held view that plants generally possess a cytosolic OPP P capable of cyclic function. The chloroplast enzymes transketolase, r ibose-5-phosphate isomerase, and ribulose-5-phosphate epimerase appear to be amphibolic for the Calvin cycle and OPPP.