No abstract

The level of the NADPH-dependent glutamate dehydrogenase activity (EC 1.4.1.4) from nitrate-grown cells of the thermophilic non-N2-flxing cyanobacterium Phormidium laminosum OH-1-p.Cl1 could be significantly enhanced by the presence of ammonium or nitrite, as well as by L-methionine-DL-sulfoximine and other sources of organic nitrogen (L-Glu, L-Gln, and methylamine). The enzyme was purified more than 4,400-fold by ultracentrifugation, ion-exchange chromatography, and affinity chromatography, and at 30°C it showed a specific activity of 32.9 gimol of NADPH oxidized per min per mg of protein. The purified enzyme showed no aminotransferase activity and catalyzed the amination of 2-oxoglutarate preferentially to the reverse catabolic reaction. The enzyme was very specific for its substrates 2-oxoglutarate (K,m = 1.25 mM) and NADPH (Km = 64 ,uM), for which hyperbolic kinetics were obtained. However, negative cooperativity (Hill coefficient h = 0.89) and [SJ0.5 of 18.2 mM were observed for ammonium. The mechanism of the aminating reaction was of a random type with independent sites. The purified enzyme showed its maximal activity at 60°C (Ea = 5.1 kcal/ mol [21.3 kJ/mol]) and optimal pH values of 8.0 and 7.5 when assayed in Tris hydrochloride and potassium phosphate buffers, respectively. The native molecular mass of the enzyme was about 280 kilodaltons. The possible physiological role of the enzyme in ammonia assimilation is discussed.