In mammals, the offending uric acid is formed as the end-product of metabolism of the purines (both those taken in the diet and those formed normally in the body). In reptiles and birds, uric acid is formed also as the major product of protein and amino acid metabolism (a role performed in mammals by the more soluble urea).
Reptiles and birds excrete uric acid with great efficiency, but it would require only slight impairment of excretion for high blood levels to build up, leading to crystallisation of uric acid in bones and joints, and the development of gout.
DAVID A BENDER
Department of Biochemistry and Molecular Biology
University College London