TETRAHYDROBIOPTERIN LOADING TEST IN XANTHINE DEHYDROGENASE AND MOLYBDENUM COFACTOR DEFICIENCIES

The objectives of this study were to find additional diagnostic inform ation for the evaluation of xanthine dehydrogenase deficiency and moly bdenum cofactor deficiency. Patients were given an oral loading test w ith 10 mg/kg 5,6,7,8-tetrahydrobiopterin. Urine excretion of pterin an d isoxanthopterin was measured by HPLC. Control subjects had a fairly constant ratio of urinary pterin/isoxanthopterin before (0.57-5.32) an d after (0.55-4.55) 5,6,7,8-tetrahydrobiopterin loading. These ratios were increased to 33 and 22 in a patient with hereditary xanthinuria a nd to 570 and 8030 in a patient with molybdenum cofactor deficiency. O bligate heterozygotes had an entirely normal test result. Evidence was obtained for the in vivo involvement of xanthine dehydrogenase in the conversion of pterin to isoxanthopterin. This test could be a sensiti ve marker for the establishment of residual enzyme activity. (C) 1996 Academic Press, Inc.