Birds, reptiles, and insects dispose of excess nitrogen atoms by excreting uric acid, rather than urea, sparing these animals the need for large quantities of water to dissolve the urea. humans and other primates also make and excrete uric acid as the catabolic degradation product of purines. uric acid has a very low water solubility. high concentrations in the blood can form insoluble crystals of sodium urate, forming painful stones in the kidneys and ureters. high concentrations in the blood can also lead to painful arthritic joint inflammation, a condition known as gout. the final step in the synthesis of uric acid is the oxidation of xanthine, catalyzed by the enzyme xanthine oxidase. this enzyme is a target for some drugs used to treat gout and a more serious condition of uric acid overproduction called lesch-nyhan syndrome. draw the structures of xanthine and uric acid, the substrate and product of xanthine oxidase.