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