The urea cycle and ammonia
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| Catabolism of amino acids generates ammonia (NH3) and ammonium ions (NH4+). Ammonia is toxic, particularly to the central nervous system (CNS). Most ammonia is detoxified at
its site of formation, by amidation of glutamate to glutamine, which is mainly derived from muscle and used as an energy source by enterocytes. The remaining nitrogen enters the portal vein either as ammonia or as alanine, both of which are used by the liver for the synthesis of urea.
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| The impaired clearance of ammonia and other nitrogenous waste products is an important cause of brain damage
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| The urea cycle is the major route by which waste nitrogen is excreted, and is described in Chapter 18. In neonates, inherited defects of any of the enzymes of the urea cycle lead to hyperammonemia, which impairs the function of the brain and causes a clinical condition known as encephalopathy. Such problems arise within the first 48 hours of life, and inevitably are made worse by protein-rich foods such as milk.
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