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Old 03-29-2010
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Question Ornithine Transcarbamyolase Deficiency

A deficiency of Ornithine Transcarbamyolase enzyme would result in elevation of the serum level of which of the following?

A- Ammonia
B- Urea
C- Arginine
D- Citrulline
E- Putrescine
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Old 03-29-2010
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Ornithine Transcarbamoylase (OTC) deficiency is a disruption of the urea cycle. The urea cycle is the means through which the body gets rid of the surplus ammonium produced as a byproduct of aminoacid metabolism.

As indicated by its name, OTC introduces a carbamoyl subgroup into ornithine, and thus a new compound is synthesized, which is citrulline, the first molecule in the cycle of urea. The carbamoyl subgroup results from the combination of ammonium with carbon dioxide. As a result, when OTC is deficient, ammonium piles up (constitutional laboratory finding in OTC deficiency -- choice A is correct), as well as other intermediate products of urea cycle (such as ornithine and arginine), but this finding is not invariably evident (choice C is not correct).

OTC deficiency provokes an apparent malfunction of the urea cycle, consequently urea is UNDERproduced (choice B is incorrect).

As already mentioned, OTC produces citrulline from carbamoyl phosphate & ornithine, thus OTC deficiency causes low levels of citrulline (choice D is incorrect).

Putrescin is a molecule also deriving from the catabolism of aminoacids in decaying tissues and is notorious for the odors produced in bacterial vaginosis, bad breath and decomposing corps. In this case, the surplus ammonium, instead of combining with carbon dioxide, it attaches to an aliphatic carbon chain. If I am correct, OTC does not take part in this process (choice E is incorrect).

Waiting impatiently for your comments and corrections!
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Old 03-29-2010
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Default Why elevated Argninine!

Quote:
Originally Posted by ath.pantelis View Post
when OTC is deficient, ammonium piles up (constitutional laboratory finding in OTC deficiency -- choice A is correct), as well as other intermediate products of urea cycle (such as ornithine and arginine), but this finding is not invariably evident (choice C is not correct).
Ornithine will build up in urea in OTC deficiency but not Arginine. Arginine will still convert to ornithine via the Arginase enzyme.
What do you think?
Of course my answer is like yours; Ammonia will build up.
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Old 03-29-2010
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This is a good observation, Dr. YoramFishke. In my opinion, the answer lies in the principles of enzyme kinetics. As time passes and ornithine piles up, arginase will reach a saturation point, beyond which no more arginine will be converted to ornithine. On the other hand, I have already mentioned that this is not a standard finding (apparently due to these "escape" biochemical pathways). So, essentially I do not disagree with you!
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Old 03-30-2010
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I think ammonia, because Ornithine Transcarbamyolase is part of the cycle & helps in detoxification of ammonia.
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Old 03-30-2010
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this is from FA:

answer is A because:

OTC is a most common urea cycle disorder (worth memorizing). its x-linked recessive. interferes with body's ability to eliminate ammonia. often evident in the first few days of life, but may present with late onset.
excess carbomyl phosphate is converted to orotic acid (part of the pyrimidine synthesis pathway)
findings in OTC deficiency: orotic acid in blood and urine, decreased BUN, and hyperammonemia symptoms.

hereditary urea cycle enzyme deficiency leads to hyperammonemia usually. so i guess when the question is a urea cycle enzyme deficiency it is safe to think about increased ammonia. what say u ppl?
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