The first reaction of pyrimidine biosynthesis is the synthesis of carbamoyl-P from HCO3- and the amide nitrogen glutamine by the cytosolic enzyme carbamoyl-P synthetase II. Carbamoyl-P is also recquired for the formation of arginine, in the urea cycle. However in this case it's synthesized by a separate mitochondrial enzyme - carbamoyl synthetase II, wich uses ammonia as its nitrogen source. (3)