Ornithine (urea) cycle: ammonia detoxification in the liver

Sources of ammonia and pathways for its detoxification

Nitrogen enters the body predominantly as part of amino acids, biogenic amines, and nucleotides. Their catabolism generates ammonia (NH₃), a key nitrogen-containing compound that must be obligatorily detoxified due to its high neurotoxicity.

Main pathways of nitrogen excretion from the body:

• synthesis of urea in the liver (ornithine cycle) — the main pathway;
• synthesis of glutamine in muscles, brain, and other tissues — a universal temporary detoxification pathway;
• excretion of ammonium salts by the kidneys (via glutamine deamination);
• formation of glutamate in the brain and alanine (in muscles and intestines) as transport forms of nitrogen.

Glutamine synthesis as a universal pathway for ammonia detoxification

The reaction occurs in virtually all tissues. The enzyme glutamine synthetase catalyzes the addition of an NH₃ molecule to the γ-carboxyl group of glutamate, forming an amide group:

Glutamate + NH₃ + ATP → Glutamine + ADP + Pi

The consumption of one high-energy ATP bond ensures the thermodynamic irreversibility of the reaction. The resulting glutamine serves as a non-toxic transport form of ammonia: it is transported via the blood to the liver (for urea synthesis) and to the kidneys (for the formation of ammonium salts).

Nitrogen transport to the liver

Nitrogen is transported from the muscles and intestines predominantly in the form of alanine. In tissues, through transamination reactions, the amino group is transferred to pyruvate, and the resulting alanine enters the blood and then the liver.

In hepatocytes, alanine undergoes transamination with α-ketoglutarate (catalyzed by ALT), yielding glutamate. Glutamate undergoes oxidative deamination by the enzyme glutamate dehydrogenase to form α-ketoglutarate and free NH₃, which then enters the ornithine cycle.

Ornithine cycle (Krebs–Henseleit urea cycle)

The full set of enzymes is localized exclusively in the liver. The first two reactions take place in the mitochondrial matrix, while the subsequent ones occur in the cytosol.

1. Carbamoyl phosphate synthesis. The enzyme carbamoyl phosphate synthetase I (mitochondrial) catalyzes: NH₃ + CO₂ + 2 ATP → carbamoyl phosphate + 2 ADP + Pi. This is the first regulatory and most energy-demanding reaction.
2. Citrulline formation. Ornithine transcarbamylase transfers the carbamoyl group to ornithine; inorganic phosphate is released, forming citrulline, which enters the cytosol.
3. Argininosuccinate synthesis. In the cytosol, argininosuccinate synthetase attaches a second nitrogen atom from aspartate to citrulline; ATP is consumed with the formation of AMP and pyrophosphate.
4. Cleavage of argininosuccinate. Argininosuccinate lyase cleaves off fumarate, yielding arginine.
5. Arginine hydrolysis. The enzyme arginase, in the presence of H₂O, cleaves arginine into urea and ornithine. Ornithine returns to the mitochondrion, completing the cycle.

Thus, a single urea molecule incorporates two nitrogen atoms: the first from free ammonia (via carbamoyl phosphate) and the second from aspartate.

Connection of the ornithine cycle with the TCA cycle and aspartate regeneration

The resulting fumarate enters the tricarboxylic acid cycle, where it is converted into malate by fumarase, and then into oxaloacetate by malate dehydrogenase. An amino group from glutamate is transferred to oxaloacetate via transamination (AST), regenerating aspartate, which re-enters the ornithine cycle.

Thus, the biological role of the cycle is not limited to urea synthesis alone: it also ensures the recycling of aspartate and coupling with the TCA cycle.

Regulation of urea synthesis

Three enzymes are regulatory:

• carbamoyl phosphate synthetase I;
• ornithine transcarbamylase;
• arginase.

Allosteric activation. Carbamoyl phosphate synthetase I is activated by N-acetylglutamate, which is synthesized by N-acetylglutamate synthase when the concentration of arginine increases as an indicator of active cycle function.

Enzyme induction. During prolonged physical exertion, fasting, a high-protein diet, and enhanced protein catabolism, the enzymes of the ornithine cycle (carbamoyl phosphate synthetase I, ornithine transcarbamylase, arginase) are induced by cortisol. Simultaneously, gluconeogenesis enzymes (phosphoenolpyruvate carboxykinase) as well as ALT and glutamate dehydrogenase are induced, ensuring the supply of ammonia and carbon skeletons.

Ammonium salts and the role of glutamine in the kidneys

Glutamine entering the kidneys is deaminated by glutaminase, releasing NH₃, which binds protons to form the NH₄⁺ ion. Ammonium salts are excreted in the urine, which simultaneously ensures nitrogen excretion and the maintenance of acid-base balance.

Hyperammonemia

A defect in any of the ornithine cycle enzymes leads to the accumulation of ammonia in the blood, known as hyperammonemia. Ammonia is primarily toxic to the central nervous system: it binds α-ketoglutarate to form glutamate (and subsequently glutamine), depleting the TCA cycle in neurons and disrupting the brain's energy supply. Clinically, hyperammonemia manifests as vomiting, seizures, and impaired consciousness progressing to coma.

Key points

• NH₃ is a toxic product of amino acid catabolism; the main detoxification pathways are the synthesis of urea, glutamine, and ammonium salts.
• Glutamine synthetase attaches NH₃ to glutamate utilizing ATP — a universal reaction in all tissues.
• The complete ornithine cycle is localized exclusively in the liver; the first two reactions occur in the mitochondria, and the rest in the cytosol.
• The two nitrogen atoms of urea originate from free NH₃ and aspartate; 4 high-energy bonds are consumed per cycle.
• Key enzymes: carbamoyl phosphate synthetase I, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinate lyase, arginase.
• The cycle is coupled to the TCA cycle via fumarate and ensures the regeneration of aspartate.
• The main allosteric activator is N-acetylglutamate; enzyme inducers include cortisol, a high-protein diet, and fasting.
• Defects in the cycle enzymes cause hyperammonemia with severe CNS damage.