Properties of glutamate dehydrogenase from Lemna minor |
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Authors: | Adelheid Ehmke Thomas Hartmann |
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Affiliation: | Botanisches Institut der Universität Bonn, D-5300 Bonn, BRD West Germany |
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Abstract: | Sterile cultures of Lemna minor grown in the presence of either nitrate, ammonium or amino acids failed to show significant changes in glutamate dehydrogenase (GDH) levels in response to nitrogen source. Crude and partially purified GDH preparations exhibit NADH and NADPH dependent activities. The ratio of these activities remain ca 12:1 during various treatments. Mixed substrate and product inhibition studies as well as electrophoretic behaviour suggest the existence of a single enzyme which is active in the presence of both coenzymes. GDH activity was found to be localized mainly in mitochondria. Kinetic studies revealed normal Michaelis kinetics with most substrates but showed deviations with NADPH and glutamate. A Hill-coefficient of 1.9 determined with NADPH indicates positive cooperative interactions, whereas a Hill-coefficient of 0.75 found with glutamate may be interpreted in terms of negative cooperative interactions. NADH dependent activity decreases rapidly during gel filtration whereas the NAD+ and NADPH activities remain unchanged. GDH preparations which have been pretreated with EDTA show almost complete loss of NADH and NAD+ activities. NADPH activity again remains unaffected. NAD+ activity is fully restored by adding Ca2+ or Mg2+, whereas the NADH activity can only be recovered by Ca2+ but not at all by Mg2+. Moderate inhibition of GDH reactions observed with various adenylates are fully reversed by adding Ca2+, indicating that the adenylate inhibition is due solely to the chelating properties of these compounds. |
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Keywords: | Lemnaceae duck weed nitrogen metabolism glutamate dehydrogenase, nitrogen source and enzyme level coenzyme specifity kinetic properties metal ion activation. |
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