[Allosteric regulation of the activity of glutamate dehydrogenase from pea seedlings by the substrate α-ketoglutarate].

Several investigations on the properties of glutamate dehydrogenase from plant sources indicate that the enzymatic activity (reductive amination) follows a Michaelis-Menten-type kinetic when velocity is plotted versus rising concentrations of the substrate α-ketoglutarate. In the course of our investigations on the effect of SO2 on pea plant enzymes we found that SO 4 (2-) , added as (NH4)2SO4 to the assay system, causes this type of activity response because of its ability to function as an activator. When (NH4)2SO4 is replaced by NH4Cl in the in vitro system however, activity response is sigmoidal. Addition of competitive inhibitor to the latter system again gives rise to a sigmoidal kinetic with reduced initial velocities. Identical kinetic behaviour is observed when either 120 fold enriched enzyme from shoots or highly purified glutamate dehydrogenase from pea roots is used, a fact which justifies the assumption that the enzyme from pea plants belongs to the MIC-type of regulatory proteins (modulator independent cooperativity). The activity response caused by other effectors, especially purine nucleotides, is discussed with regard to the above findings.