An end product of purine metabolism in humans and higher primates. Uric acid is excreted as such in the urine, and it is very poorly soluble in aqueous solutions, causing it to crystallize when concentrations of the compound in the urine are abnormally high. This leads to crystalluria (excretion of crystals in urine), hematuria (blood in the urine), infection, or urinary tract stones. Other mammals do not experience these problems, as they express an enzyme, uricase, which catalyzes the conversion of uric acid to allantoin, which is highly soluble. A nonprimate exception is the Dalmatian dog, in which high rates of excretion of uric acid are a consequence of defective renal tubular reabsorption; thus, the uric acid is excreted before it can be oxidized. In birds and reptiles, uric acid is an excretory end product of the metabolism of proteins.