Effect of ammonia on the glutamate dehydrogenase catalyzed oxidative deamination of L-glutamate: production of an ammonia-containing intermediate in the "burst" phase.

We have studied the effects of ammonium acetate on the transient "burst" phase of the oxidation of L-glutamate by glutamate dehydrogenase. Two measurable changes are observed in the "burst" phase as ammonium acetate concentration is increased: (i) an increase in the apparent first-order rate constant, kapp, and (ii) a decrease in the amplitude of the absorbance change measured at 320 nm. The increase in kapp shows a hyperbolic dependence on ammonium acetate concentration and is independent of glutamate concentration. The results demonstrate the existence of an intermediate immediately following hydrogen transfer. The intermediate contains enzyme, reduced coenzyme, ammonia, and alpha-ketoglutarate moieties and is in equilibrium with the known complex consisting of enzyme, reduced coenzyme, and alpha-ketoglutarate. At high concentrations of ammonium acetate, the equilibrium favors the ammonia containing complex.