At which stage of ornithine cycle arginase is used

Options

A. ornithine `to` urea
B. arginine `to` ornithine
C. ornithine `to` citrulline
D. citrulline `to` arginosuccinic acid

Answer

To determine at which stage of the ornithine cycle arginase is used, let's analyze the steps of the ornithine cycle (also known as the urea cycle) and identify where arginase plays a role.

### Step-by-Step Solution:

1. **Understanding the Ornithine Cycle**:
The ornithine cycle is a series of biochemical reactions that convert ammonia to urea, which is then excreted from the body. It involves several key intermediates, including ornithine, citrulline, argininosuccinate, and arginine.

2. **Identifying the Role of Arginase**:
Arginase is an enzyme that catalyzes the conversion of arginine into ornithine and urea. This reaction is crucial as it is one of the final steps in the urea cycle.

3. **Analyzing the Options**:
- **Option A**: Ornithine to urea - This is not correct as arginase does not catalyze this step.
- **Option B**: Arginine to ornithine - This is the correct answer, as arginase converts arginine into ornithine and urea.
- **Option C**: Ornithine to citrulline - This step is catalyzed by ornithine transcarbamylase, not arginase.
- **Option D**: Citrulline to argininosuccinic acid - This step is catalyzed by argininosuccinate synthetase, not arginase.

4. **Conclusion**:
The correct answer is **Option B: Arginine to Ornithine**. This is the stage of the ornithine cycle where arginase is utilized.