Effects of Phosphate and other Ionic Compounds on the Activity of Crystalline Beef Liver Glutamate Dehydrogenase

At pH 9.0 the activity of crystalline glutamate dehydrogenase was increased by the addition to the incubation mixture of various electrolytes including sodium chloride, potassium phosphate, Tris, 2‐amino‐2‐methyl‐1, 3‐propanediol and methionine. Potassium phosphate was most stimulatory and, at an optimum concentration, increased activity three to four‐fold. No stimulation was obtained at pH 7.6. Potassium phosphate and the other electrolytes abolished activation by high concentrations of NAD or NADP and altered the inhibitory effects obtained by compounds which presumably do not interact at the NAD binding site. These electrolytes also overcame the inhibition by GTP, a ligand which binds at the same site as NAD, but they did not suppress the reactivity with ADP. It is proposed that at pH 9.0 potassium phosphate and other electrolytes alter glutamate dehydrogenase so that the regulatory site which binds NAD or GTP is no longer available to these ligands.