AMMONIUM ION-DEPENDENT ISOMERIZATION OF GLUTAMATE-DEHYDROGENASE IN RELATION TO GLUTAMATE SYNTHESIS IN MAIZE

The glutamate dehydrogenase [GDH, EC 1.4.1.2] from maize seeds that we re germinated in the presence of different concentrations of NH4Cl had charge isomers in the pH 5.8-7.5 range. In a three-phase process, dif ferent NH,CI concentrations sequentially altered the binomial distribu tion of the GDH isoenzyme population. The first phase of GDH isomeriza tion was by low ( < 2 mM); the second was by medium (2-58 mM); and the third was by high (> 58 mM) NH4Cl concentrations. Ammonium ion isomer ized the GDH by regulating the hexameric subunit assembly reaction. Th e isomerization Was abolished by methionine sulphoximine. The GDH isoe nzyme population distribution that was induced by 25 mM NH4Cl closely approached the theoretical binomial distribution, and it also gave the highest GDH activity. An analysis of the free amino acids of maize se eds, and comparison of the activities of GDH and glutamate synthase (G OGAT), in each of the three phases of GDH isomerization, showed that G DH was 25% more efficient than GOGAT in the synthesis of L-Glu.