In animals which inhabit extremely arid environments, ammonia is converted into uric acid. Uric acid is least toxic, relatively insoluble and is easily precipitated. Hence, it can be excreted in solid form without loss of substantial amount of water. Pulmonate snails, terrestrial insects, squamate reptiles (lizards and snakes) and birds excrete a major portion of their waste nitrogen in the form of semi-solid or solid uric acid and hence are referred to as uricotelic animals.
Figure: Uric Acid
The synthesis of uric acid from ammonia in these animals occurs by the inosinic acid pathway. This pathway was first elucidated by Buchanan and his co- workers in the 1950s in pigeon liver. Uric acid is a member of the purines. While the details of the pathway are beyond the scope of this unit, in Figure the different fragments from which the uric acid molecule is assembled is shown. The dashed lines which divide the uric acid molecule in the Figure indicate the building blocks from which its biosynthesis takes place.