TY - CHAP M1 - Book, Section TI - Urea Cycle Disorders A1 - Burrage, Lindsay C. A1 - Lee, Brendan A1 - Nagamani, Sandesh C.S. A2 - Kline, Mark W. PY - 2018 T2 - Rudolph's Pediatrics, 23e AB - Animals excrete excess nitrogen in 3 major forms: ammonia, uric acid, and urea. Mammals are ureotelic organisms, which means that they are dependent on the synthesis and excretion of urea to maintain nitrogen balance. The urea cycle (also called the ornithine cycle), first described in 1932 by Hans Krebs and Kurt Henseleit, converts waste nitrogen derived from dietary protein and amino acid catabolism into urea. The urea cycle consists of a cofactor synthesizing enzyme (N-acetylglutamate synthase [NAGS]), 5 catalytic enzymes (carbamoyl-phosphate synthase 1 [CPS1], ornithine transcarbamylase [OTC], argininosuccinate synthase 1 [ASS1], argininosuccinate lyase [ASL], and arginase 1 [ARG1]), and 2 transporters (mitochondrial aspartate/glutamate carrier SLC25A13 or citrin and mitochondrial ornithine transporter ORNT1 or SLCA15) that facilitate the transfer of nitrogen from ammonia and aspartate to urea (Fig. 141-1). The initial 2 catalytic reactions occur within the mitochondria, whereas the remaining reactions, including the generation of urea, occur in the cytosol. The complete urea cycle exists only in the liver, although certain enzymatic components of the urea cycle (eg, ASS1 and ASL) are also expressed in a variety of other tissues. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/03/28 UR - accesspediatrics.mhmedical.com/content.aspx?aid=1182928800 ER -