The two nitrogen atoms, that are incorporated into the urea cycle. The source of the nitrogen are

(Aspartate & NH3) (448 - R S 7th) (251 - Lippincott's)UREA CYCLE* One nitrogen of the urea molecule is supplied by free NH3 and other nitrogen by aspartate* Glutamate is the immediate precursor of both ammonia through oxidative deamination by glutamate dehydrogenase) and aspartate nitrogen (through transmination of oxaloacetate by aspartate aminotransferase* Urea is produced by the hydrolysis of arginine* The urea cycle is linked to the TCA cycle through the production of fumarate. Amino acid catabolism is therefore directly coupled to energy production