Hyperornithinemia, Hyperammonemia, and Homocitrullinuria

THE Krebs-Henseleit urea cycle is the only biochemical pathway for the conversion of ammonia to urea in man (Fig 1). It is known to involve five enzymatic reactions and, so far, four genetically determined disease states have been described, each of which is characterized by a deficiency of one of the urea cycle enzymes.1All of these states are associated with varying degrees of hyperammonemia which, as would be expected, are most severe in the disorders of steps 1 and 2, and least severe in argininosuccinic aciduria where it usually occurs only after a protein load. The clinical picture of all these disorders includes a reluctance to eat high protein foods, intermittent ataxia, irritability, lethargy or even coma, and mental retardation. There are several instances in which a low protein diet simultaneously caused clinical improvement and reduction or elimination of hyperammonemia. For this and other reasons, it is likely