Which statement about the enzymatic assay for ammonia is true?

Which statement about the enzymatic assay for ammonia is true?

• A. The reaction is catalyzed by glutamate dehydrogenase
• B. NAD+ is always required as a cofactor
• C. Ammonium ion must be isolated before the enzymatic step
• D. Absorbance is measured at 520 nm

Answer: (A)

The essential idea is that the ammonia assay uses glutamate dehydrogenase to couple ammonia with α-ketoglutarate to form glutamate, using NADH in the process. As ammonia is present, glutamate dehydrogenase catalyzes the amination of α-ketoglutarate, and NADH is oxidized to NAD+. The amount of NADH that disappears is proportional to the ammonia concentration, and this change is measured spectrophotometrically. That makes the statement true because the enzyme driving the reaction is glutamate dehydrogenase. The other points don’t fit: NAD+ is not the cofactor used here (NADH is consumed, forming NAD+), so requiring NAD+ is incorrect. There’s no need to isolate ammonium before the enzymatic step—the assay measures ammonia directly in the sample. And the absorbance is not read at 520 nm; NADH absorption is monitored at 340 nm, the wavelength at which NADH has its peak absorbance.

The essential idea is that the ammonia assay uses glutamate dehydrogenase to couple ammonia with α-ketoglutarate to form glutamate, using NADH in the process. As ammonia is present, glutamate dehydrogenase catalyzes the amination of α-ketoglutarate, and NADH is oxidized to NAD+. The amount of NADH that disappears is proportional to the ammonia concentration, and this change is measured spectrophotometrically.

That makes the statement true because the enzyme driving the reaction is glutamate dehydrogenase. The other points don’t fit: NAD+ is not the cofactor used here (NADH is consumed, forming NAD+), so requiring NAD+ is incorrect. There’s no need to isolate ammonium before the enzymatic step—the assay measures ammonia directly in the sample. And the absorbance is not read at 520 nm; NADH absorption is monitored at 340 nm, the wavelength at which NADH has its peak absorbance.