Question:hard
Glutamine is increased in CSF, blood, and urine. This is due to deficiency of which enzyme?
Glutamine is increased in CSF, blood, and urine. This is due to deficiency of which enzyme?
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A proximal urea cycle block raises glutamine - look at the rate-limiting enzyme.
Updated On: Jun 23, 2026
- CPS-I
- Arginase
- OTC
- Argininosuccinate synthetase
Show Solution
The Correct Option is A
Solution and Explanation
Step 1: Rising glutamine across CSF, blood, and urine is the body's way of trapping excess ammonia, signalling a block at the very start of the urea cycle.
Step 2: The first committed, rate-limiting step is run by carbamoyl phosphate synthetase I, which fuses ammonia, $ATP$, and $HCO_3^-$ into carbamoyl phosphate and needs N-acetylglutamate as an allosteric switch.
Step 3: If CPS-I fails, ammonia never gets incorporated and is shunted into glutamine instead, raising glutamine levels everywhere. This proximal defect fits better than the more distal enzymes OTC, argininosuccinate synthetase, or arginase.\[\boxed{CPS\text{-}I}\]
Step 2: The first committed, rate-limiting step is run by carbamoyl phosphate synthetase I, which fuses ammonia, $ATP$, and $HCO_3^-$ into carbamoyl phosphate and needs N-acetylglutamate as an allosteric switch.
Step 3: If CPS-I fails, ammonia never gets incorporated and is shunted into glutamine instead, raising glutamine levels everywhere. This proximal defect fits better than the more distal enzymes OTC, argininosuccinate synthetase, or arginase.\[\boxed{CPS\text{-}I}\]
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