Ammonia Metabolism Flashcards
Is there a lot of free Nitrogen in the body?
There is not a lot of free N in the body because it is toxic (esp to neuronal cells). Most N is attached to C but if it isn’t it predominates as NH4 which cannot cross membranes.
How are amino acids transported from the gut lumen to the intestinal epithelial cells?
Secondary active transport–Cell expends energy to pump Na from the lumen into the blood
What are the key amino acids in metabolism?
GAAG
- Glutamate (a-ketoglutarate): the amino group pool of the cell, synthesis/ catabolism of AA takes from here.
- Aspartate (oxaloacetate): Donates N to the urea cycle
- Alanine (pyruvate): Plays a key role in Gluconeogenesis
- Glutamine (glutamate): Transports N to liver for urea cycle
What does insulin do?
It promotes storage pathways.
How are AA digested in the FED state?
Dietary protein > AA then transported to…
1. Cells to make protein
2. Liver–excess AA are used to make triacylgycerols > VLDL (packaging as TAG in adipose, used as fuel by muscle)
3. Glucose to glycogen
4. Blood
OR
AA enter cell, join AA pool, translated to proteins, Proteolysis recycles back to AA pool
What happens in the fasted state?
Glucagon, cortisol, epinephrine and norepinephrine mobilize stored foods. The pool of AA in the blood come form breakdown of proteins in tissues. While proteins continue to be made Alanine and glutamine are transported to the liver for the urea cycle and gluconeogenesis.
What does the alanine/glucose cycle do?
It allows AA to be used as fuel by muscle cells.
- alpha KG acts as the universal amine acceptor forming glutamate.
- Glutamate dones the amine to pyruvate, forming alanine.
- Alanine is transported to the liver
- Alanine’s C is used for gluconeogenesis and N is converted to urea for excretion.
What can Glutamate dehydrogenase and glutamine synthase do in muscle cells?
They can add free ammonium to “fix” it as glutamine using ATP. Glutamine then transports N to the liver, where it’s made into urea and alpha KG.
What is GDH?
One of three enzymes that can fix free ammonium.
Reversible
Can use either NAD/NADH or NAD/NADPH as electron acceptor/donor.
What is the function of the uric acid cycle?
To convert N to urea, which can be excreted as urine.
Which product’s role in the uric acid cycle is analogous to oxaloacetate in the TCA cycle?
Ornithine–carries phosphate than is regenerated
How does N enter the cycle?
As free ammonium or aspartate.
What is the main regulator of the urea cycle?
- Availability of substrates
- Allosteric regulation of CPS-1
- Transcriptional regulation of urea cycle enzymes
How does Arginine regulate the uric acid cycle?
- Arginine increases synthesis of NAG by N acetyl glutamate synthase. NAG is an allosteric activator of CPS I.
What happens when arginine builds up?
- It increases synthesis of NAG which activates CPS1
2. It increases arginase activity
When is the transcription of urea cycle enzymes increased?
As long term adaption to conditions of excess protein breakdown: High protein diet, Starvation
How do urea cycle disorders manifest?
As hyperammonaemia (increased NH4) or hyperglutaminaemia (elevated glutamate)
What happens in cases of Glutamate depletion?
Glutamate is a precursor for neurotransmitters so depletion prevents synthesis of neurotransmitters. Symptoms: Refusal to eat Seizures Irritability Lethargy Ataxia Tremors FTT
How many inherited disorders are there related to hte urea cycle?
6 enzymes, 6 disorders and all manifest as ELEVATED BLOOD NH4+
What are the characteristics of urea cycle disorders down stream of CPS1?
Elevated urinary orotic acid–Cargonyl phosphate is also a substrate for pyrimidine synthesis which makes orotic acid which leads to elevated urinary orotic acid
What is the primary characteristic of urea cycle disorders downstream of CPS1?
- Elevated urinary orotic acid if OTC is defect
2. Moderately elevated urinary orotic acid if htere’s a defect down stream of orotic acid.
What is HHH?
A syndrome that occurs if the ornithine/citruline antiporter is defective. Causes hyperammononaemia, hyperornithaemia, and homocitrulinaemai. (Lethargy, irritability, seizures and confusion.)
What is a BUN test?
A test to measure free ammonium and blood urea nitrogen.
How do you measure free ammonia in a patient?
- INcubate their blood with recombinant GDH, alpha KG and NADPH.
- NADH absorbs UV light so absorption decreases as it is oxidized.
- The decrease in light absorption is proportional to the concentration of NH4+.
* *If there is a lot of NH4 in the blood stream you will have more NADPH
