Purification and characterization of the NAD-dependent glutamate dehydrogenase in the ectomycorrhizal fungus Laccaria bicolor (Maire) Orton

The NAD-dependent glutamate dehydrogenase (GDH) (EC 1.4.1.2) from Laccaria bicolor was purified 410-fold to apparent electrophoretic homogeneity with a 40% recovery through a three-step procedure involving ammonium sulfate precipitation, anion-exchange chromatography on DEAE-Trisacryl, and gel filtration, The molecular weight of the native enzyme determined by gel filtration was 470 kDa, whereas sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave rise to a single band of 116 kDa, suggesting that the enzyme is composed of four identical subunits, The enzyme was specific for NAD(H). The pH optima were 7.4 and 8.8 for the amination and deamination reactions, respectively, The enzyme was found to be highly unstable, with virtually no activity after 20 days at -75 degrees C, 4 days at 4 degrees C, and 1 h at 50 degrees C, The addition of ammonium sulfate improved greatly the stability of the enzyme and full activity was still observed after several months at -75 degrees C, NAD-GDH activity was stimulated by Ca2+ and Mg2+ but strongly inhibited by Cu2+ and slightly by the nucleotides AMP, ADP, and ATP, The Michaelis constants for NAD, NADH, 2-oxoglutarate, and ammonium were 282 mu M, 89 mu M, 1.35 mM, and 37 mM, respectively. The enzyme had a negative cooperativity for glutamate (Hill number of 0.3), and its K-m value increased from 0.24 to 3.6 mM when the glutamate concentration exceeded 1 mM, These affinity constants of the substrates, compared with those of the NADP-GDH of the fungus, suggest that the NAD-GDH is mainly involved in the catabolism of glutamate, while the NADP-GDH is involved in the catalysis of this amino acid. (C) 1997 Academic Press.