Resolution and characterization of multiple cytosolic phosphatases capable of hydrolyzing fructose-1,6-bisphosphate in spinach and soybean leaves |
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Authors: | David M Pharr Steven C Huber |
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Institution: | Departments of Horticultural and Crop Science, U. S. Department of Agriculture, Agricultural Research Service, North Carolina State Univ., Raleigh, NC, 27650, USA. |
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Abstract: | The apparent activity of cytoplasmic fructose bisphosphatase (EC 3.1.3.11) in crude extracts of spinach ( Spinacia oleracea L.) and soybean ( Glycine max L.] Merr.) leaves was only partially dependent on Mg2+. At least two major non-chloroplastic fructose bisphosphatases that differed in dependence on Mg2+ were chromatographically resolved from spinach leaves. The Mg2+-dependent enzyme had an apparent Michaelis constant of 4 μM for fructose-1,6-P2, was highly specific, and was strongly inhibited by fructose-2,6-P2. Enzyme activity was inhibited by physiological levels of fructose-6-P. Both species also contained at least one major enzyme, the activity of which was independent of Mg2+. These enzymes had pH optima near neutrality, Michaelis constants of 25 to 30 μM for fructose-1,6-P2, and were inhibited by AMP. Although hexose monophosphates were not metabolized, the enzymes were not specific for fructose-1,6-P2: phosphate was released from phosphoenolpyruvate and ribulose-1, 5-P2, and with fructose-1,6-P2, as substrate, Pi release was about 1.5-fold greater than fructose-6-P production. It is concluded that only the Mg2+-dependent fructose bisphosphatase, previously characterized, functions in the photosynthetic sucrose formation pathway. Inhibition of the Mg2+-dependent enzyme by fructose-6-P may be involved in regulation of sucrose formation. |
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Keywords: | Fruetose bisphosphatase fructose-2 6-bisphosphate |
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