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NADH-Nitrate Reductase Inhibitor from Soybean Leaves
1Department of Biochemistry, Michigan State University, East Lansing, Michigan 48824.
Plant Physiology
|August 1, 1978
Summary
A novel NADH-nitrate reductase inhibitor protein was isolated from soybean leaves. This inhibitor
Area of Science:
- Plant Biochemistry
- Enzyme Regulation
- Molecular Biology
Background:
- Nitrate reductase (NR) activity in plants is crucial for nitrogen assimilation.
- Light and dark conditions significantly influence NR activity.
- The precise regulatory mechanisms controlling NR activity remain incompletely understood.
Purpose of the Study:
- To isolate and characterize a protein inhibitor of NADH-nitrate reductase from soybean leaves.
- To elucidate the role of this inhibitor in regulating NR activity under different light conditions.
- To investigate the biochemical properties and mode of action of the isolated inhibitor.
Main Methods:
- Isolation and purification of the inhibitor using DEAE-cellulose chromatography, Sephadex G-100 filtration, and ammonium sulfate precipitation.
- Enzyme assays to measure nitrate reductase activity and inhibition.
- Electrophoresis (polyacrylamide gel) and molecular weight determination (Sephadex G-75) to characterize the inhibitor's properties.
Main Results:
- A heat-labile protein inhibitor of NADH-nitrate reductase, with a molecular weight of approximately 31,000 Da and two identical subunits, was purified 2,500-fold.
- The inhibitor inactivated NADH-NR but not NADPH-NR or bacterial NR, suggesting a specific interaction.
- Inhibition was noncompetitive with nitrate, decreasing Vmax, and the inhibitor was photoinactivated in light but regained activity in the dark.
Conclusions:
- A specific protein inhibitor regulates soybean NADH-nitrate reductase activity, particularly in response to light-dark cycles.
- The inhibitor's activity is modulated by light, suggesting a role in diurnal regulation of nitrogen assimilation.
- These findings highlight the importance of protein-protein interactions and post-translational modifications in controlling enzyme function in plants.
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