1
• Interconversion of GLN, GLU, and α-KG are important in
the regulation of blood NH3 concentration.
Ammonia is highly toxic to the CNS.Its level in the blood should be low.
Enzymes involved:
Transaminase GDH
GLN synthetase Glutaminase
• Sources of Ammonia
1) Dietary protein
~60% dietary N are converted into urea in the liver
2) Enzymes of GIT bacteria
hydrolysis of GLN by intestinal bacteria. Bacteria also
degrade urea present in intestinal lumen.
3) Catabolism of endogenous amino acids
generating free NH3
emcrit.org/images/part1/ammonia%20produce.jpeg
2
Biomed Pharmacother 56:446-457, 2002
putrescine
spermidine
spermine
3
Glucagon
+
(why?)
4
Reactions of the urea cycle
Lippincott’s Illustrated Reviews: Biochemistry, 2011
*
Urea cycle and TCA cycle interface
†
† via shuttle
5
http://www.ojrd.com/content/figures/1750-1172-7-32-1-l.jpg
Citrin=aspartate-glutamate carrier
Citrin deficiency causes citrullinemia
cytosolic
mitochondrial
Cytosolic & mitochondrial
Overall stoichiometry of the urea cycle
Aspartate + NH3 + CO2 + 3 ATP + H2O
urea + fumarate + 2ADP + AMP + 2Pi + PPi
6
Saheki & Kobayahi. J Human Genet 47:333-341, 2002
Fum Citrin deficiency causes
inefficient ASS activity and
limits urea cycle activity.
Mutation of ASS can also
occur, leading to type II
citrullinemia (and
hyperammonemia).
Metabolic fates of arginine. Arginine is derived from dietary sources, protein catabolism, and endogenous synthesis. The
citrulline–NO cycle (green) is responsible for the regeneration of arginine in various tissues. Arginine serves as the
precursor for many biologically important molecules; a decrease in arginine may result in decreased production of
compounds for which it serves as a precursor. ADC, arginine decarboxylase; Arg1, arginase 1; ASA, argininosuccinic acid;
ASL, argininosuccinate lyase; ASS, argininosuccinate synthase; GABA, γ-amino butyric acid; GATM, glycine
amidinotransferase; NO, nitric oxide; NOS, NO synthase; OTC, ornithine transcarbamylase. Reproduced with permission
from publishers John Wiley and Sons.
Argininosuccinate lyase (ASL)
deficiency can lead to life-
threatening hyperammonemia.
Long-term complications may
include liver dysfunction,
cognitive deficit and
hypertension. Hypertension
may be due to secondary nitric
oxide deficiency.
Dietary therapy is key to
optimum health including
protein restriction and arginine
base supplementation. Liver
transplant may be required.
Nagamani et al. Genetics in Medicine 14:501-507, 2012
7
Km of CPS I for NH3 is high
(low affinity, high capacity, not saturated easily.
Therefore, is the main channel of GLN metabolism)
Km of GLN Synthetase
for NH3 is low
(high affinity, low capacity)
Fail safe design to take care of body ammonia
escaping
emcrit.org/images/part1/ammonia%20produce.jpeg
8
Liver is the main organ of amino acid catabolism (except BCAA).
When liver function is impaired, the following nitrogen related metabolism may occur
which may play role in the development of hepatic encephalopathy or hepatic coma:
a)Reduced capacity of urea cycle, leading to elevated NH3
Ammonia is toxic to CNS because, more GLN will be formed leading to increase
osmotic pressure and cell swelling (edema). Also more GABA will be form from GLU (via
glutamic acid decarboxylase) which is an inhibitory neurotransmitter.
b) Elevated level of many amino acids including those which are neurotransmitter
precursors
AA precursor Neurotransmitter
PHE Norepinephrine (noradrenaline)
TYR Epinephrine (adrenaline)
Dopamine
TRP Serotonin
HIS Histamine
Increase supply of precursors may disrupt normal neurotransmitter metabolism
and lead to formation of false neurotransmitters
9
lactulose*
Lactulose is a nondigestible disaccharide (fructose and galactose). Upon GIT
fermentation by bacteria produces SCFA therby lowering GIT pH and
transforms freely diffusible NH3 into NH4+ which cannot be diffuse back
into the blood.
Biomed Pharmacother 56: 427-438, 2002
Glutamine, Arginine and Nitric Oxide
10
Overloading of urea cycle could result from:
- NH4Cl consumption (accident)
- Increase protein degradation
- Arginine (ARG) deficient diet*
- High lysine (LYS) diet*
These conditions will lead to OROTIC ACIDURIA
which could be reversed by supplementing the diet with:
Citrulline, Ornithine or Arginine
* Often seen in chicken. Even more serious for cats because ARG is
essential.
Brosnan & Brosnan. J Nutr 137:1656S-1661S, 2007
http://www.chembio.uoguelph.ca/educmat/chm452/lectur30.htm
?
Overloading of urea cycle
activates cytoplasmic
handling of GLN leading to
increase orotate.
11
Biomed Pharmacother 56: 427-438, 2002
biochem4.okstate.edu/.../nucleotides.htm
(cytosine, thymine, uracil)
Pyrimidine synthesis in
cytosol from carbamoyl
asparate
12
Highlights for ARGININE synthesis
• ARG is non-essential but could be conditionally essential
• ARG synthesis involves the GIT and the kidney
• Pyrroline-5-carboxylate synthetase is only found in GIT
• Conversion of citrulline to ARG occurs in kidney
Requirement consideration – cats & patients with massive
intestine resection.
Cats – have very low intestinal activities of
Pyrroline-5-carboxylase synthetase &
Ornithine aminotransferase
As a consequence, citrulline supply is low. A single ARG-free
meal can result in hyperammonemia, convulsion and even death.
13
Role of the kidney
For ARG formation (~60% of net synthesis)
Takes up GLN for acid-base homeostasis
- sulfuric acids are formed from
dietary protein
- the hydrogen ions will be neutralized by
blood bicarbonate
Nutrition & Metabolism (2003, Blackwell Publishing)
Amino Acids 26:345-351, 2004
OAT: ornithine animotransferase
14
Biomed Pharmacother 56:439-445, 2002
http://www.djur.cob.lu.se/Cellprolif/Research/research_area_1.html
*
J Nutr Biochem 17: 571-588, 2006
Arterioscler Thromb Vasc Biol 28:s11-16, 2008
15
Amino acids as ingredients in supplements, functional
foods & beverages
Rationale:
- stimulate protein synthesis
- for various purposes including performance
enhancement (in sports), treatment of
bed sores, sarcopenia or after certain surgery.
Amino acids in single or combination:
BCAA
ARG
GLN
TAU
Observed safe level (OSL)
ARG – 20 g/d
GLN – 14 g/d