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Plant Physiology 62:287-290 (1978)
© 1978 American Society of Plant Biologists

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6-Phosphogluconate Dehydrogenase Isoenzymes from the Developing Endosperm of Ricinus communis L. 1

P. David Simcox2 and David T. Dennis

Department of Biology, Queen's University, Kingston, Ontario, K7L 3N6 Canada

The cytosolic and proplastid isoenzymes of 6-phosphogluconate dehydrogenase were purified from the developing endosperm of the castor bean (Ricinis communis L.). No differences in physical or kinetic properties were found for the purified isoenzymes. Each was composed of two identical 55,000 subunits. They had identical pH optima of 7.8 to 8.0 and similar MgCl2 stimulation for the oxidative decarboxylation of 6-phosphogluconate. The KM values for 6-phosphogluconate were 12 and 9.6 micromolar and for NADP+ were 4.1 and 5.4 micromolar for the cytosolic and proplastid isoenzymes, respectively. Therefore, the synthesis of two 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.

2 Present Address: Diamond Shamrock Corp., P.O. Box 340, Painesville, Ohio, 44077.

1 Funding was provided by the National Research Council of Canada.






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Copyright © 1978 by the American Society of Plant Biologists