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Induction of Glyconeogenic Enzymes by Gibberellin A₃ in Endosperm of Castor Bean Seedlings ¹

Department of Biology, University of California, Los Angeles, California 90024

Seedlings of castor bean (Ricinus communis cv. Hale) were exposed to gibberellin A₃ (GA₃) (100 micromolar) for periods up to 20 hours. Endosperm homogenates were fractionated on linear sucrose gradients and enzymes in mitochondria, glyoxysome, and cytosol fractions were assayed. Gibberellin treatment resulted in increases in the activities of enzymes in all three compartments. There were also enzymes in all three compartments which were not affected by exogenous applications of GA₃. The isozymes of L-asparate--ketoglutarate aminotransferase in both mitochondria and glyoxysomes were induced coordinately, whereas the isozymes of citrate synthase and malate dehydrogenase were not. All gluconeogenic enzymes in glyoxysomes are induced by GA₃. With the exception of the mitochondrial malate dehydrogenase isozyme, all enzymes of the tricarboxylic acid cycle believed to participate in glyconeogenesis were increased. The cytosolic enzymes malate dehydrogenase, phosphoenolpyruvate carboxykinase, and fructose bisphosphatase were induced, but the levels of pyruvate kinase and enolase were not affected by GA₃ treatment.