Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Plant Physiology 77:358-364 (1985)
© 1985 American Society of Plant Biologists

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Articles

Synthesis and Degradation of Fructose 2,6-Bisphosphate in Endosperm of Castor Bean Seedlings 1

Nicholas J. Kruger2 and Harry Beevers

Biology Department, University of California, Santa Cruz, California 95064

The aim of this work was to examine the possibility that fructose 2,6-bisphosphate (Fru-2,6-P2) plays a role in the regulation of gluconeogenesis from fat. Fru-2,6-P2 is known to inhibit cytoplasmic fructose 1,6-bisphosphatase and stimulate pyrophosphate:fructose 6-phosphate phosphotransferase from the endosperm of seedlings of castor bean (Ricinus communis). Fru-2,6-P2 was present throughout the seven-day period in amounts from 30 to 200 picomoles per endosperm. Inhibition of gluconeogenesis by anoxia or treatment with 3-mercaptopicolinic acid doubled the amount of Fru-2,6-P2 in detached endosperm. The maximum activities of fructose 6-phosphate,2-kinase and fructose 2,6-bisphosphatase (enzymes that synthesize and degrade Fru-2,6-P2, respectively) were sufficient to account for the highest observed rates of Fru-2,6-P2 metabolism. Fructose 6-phosphate,2-kinase exhibited sigmoid kinetics with respect to fructose 6-phosphate. These kinetics became hyperbolic in the presence of inorganic phosphate, which also relieved a strong inhibition of the enzyme by 3-phosphoglycerate. Fructose 2,6-bisphosphatase was inhibited by both phosphate and fructose 6-phosphate, the products of the reaction. The properties of the two enzymes suggest that in vivo the amounts of fructose-6-phosphate, 3-phosphoglycerate, and phosphate could each contribute to the control of Fru-2,6-P2 level. Variation in the level of Fru-2,6-P2 in response to changes in the levels of these metabolites is considered to be important in regulating flux between fructose 1,6-bisphosphate and fructose 6-phosphate during germination.

2 Present address: Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada.

1 Supported by Grant PCM 78-19575 from the United States National Science Foundation.






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Copyright © 1985 by the American Society of Plant Biologists