6-Phosphogluconate Dehydrogenase Isoenzymes from the Developing Endosperm of Ricinus communis L.

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Abstract
The cytosolic and proplastid isoenzymes of 6-phosphogluconate dehydrogenase were purified from the developing endosperm of the castor bean (R. communis L.). No differences in physical or kinetic properties were found for the purified isoenzymes. Each was composed of 2 identical 55,000 subunits. They had identical pH optima of 7.8-8.0 and similar MgCl2 stimulation for the oxidative decarboxylation of 6-phosphogluconate. The Km values for 6-phosphogluconate were 12 and 9.6 .mu.M and for NADP+ were 4.1 and 5.4 .mu.M for the cytosolic and proplastid isoenzymes, respectively. Therefore, the synthesis of 2 distinct 6-phosphogluconate dehydrogenase isoenzymes does not appear to have any kinetic significance for the developing seed. However, changes in the proplastid contribution toward carbohydrate metabolism occur in the developing seed and may necessitate independent gene expression to allow for a unique and flexible subcellular distribution of isoenzymes during development.