Amino acid metabolism: renal ammoniagenesis: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Amino acid metabolism: renal ammoniagenesis

Sample item

In chronic metabolic acidosis the kidney excretes more ammonium in the urine. Which substrate and enzyme in the proximal tubular cell supply most of this ammonia?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Glutamine, deamidated by glutaminase

Proximal tubular cells take up glutamine and release ammonia by glutaminase, then glutamate dehydrogenase releases a second ammonia. Ammonia diffuses into the lumen and is trapped as NH4+, carrying out H+ and generating new bicarbonate. Glutaminase activity is induced in chronic acidosis.

Option by option

  • A. Urea, hydrolysed by urease

    Humans do not have urease, and urea is excreted unchanged by the kidney.

  • B. Asparagine, hydrolysed by asparaginase

    Asparagine is a minor substrate and asparaginase is not the main renal source of ammonia.

  • C. Arginine, cleaved by arginase

    Arginase releases urea in the liver urea cycle, not ammonia in the kidney.

  • D. Glutamine, deamidated by glutaminase (correct)

    Correct. Glutamine deamidation by glutaminase is the main source of urinary ammonium.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Glutamine, deamidated by glutaminase. Proximal tubular cells take up glutamine and release ammonia by glutaminase, then glutamate dehydrogenase releases a second ammonia.

Why is "Urea, hydrolysed by urease" not the answer?

Humans do not have urease, and urea is excreted unchanged by the kidney.

Why is "Asparagine, hydrolysed by asparaginase" not the answer?

Asparagine is a minor substrate and asparaginase is not the main renal source of ammonia.

Why is "Arginine, cleaved by arginase" not the answer?

Arginase releases urea in the liver urea cycle, not ammonia in the kidney.

What concept does this question test?

It tests Amino acid metabolism: renal ammoniagenesis in Biochemistry. Ammonia diffuses into the lumen and is trapped as NH4+, carrying out H+ and generating new bicarbonate. Glutaminase activity is induced in chronic acidosis.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Amino acid metabolism: renal ammoniagenesis?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.