Argininosuccinate Synthase (ASS) Deficiency (also known as Citrullinemia type I)
The enzyme ASS1 (or ASS) uses the citrulline produced by OTC and combines it with the amino acid aspartate to make a compound called argininosuccinate. Patients with complete deficiency of ASS (most severe type of this disorder) present with high levels of ammonia soon after birth. The blood level of citrulline in these patients is typically many times higher than the normal. The specific diagnosis can be made by plasma amino acid analysis based on extremely elevated citrulline levels and/or by enzyme analysis of cultured skin cells obtained from a skin biopsy, or by genetic testing.