Suppose that you label a pool of ammonia in the mitochondria of hepatocytes with

Question

Suppose that you label a pool of ammonia in the mitochondria of hepatocytes with the stable isotope 15N. After introducing the^15 NH4+ in the mitochondria, you wait a sufficient amount of time such that the urea cycle completes multiple turns. You then collect the urea in the cytosol A) How many 15N labels would you expect to detect in urea? Please justify your answer in 1 sentence. B) You feed the hepatocytes N-acetylglutamate. would you expect higher or lower levels of 15N label in urea? Explain your answer in one sentence You do the same experiment described at the at the beginning of Q2 (give hepatocytes^15 NH4+ and isolate cytosolic contents). The hepatocytes are from a patient with an ornithine transcarbamylase deficiency. 1) Would you expert higher or lower levels of 15N label in urea and 2) if you supplement

Explanation / Answer

Answer

A) Half of the nitrogen in the Urea will be labeled because one nitrogen of urea coming from ammonia ( that is labeled) and other nitrogen comes from L-aspartate in the cytosol So one nitrogen in the urea will be labeled with 15 N and other will not be labeled.

B). the level of 15N in urea remain same even at a higher concentration of N-acetyl-glutamate because only one nitrogen is added by N-acetyle glutamate in the mitochondria and other is added by argininosuccinate synthase from L-aspartate in the cytosol.   

C). The level of N15 nitrogen in the urea will not be affected by ornithine transcarbamylase deficiency but overall production of urea will hamper So level N15 ration in the urea will remain same. if we supplement the enzyme urea production level resume to normal and leveling of N15 in the urea will go up with production.

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