Protein load in argininosuccinic aciduria: thoughts on its biochemical implications.

A patient with argininosuccinc aciduria was charged with 50 grams of protein, which was followed by considerable hyperammonemia. There was no response in further urea formation; but there was a considerable production of orotic acid, a precursor of pyrimidines. This makes orotic acid to an important diagnostic tool for the diagnosis of impaired urea formation. The patient's plasma amino acid pattern led to the suggestion that orotic acid synthesis is initiated by increased de novo formation of carbamyl phosphate in the cytosol and not by deviation of already existing intramitochondrial carbamyl phosphate.