Ammonia for the urea cycle is supplied by the action of the following enzymes?
**Core Concept**
The urea cycle is a crucial metabolic pathway that converts ammonia into urea for excretion. Ammonia is a toxic compound that needs to be detoxified, and the urea cycle plays a vital role in this process. The cycle involves the conversion of ammonia into carbamoyl phosphate, which is then used to synthesize urea.
**Why the Correct Answer is Right**
Glutamate dehydrogenase is the enzyme responsible for converting glutamate into alpha-ketoglutarate, releasing ammonia in the process. This reaction is a key step in supplying ammonia for the urea cycle. The enzyme glutamate dehydrogenase catalyzes the reversible reaction, allowing ammonia to be generated from glutamate. This process is essential for the proper functioning of the urea cycle, as ammonia is a critical substrate for the cycle.
**Why Each Wrong Option is Incorrect**
**Option A:** Aspartate aminotransferase is involved in the transamination of aspartate to form oxaloacetate, but it is not directly involved in supplying ammonia for the urea cycle.
**Option C:** Argininosuccinate synthase is an enzyme involved in the urea cycle, but it catalyzes the condensation of citrulline and aspartate to form argininosuccinate, rather than supplying ammonia.
**Option D:** Argininosuccinate lyase is also an enzyme involved in the urea cycle, but it catalyzes the hydrolysis of argininosuccinate to form arginine and fumarate, rather than supplying ammonia.
**Clinical Pearl / High-Yield Fact**
Glutamate dehydrogenase is a key enzyme in the regulation of ammonia levels in the body, and its activity is often increased in conditions of high ammonia production, such as liver disease.
**✓ Correct Answer: B. Glutamate dehydrogenase**