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Liver Disorders

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Urea Cycle Enzyme Deficiency

Ammonia is a byproduct of protein metabolism. Elevated levels of ammonia (hyperammonemia) in the body are toxic to the central nervous system and can lead to death in neonates, infants and young children. The urea cycle (Krebs-Henseleit) is a series of five enzymatic reactions in the liver to detoxify ammonia and prepare it for excretion as urea. Normal urea synthesis requires five enzymes: arginase, argininosuccinate lyase (AL), argininosuccinate synthetase (AS), carbamylphosphate synthetase (CPS) and ornithine transcarbamylase (OTC); a deficiency in any one may lead to hyperammonemia. Genetic defects and inborn metabolic errors are linked to enzymatic deficiencies affecting an estimated 1 in 30,000 newborns.

Neonates appear normal at birth, but within a few days show symptoms that include lethargy, food denial and nausea. Convulsions and deep coma are common. Infants and children present with irritability, ataxia, vomiting and neurologic abnormalities. When infection remains unexplained, it is important to assess plasma ammonia levels to avoid misdiagnosis. Treatment includes fluids, calories and electrolytes to reduce further protein break down. Protein should be minimized.