Decreased xanthine oxidase activities and increased urinary oxypurines in heterozygotes for hereditary xanthinuria.

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Two brothers with hereditary xanthinuria (xanthine oxidase deficiency ) and several members of their family were studied . In both subjects , plasma and urinary concentrations of uric acid were low whereas xanthine and hypoxanthine concentrations were markedly elevated . Xanthine oxidase activity was virtually absent in the patients ' duodenal mucosa , a finding that established the diagnosis of hereditary xanthinuria . In their parents (obligate heterozygotes ), the duodenal xanthine oxidase activity was about 50 % of that in control subjects (father 9 .3 and mother 12 .8 mU /g tissue compared with 21 .3 +/- 5 .0 mU /g tissue , mean +/- SD ). The residual xanthine oxidase from the parents exhibited normal kinetics with respect to hypoxanthine . The parents ' urinary xanthine and hypoxanthine concentrations were significantly greater than those of control subjects , while their plasma concentrations of oxypurines were normal . Similar findings were observed in at least 6 other relatives , a finding that suggested that they were heterozygotes . This study suggests that obligate hereditary xanthinuria heterozygotes have only 50 % of the xanthine oxidase activity of controls ; this deficiency results in a partial metabolic blockage at this enzymatic step in heterozygotes .