Abstract
The assimilatory nitrate reductase (NR) from the cyanobacteriumAnabaena doliolum was membrane bound and solubilized by sonication. The Km value of the enzyme was 870 µM for nitrate with dithionite-reduced methyl viologen (MV) as electron donor. The pH optimum was 10.5 in the MV assay. Nitrate acted as an inducer and ammonium as repressor of the enzyme synthesis. In the presence ofl-methionine-d,l-sulfoximine (MSX) or azaserine, inhibitors of the glutamine synthetase-glutamate synthase (GS-GOGAT) pathway, ammonium did not exhibit any inhibitory effect on the enzyme. The photosynthetic nature of NR was shown with PS II inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). This enzyme fromA. doliolum has been shown to be a light-driven process, requiring de novo protein synthesis. It was inhibited by chlorate, the structural analog of nitrate;p-chloromercuribenzoate, a thiol reagent; sodium tungstate; and certain cations.
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