Citrulline

Citrulline is a nonproteinogenic amino acid formed in the urea cycle and recycled into arginine. It also appears in nitric oxide synthesis and protein metabolism.

Urea cycle: A liver-centered metabolic pathway that converts toxic ammonia into urea for excretion. Citrulline is formed from ornithine and carbamoyl phosphate early in this cycle.
Ornithine: A nonproteinogenic amino acid that participates in the urea cycle and polyamine synthesis. Ornithine supplies the carbon skeleton from which citrulline is formed.
Plasma amino acid test: A blood test that measures concentrations of amino acids to help evaluate metabolic and inherited disorders. Measured citrulline can help localize defects in the urea cycle.
Dietary supplement: A product taken to add nutrients or other substances to the diet. Citrulline supplements are studied within the broader, unevenly regulated supplement landscape.
Argininosuccinate: An intermediate formed when citrulline combines with aspartate in the urea cycle. Its formation is the next reaction that consumes citrulline in the cycle.
Carbamoyl phosphate: A high-energy molecule that donates a carbamoyl group in several metabolic pathways. It contributes the carbamoyl group used to make citrulline.
Citrullinemia: An inherited metabolic disorder in which citrulline accumulates because of impaired urea-cycle function. Its elevated blood citrulline illustrates how the metabolite can signal a pathway block.
Exercise performance: The measurable capacity to perform physical activity, including endurance, strength, and repeated effort. Trials examine whether citrulline meaningfully improves exercise outcomes beyond changing blood metabolites.
Argininosuccinate lyase: An enzyme that cleaves argininosuccinate into arginine and fumarate. The enzyme completes citrulline’s conversion toward arginine.
Aspartate: A protein-forming amino acid that also donates nitrogen in the urea cycle. Aspartate supplies the second nitrogen that is ultimately removed as urea.

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