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71 FAMILY STUDY OF HEREDITARY XANTHINURIA -DECREASED DUODENAL XANTHINE OXIDASE ACTIVITY AND INCREASED URINARY EXCRETION OF XANTHINE AND HYPOXANTHINE IN HETEROZYGOTES

We studied two brothers with hereditary xanthinuria (xanthine oxidase deficiency) and their family members. The two brothers had extremely low concentrations of urate but markedly high concentrations of xanthine and hypoxanthine in plasma and urine. Xanthine oxidase activities were virtually absent in the duodenal mucosa. In their parents (presumed obligate heterozygotes), the activities of xanthine oxidase were about half that of normal subjects. Although plasma xanthine and hypoxanthine concentrations of the parents were normal, urinary xanthine and hypoxanthine excretions were significantly higher than those of normal subjects (xanthine, father 17.1 mg/g creatinine and mother 27.4 vs. normal controls 5.7 to 11.0; hypoxanthine, father 14.0 and mother 27.3 vs. controls 4.0 to 8.4). Similar changes in the metabolite concentrations were seen in at least 6 other relatives, suggesting they were heterozygotes. This study indicates that the presumed obligate heterozygotes of xanthine oxidase deficiency retained about half normal enzyme activities causing the partial metabolic blockage in vivo at this enzyme step.