Studies on the regulation of assimilatory nitrate reductase in Ankistrodesmus braunii

In the green alga Ankistrodesmus braunii, all the activities associated with the nitrate reductase complex (i.e., NAD(P)H-nitrate reductase, NAD(P)H-cytochrome c reductase and FMNH₂-or MVH-nitrate reductase) are nutritionally repressed by ammonia or methylamine. Besides, ammonia or methylamine promote in vivo the reversible inactivation of nitrate reductase, but not of NAD(P)H-cytochrome c reductase. Subsequent removal of the inactivating agent from the medium causes reactivation of the inactive enzyme. Menadione has a striking stimulation on the in vivo reactivation of the inactive enzyme. The nitrate reductase activities, but not the diaphorase activity, can be inactivated in vitro by preincubating a partially purified enzyme preparation with NADH or NADPH. ADP, in the presence of Mg²⁺, presents a cooperative effect with NADH in the in vitro inactivation of nitrate reductase. This effect appears to be maximum at a concentration of ADP equimolecular with that of NADH.Abbreviations ADP Adenosine-5-diphosphate - AMP Adenosine-5-monophosphate - ATP Adenosine-5-triphosphate - FAD Flavin adenine dinucleotide - FMNH₂ Flavin adenine mononucleotide, reduced form - GDP Guanosine-5-diphosphate - MVH Methyl viologen, reduced form - NADH Nicotinamide adenine dinucleotide, reduced form - NADPH Nicotinamide adenine dinucleotide phosphate, reduced form