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Urea cycle

The urea cycle (also called the ornithine cycle or Krebs-Henseleit cycle) is the metabolic pathway of five enzymatic reactions and associated transporters by which the mammalian liver converts toxic ammonia, generated mainly by amino acid catabolism, into urea, which the kidneys then excrete in urine.¹^•² It was the first major biochemical cycle to be discovered, described by Hans Krebs and Kurt Henseleit in 1932, and genetic defects in its components cause the urea cycle disorders, a group of inborn errors of metabolism marked by ammonia accumulation that is especially toxic to the brain.³^•⁴

Key fact	Detail
Discovery	Described by Hans Krebs and Kurt Henseleit, published in 1932 as the "ornithine cycle"; the first of the major biochemical cycles.³^•⁵
Net reaction	Two moles of ammonia and one mole of bicarbonate are converted to one mole of urea at the expense of three moles of ATP.⁶
Location	The liver is the only site of urea synthesis; the first two enzymatic steps occur in the mitochondrial matrix and the remaining steps in the cytoplasm.¹^•²
Rate-limiting step	Carbamoyl phosphate synthetase 1 (CPS1), which requires the obligate activator N-acetylglutamate.¹^•²
Disorders	Eight types of urea cycle disorders result from defects in the pathway; ornithine transcarbamylase deficiency is the only X-linked recessive one.²
Clinical test	Blood urea nitrogen (BUN) measures the urea level in blood and is used to assess kidney function.¹
Treatment	Nitrogen scavengers, dietary protein restriction, arginine or citrulline supplementation, calorie support, and liver transplant; carglumic acid became available for N-acetylglutamate synthase deficiency in 2011.⁴

Function and nitrogen sources

The cycle's purpose is detoxification. Ammonia originates from protein catabolism, whether from a high-protein diet, deamination, or prolonged starvation, and is also produced naturally by gut flora.¹ In muscle and peripheral tissues, glutamate accepts free ammonia to form glutamine, which is exported to the liver; there, glutaminase releases glutamate and ammonia, and glutamate dehydrogenase yields additional ammonia for urea synthesis.¹ Glutamine synthetase also operates in the brain and skeletal muscle, detoxifying ammonia outside the liver when circulating ammonia is high.⁷

The two nitrogen atoms of each urea molecule enter at defined points: ammonia supplies the first amine group at the CPS1 step, and aspartate, formed by transamination of oxaloacetate and glutamate, supplies the second.¹

The five reactions

The pathway runs as follows, with the first two steps in the mitochondrial matrix and the rest in the cytoplasm:²

Carbamoyl phosphate synthesis. Carbamoyl phosphate synthetase 1 converts carbon dioxide, water, and ammonia into carbamoyl phosphate, consuming two ATP: $\mathrm{CO_2 + H_2O + 2\ ATP \rightarrow carbamoyl\ phosphate + 2\ ADP + P_i}$ . CPS1 is the rate-limiting step and requires the obligate activator N-acetylglutamate, which is generated from glutamate and acetyl-CoA by N-acetylglutamate synthase, an enzyme that arginine can upregulate.¹
Citrulline formation. Ornithine transcarbamoylase combines carbamoyl phosphate with ornithine to form citrulline, which ornithine translocase carries from the mitochondria into the cytoplasm.¹
Argininosuccinate synthesis. Argininosuccinate synthetase joins citrulline with aspartate in an ATP-requiring reaction: $\mathrm{citrulline + aspartate + ATP \rightarrow argininosuccinate}$ .¹
Arginine formation. Argininosuccinate lyase cleaves argininosuccinate into arginine and fumarate; the fumarate feeds into the tricarboxylic acid cycle and tyrosine catabolism.¹
Urea release. Arginase hydrolyzes arginine to urea and ornithine: $\mathrm{arginine + H_2O \rightarrow urea + ornithine}$ . The regenerated ornithine returns to step 2, closing the cycle.¹

The overall stoichiometry, worked out in later enzymatic studies, converts two moles of ammonia and one mole of bicarbonate to urea at the expense of three moles of ATP.⁶

Discovery and history

Krebs and Henseleit, 1932. Hans Krebs (1900-1981), then a physician and researcher in Freiburg, Germany, and his doctoral fellow Kurt Henseleit (1907-1973) published "Studies on urea formation in the animal organism" in 1932, laying the ground for what is now called the Krebs-Henseleit cycle.⁵ Their theory was proposed to explain two facts: the catalytic effect of ornithine on urea synthesis, and the abundant occurrence of arginase in the livers of ureotelic organisms; a third fact, the liver's ability to form urea from citrulline and ammonia, followed from the theory.⁸ Ornithine acts like a catalyst, and citrulline, arginine, and ornithine participate in a cyclic sequence whose net effect is urea formation from carbon dioxide and two molecules of ammonia.⁹

The discovery was conceptually significant beyond urea itself: it was the first of the major biochemical cycles, and it introduced the idea that metabolic organization is cyclical rather than linear, an insight that foreshadowed the Krebs citric acid cycle of 1937.³ Krebs later won the 1953 Nobel Prize in Physiology or Medicine for the tricarboxylic acid cycle.⁵ Krebs and Henseleit had described the cycle only in intact liver cells; in 1946 Philip Pacy Cohen and Mika Hayano demonstrated it in liver homogenates by adding glutamic acid, ATP, cytochrome C, and magnesium ions under aerobic conditions.⁶

Urea cycle disorders

Defects in the pathway's components cause eight types of urea cycle disorders, in which nitrogen accumulates as ammonia, a compound especially toxic to the brain.²^•⁴ The pathway comprises six enzymes and two transporters, and a deficiency of any of them disrupts nitrogen detoxification.⁴ Ornithine transcarbamylase deficiency is the only X-linked recessive enzyme deficiency among them.¹ In newborns with clinical distress where sepsis is suspected, hyperammonemia must always form part of the initial differential diagnosis, and treatment should not be delayed while the final diagnosis is reached.¹⁰^•¹¹

Treatment. For decades, treatment consisted of nitrogen scavengers, dietary protein restriction, arginine or citrulline supplementation, calorie support, and liver transplant.⁴ In 2011, carglumic acid became available as a substitute for N-acetylglutamate in N-acetylglutamate synthase deficiency.⁴ The decade after that saw the development of enzyme therapy for arginase deficiency and gene therapy for ornithine transcarbamylase deficiency.⁴

Comparative physiology

The cycle marks one of three strategies vertebrates use to dispose of nitrogen. Aquatic species can excrete ammonia directly through the gills and are called ammonotelic; semiaquatic and terrestrial animals detoxify ammonia, at a considerable energetic cost, through the synthesis of urea (mammals, some fish) or uric acid (birds), or both (reptiles).¹² Mammals, amphibians, and elasmobranch fishes such as sharks and rays are ureotelic; birds, terrestrial reptiles, and most insects are uricotelic.¹³^•¹⁴

The uricotelic strategy differs biochemically, not just in the end product. Uricotelic species lack the mitochondrial enzyme CPS1, which catalyzes the first reaction of ammonia detoxification, and instead rely on glutamine synthetase as a primary means of ammonia detoxification; the glutamine nitrogen is then used in purine ring biosynthesis to build uric acid.⁷ In avian liver, glutamine synthetase is broadly distributed, congruent with the distribution of CPS1 in rat liver, whereas in mammalian liver it is restricted to the perivenular region.⁷

The cycle is not confined to mammals. The gulf toadfish, Opsanus beta, is one of the teleosts expressing a full complement of ornithine-urea cycle enzymes in the liver, and it can switch between ammonotelic and ureotelic excretion.¹²^•¹⁵ Elasmobranchs convert ammonia waste to urea through the ornithine-urea cycle and retain approximately 400 mM urea in body fluids as an organic osmolyte, keeping plasma osmolarity close to that of seawater.¹³ In ruminants, urea excretion is heavily influenced by rumen microbial metabolism and a urea nitrogen recycling loop, in which urea returns to the rumen, microbes hydrolyze it, and the liberated ammonia supports microbial amino acid synthesis.¹⁶^•¹²

Regulation and other roles

Urea cycle enzymes are highly regulated in the liver, and several are also expressed in many other tissues, where they participate in the synthesis of nitric oxide, polyamines, proline, and glutamate.¹⁷ Mammals respond to increased protein intake with an increase in glomerular filtration rate and renal plasma flow, raising urea excretory capacity.¹⁸ In mammals, urea excretion itself may enhance the kidney's urine-concentrating ability.¹⁹

References

Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Metabolism and metabolic pathways › Amino acid and nitrogen metabolism › Urea cycle and nitrogen disposal › Urea cycle reactions and intermediate metabolites

Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026

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