Nitrogen Homeostasis and Urea Cycle

a-ketoglutarate

In the kidney, lymphocytes, neurons, and gut, glutamine is oxidized to —

urea

In the liver, nitrogen from AAs is converted into _

alanine, glutamine

In the muscle, some amino acids are converted to — and — and are released into the blood along with other AAs

glutamate

In the liver, most amino acids transfer the alpha amino group to alpha ketoglutarate to form —.

serine, threonine

All amino acids except these 2 undergo transamination

aspartate transaminase

Catalyzes the transfer of amino group of aspartate to a-ketoglutarate to form oxaloacetate and glutamate

Alanine + α-ketoglutarate⟷Pyruvate + Glutamate

reversible reaction catalyzed by alanine aminotransferase

nitrogen

Aminotransferases shuttle —

liver

ALT and AST are released into plasma after cellular damage to the —

B6

Aminotransferases require vitamin — for activity.

ammonium ion, α–ketoglutarate

Oxidative deamination of glutamate releases — and produces —

glutamate dehydrogenase

Oxidative deamination of glutamate to produce a-ketoglutarate and ammonium is catalyzed by —

NAD+ or NADPH

Glutamate dehydrogenase coenzyme (2 options)

glutamine synthetase

This enzyme uses ATP and adds ammonia to glutamate to produce glutamine (amide)

glutamine synthetase, glutamate dehydrogenase, carbamoyl phosphate synthetase 1

the 3 enzymes that can “fix” ammonia to an organic molecule

glutaminase

Hydrolyzes glutamine to produce glutamate and ammonium ion

glutamine, alanine

The 2 major transporters of nitrogen in the blood to the liver

liver

ammonia is converted to urea in the —

ALT

In the muscle, pyruvate produced by glycolysis is transaminated to generate alanine by —.

alpha-ketoglutarate

Alanine is transported in the blood to the liver, where it is transaminated with — to give pyruvate and glutamate.

GDH

Oxidative deamination of glutamate by — releases ammonia which is converted to urea in the liver.

glutaminase, glutamate dehydrogenase

glutamine —?—> glutamate —?—> a-ketoglutarate

In healthy adults, the amount of N consumed=N excreted

Nitrogen balance

positive N balance=growing children, pregnancy, recovering from illness/injury

negative N balance=starvation, deficiency in essential amino acid, disease like Kwashiokor

Circumstances under which one would not be in nitrogen balance

85-90% as urea in urine

small amounts of nitrogenous compounds (uric acid, creatinine, ammonia) also excreted in urine

some nitrogen is also lost in sweat and feces

How nitrogen is excreted:

structure of urea

urea cycle

arginine is synthesized during:

ammonia, aspartate

the sources of the 2 nitrogens in urea

ornithine

Each urea cycle regenerates —

citric acid cycle

The urea cycle shares intermediates with the —

carbamoyl phosphate synthetase-1

Rate limiting enzyme of urea cycle

N-acetylglutamate

CPS1 is allosterically activated by —

arginine

— allosterically activates N-acetylglutamate synthetase

NH4+

Defects in any step in the urea cycle lead to an increase in — in the blood

x-linked recessive

Inheritance pattern of ornithine transcarbamoylase deficiency

screen for elevated blood ammonia, confirm with liver biopsy

How urea cycle disorders are diagnosed after birth:

Fair if caught early to prevent intellectual impairment; 100% mortality if undiagnosed

Prognosis of urea cycle disorders

lethargy, poor feeding, vomiting, convulsions, ataxia, hypothermia, hyperventilation, eventually coma

Symptoms of urea cycle disorder in newborn:

Renal issue

If BUN in newborn is elevated:

liver malfunction

If BUN in newborn is decreased:

plasma ammonia, glutamine, alanine all ELEVATED

citrulline, arginine, BUN all LOW

Biochemical profile of CPS1 deficiency in newborn (type 1 hyperammonemia):

plasma ammonia, glutamine, alanine, citrulline, arginine, BUN

low protein diet with increased dietary intake of citrulline; IV sodium benzoate and sodium phenylacetate

Treatment of CPS1 deficiency

can be successfully treated with N-carbamoylglutamate (Carbaglu®), a functional analogue of NAG to activate CPS1;

arginine allosterically activates available N-acetylglutamates

Sodium benzoate or sodium phenylbutyrate remove ammonia

Treatment of N-acetylglutamate synthetase deficiency:

ornithine transcarbamoylase deficiency

Most common urea cycle disorder

elevated urinary orotic acid

Key diagnostic difference in OTC deficiency vs other UCDs

Carbamoyl phosphate produced in mitochondria during urea cycle leaks into the cytosol and gets converted to orotic acid (orotate), builds up in the blood, and is excreted in the urine (only for this urea cycle defect).

why urinary orotic acid is elevated in OTC deficiency:

Argininosuccinate Synthetase deficiency (Citrullinemia)

What deficiency? elevated plasma citrulline (very high), ammonia and glutamine

Argininosuccinate Lyase deficiency (Arginosuccinic aciduria)

What deficiency? Elevated plasma argininosuccinate, citrulline, glutamine, ammonia

Sodium benzoate and sodium phenylbutyrate bind covalently to AAs and produce nitrogen containing compounds which are excreted in the urine.

Why Na-Benzoate and Na-Phenylbutyrate are used for treating Hyperammonemia

hemodialysis

Only effective means to lower life threatening hyperammonemia

plasma glutamine

Measurement of — is the single most important guide to therapeutic interventions in UCDs

removing excess ammonia and replacing missing intermediates from cycle, limit protein intake

Therapeutic interventions in UCD involve 3 key components:

Glutamate dehydrogenase reaction (glutamate←→a-ketoglutarate + NH₄⁺) is shifted toward glutamate formation to lower ammonia. This depletes TCA cycle participants a-ketoglutarate (and therefore oxaloacetate), resulting in decreased oxidation and ATP production.

How hyperammonemia depletes energy availability in the brain

astroglial

These cells regulate ammonia levels via glutamate, glutamine, and NH_4⁺ interrelationships in the brain

neurotransmitter, GABA

In neurons glutamine is converted to glutamate, which is secreted as a — or converted to inhibitory neurotransmitter —.

GABA

Elevated ammonia depletes glutamate and decreases — synthesis, impairing CNS function

Glutamine is intracellular organic osmolyte, when levels rise, water enters astroglia and results in brain edema.

When glutamine level rises in the brain, brain edema occurs. Why?