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Articles

Nitrate Reductases from Wild-Type and nr₁-Mutant Soybean (Glycine max [L.] Merr.) Leaves ¹

II. Partial Activity, Inhibitor, and Complementation Analyses

Department of Agronomy, University of Illinois, Urbana, Illinois 61801, United States Department of Agriculture, Agricultural Research Service, University of Illinois, Urbana, Illinois 61801

Soybean (Glycine max [L.] Merr.) leaves have been shown to contain three forms of nitrate reductase (NR). Two of the forms, which are present in leaves of wild-type (cv. Williams) plants grown in the absence of NO₃^–, are termed constitutive and designated c₁NR and c₂NR. The third form, which is present in NO₃^–-grown mutant (nr₁) plants lacking the constitutive forms, is termed inducible and designated iNR. Samples of c₁NR, c₂NR, and iNR obtained from appropriately treated plants were analyzed for the presence of partial activities, response to inhibitors, and ability to complement a barley NR which lacks the molybdenum cofactor (MoCo) but is otherwise active.

The three forms were similar to most assimilatory NR enzymes in that they (a) exhibited NADH-cytochrome c reductase, reduced flavin mononucleotide-NR, and reduced methyl viologen-NR partial activities; (b) were inhibited by p-hydroxymercuribenzoate at the site of initial electron transport through each enzyme; (c) were more inhibited by CN^– in their reduced enzyme state as compared with their oxidized state; and (d) complemented a MoCo-defective NR (e.g. contained cofactors with characteristics similar to the MoCo found in barley NR and commercial xanthine oxidase). However, among themselves, they showed dissimilarities in their response to treatment with HCO₃^– and CN^–, and in their absolute ability to complement the barley NR. The site of effect for these treatments was the terminal cofactor-containing portion of each enzyme. This indicated that, although a terminal cofactor (presumably a MoCo) was present in each form, structural or conformational differences existed in the terminal cofactor-protein complex of each form.

¹ Supported by United States Department of Agriculture Competitive Research Grant, Agreement No. 82-CRCR-1-1038.