Purification and characterization of soybean nodule nitrite reductase [bacteroid, Glycine max, nitrate assimilation, Rhizobium japonicum]

A nodule cytosol nitrite reductase was isolated from soybean [Glyine max (L.) Mer. cv. Tracy] grown in the presence of nitrate. Enzyme activity increased when increased amounts of nitrate were supplied to the plant. A purification procedure involving ammonium sulfate precipitation, gel filtration, DEAE Sephadex and Blue Sepharose chromatography resulted in an activity capable of forming 6.7 μmol ammonia (mg protein)−1 min−1. This represented a 235‐fold increase in specific activity. The molecular weight, estimated by gel filtration, was 55 000. The pH optimum for activity was 7.1. Ammonia formed stoichiometrically as nitrice was consumed. From Lineweaver‐Burk plots, Km values of 0.5mM for nitrite and 0.2mM for methyl viologen were calculated. Spectral data suggest the association of a heme chromophore with the enzyme.