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. See urea cycle reactions here.

Uric acid is an intermediate in purine nucleotide metabolism (Figure 22.7) and is quite insoluble in water. Consequently, increasing concentrations of it causes it to precipitate as crystals of sodium urate, and cause the painful condition of gout.

Uric acid also has antioxidant properties.