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GI > Kiersti's High Yield of Urea Cycle > Flashcards

Kiersti's High Yield of Urea Cycle Flashcards

1
Q

When do we have a positive nitrogen balance? (N intake > N excretion)

A
  • During growth (from infancy to teens, and during pregnancy)
  • During recovery from illness or malnutrition
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2
Q

When do we have a negative nitrogen balance?

A

illness

malnutrition

severe burn victims

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3
Q

Urea is the main way that nitrogen is excreted. What are the other ways?

A

NH4+

Creatinine

Uric Acid

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4
Q

What happens to BUN, Blood creatinine levels, and blood uric acid levels during pregnancy?

A

They all DECREASE!

(Note that during pregancy GFR increases)

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5
Q

BUN increases in………………. damage and decreases during………….. damage.

A

kidney

liver

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6
Q

What are the main steps in amino acid degredation?

A
  • Removal of nitrogen
  • Clearance of NH4+ (urea cycle)
  • Utilization of the carbon backbone
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7
Q

What are the 3 types of reactions to remove nitrogen from amino acids?

A

Transamination (main way!)

deamination

deamidation

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8
Q

Deamination is the removal of the alpha amino group of certain amino acids as NH4+. These amino acids are….

A

Glutamate ***

Glycine

Serine

Threonine

Histidine

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9
Q

Deamidation is the removal of an amide group as NH4+ from which amino acids?

This provides most of the NH4+ in the urine. It is most important in which organ?

A

Glutamine and asparagine

Kidney

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10
Q

Transamination reactions are reversible! The amino nitrogen is generally collected on what?

A

Glutamate!

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11
Q

What is the coenzyme for transamination reactions?

A

Pyridoxal phosphate (PLP)

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12
Q

Which aminotransferase/Transaminase is more specific, ALT or AST?

A

ALT

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13
Q

Glutamate plays a central role in nitrogen removal. How?

A
  • it collects nitrogen via transamination
  • provides NH4+ to the urea cycle both directly and indirectly
  • Its a precursor for N-acetyl glutamate (the allosteric activator of the urea cycle)
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14
Q

How does glutamate provide NH4+ to the urea cycle directly?

A

via deamination by glutamate dehydrogenase.

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15
Q

How does glutamate provide NH4+ to the urea cycle indirectly?

A

by transaminating oxaloacetate into aspartate (which then can provide an amino group to urea).

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16
Q

What are the 2 major mechanisms to control blood ammonia levels?

A
  • Transamination rxns to “collect” nitrogen on glutamate
  • “fixing” free ammonia into organic molecules
17
Q

What are the enzymes used for the 3 rxns that “fix” free ammonia into organic molecules?

A
  • Glutamate dehydrogenase
  • Glutamine synthetase (main detoxyfication rxn in brain)
  • CPSI (1st step in urea synthesis)
18
Q
A
19
Q

Glutamine synthetase catalyzes the main detoxification rxn in the brain. What is this rxn?

A

Glutamate + NH4+ + ATP –> Glutamine

20
Q

Where is the only place the urea cycle takes place?

A

The liver!

21
Q

How is nitrogen carried to the liver?

A

Alanine and glutamine

22
Q

The transport of alanine to the liver serves what 2 main purposes?

A
  1. Removal of nitrogen through the urea cycle
  2. Gluconeogenesis
23
Q

The main sources of alanine is the muscle. What are the other sources?

A

Kidney and intestines

* Due to converting glutamine -> alanine

Glutamine -> Glutamate -> alanine

24
Q

Glutamine -> Glutamate -> alanine

What enzyme makes glutamine into glutamate?

What makes glutamate into alanine?

A

glutaminase

ALT

25
Q

Which organs are the main glutamine producers?

A

Muscle and brain

26
Q

Which organs are the main glutamine users?

A

Kidney, gut, immune cells and liver

27
Q

What enzyme catalyzes the rate limiting step in the urea cycle?

A

CPSI

28
Q

What is the allosteric activator of CPSI?

A

N-Acetyl Glutamate

* Note that Arginine is the allosteric activator for the enzyme that makes N-Acetyl Glutamate!

29
Q

Which urea cycle intermediate enters the TCA cycle to regenerate oxaloacetate and then aspartate?

A

Fumarate

30
Q

Which urea cycle intermediate leaves the mitochondria?

A

Citrulline

31
Q

Will nitrogen excretion increase or decrease during fasting and starvation?

A

During fasting, nitrogen excretion will increase as the body degrades muscle to amino acids, to transport to the liver to make glucose.

Then during starvation, levels will fall as the brain switches from glucose to ketone bodies! (so muscle degredation decreases)

32
Q

What results from hyperammonemia?

A

hepatic encephalopathy, cerebral edema, seizures, nausea, vomiting, lethargy, death.

33
Q

Primary hyperammonemias are due to what?

A

Defective enzymes!

34
Q

Secondary hyperammonemias are due to what?

A

Mainly from hepatic failure, meaning that hepatocytes can’t carry out the urea cycle.

Also can be caused by genetic defects not related to urea cycle enzymes.

35
Q

All primary hyperammonemias are autosomal recessive, except which one?

A

Type II hyperammonemia (OTC) which is X-Linked recessive

36
Q

Type II hyperammonemia (due to the defective OTC enzyme) leads to a build up of carbamoyl phosphate. When this occurs, it goes to the cytoplasm to be made into what?

A

Orotate

* orotate levels will be increased in urine!

37
Q

To manage urea cycle disorders, the patient should reduce the ammonia load on the urea cycle by a low protein diet, avoiding fasting, and taking scavenger drugs. What are the scavenger drugs we need to know?

A

Benzoate and Phenylbutyrate

* Scavenger drugs conjugate amino acids and target them for urinary excretion!

38
Q

Patients with urea cycle disorders should be provided with what urea cycle intermediates?

A
  • Arginine (if arginosuccinate lyase is deficient. It will generate more ornithine for the cycle to continue)
  • Citrulline (in type I and II hyperammonemia. It captures aspartate, so at least one nitrogen can be excreted)

GI (9 decks)

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