Utilization and excretion of AA and Nitrogen Flashcards
Protein metabolism
1.become tissue protein
2. become amino acids (acetyl coA and CAC; ketones; carbohydrates)
3.non-protein nitrogen derivatives such as DNA
Protein breakdown
- 25-95% of AA catabolized in enterocytes or by bacteria
-5-75% absorbed in portal vein and metabolized in liver
Ammonia/ammonium
produced from the oxidation of AAs
*in equilibrium with each other in aqueous solutions
Transport of ammonia
moves easily across biological membranes
**ammonium difficult to transport
Deamination of AAS
bacterial urease in the colon converts the urea into NH3 (ammonia)
Ammonia in portal vein vs. hepatic vein
Portal vein: 250microM
*very high levels of ammonia in portal vein, meaning the liver needs to convert it to urea
Hepatic vein: 30 microM
Pyruvate/alanine part of urea cycle
- Pyruvate converted to alanine through transamination by alanine transaminase (ALT). Alanine carries N to liver
- Alanine deaminated in liver into NH2 and pyruvate
- NH2 used to make urea and pyruvate used to make glucose
Cahill cycle
-occurs in post-absorptive state
-free AAs (especially alanine and glutamine) are released from muscle into circulation
>Alanine is key gluconeogenic AA, and NH2 is converted to urea
AA extraction and release
Occurs post meal
-Release AA- splanchnic tissues
-extract AA- peripheral muscles
Urea
-water soluble
-non-toxic
-synthesized in liver (mammals) but requires energy
-not ionized at physiological pH meaning it is easily transported
Biosynthesis of urea
1.Transmination
2. Oxidative deamination of glutamate
3. Ammonia transport
4. Urea cycle
Transamination
-freely reversible
-Requires pyridoxal phosphate (derivative of vit B6)
-All of the amino Nitrogen can be concentrated in L-glutamate for transport
Oxidative deamination
Glutamate undergoes oxidative deamination
-Transfer amino nitrogen to alpha-ketoglutarate catalyzed by hepatic L-glutamate dehydrogenase (GDH)
-release nitrogen as NH3
**freely reversible
Liver GDH
-inhibited by ATP, GTP, and NADH
-activated by ADP
Ammonia Transport
Ammonia can be produced by enteric bacteria and extra-hepatic tissues.
Glutamine synthetase fixes NH4 as glutamine. Glutamine holds ammonia in a non-toxic form
Glutamine to Glutamate reaction
Irreversible in the direction of glutamate and NH4 formation
> allows for ammonia to be free in the liver so that it can be synthesized into urea
Urea cycle
1.Condensation of CO2 ammonia and ATP into carbomyl phosphate
2.Carbomyl phosphate and ornithine= citrulline
3. citrulline and aspartate into arginosuccinate
4.cleavage of arginosuccinate into arginine and fumarate
5. cleavage of arginine into urea and ornithine
Rate limiting step of urea cycle
Condensation of CO2 ammonia and ATP into carbomyl phosphate
>occurs by carbomyl phosphate synthase I in mitochondria
Blood urea nitrogen
-BUN, PUN, MUN- indicate liver and kidney health
-urea travels from the liver to the kidneys to be excreted
Decreased BUN
-chronic, severe liver disease
-low protein diets
-drugs causing diuresis
-overhydration
-increased protein synthesis
Increased BUN
-blood loss
-dehydration
-recent high protein meal
-kidney infection or inflammation
-bladder or urethral obstruction
BUN and AA adequacy
-There is an AA threshold, which is the max amount of AA needed. If above this threshold, then would expect there to be an increased level of urea in the blood
Energy cost of urea synthesis
6.5 ATP/mol of urea
**takes a lot of energy to get rid of excess protein
Gross energy
-total possible energy from 100% combustion of feed
Digestible energy
-Energy when taking into account the energy lost in feces
Metabolizable energy
Energy left after accounting for loss in feces, urine, gas
**energy actually available for use
Net energy
the total energy left after removing energy lost to feces, urine, gas, and heat (from metabolic processes)
Urea cycles role in heat increment
-urea cycle plays a large role in het increment therefore high crude protein ingredients will result in increased urea cycle and therefore less energy available from the original food intake
Pet food energy use
Use metabolizable energy adjusted for N
