Two male siblings with xanthinuria due to xanthine oxidase deficiency are reported. The propositus, a 29-year-old man, had gastroduodenal ulcer. Routine laboratory studies showed an extremely low serum uric acid level (0.1mg/dl)and decreased urinary excreti on of uric acid (4.9 mg /dl 36.8 mg/day), indicating reduced uric acid synthesis. Similar findings were obtained in his elder brother. Further biochemical analyses showed elevated serum levels of xanthine (proposi tus 0.44 mg /dl and his brother 0.40 mg /dl vs. normal controls < 0.02 mg/dl)and hypoxanthine ( 0.14 mg/dl and 0.10 mg /dl vs.0.03 mg/dl), and concomitantly increased urinary excretions of xanthine (51.2 mg/dl and 29.0 mg/dl vs.1.0 mg/dl)and hypoxanthine (15.5 mg /dl and 9.9 mg /dl vs.0.3 mg/dl). Xanthine oxidase activity in duodenal mucosa extracts was undetectable in the propositus and extremely low in his brother ( 0 and 0.2 mU/g tissue vs. normal controls 20.6mU/g tissue). Thus, the diagnosis of hereditary xanthinuria due to xanthine oxidase deficiency was established.
