Carbamoyl-phosphate synthetase ll occurs in the cytosol of all nucleated cells. Carbomyl-phosphate synthetase I that uses free ammonia (not glutamine) as the source of the nitrogen for urea synthesis (Chapter 13) occurs in liver mitochondria. Carbomyl-phosphate synthetase II is inhibited by uridine diphosphate (UDP) and uridine triphosphate (UTP), end products of pyrimidine biosynthesis. Carbamoyl-phosphate synthetase I is not affected by UDP or UTP (Table 25-3). Carbamoylphosphate synthetase I, but not ll, is actived by N-acetylglutamate.