Argininosuccinate synthetase

Argininosuccinate synthetase (ASS) is an enzyme that catalyzes the synthesis of argininosuccinate from citrulline and aspartate

ASS is responsible for the third step of the urea cycle and one of the reactions of the Citrulline-NO cycle.

Gene
The gene that encodes for this enzyme, ASS, is located on chromosome 9. In humans, ASS is expressed mostly in the cells of liver and kidney.

Pathology
Citrullinemia: At least 50 mutations that cause type I citrullinemia have been identified in the ASS gene. Most of these mutations substitute one amino acid for another in ASS. These mutations likely affect the structure of the enzyme and its ability to bind to citrulline, aspartate, and other molecules. A few mutations lead to the production of an abnormally short enzyme that cannot effectively play its role in the urea cycle.

Defects in ASS disrupt the third step of the urea cycle, preventing the liver from processing excess nitrogen into urea. As a result, nitrogen (in the form of ammonia) and other byproducts of the urea cycle (such as citrulline) build up in the bloodstream. Ammonia is toxic, particularly to the nervous system. An accumulation of ammonia during the first few days of life leads to poor feeding, vomiting, seizures, and the other signs and symptoms of type I citrullinemia.