Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. The Urea Cycle
The Energetic Cost of Urea Synthesis

The equation above shows that the synthesis of one urea molecule consumes four high-energy phosphate groups. Two ATP molecules are required for The formation of carbamoyl phosphate, and one is needed for the formation of argininosuccinate. However, in the latter reaction, ATP undergoes pyrophosphate Cleavage (Sec. 14.17) to yield AMP and pyrophosphate, which is subsequently hydrolyzed to form two orthophosphate molecules. Consequently, a total of four ATP molecules are consumed to produce a single molecule of urea.

By excreting urea instead of ammonia, ureotelic animals pay a price for this advantage, losing an estimated 15% of the energy of the Amino Acids that serve as the source of this urea. In some ruminants, these energy losses are compensated for to some extent. In cows, for example, a significant portion of urea passes from the Blood into the first compartment of The Stomach (the rumen). The Bacteria living there use it as a source of NH₃ for the synthesis of amino acids, which are then absorbed and utilized by the host's body. In the camel, urea enters the gastrointestinal tract and is recycled in the same manner, sparing the animal the Water loss that would otherwise be inevitable during The excretion of urea in urine. This is one of the biochemical and physiological adaptations that enable the camel to survive on very little water. Neither ruminants nor any other animal groups are capable, on their own without the aid of microorganisms, of using urea as a source of amino groups for Amino acid synthesis. This is because they lack the Enzymes required to hydrolyze or utilize urea.