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Urea Cycle
Hepatic mitochondrial-cytosolic cycle that converts toxic ammonia derived from amino acid deamination into urea for renal excretion via five enzymatic steps starting with carbamoyl phosphate synthetase I.
Also: Krebs-Henseleit cycle, Ureagenesis
The cycle integrates two nitrogens per urea: one from ammonia via CPS-I, one from aspartate. Intermediates include ornithine, citrulline (mitochondrial export), argininosuccinate, and arginine. Disorders (OTC deficiency, ASS, ASL, arginase) cause hyperammonemic encephalopathy and require protein restriction with nitrogen-scavenger therapy. Citrulline supplementation bypasses OTC deficiency proximal blocks.
How one textbook covers it
Modern Nutrition in Health and Disease, 12th ed. — Ch 1: Proteins and Amino Acids
The cycle integrates two nitrogens per urea: one from ammonia via CPS-I, one from aspartate. Intermediates include ornithine, citrulline (mitochondrial export), argininosuccinate, and arginine. Disorders (OTC deficiency, ASS, ASL, arginase) cause hyperammonemic encephalopathy and require protein restriction with nitrogen-scavenger therapy. Citrulline supplementation bypasses OTC deficiency proximal blocks.
Related terms
Ammonia, Arginine, Citrulline