Hepatic mitochondrial → cytosolic cycle converting two molecules of toxic NH3 into one neutral urea. Mitochondrial: NH3 + CO2 → carbamoyl phosphate (CPS1, rate-limiting, N-acetylglutamate-activated) → citrulline (OTC). Cytosolic: citrulline + aspartate → argininosuccinate (ASS1) → arginine + fumarate (ASL) → urea + ornithine (arginase). Ornithine re-enters mitochondria. Genetic defects in OTC (X-linked) and CPS1 are the most common urea-cycle disorders — present as hyperammonemic encephalopathy.
Organ Systems
digestive
renal
Pathway Steps
ammonia → carbamoyl-phosphate — via CPS1 — rate-limiting; mitochondrial; NAG-activated. CPS1 is the rate-limiting, mitochondrial entry step and absolutely requires N-acetylglutamate (NAG) as an allosteric activator — so NAG-synthase activity (stimulated by arginine) gates the whole cycle. It fixes the first nitrogen from free ammonia.
carbamoyl-phosphate → citrulline — via OTC (ornithine transcarbamylase) — X-linked deficiency is most common UCD. Ornithine transcarbamylase condenses carbamoyl-phosphate with ornithine; the citrulline product is exported to the cytosol. OTC deficiency is X-linked and the most common urea-cycle disorder, causing hyperammonemia with orotic aciduria.
citrulline → argininosuccinate — via argininosuccinate synthase (ASS1). Argininosuccinate synthetase joins citrulline with aspartate (the second nitrogen source), consuming ATP (→ AMP + PPi). This cytosolic step is also how citrulline supplementation raises arginine and nitric-oxide substrate more effectively than arginine itself.
argininosuccinate → l-arginine — via argininosuccinate lyase (ASL); also releases fumarate to TCA. Argininosuccinate lyase cleaves argininosuccinate into arginine plus fumarate — the fumarate links the urea cycle to the TCA cycle (the aspartate–argininosuccinate shunt), recovering the carbon skeleton.
l-arginine → urea — via arginase — also regenerates ornithine. Arginase hydrolyzes arginine to urea and regenerates ornithine (re-imported to mitochondria to continue the cycle). Arginase competes with nitric-oxide synthase for arginine, coupling ureagenesis to vascular NO availability.
Known Modulators
arginine (substrate) — urea cycle + NO precursor. L-arginine; substrate for NOS → NO + ornithine; supplementation for vascular health (modest)
l citrulline (substrate) — urea cycle intermediate → arginine. L-citrulline; bypasses arginine GI metabolism → more bioavailable arginine elevation
ornithine (substrate) — urea cycle (arginase product). L-ornithine; ammonia handling claims; OKG (ornithine-α-ketoglutarate) form for ICU nutrition
citrulline (substrate) — urea-cycle intermediate; NO precursor