Which enzyme is used in the enzymatic ammonia assay?

• A. Glutamate dehydrogenase
• B. Catalase
• C. Urease
• D. Lactate dehydrogenase

Answer: (A)

Glutamate dehydrogenase is used in the enzymatic ammonia assay. In this method, ammonia reacts with α-ketoglutarate in a reaction catalyzed by glutamate dehydrogenase, forming glutamate while NADH is oxidized to NAD+. The amount of NADH that disappears is proportional to the amount of ammonia in the sample, and you typically measure this change by the decrease in absorbance at 340 nm. This direct link between ammonia and NADH consumption makes it a sensitive and specific way to quantify ammonia.

The other enzymes don’t perform this ammonia-detection step: catalase breaks down hydrogen peroxide; urease hydrolyzes urea into ammonia and carbon dioxide (not the same as measuring existing ammonia in a sample); lactate dehydrogenase interconverts lactate and pyruvate with NADH/NAD+.