Ammonia is a universal participant in amino acid synthesis and degradation, but its accumulation has toxic consequences. Because terrestrial animals must conserve water, they convert ammonia to a form that can be excreted without large water losses. Birds, terrestrial reptiles, and insects convert most of their excess ammonia to uric acid, an oxidized purine. Most mammals excrete the bulk of their nitrogen as urea.

Urea is synthesized almost exclusively in the liver and then transported to the kidneys for excretion. The process that generates urea is called the urea cycle and is depicted in Figure 20.13. Urea is synthesized almost exclusively in the liver and then transported to the kidneys for excretion. The enzyme arginase is responsible for the cyclic nature of the urea cycle and for production of urea, as follows:

Arginine + H2O <=> Urea + Ornithine

Virtually all organisms synthesize arginine from ornithine by the reactions shown in Figure 20.13. However, only ureotelic organisms (those excreting most of their nitrogen as urea) contain arginase and, hence, only those organisms carry out the cyclic pathway.