Plant Physiol. Drug Metab Dispos
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Plant Physiology 93:537-540 (1990)
© 1990 American Society of Plant Biologists

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Metabolism and Enzymology

Spinach Nitrate Reductase 1

Effects of Ionic Strength and pH on the Full and Partial Enzyme Activities

Michael J. Barber and Brian A. Notton

Department of Biochemistry and Molecular Biology, University of South Florida, College of Medicine, Tampa, Florida 33612, University of Bristol, Department of Agricultural Sciences, Institute of Arable Crops Research, Long Ashton Research Station, Long Ashton, Bristol, England

Initial velocity studies of immunopurified spinach nitrate reductase have been performed under conditions of controlled ionic strength and pH and in the absence of chloride ions. Increased ionic strength stimulated NADH:ferricyanide reductase and reduced flavin:nitrate reductase activities and inhibited NADH:nitrate reductase, NADH:cytochrome c reductase and reduced methyl viologen:nitrate reductase activities. NADH:dichlorophenolindophenol reductase activity was unaffected by changes in ionic strength. All of the partial activities, expressed in terms of micromole 2 electron transferred per minute per nanomole heme, were faster than the overall full, NADH:nitrate reductase activity indicating that none of the partial activities included the rate limiting step in electron transfer from NADH to nitrate. The pH optimum for NADH:nitrate reductase activity was determined to be 7 while values for the various partial activities ranged from 6.5 to 7.5. Chlorate, bromate, and iodate were determined to be alternate electron acceptors for the reduced enzyme. These results indicate that unlike the enzyme from Chlorella vulgaris, intramolecular electron transfer between reduced heme and Mo is not rate limiting for spinach nitrate reductase.

1 This work was supported in part by grants GM 32696 from the National Institutes of Health, 88-37120-3871 from the U.S. Department of Agriculture and NATO Collaborative Research Grant 04-0015-86. Long Ashton Research Station is funded by the U.K. Agricultural Research Council.






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Copyright © 1990 by the American Society of Plant Biologists