Nitrogen Balance

Question 1

What does anabolic mean in terms of amino acids?

Answer 1

Positive nitrogen balance = Net gain in amino acids

Question 2

What does catabolic mean in terms of amino acids?

Answer 2

Negative nitrogen balance = Net loss in amino acids

Question 3

What is the most common cause of a positive nitrogen balance?

Answer 3

Pregnancy

Question 4

What is the most common cause of a negative nitrogen balance?

Answer 4

Malnutrition

Question 5

Describe the glucose-alanine cycle.

Answer 5

1. Pyruvate is made into alanine by transamination. Excess alanine is released into the blood and transported to the liver
2. In the liver, alanine is converted back to pyruvate by oxidative deamination
3. The pyruvate is used a source for carbons for glucose production via gluconeogenesis
4. Glucose enters the blood and is used by the muscles to produce pyruvate via glycolysis. This can then be used again to remove excess ammonia - this is the glucose-alanine cycle

Question 6

How much ATP and PO4- is consumed in one turn of the urea cycle?

Answer 6

3 ATP equivalents

4 high energy nucleotides i.e. PO4-

Question 7

Which is the only product generated by the urea cycle?

Answer 7

Urea. All other components are recycled.

Question 8

What is the implication in a patient if they have deficiencies of any of the enzymes involved in the urea cycle?

Answer 8

Higher levels of ammonia. Absence of any of the enzymes is not compatible with life.

Question 9

How does high levels of ammonia lead to neurotoxicity?

Answer 9

1. Ammonia is able to cross the blood brain barrier easily.
2. Once inside, it is converted to glutamate under the action of glutamate dehydrogenase
3. This causes a depletion in alpha-ketoglutarate which causes a depletion in oxaloacetate
4. This causes the Kreb’s cycle to stop, resulting in irreparable cell damage and neural cell death