Urea cycle defects causing citrullinemia

Ammonia is detoxified through its conversion to urea by the enzymes in the small boxes, whose abbreviations are spelled out below. ASS catalyzes the synthesis of argininosuccinate from citrulline and aspartate. Deficiency of ASS (highlighted red) causes hyperammonemia along with marked accumulation of plasma glutamine and citrulline and decreased plasma arginine concentrations. Urinary orotic acid may be elevated. Deficiency of citrin causes hyperammonemia along with variable accumulation of plasma glutamine and citrulline, but to a lesser degree compared with that of deficiency of ASS. Plasma arginine concentrations are normal or slightly elevated. NAGS, N-acetylglutamate synthase; CPS1, Carbamoyl phosphate synthetase 1; OTC, Ornithine transcarbamylase; ASS Argininosuccinate synthetase; ASL, Argininosuccinate lyase; ARG1, Arginase 1. (Adapted from: Zschocke and Hoffmann. Vademecum Metabolicum. 3rd revised edition. 2011. Contributed by Dr. Georg F Hoffmann.)