Abstract
Both argininosuccinate synthase deficiency (ASS deficiency or CTLNI, OMIM # 215700) and citrin deficiency (CTLNII, OMIM # 603471, 605814) are urea cycle disorders. Argininosuccinate synthase (EC6.3.4.5) is the enzyme that ligates aspartate and citrulline to form argininosuccinic acid in the urea cycle. Deficiency in this enzyme causes massively elevated concentrations of citrulline and decreased arginine concentrations. Citrin is a membrane bound aspartate and glutamate carrier protein that transports aspartate out of the mitochondria, making aspartate available for ligation to citrulline in the next step of the urea cycle. Deficiency or dysfunction of this transporter results in decreased aspartate in the cytoplasm, which limits ASS enzyme activity.
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