Metabolic Fate of Ammonia and MCQs NEET, GATE, GPAT UPSC and NET Exam

Metabolic Fate of Ammonia and MCQs NEET, GATE, GPAT UPSC and NET Exam

Ammonia is produced in most of the tissues. Since ammonia is very toxic in nature, it is immediately converted into non-toxic metabolites like glutamate, glutamine or alanine and then ultimately to urea. For the conversion of ammonia to urea, it must be transported to liver. Since ammonia is toxic in nature so it is never transported in free form in blood. So for the transport of ammonia from tissues to the liver, there are two mechanisms

Transport of ammonia from peripheral tissues to the liver

Although most of amino acid degradation takes place in liver, some other tissue can also degrade. For example- muscle uses branched chain amino acid as a source of fuel during long term exercise and fasting. The amino group of amino acid is removed in the form of ammonia. Since muscle lacks enzymes of urea cycle, so it must be released in non-toxic form so that it can be transported to the liver. Ammonia is transported from muscle to the  liver in two forms: glutamine and alanine

Transport of ammonia in the form of glutamine

In tissues like brain, liver and kidney free ammonia is combined with glutamate to form glutamine through glutamine synthetase enzyme. The glutamine so formed is a neutral non-toxic compound which can pass through the cell membrane, but glutamate alone cannot be transported. The glutamine is carried to the liver where it is again dissociated into glutamate and ammonia by glutaminase enzyme. The ammonia yielded is then converted to urea. Glutamine is a major transport and storage form of ammonia in the body.

Transport of ammonia in the form of alanine

Alanine plays an important role in transporting ammonia to the liver through glucose-alanine cycle. In muscle, amino group of amino acids are collected in the form of glutamate through transamination reaction. The glutamate is further converted in two compounds:

  • Glutamate can be converted to glutamine as studied earlier
  • Glutamate can transfer its alpha-amino group to the pyruvate to form alanine by alanine amino transferase (ALT). alanine so formed passes into the blood and reaches liver.

In liver, by the action of reverse reaction, ALT transfers the amino group from alanine to alpha-ketoglutarate and forms glutamate and pyruvate. Glutamate then releases ammonia which is further converted to urea and pyruvate generated is again converted to glucose by gluconeogenesis.

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Multiple Choice Questions (MCQs)

1. What is the need for the conversion of ammonia in other metabolites?

A. It is toxic in nature

B. It cannot mix with other components of urine

C. Both

D. None

2. For the conversion of ammonia to urea, where is ammonia transported?

A. Liver

B. Kidney

C. Muscle

D. All of the above

3. In which form, the ammonia is transported into the blood?

A. Alanine

B. Pyruvate

C. Glutamine

D. Both A and c

4. Where do amino acid degradation takes place?

A. Liver

B. Kidney

C. Muscle

D. Brain

5. Which type of amino acid is used in the muscle as source of fuel? A. Branched chain amino acid

B. Linear amino acid

C. Both

D. None

6. During which condition, amino acid acts as fuel in muscle?

A. Exercise

B. Fasting

C. During rest

D. Both A and B

7. Why is ammonia not converted into urea in muscle?

A. It lacks enzyme of glucose alanine pathway

B. Lacks enzyme of urea cycle

C. Muscle is busy in doing contraction and relaxation process

D. Both B and C

8. Match the following-

a. glutamate to glutamine 1. ALT

b. glutamine to glutamate 2. Glutaminase

c. glutamate to alanine 3. glutamine synthetase

9. In which tissue, ammonia combines with glutamate?

A. Kidney

B. Brain

C. Liver

D. All of the above

10. Combination of ammonia and glutamate forms which compound?

A. Alanine

B. Pyruvate

C. Glucose

D. None of the above

11. Which of the following compound is major storage form of ammonia?

A. Alanine

B. Pyruvate

C. Glucose

D. Glutamine

12. Which of the following statement is NOT true?

A. Production of ammonia increases metabolic acidosis

B. Increased level of ammonia can affect the brain

C. There are 3 isoenzymes of mitochondrial glutaminase

D. For the synthesis of glutamine, ammonia is required

13. Alanine transports ammonia to liver through which pathway?

A. Glycolysis

B. Glucose-alanine cycle

C. Urea cycle

D. None of the above

14. Which enzyme converts pyruvate to alanine?

A. ALT

B. AST

C. Pyruvate kinase

D. Pyruvate decarboxylase

15. Pyruvate is converted into glucose through which pathway?

A. Glycolysis

B. Glucose-alanine cycle

C. Urea cycle

D. Gluconeogenesis

ANSWERS:-

1. Toxic in nature

2. Liver

3. Both A and c

4. Liver

5. Branched chain amino acid

6. Both A and B

7. Lacks enzyme of urea cycle

8. a – 3 b – 2 c – 1

9. All of the above

10. None of the above

11. Glutamine

12. There are 3 isoenzymes of mitochondrial glutaminase

13. Glucose-alanine cycle

14. ALT

15. Gluconeogenesis

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REFERENCE:- Pankaja Naik- Biochemistry; 4th edition; page no:- 270-271

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