Which statement can be associated with the enzymatic assay of ammonia?

Which statement can be associated with the enzymatic assay of ammonia?

• A. Increase in absorbance monitored at 340 nm
• B. Nicotinamide-adenine dinucleotide (NAD+) required as a cofactor
• C. Ammonium ion isolated from specimen before the enzymatic step
• D. Reaction catalyzed by glutamate dehydrogenase

Answer: (D)

Glutamate dehydrogenase is used in ammonia assays because it catalyzes the amination of alpha-ketoglutarate with ammonium to form glutamate, using NADH as the cofactor. The reaction is alpha-ketoglutarate + NH4+ + NADH + H+ → glutamate + NAD+. Since NADH is oxidized to NAD+, the assay tracks a decrease in absorbance at 340 nm, not an increase. That direct link between ammonia processing and NADH consumption is what makes this enzyme the basis of the test. The other statements don’t fit because the assay relies on NADH as the cofactor (NAD+ would be produced, not consumed), and you don’t need to isolate ammonium from the sample before the enzymatic step. Monitoring is through NADH decline, not an absorbance rise, and the enzyme driving the reaction is glutamate dehydrogenase.

Glutamate dehydrogenase is used in ammonia assays because it catalyzes the amination of alpha-ketoglutarate with ammonium to form glutamate, using NADH as the cofactor. The reaction is alpha-ketoglutarate + NH4+ + NADH + H+ → glutamate + NAD+. Since NADH is oxidized to NAD+, the assay tracks a decrease in absorbance at 340 nm, not an increase. That direct link between ammonia processing and NADH consumption is what makes this enzyme the basis of the test.

The other statements don’t fit because the assay relies on NADH as the cofactor (NAD+ would be produced, not consumed), and you don’t need to isolate ammonium from the sample before the enzymatic step. Monitoring is through NADH decline, not an absorbance rise, and the enzyme driving the reaction is glutamate dehydrogenase.