Which of the following aminoacid in a protein is involved in urea cycle and is required for synthesis of NO?
**Core Concept**
The urea cycle is a critical pathway in the liver that converts ammonia into urea for excretion. Nitric oxide (NO) is a potent vasodilator synthesized from the amino acid L-arginine. The connection between these two processes lies in the enzyme responsible for NO synthesis, which also plays a role in the urea cycle.
**Why the Correct Answer is Right**
L-arginine serves as the substrate for nitric oxide synthase (NOS), an enzyme that catalyzes the conversion of L-arginine to L-citrulline and NO. This reaction is essential for the synthesis of NO, which plays a crucial role in various physiological processes, including vasodilation and neurotransmission. Moreover, L-arginine is also a key player in the urea cycle, as it is converted to L-ornithine and then to citrulline, which enters the urea cycle.
**Why Each Wrong Option is Incorrect**
* **Option A:** L-glutamate is involved in various neurotransmitter systems and serves as a precursor for glutamine in the urea cycle, but it is not directly involved in NO synthesis.
* **Option B:** L-aspartate is a precursor for oxaloacetate in gluconeogenesis and the citric acid cycle but is not directly involved in the urea cycle or NO synthesis.
* **Option C:** L-histidine is involved in the synthesis of histamine and is not directly related to the urea cycle or NO synthesis.
**Clinical Pearl / High-Yield Fact**
Remember that L-arginine supplementation can increase NO production and has been explored as a potential therapeutic agent for conditions such as erectile dysfunction and pulmonary hypertension.
**Correct Answer: C. L-arginine. L-arginine.**