Two siblings with classical xanthinuria type 1: significance of allopurinol loading test.

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Two brothers with classical xanthinuria who lacked xanthine dehydrogenase activity were encountered . Their hypouricemia was caused by underproduction of uric acid . In their duodenal mucosa , no xanthine dehydrogenase (oxidase ) activity was detected . The patients had no symptoms except for duodenal ulcer in one case . The conversion of allopurinol to oxipurinol during an allopurinol loading test for determining the type of classical xanthinuria revealed that the patients had classical type 1 xanthinuria , because aldehyde oxidase activity was present . Furthermore , the allopurinol loading test was conducted to determine the optimal examination times and specimens required for this test .