The
urea cycle consists of six consecutive enzymatic reactions that convert waste
nitrogen into
urea.
Urea cycle disorders are a group of inborn errors of hepatic metabolism that often result in life threatening
hyperammonemia and hyperglutaminemia. Deficiencies of all of the
enzymes of the cycle have been described and although each specific disorder results in the accumulation of different precursors,
hyperammonemia and hyperglutaminemia are common biochemical hallmarks of these disorders.
Arginase is the
enzyme involved in the last step of the
urea cycle. It catalyzes the conversion of
arginine to
urea and
ornithine. The latter reenters the mitochondrion to continue the cycle.
Hyperargininemia is an autosomal recessive disorder caused by a defect in the
arginase I
enzyme. Unlike other
urea cycle disorders, this condition is not generally associated with a hyperammonemic
encephalopathy in the neonatal period. It typically presents later in childhood between 2 and 4 years of age with predominantly neurological features. If untreated, it progresses with gradual developmental regression. A favorable outcome can be achieved if dietary treatment and alternative pathway
therapy are instituted early in the disease course. With this approach, further neurological deterioration is prevented and partial recovery of skills ensues. Early diagnosis of this disorder through newborn screening programs may lead to a better outcome. This review article summarizes the clinical characterization of this disorder; as well as its biochemical, enzymatic, and molecular features. Treatment, prenatal diagnosis and diagnosis through newborn screening are also discussed.