A patient is infused with 15N-labelled aspartate before an assay of newly formed urea. Where will the labelled nitrogen appear?
- A In neither nitrogen atom of urea
- B In both nitrogen atoms of urea
- C In one nitrogen atom of urea, donated at the argininosuccinate synthetase step ✓
- D Only in the carbon skeleton, never in the urea nitrogen
Correct answer: C. In one nitrogen atom of urea, donated at the argininosuccinate synthetase step
Explanation
Urea contains two nitrogens: the first arrives as free ammonia incorporated by CPS-I, while the second is contributed by the amino group of aspartate during the argininosuccinate synthetase reaction. Aspartate therefore donates exactly one labelled nitrogen per urea molecule. This dual-source design explains why defects in aspartate supply, such as citrin deficiency, also produce hyperammonemia despite normal CPS-I activity.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.